Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

A single bout of aerobic exercise does not alter inhibitory control preparatory set cerebral hemodynamics: Evidence from the antisaccade task

A single bout of exercise improves executive function (EF) and is a benefit - in part -attributed to an exercise-mediated increase in cerebral blood flow enhancing neural efficiency. Limited work has used an event-related protocol to examine postexercise changes in preparatory phase cerebral hemodynamics for an EF task. This is salient given the neural efficiency hypothesis' assertion that improved EF is related to decreased brain activity. Here, event-related transcranial Doppler ultrasound was used to measure pro- (saccade to target) and antisaccades (saccade mirror-symmetrical target) preparatory phase middle cerebral artery velocity (MCAv) prior to and immediately after 15-min of aerobic exercise. Antisaccades produced longer reaction times (RT) and an increased preparatory phase MCAv than prosaccades - a result attributed to greater EF neural activity for antisaccades. Antisaccades selectively produced a postexercise RT reduction (ps < 0.01); however, antisaccade preparatory phase MCAv did not vary from pre- to postexercise (p=0.53) and did not correlate with the antisaccade RT benefit (p = 0.31). Accordingly, results provide no evidence that improved neural efficiency indexed via functional hyperemia is linked to a postexercise EF behavioural benefit. Instead, results support an evolving view that an EF benefit represents the additive interplay between interdependent exercise-mediated neurophysiological changes.

Atrial fibrillation, hypertension, and the cerebral vasodilatory reserve

Schematic representation for the cascade model of atrial fibrillation, hypertension, the cerebral vasodilatory reserve, and cognitive decline.

Loop gain response to increased cerebral blood flow at high altitude

PURPOSE

To compare loop gain (LG) before and during pharmacological increases in cerebral blood flow (CBF) at high altitude (HA). Loop gain (LG) describes stability of a negative-feedback control system; defining the magnitude of response to a disturbance, such as hyperpnea to an apnea in periodic breathing (PB). "Controller-gain" sensitivity from afferent peripheral (PCR) and central-chemoreceptors (CCR) plays a key role in perpetuating PB. Changes in CBF may have a critical role via effects on central chemo-sensitivity during sleep.

METHODS

Polysomnography (PSG) was performed on volunteers after administration of I.V. Acetazolamide (ACZ-10mg/kg) + Dobutamine (DOB-2-5 μg/kg/min) to increase CBF (via Duplex-ultrasound). Central sleep apnea (CSA) was measured from NREM sleep. The duty ratio (DR) was calculated as ventilatory duration (s) divided by cycle duration (s) (hyperpnea/hyperpnea + apnea), LG = 2π/(2πDR-sin2πDR).

RESULTS

A total of 11 volunteers were studied. Compared to placebo-control, ACZ/DOB showed a significant increase in the DR (0.79 ± 0.21 vs 0.52 ± 0.03, P = 0.002) and reduction in LG (1.90 ± 0.23 vs 1.29 ± 0.35, P = 0.0004). ACZ/DOB increased cardiac output (CO) (8.19 ± 2.06 vs 6.58 ± 1.56L/min, P = 0.02) and CBF (718 ± 120 vs 526 ± 110ml/min, P < 0.001). There was no significant change in arterial blood gases, minute ventilation (VE), or hypoxic ventilatory response (HVR). However, there was a reduction of hypercapnic ventilatory response (HCVR) by 29% (5.9 ± 2.7 vs 4.2 ± 2.8 L/min, P = 0.1).

CONCLUSION

Pharmacological elevation in CBF significantly reduced LG and severity of CSA. We speculate the effect was on HCVR "controller gain," rather than "plant gain," because PaCO2 and VE were unchanged. An effect via reduced circulation time is unlikely, as the respiratory-cycle length did not change.

Cerebral autoregulation, spreading depolarization, and implications for targeted therapy in brain injury and ischemia

Cerebral autoregulation is an intrinsic myogenic response of cerebral vasculature that allows for preservation of stable cerebral blood flow levels in response to changing systemic blood pressure. It is effective across a broad range of blood pressure levels through precapillary vasoconstriction and dilation. Autoregulation is difficult to directly measure and methods to indirectly ascertain cerebral autoregulation status inherently require certain assumptions. Patients with impaired cerebral autoregulation may be at risk of brain ischemia. One of the central mechanisms of ischemia in patients with metabolically compromised states is likely the triggering of spreading depolarization (SD) events and ultimately, terminal (or anoxic) depolarization. Cerebral autoregulation and SD are therefore linked when considering the risk of ischemia. In this scoping review, we will discuss the range of methods to measure cerebral autoregulation, their theoretical strengths and weaknesses, and the available clinical evidence to support their utility. We will then discuss the emerging link between impaired cerebral autoregulation and the occurrence of SD events. Such an approach offers the opportunity to better understand an individual patient's physiology and provide targeted treatments.

Cerebrospinal fluid pH regulation

The cerebrospinal fluid (CSF) fills the brain ventricles and the subarachnoid space surrounding the brain and spinal cord. The fluid compartment of the brain ventricles communicates with the interstitial fluid of the brain across the ependyma. In comparison to blood, the CSF contains very little protein to buffer acid-base challenges. Nevertheless, the CSF responds efficiently to changes in systemic pH by mechanisms that are dependent on the CO2/HCO3- buffer system. This is evident from early studies showing that the CSF secretion is sensitive to inhibitors of acid/base transporters and carbonic anhydrase. The CSF is primarily generated by the choroid plexus, which is a well-vascularized structure arising from the pial lining of the brain ventricles. The epithelial cells of the choroid plexus host a range of acid/base transporters, many of which participate in CSF secretion and most likely contribute to the transport of acid/base equivalents into the ventricles. This review describes the current understanding of the molecular mechanisms in choroid plexus acid/base regulation and the possible role in CSF pH regulation.

Circadian and Diurnal Regulation of Cerebral Blood Flow

Circadian and diurnal variation in cerebral blood flow directly contributes to the diurnal variation in the risk of stroke, either through factors that trigger stroke or due to impaired compensatory mechanisms. Cerebral blood flow results from the integration of systemic hemodynamics, including heart rate, cardiac output, and blood pressure, with cerebrovascular regulatory mechanisms, including cerebrovascular reactivity, autoregulation, and neurovascular coupling. We review the evidence for the circadian and diurnal variation in each of these mechanisms and their integration, from the detailed evidence for mechanisms underlying the nocturnal nadir and morning surge in blood pressure to identifying limited available evidence for circadian and diurnal variation in cerebrovascular compensatory mechanisms. We, thus, identify key systemic hemodynamic factors related to the diurnal variation in the risk of stroke but particularly identify the need for further research focused on cerebrovascular regulatory mechanisms.

The conducted vascular response as a mediator of hypercapnic cerebrovascular reactivity: A modelling study

It is well established that the cerebral blood flow (CBF) shows exquisite sensitivity to changes in the arterial blood partial pressure of CO2 ( [Formula: see text] ), which is reflected by an index termed cerebrovascular reactivity. In response to elevations in [Formula: see text] (hypercapnia), the vessels of the cerebral microvasculature dilate, thereby decreasing the vascular resistance and increasing CBF. Due to the challenges of access, scale and complexity encountered when studying the microvasculature, however, the mechanisms behind cerebrovascular reactivity are not fully understood. Experiments have previously established that the cholinergic release of the Acetylcholine (ACh) neurotransmitter in the cortex is a prerequisite for the hypercapnic response. It is also known that ACh functions as an endothelial-dependent agonist, in which the local administration of ACh elicits local hyperpolarization in the vascular wall; this hyperpolarization signal is then propagated upstream the vascular network through the endothelial layer and is coupled to a vasodilatory response in the vascular smooth muscle (VSM) layer in what is known as the conducted vascular response (CVR). Finally, experimental data indicate that the hypercapnic response is more strongly correlated with the CO2 levels in the tissue than in the arterioles. Accordingly, we hypothesize that the CVR, evoked by increases in local tissue CO2 levels and a subsequent local release of ACh, is responsible for the CBF increase observed in response to elevations in [Formula: see text] . By constructing physiologically grounded dynamic models of CBF and control in the cerebral vasculature, ones that integrate the available knowledge and experimental data, we build a new model of the series of signalling events and pathways underpinning the hypercapnic response, and use the model to provide compelling evidence that corroborates the aforementioned hypothesis. If the CVR indeed acts as a mediator of the hypercapnic response, the proposed mechanism would provide an important addition to our understanding of the repertoire of metabolic feedback mechanisms possessed by the brain and would motivate further in-vivo investigation. We also model the interaction of the hypercapnic response with dynamic cerebral autoregulation (dCA), the collection of mechanisms that the brain possesses to maintain near constant CBF despite perturbations in pressure, and show how the dCA mechanisms, which otherwise tend to be overlooked when analysing experimental results of cerebrovascular reactivity, could play a significant role in shaping the CBF response to elevations in [Formula: see text] . Such in-silico models can be used in tandem with in-vivo experiments to expand our understanding of cerebrovascular diseases, which continue to be among the leading causes of morbidity and mortality in humans.

Long-term effects of multiple concussions on prefrontal cortex oxygenation during a hypercapnic challenge in retired contact sport athletes

This exploratory study aimed to investigate the long-term effects of multiple concussions on prefrontal cortex oxygenation during a five-minute hypercapnic challenge using Near Infrared Spectroscopy (NIRS). 55 physically active retired contact sport male athletes with three or more previous concussions (mTBI) were recruited along with 29 physically active males with no concussions history (CTRL). Participants completed five minutes of seated rest prior to the five-minute hypercapnic challenge (20-second breath-hold, 40-second recovery breathing; five times). NIRS measured right and left side oxygenated (O2Hb), deoxygenated (HHb), total (tHb) haemoglobin, and haemoglobin difference (HbDiff) with all parameters analysed through changes in average maximal and minimal values (ΔMAX), Z-scores, and standard deviations. Right prefrontal cortex HbDiff ΔMAX was significantly higher in the mTBI compared to CTRL (p = 0.045) group. Left prefrontal cortex O2Hb ΔMAX (p = 0.040), HHb Z-Scores (p = 0.008), and HbDiff ΔMAX(p = 0.014) were significantly higher in the mTBI group. Within-group right vs left analyses demonstrated significantly lower left HbDiff ΔMAX (p = 0.048) and HbDiff Z-scores (p = 0.002) in the mTBI group, while the CTRL group had significantly lower left HHb Z-scores (p = 0.003) and left tHb Z-scores (p = 0.042). This study provides preliminary evidence that athletes with a history of three or more concussions may have impaired prefrontal cortex oxygenation parameters during a hypercapnic challenge.

Thermodynamic limitations on brain oxygen metabolism: physiological implications

Recent thermodynamic modelling indicates that maintaining the brain tissue ratio of O2 to CO2 (abbreviated tissue O2 /CO2 ) is critical for preserving the entropy increase available from oxidative metabolism of glucose, with a fall of that available entropy leading to a reduction of the phosphorylation potential and impairment of brain energy metabolism. This provides a novel perspective for understanding physiological responses under different conditions in terms of preserving tissue O2 /CO2 . To enable estimation of tissue O2 /CO2 in the human brain, a detailed mathematical model of O2 and CO2 transport was developed, and applied to reported physiological responses to different challenges, asking: how well is tissue O2 /CO2 preserved? Reported experimental results for increased neural activity, hypercapnia and hypoxia due to high altitude are consistent with preserving tissue O2 /CO2 . The results highlight two physiological mechanisms that control tissue O2 /CO2 : cerebral blood flow, which modulates tissue O2 ; and ventilation rate, which modulates tissue CO2 . The hypoxia modelling focused on humans at high altitude, including acclimatized lowlanders and Tibetan and Andean adapted populations, with a primary finding that decreasing CO2 by increasing ventilation rate is more effective for preserving tissue O2 /CO2 than increasing blood haemoglobin content to maintain O2 delivery to tissue. This work focused on the function served by particular physiological responses, and the underlying mechanisms require further investigation. The modelling provides a new framework and perspective for understanding how blood flow and other physiological factors support energy metabolism in the brain under a wide range of conditions. KEY POINTS: Thermodynamic modelling indicates that preserving the O2 /CO2 ratio in brain tissue is critical for preserving the entropy change available from oxidative metabolism of glucose and the phosphorylation potential underlying energy metabolism. A detailed model of O2 and CO2 transport was developed to allow estimation of the tissue O2 /CO2 ratio in the human brain in different physiological states. Reported experimental results during hypoxia, hypercapnia and increased oxygen metabolic rate in response to increased neural activity are consistent with maintaining brain tissue O2 /CO2 ratio. The hypoxia modelling of high-altitude acclimatization and adaptation in humans demonstrates the critical role of reducing CO2 with increased ventilation for preserving tissue O2 /CO2 . Preservation of tissue O2 /CO2 provides a novel perspective for understanding the function of observed physiological responses under different conditions in terms of preserving brain energy metabolism, although the mechanisms underlying these functions are not well understood.

10-min exposure to a 2.5% hypercapnic environment increases cerebral blood blow but does not impact executive function

Space travel and exploration are associated with increased ambient CO2 (i.e., a hypercapnic environment). Some work reported that the physiological changes (e.g., increased cerebral blood flow [CBF]) associated with a chronic hypercapnic environment contributes to a "space fog" that adversely impacts cognition and psychomotor performance, whereas other work reported no change or a positive change. Here, we employed the antisaccade task to evaluate whether transient exposure to a hypercapnic environment influences top-down executive function (EF). Antisaccades require a goal-directed eye movement mirror-symmetrical to a target and are an ideal tool for identifying subtle EF changes. Healthy young adults (aged 19-25 years) performed blocks of antisaccade trials prior to (i.e., pre-intervention), during (i.e., concurrent) and after (i.e., post-intervention) 10-min of breathing factional inspired CO2 (FiCO2) of 2.5% (i.e., hypercapnic condition) and during a normocapnic (i.e., control) condition. In both conditions, CBF, ventilatory and cardiorespiratory responses were measured. Results showed that the hypercapnic condition increased CBF, ventilation and end-tidal CO2 and thus demonstrated an expected physiological adaptation to increased FiCO2. Notably, however, null hypothesis and equivalence tests indicated that concurrent and post-intervention antisaccade reaction times were refractory to the hypercapnic environment; that is, transient exposure to a FiCO2 of 2.5% did not produce a real-time or lingering influence on an oculomotor-based measure of EF. Accordingly, results provide a framework that - in part - establishes the FiCO2 percentage and timeline by which high-level EF can be maintained. Future work will explore CBF and EF dynamics during chronic hypercapnic exposure as more direct proxy for the challenges of space flight and exploration.

Heart-brain interaction in cardiogenic dementia: pathophysiology and therapeutic potential

Diagnosis and treatment of patients with cardiovascular and neurologic diseases primarily focus on the heart and brain, respectively. An increasing number of preclinical and clinical studies have confirmed a causal relationship between heart and brain diseases. Cardiogenic dementia is a cognitive impairment caused by heart dysfunction and has received increasing research attention. The prevention and treatment of cardiogenic dementia are essential to improve the quality of life, particularly in the elderly and aging population. This study describes the changes in cognitive function associated with coronary artery disease, myocardial infarction, heart failure, atrial fibrillation and heart valve disease. An updated understanding of the two known pathogenic mechanisms of cardiogenic dementia is presented and discussed. One is a cascade of events caused by cerebral hypoperfusion due to long-term reduction of cardiac output after heart disease, and the other is cognitive impairment regardless of the changes in cerebral blood flow after cardiac injury. Furthermore, potential medications for the prevention and treatment of cardiogenic dementia are reviewed, with particular attention to multicomponent herbal medicines.

Exposure to passive heat and cold stress differentially modulates cerebrovascular-CO2 responsiveness

Heat and cold stress influence cerebral blood flow (CBF) regulatory factors (e.g., arterial CO2 partial pressure). However, it is unclear whether the CBF response to a CO2 stimulus (i.e., cerebrovascular-CO2 responsiveness) is maintained under different thermal conditions. This study aimed to compare cerebrovascular-CO2 responsiveness between normothermia, passive heat, and cold stress conditions. Sixteen participants (8 females; 25 ± 7 yr) completed two experimental sessions (randomized) comprising normothermic and either passive heat or cold stress conditions. Middle and posterior cerebral artery velocity (MCAv, PCAv) were measured during rest, hypercapnia (5% CO2 inhalation), and hypocapnia (voluntary hyperventilation to an end-tidal CO2 of 30 mmHg). The linear slope of the cerebral blood velocity (CBv) response to changing end-tidal CO2 was calculated to measure cerebrovascular-CO2 responsiveness, and cerebrovascular conductance (CVC) was used to examine responsiveness independent of blood pressure. CBv-CVC-CO2 responsiveness to hypocapnia was greater during heat stress compared with cold stress (MCA: +0.05 ± 0.08 cm/s/mmHg/mmHg, P = 0.04; PCA: +0.02 ± 0.02 cm/s/mmHg/mmHg, P = 0.002). CBv-CO2 responsiveness to hypercapnia decreased during heat stress (MCA: -0.67 ± 0.89 cm/s/mmHg, P = 0.02; PCA: -0.64 ± 0.62 cm/s/mmHg; P = 0.01) and increased during cold stress (MCA: +0.98 ± 1.33 cm/s/mmHg, P = 0.03; PCA: +1.00 ± 0.82 cm/s/mmHg; P = 0.01) compared with normothermia. However, CBv-CVC-CO2 responsiveness to hypercapnia was not different between thermal conditions (P > 0.08). Overall, passive heat, but not cold, stress challenges the maintenance of cerebral perfusion. A greater cerebrovascular responsiveness to hypocapnia during heat stress likely reduces an already impaired cerebrovascular reserve capacity and may contribute to adverse events (e.g., syncope).NEW & NOTEWORTHY This study demonstrates that thermoregulatory-driven perfusion pressure changes, from either cold or heat stress, impact cerebrovascular responsiveness to hypercapnia. Compared with cold stress, heat stress poses a greater challenge to the maintenance of cerebral perfusion during hypocapnia, challenging cerebrovascular reserve capacity while increasing cerebrovascular-CO2 responsiveness. This likely exacerbates cerebral hypoperfusion during heat stress since hyperthermia-induced hyperventilation results in hypocapnia. No regional differences in middle and posterior cerebral artery responsiveness were found with thermal stress.

Update on vascular control of central chemoreceptors

At least four mechanisms have been proposed to elucidate how neurons in the retrotrapezoid (RTN) region sense changes in CO2 /H+ to regulate breathing (i.e., function as respiratory chemosensors). These mechanisms include: (1) intrinsic neuronal sensitivity to H+ mediated by TASK-2 and GPR4; (2) paracrine activation of RTN neurons by CO2 -responsive astrocytes (via a purinergic mechanism); (3) enhanced excitatory synaptic input or disinhibition; and (4) CO2 -induced vascular contraction. Although blood flow can influence tissue CO2 /H+ levels, there is limited understanding of how control of vascular tone in central CO2 chemosensitive regions might contribute to respiratory output. In this review, we focus on recent evidence that CO2 /H+ -induced purinergic-dependent vasoconstriction in the ventral parafacial region near RTN neurons supports respiratory chemoreception. This mechanism appears to be unique to the ventral parafacial region and opposite to other brain regions, including medullary chemosensor regions, where CO2 /H+ elicits vasodilatation. We speculate that this mechanism helps to maintain CO2 /H+ levels in the vicinity of RTN neurons, thereby maintaining the drive to breathe. Important next steps include determining whether disruption of CO2 /H+ vascular reactivity contributes to or can be targeted to improve breathing problems in disease states, such as Parkinson's disease.

Leptin signaling in the dorsomedial hypothalamus couples breathing and metabolism in obesity

Mismatch between CO2 production (Vco2) and respiration underlies the pathogenesis of obesity hypoventilation. Leptin-mediated CNS pathways stimulate both metabolism and breathing, but interactions between these functions remain elusive. We hypothesized that LEPRb+ neurons of the dorsomedial hypothalamus (DMH) regulate metabolism and breathing in obesity. In diet-induced obese LeprbCre mice, chemogenetic activation of LEPRb+ DMH neurons increases minute ventilation (Ve) during sleep, the hypercapnic ventilatory response, Vco2, and Ve/Vco2, indicating that breathing is stimulated out of proportion to metabolism. The effects of chemogenetic activation are abolished by a serotonin blocker. Optogenetic stimulation of the LEPRb+ DMH neurons evokes excitatory postsynaptic currents in downstream serotonergic neurons of the dorsal raphe (DR). Administration of retrograde AAV harboring Cre-dependent caspase to the DR deletes LEPRb+ DMH neurons and abolishes metabolic and respiratory responses to leptin. These findings indicate that LEPRb+ DMH neurons match breathing to metabolism through serotonergic pathways to prevent obesity-induced hypoventilation.

Detection of Blood CO2 Influences on Cerebral Hemodynamics Using Transfer Entropy

Cerebral hemodynamics describes an important physiological system affected by components such as blood pressure, CO2 levels, and endothelial factors. Recently, novel techniques have emerged to analyse cerebral hemodynamics based on the calculation of entropies, which quantifies or describes changes in the complexity of this system when it is affected by a pathological or physiological influence. One recently described measure is transfer entropy, which allows for the determination of causality between the various components of a system in terms of their flow of information, and has shown positive results in the multivariate analysis of physiological signals. This study aims to determine whether conditional transfer entropy reflects the causality in terms of entropy generated by hypocapnia on cerebral hemodynamics. To achieve this, non-invasive signals from 28 healthy individuals who undertook a hyperventilation maneuver were analyzed using conditional transfer entropy to assess the variation in the relevance of CO2 levels on cerebral blood velocity. By employing a specific method to discretize the signals, it was possible to differentiate the influence of CO2 levels during the hyperventilation phase (22.0% and 20.3% increase for the left and right hemispheres, respectively) compared to normal breathing, which remained higher during the recovery phase (15.3% and 15.2% increase, respectively).

Impaired cerebrovascular CO2 reactivity at high altitude in prematurely born adults

Premature birth impairs cardiac and ventilatory responses to both hypoxia and hypercapnia, but little is known about cerebrovascular responses. Both at sea level and after 2 days at high altitude (3375 m), 16 young preterm-born (gestational age, 29 ± 1 weeks) and 15 age-matched term-born (40 ± 0 weeks) adults were exposed to two consecutive 4 min bouts of hyperoxic hypercapnic conditions (3% CO2 -97% O2 ; 6% CO2 -94% O2 ), followed by two periods of voluntary hyperventilation-induced hypocapnia. We measured middle cerebral artery blood velocity, end-tidal CO2 , pulmonary ventilation, beat-by-beat mean arterial pressure and arterialized capillary blood gases. Baseline middle cerebral artery blood velocity increased at high altitude compared with sea level in term-born (+24 ± 39%, P = 0.036), but not in preterm-born (-4 ± 27%, P = 0.278) adults. The end-tidal CO2 , pulmonary ventilation and mean arterial pressure were similar between groups at sea level and high altitude. Hypocapnic cerebrovascular reactivity was higher at high altitude compared with sea level in term-born adults (+173 ± 326%, P = 0.026) but not in preterm-born adults (-21 ± 107%, P = 0.572). Hypercapnic reactivity was altered at altitude only in preterm-born adults (+125 ± 144%, P < 0.001). Collectively, at high altitude, term-born participants showed higher hypocapnic (P = 0.012) and lower hypercapnic (P = 0.020) CO2 reactivity compared with their preterm-born peers. In conclusion, exposure to high altitude revealed different cerebrovascular responses in preterm- compared with term-born adults, despite similar ventilatory responses. These findings suggest a blunted cerebrovascular response at high altitude in preterm-born adults, which might predispose these individuals to an increased risk of high-altitude illnesses. KEY POINTS: Cerebral haemodynamics and cerebrovascular reactivity in normoxia are known to be similar between term-born and prematurely born adults. In contrast, acute exposure to high altitude unveiled different cerebrovascular responses to hypoxia, hypercapnia and hypocapnia. In particular, cerebral vasodilatation was impaired in prematurely born adults, leading to an exaggerated cerebral vasoconstriction. Cardiovascular and ventilatory responses to both hypo- and hypercapnia at sea level and at high altitude were similar between control subjects and prematurely born adults. Other mechanisms might therefore underlie the observed blunted cerebral vasodilatory responses in preterm-born adults at high altitude.

Effects of Pre-Exercise Voluntary Hyperventilation on Metabolic and Cardiovascular Responses During and After Intense Exercise

Purpose: We investigated the effects of pre-exercise voluntary hyperventilation and the resultant hypocapnia on metabolic and cardiovascular responses during and after high-intensity exercise. Methods: Ten healthy participants performed a 60-s cycling exercise at a workload of 120% peak oxygen uptake in control (spontaneous breathing), hypocapnia and normocapnia trials. Hypocapnia was induced through 20-min pre-exercise voluntary hyperventilation. In the normocapnia trial, voluntary hyperpnea was performed with CO2 inhalation to prevent hypocapnia. Results: Pre-exercise end-tidal CO2 partial pressure was lower in the hypocapnia trial than the control or normocapnia trial, with similar levels in the control and normocapnia trials. Average V ˙ O 2 during the entire exercise was lower in both the hypocapnia and normocapnia trials than in the control trial (1491 ± 252vs.1662 ± 169vs.1806 ± 149 mL min-1), with the hypocapnia trial exhibiting a greater reduction than the normocapnia trial. Minute ventilation during exercise was lower in the hypocapnia trial than the normocapnia trial. In addition, minute ventilation during the first 10s of the exercise was lower in the normocapnia than the control trial. Pre-exercise hypocapnia also reduced heart rates and arterial blood pressures during the exercise relative to the normocapnia trial, a response that lasted through the subsequent early recovery periods, though end-tidal CO2 partial pressure was similar in the two trials. Conclusions: Our results suggest that pre-exercise hyperpnea and the resultant hypocapnia reduce V ˙ O 2 during high-intensity exercise. Moreover, hypocapnia may contribute to voluntary hyperventilation-mediated cardiovascular responses during the exercise, and this response can persist into the subsequent recovery period, despite the return of arterial CO2 pressure to the normocapnic level.

Cerebrovascular Function is Altered in Hemodialysis Patients

Key Points

Hemodialysis patients have impaired cerebrovascular reactivity. Hemodialysis patients have cerebral structural deficits.

Background

Hemodialysis patients have declines in cerebral blood flow (CBF) and cerebral oxygenation during hemodialysis that may lead to ischemic brain injury. Cerebrovascular reactivity (CVR) may indicate which individuals are more susceptible to intradialytic hypoperfusion and ischemia. We hypothesized that hemodialysis patients would have decreased CVR and increased CBF relative to controls and deficits in CVR would be related to brain structural deficits.

Methods

We measured cortical thickness and white matter hyperintensity (WMH) volume from T1 and T2 fluid attenuation inversion recovery images, respectively; CVR from a breath hold blood oxygen level–dependent CVR functional magnetic resonance imaging (fMRI); and arterial transit time and CBF from arterial spin labeling. Cerebrovascular and structural deficits in gray matter and white matter (GM and WM) were tested by averaging across the tissue and with a pothole analysis. Finally, we correlated cortical thickness and WMH volume with GM and WM cerebrovascular variables to assess the relationship between brain structure and cerebrovascular health.

Results

In ten hemodialysis patients, cortical thickness was found to be decreased (P = 0.002), WMH volume increased (P = 0.004), and WM CBF increased (P = 0.02) relative to ten controls. Pothole analysis indicated a higher number of increased GM and WM CBF voxels (P = 0.03, P = 0.02) and a higher number of decreased GM and WM CVR voxels (P = 0.02, P = 0.01).

Conclusions

This pilot study demonstrates that hemodialysis patients have decreased CVR and increased CBF relative to controls, along with reduced brain integrity. Further investigation is required to fully understand whether these cerebrovascular deficits may lead to structural changes.

High ventilation breathwork practices: An overview of their effects, mechanisms, and considerations for clinical applications

High Ventilation Breathwork (HVB) refers to practices employing specific volitional manipulation of breathing, with a long history of use to relieve various forms of psychological distress. This paper seeks to offer a consolidative insight into potential clinical application of HVB as a treatment of psychiatric disorders. We thus review the characteristic phenomenological and neurophysiological effects of these practices to inform their mechanism of therapeutic action, safety profiles and future clinical applications. Clinical observations and data from neurophysiological studies indicate that HVB is associated with extraordinary changes in subjective experience, as well as with profound effects on central and autonomic nervous systems functions through modulation of neurometabolic parameters and interoceptive sensory systems. This growing evidence base may guide how the phenomenological effects of HVB can be understood, and potentially harnessed in the context of such volitional perturbation of psychophysiological state. Reports of putative beneficial effects for trauma-related, affective, and somatic disorders invite further research to obtain detailed mechanistic knowledge, and rigorous clinical testing of these potential therapeutic uses.

Peritoneal dialysis and acute kidney injury in acute brain injury patients

Acute kidney injury (AKI) is a heterogeneous syndrome with multiple etiologies. It occurs frequently in the neurocritical intensive care unit and is associated with greater morbidity and mortality. In this scenario, AKI alters the kidney-brain axis, exposing patients who receive habitual dialytic management to greater injury. Various therapies have been designed to mitigate this risk. Priority has been placed by KDIGO guidelines on the use of continuous over intermittent acute kidney replacement therapies (AKRT). On this background, continuous therapies have a pathophysiological rationale in patients with acute brain injury. A low-efficiency therapy such as PD and CRRT could achieve optimal clearance control and potentially reduce the risk of secondary brain injury. Therefore, this work will review the evidence on peritoneal dialysis as a continuous AKRT in neurocritical patients, describing its benefits and risks so it may be considered as an option when deciding among available therapeutic options.

Mild and moderate chronic hypercapnia elicit distinct transcriptomic responses of immune function in cardiorespiratory nuclei

Chronic hypercapnia (CH) is a hallmark of respiratory-related diseases, and the level of hypercapnia can acutely or progressively become more severe. Previously, we have shown time-dependent adaptations in steady-state physiology during mild (arterial Pco2 ∼55 mmHg) and moderate (∼60 mmHg) CH in adult goats, including transient (mild CH) or sustained (moderate CH) suppression of acute chemosensitivity suggesting limitations in adaptive respiratory control mechanisms as the level of CH increases. Changes in specific markers of glutamate receptor plasticity, interleukin-1ß, and serotonergic modulation within key nodes of cardiorespiratory control do not fully account for the physiological adaptations to CH. Here, we used an unbiased approach (bulk tissue RNA sequencing) to test the hypothesis that mild or moderate CH elicits distinct gene expression profiles in important brain stem regions of cardiorespiratory control, which may explain the contrasting responses to CH. Gene expression profiles from the brain regions validated the accuracy of tissue biopsy methodology. Differential gene expression analyses revealed greater effects of CH on brain stem sites compared with the medial prefrontal cortex. Mild CH elicited an upregulation of predominantly immune-related genes and predicted activation of immune-related pathways and functions. In contrast, moderate CH broadly led to downregulation of genes and predicted inactivation of cellular pathways related to the immune response and vascular function. These data suggest that mild CH leads to a steady-state activation of neuroinflammatory pathways within the brain stem, whereas moderate CH drives the opposite response. Transcriptional shifts in immune-related functions may underlie the cardiorespiratory network's capability to respond to acute, more severe hypercapnia when in a state of progressively increased CH.NEW & NOTEWORTHY Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Transient intracranial pressure elevations (B waves) are associated with sleep apnea

BACKGROUND

Repetitive transient intracranial pressure waveform elevations up to 50 mmHg (ICP B-waves) are often used to define pathological conditions and determine indications for ICP-reducing treatment. We recently showed that nocturnal transient ICP elevations are present in patients without structural brain lesions or hydrocephalus in whom they are associated with sleep apnea. However, whether this signifies a general association between ICP macropatterns and sleep apnea remains unknown.

METHODS

We included 34 patients with hydrocephalus, or idiopathic intracranial hypertension (IIH), who were referred to the Neurosurgical Department, Copenhagen, Denmark, from 2017 to 2021. Every patient underwent diagnostic overnight ICP monitoring for clinical indications, with simultaneous polysomnography (PSG) sleep studies. All transient ICP elevations were objectively quantified in all patients. Three patients were monitored with continuous positive airway pressure (CPAP) treatment for an additional night.

RESULTS

All patients had transient ICP elevations associated with sleep apnea. The mean temporal delay from sleep apnea to transient ICP elevations for all patients was 3.6 s (SEM 0.2 s). Ramp-type transient ICP elevations with a large increase in ICP were associated with rapid eye movement (REM) sleep and sinusoidal-type elevations with non-REM (NREM) sleep. In three patients treated with CPAP, the treatment reduced the number of transient ICP elevations with a mean of 37%. CPAP treatment resulted in insignificant changes in the average ICP in two patients but elevated the average ICP during sleep in one patient by 5.6 mmHg.

CONCLUSION

The findings suggest that sleep apnea causes a significant proportion of transient ICP elevations, such as B-waves, and sleep apnea should be considered in ICP evaluation. Treatment of sleep apnea with CPAP can reduce the occurrence of transient ICP elevations. More research is needed on the impact of slow oscillating mechanisms on transient ICP elevations during high ICP and REM sleep.

Carbohydrate ingestion attenuates the reduction in complex cognitive function and cerebral blood flow during prolonged passive heat stress in humans

PURPOSE

To determine whether carbohydrate ingestion would reduce cognitive dysfunction in humans following long duration passive heat stress (PHS) versus consuming electrolytes alone.

METHODS

Fifteen young (27 ± 4 y) healthy adults were exposed to 120 min of PHS through the use of a liquid perfused suit (50 °C) on two randomized visits. Subjects consumed fluids supplemented with electrolytes (E) or electrolytes + carbohydrates (E + C). Pre- and post-heat stress, body mass (BM) and plasma osmolality (pOsm) were measured. Heart rate (HR), blood pressure (BP), Physiological Strain Index (PSI), core temperature (Tc), plasma glucose, respiration rate (RR), end-tidal CO2 (PetCO2) and internal carotid artery (ICA) blood flow were recorded at baseline and every 15 min of heat stress. Cognitive function was assessed via the Automated Neuropsychological Assessment Metric at baseline and at 30- and 120 min during heat stress.

RESULTS

There were no significant differences between fluid conditions for BM, pOsm, PSI, Tc, RR or PetCO2. Plasma glucose was ∼75% greater in the E + C condition compared to the E condition after 90 min of PHS (P < 0.05). Cognitive function (120 min) was impaired following PHS only in E condition (P < 0.05) and performance on complex cognitive tasks were better by ∼22-340% in the E + C vs. E (P < 0.05). Compared to the E condition, HR and BP were lower and ICA blood flow, vascular conductance, and glucose delivery was ∼90% greater in the E + C after 90 min of PHS (P < 0.05).

CONCLUSIONS

These data are the first to demonstrate that carbohydrate ingestion may have a protective effect on cognitive function during long duration PHS. Furthermore, this protection was associated with preserved ICA blood flow and glucose delivery to the brain.

Evaluation of Potential Effects of Increased Outdoor Temperatures Due to Global Warming on Cerebral Blood Flow Rate and Respiratory Function in Chronic Obstructive Disease and Anemia

Global warming due to increased outdoor carbon dioxide (CO2) levels may cause several health problems such as headaches, cognitive impairment, or kidney dysfunction. It is predicted that further increases in CO2 levels will increase the morbidity and mortality of patients affected by a variety of diseases. For instance, patients with Chronic Obstructive Pulmonary Disease (COPD) may suffer cognitive impairments or intracranial bleeding due to an increased cerebral blood flow rate. Predicting the harmful effects of global warming on human health will help to take measures for potential problems. Therefore, the quantification of physiological parameters is an essential step to investigate the effects of global warming on human health. In this study, the effects of increased outdoor temperatures due to climate change on cerebral blood flow rate and respiratory function in healthy subjects and COPD patients with anemia and respiratory acidosis are evaluated utilizing numerical simulations. The numerical model simulates cardiac function and blood circulation in systemic, pulmonary and cerebral circulations, cerebral autoregulatory functions, respiratory function, alveolar gas exchange, oxygen (O2) and CO2 contents, and hemoglobin levels in the blood. The simulation results show that although the cardiovascular function is not significantly altered, the respiratory function and cerebral blood flow rates are altered remarkably.

Peripheral and Cerebral Vasodilation in Black and White Women: Examining the Impact of Psychosocial Stress Exposure Versus Internalization and Coping

BACKGROUND

Black women have among the highest rates of cardiovascular and cerebrovascular disease prevalence and mortality in part due to blunted vascular function. Psychosocial stress likely also contributes but its relationship to vascular function remains incompletely understood. Recent studies suggest that stress internalization and coping strategies are more important than stress exposures alone. We hypothesized that Black women would have blunted peripheral and cerebral vasodilation and that, among Black women, this would be inversely related with psychosocial stress internalization/coping but not stress exposures.

METHODS

Healthy Black (n=21; 20±2 years) and White (n=16; 25±7 years) women underwent testing for forearm reactive hyperemia, brachial artery flow-mediated dilation (FMD), and cerebrovascular reactivity. Psychosocial stress exposures (adverse childhood experiences; past week discrimination) and internalization/coping techniques (John Henryism Active Coping Scale; Giscombe Superwoman Schema Questionnaire) were assessed.

RESULTS

Reactive hyperemia and cerebrovascular reactivity were not different between groups (P>0.05), whereas FMD was lower in Black women (P=0.007). Neither adverse childhood experiences nor past week discrimination were associated with FMD in either group (P>0.05 for all). John Henryism Active Coping Scale scores were negatively associated with FMD in Black women (P=0.014) but positively associated with FMD in White women (P=0.042). Superwoman Schema-Succeed was negatively associated (P=0.044) and Superwoman Schema-Vulnerable tended to be negatively associated (P=0.057) with FMD in Black women.

CONCLUSIONS

These findings indicate that blunted FMD in Black women may be due more to stress internalization and maladaptive coping than stress exposures alone.

A test of the interaction between central and peripheral respiratory chemoreflexes in humans

How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured the peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic CO2 tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Twenty participants (10F) completed three repetitions of modified rebreathing tests with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) clamped at 150, 70, 60 and 45 mmHg. End-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ),P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , ventilation (V ̇ $\dot{V}$ E ) and calculated oxygen saturation (SC O2 ) were measured breath-by-breath by gas-analyser and pneumotach. TheV ̇ $\dot{V}$ E -P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship of repeat-trials were linear-interpolated, combined, averaged into 1 mmHg bins, and fitted with a double-linear function (V ̇ $\dot{V}$ E S, L min-1 mmHg-1 ). PChS was computed at intervals of 1 mmHg ofP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding SC O2 ; and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). These processing steps were repeated at eachP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ to produce the PChS vs. isocapnicP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship. These were fitted with linear and polynomial functions, and Akaike information criterion identified the best-fit model. One-way repeated measures analysis of variance assessed between-condition differences.V ̇ $\dot{V}$ E S increased (P < 0.0001) with isoxicP ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ from 3.7 ± 1.5 L min-1 mmHg-1 at 150 mmHg to 4.4 ± 1.8, 5.0 ± 1.6 and 6.0 ± 2.2 Lmin-1 mmHg-1 at 70, 60 and 45 mmHg, respectively. Mean SC O2 fell progressively (99.3 ± 0%, 93.7 ± 0.1%, 90.4 ± 0.1% and 80.5 ± 0.1%; P < 0.0001). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear model in 75%. Despite increasing central chemoreflex activation, PChS increased linearly withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ indicative of an additive central-peripheral chemoreflex response. KEY POINTS: How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic carbon dioxide tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Participants performed three repetitions of modified rebreathing with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fixed at 150, 70, 60 and 45 mmHg. PChS was computed at intervals of 1 mmHg of end-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding calculated oxygen saturation (SC O2 ); and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear (rather than polynomial) model in 15 of 20. Most participants did not exhibit a hypo- or hyper-additive effect of central chemoreceptors on the peripheral chemoreflex indicating that the interaction was additive.

The jugular venous-to-arterial P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ difference during rebreathing and end-tidal forcing: Relationship with cerebral perfusion

We examined two assumptions of the modified rebreathing technique for the assessment of the ventilatory central chemoreflex (CCR) and cerebrovascular CO2 reactivity (CVR), hypothesizing: (1) that rebreathing abolishes the gradient between the partial pressures of arterial and brain tissue CO2 [measured via the surrogate jugular venousP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ and arterialP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ difference (Pjv-a CO2 )] and (2) rebreathing eliminates the capacity of CVR to influence the Pjv-a CO2 difference, and thus affect CCR sensitivity. We also evaluated these variables during two separate dynamic end-tidal forcing (ETF) protocols (termed: ETF-1 and ETF-2), another method of assessing CCR sensitivity and CVR. Healthy participants were included in the rebreathing (n = 9), ETF-1 (n = 11) and ETF-2 (n = 10) protocols and underwent radial artery and internal jugular vein (advanced to jugular bulb) catheterization to collect blood samples. Transcranial Doppler ultrasound was used to measure middle cerebral artery blood velocity (MCAv). The Pjv-a CO2 difference was not abolished during rebreathing (6.2 ± 2.6 mmHg; P < 0.001), ETF-1 (9.3 ± 1.5 mmHg; P < 0.001) or ETF-2 (8.6 ± 1.4 mmHg; P < 0.001). The Pjv-a CO2 difference did not change during the rebreathing protocol (-0.1 ± 1.2 mmHg; P = 0.83), but was reduced during the ETF-1 (-3.9 ± 1.1 mmHg; P < 0.001) and ETF-2 (-3.4 ± 1.2 mmHg; P = 0.001) protocols. Overall, increases in MCAv were associated with reductions in the Pjv-a CO2 difference during ETF (-0.095 ± 0.089 mmHg cm-1  s-1 ; P = 0.001) but not during rebreathing (-0.028 ± 0.045 mmHg · cm-1  · s-1 ; P = 0.067). These findings suggest that, although the Pjv-a CO2 is not abolished during any chemoreflex assessment technique, hyperoxic hypercapnic rebreathing is probably more appropriate to assess CCR sensitivity independent of cerebrovascular reactivity to CO2 . KEY POINTS: Modified rebreathing is a technique used to assess the ventilatory central chemoreflex and is based on the premise that the rebreathing method eliminates the difference between arterial and brain tissueP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Therefore, rebreathing is assumed to isolate the ventilatory response to central chemoreflex stimulation from the influence of cerebral blood flow. We assessed these assumptions by measuring arterial and jugular venous bulbP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ and middle cerebral artery blood velocity during modified rebreathing and compared these data against data from another test of the ventilatory central chemoreflex using hypercapnic dynamic end-tidal forcing. The difference between arterial and jugular venous bulbP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ remained present during both rebreathing and end-tidal forcing tests, whereas middle cerebral artery blood velocity was associated with theP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ difference during end-tidal forcing but not rebreathing. These findings offer substantiating evidence that clarifies and refines the assumptions of modified rebreathing tests, enhancing interpretation of future findings.

Respiratory muscle training induces additional stress and training load in well-trained triathletes-randomized controlled trial

Background: Respiratory muscle training (RMT) has been investigated in the context of improved athletic performance and pulmonary function. However, psychophysiological costs of RMT remain understudied. Voluntary isocapnic hyperpnoea (VIH) and inspiratory pressure threshold loading (IPTL) are widely applied RMT methods. The main purposes of this study were to assess whether RMT induces additional load on well-trained triathletes and determine differences in RMT-induced load between sexes and applied methods. Materials and Methods: 16 well-trained triathletes (n = 16, 56% males) underwent 6 weeks of VIH or IPTL program with progressive overload. Blood markers, subjective measures, cardiac indices, near-infrared spectroscopy indices, inspiratory muscle fatigue, and RMT-induced training load were monitored pre-, in and post-sessions. We used multiple ANOVA to investigate effects of sex, training method, and time on measured parameters. Results: There were significant interactions for acid-base balance (p = 0.04 for sex, p < 0.001 for method), partial carbon dioxide pressure (p = 0.03 for sex, p < 0.001 for method), bicarbonate (p = 0.01 for method), lactate (p < 0.001 for method), RMT-induced training load (p = 0.001 for method for single session, p = 0.03 for method per week), average heart rate (p = 0.03 for sex), maximum heart rate (p = 0.02 for sex), intercostales muscle oxygenation (p = 0.007 for testing week), and intercostales muscle oxygenation recovery (p = 0.003 for testing week and p = 0.007 for method). Conclusion: We found that RMT induced additional load in well-trained triathletes. Elicited changes in monitored variables depend on sex and training method. VIH significantly increased subjective training load measures. IPTL was associated with disbalance in blood gasometry, increase in lactate, and reports of headaches and dizziness. Both methods should be applied with consideration in high-performance settings.

Differences in time-frequency characteristics between healthy controls and TBI patients during hypercapnia assessed via fNIRS

Damage to the cerebrovascular network is a universal feature of traumatic brain injury (TBI). This damage is present during different phases of the injury and can be non-invasively assessed using functional near infrared spectroscopy (fNIRS). fNIRS signals are influenced by partial arterial carbon dioxide (PaCO2), neurogenic, Mayer waves, respiratory and cardiac oscillations, whose characteristics vary in time and frequency and may differ in the presence of TBI. Therefore, this study aims to investigate differences in time-frequency characteristics of these fNIRS signal components between healthy controls and TBI patients and characterize the changes in their characteristics across phases of the injury. Data from 11 healthy controls and 21 TBI patients were collected during the hypercapnic protocol. Results demonstrated significant differences in low-frequency oscillations between healthy controls and TBI patients, with the largest differences observed in Mayer wave band (0.06 to 0.15 Hz), followed by the PaCO2 band (0.012 to 0.02 Hz). The effects within these bands were opposite, with (i) Mayer wave activity being lower in TBI patients during acute phase of the injury (d = 0.37 [0.16, 0.57]) and decreasing further during subacute (d = 0.66 [0.44, 0.87]) and postacute (d = 0.75 [0.50, 0.99]) phases; (ii) PaCO2 activity being lower in TBI patients only during acute phase of the injury (d = 0.36 [0.15, 0.56]) and stabilizing to healthy levels by the subacute phase. These findings demonstrate that TBI patients have impairments in low frequency oscillations related to different mechanisms and that these impairments evolve differently over the course of injury.

Distinct cortical haemodynamics during squat-stand and continuous aerobic exercise do not influence the magnitude of a postexercise executive function benefit

A single bout of aerobic exercise benefits executive function (EF). A potential mechanism for this benefit is an exercise-mediated increase in cerebral blood flow (CBF) that elicits vascular endothelial shear-stress improving EF efficiency. Moderate intensity continuous aerobic exercise (MCE) asymptotically increases CBF, whereas continuous body weight squat-stand exercise (SSE) provides a large amplitude oscillatory response. Some work has proposed that an increase in CBF oscillation amplitude provides the optimal shear-stress for improving EF and brain health. We examined whether a large amplitude oscillatory CBF response associated with a single bout of SSE imparts a larger postexercise EF benefit than an MCE cycle ergometer protocol. Exercise changes in middle cerebral artery velocity (MCAv) were measured via transcranial Doppler ultrasound to estimate CBF, and pre- and postexercise EF was assessed via the antisaccade task. MCE produced a steady state increase in MCAv, whereas SSE produced a large amplitude MCAv oscillation. Both conditions produced a postexercise EF benefit that null hypothesis and equivalence tests showed to be comparable in magnitude. Accordingly, we provide a first demonstration that a single bout of SSE benefits EF; however, the condition's oscillatory CBF response does not impart a larger benefit than a time- and intensity-matched MCE protocol.

Normative cerebral microvascular blood flow waveform morphology assessed with diffuse correlation spectroscopy

Microvascular cerebral blood flow exhibits pulsatility at the cardiac frequency that carries valuable information about cerebrovascular health. This study used diffuse correlation spectroscopy to quantify normative features of these waveforms in a cohort of thirty healthy adults. We demonstrate they are sensitive to changes in vascular tone, as indicated by pronounced morphological changes with hypercapnia. Further, we observe significant sex-based differences in waveform morphology, with females exhibiting higher flow, greater area-under-the-curve, and lower pulsatility. Finally, we quantify normative values for cerebral critical closing pressure, i.e., the minimum pressure required to maintain flow in a given vascular region.

Neurologic Complications of Cardiac and Pulmonary Disease

OBJECTIVE

The heart and lungs work as a functional unit through a complex interplay. The cardiorespiratory system is responsible for the delivery of oxygen and energy substrates to the brain. Therefore, diseases of the heart and lungs can lead to various neurologic illnesses. This article reviews various cardiac and pulmonary pathologies that can lead to neurologic injury and discusses the relevant pathophysiologic mechanisms.

LATEST DEVELOPMENTS

We have lived through unprecedented times over the past 3 years with the emergence and rapid spread of the COVID-19 pandemic. Given the effects of COVID-19 on the lungs and heart, an increased incidence of hypoxic-ischemic brain injury and stroke associated with cardiorespiratory pathologies has been observed. Newer evidence has questioned the benefit of induced hypothermia in patients with out-of-hospital cardiac arrest. Further, global collaborative initiatives such as the Curing Coma Campaign are underway with the goal of improving the care of patients with coma and disorders of consciousness, including those resulting from cardiac and pulmonary pathologies.

ESSENTIAL POINTS

The neurologic complications of cardiorespiratory disorders are common and present in various forms such as stroke or hypoxic and anoxic injury related to cardiac or respiratory failure. With the emergence of the COVID-19 pandemic, neurologic complications have increased in recent years. Given the intimate and interdependent dynamics of the heart, lungs, and brain, it is crucial for neurologists to be aware of the interplay between these organs.

Evaluation of Cerebral Vasomotor Reactivity by Transcranial Doppler Ultrasound in Patients with Diabetic Retinopathy

The aim of this study was to assess the correlation between cerebral vasomotor reactivity (CVR) and the grade of diabetic retinopathy. A total of 43 diabetic patients with matched severity of diabetic retinopathy between their right and left eyes were included in this study. Diabetic retinopathy was graded in three groups. Right and left middle cerebral artery CVR was assessed by the breath-holding index (BHI) using transcranial Doppler ultrasound (TCD). The mean age of the patients was 56.51 ± 9.34 years with a mean duration of having diabetes mellitus of 14.49 ± 8.06 years. Diabetic retinopathy was graded as mild, moderately severe, and severe in 27.9%, 34.9%, and 37.2% of the patients, respectively. The grade of diabetic retinopathy was associated with the HbA1c level (p < .049), microalbuminuria (p < .024), and BHI (p = .001). In patients with severe diabetic retinopathy, the right-sided BHI was significantly lower as compared to those with mild or moderately severe retinopathy (p = .001 and p = .008, respectively). The left-sided BHI value in patients with severe diabetic retinopathy was significantly lower as compared to those with mild or moderately severe retinopathy (p = .001 and p = .012, respectively). In subjects with moderately severe diabetic retinopathy, both-sided BHI was significantly reduced compared to those with mild retinopathy (p = .001). Our results indicate that the grade of diabetic retinopathy was associated with impaired CVR.

Effects of hypocapnia and hypercapnia on human somatosensory processing

The present study investigated the effects of hypocapnia and hypercapnia on human somatosensory processing by utilizing somatosensory evoked magnetic fields (SEFs) with magnetoencephalography (MEG). Thirteen volunteers participated in two experiments separately to measure respiratory and cardiovascular data and SEFs. Both experiments consisted of a combination of normal and rapid respiratory rhythms and two inspiratory gas conditions (air and a hypercapnic gas); normal breathing with air (NB), rapid breathing with air (RB), normal breathing with the hypercapnic gas (NB+Gas), and rapid breathing with gas (RB+Gas). Partial pressures of end-tidal CO₂ (P_ETCO2) increased during inhaling the hypercapnic gas and decreased during RB, but the RB+Gas condition continued to cause elevated PETCO₂ compared with the baseline. Subsequently, middle cerebral artery blood (MCA) velocity using transcranial Doppler changed as well, while mean MCA velocity increased under the RB+Gas condition. The peak amplitude of the M60 component in SEFs was also significantly larger under with-gas than without-gas conditions, irrespective of the respiratory frequency. These results suggest that there is a close relationship between cerebral blood flow and neural activity of the M60 component in SEFs. This study provides evidence to further understanding on one of the neural mechanisms of hypercapnia.

Can Alterations in Cerebrovascular CO2 Reactivity Be Identified Using Transfer Function Analysis without the Requirement for Carbon Dioxide Inhalation?

The present study aimed to examine the validity of a novel method to assess cerebrovascular carbon dioxide (CO₂) reactivity (CVR) that does not require a CO₂ inhalation challenge, e.g., for use in patients with respiratory disease or the elderly, etc. In twenty-one healthy participants, CVR responses to orthostatic stress (50° head-up tilt, HUT) were assessed using two methods: (1) the traditional CO₂ inhalation method, and (2) transfer function analysis (TFA) between middle cerebral artery blood velocity (MCA V) and predicted arterial partial pressure of CO₂ (PaCO₂) during spontaneous respiration. During HUT, MCA V steady-state (i.e., magnitude) and MCA V onset (i.e., time constant) responses to CO₂ inhalation were decreased (p < 0.001) and increased (p = 0.001), respectively, indicative of attenuated CVR. In contrast, TFA gain in the very low-frequency range (VLF, 0.005-0.024 Hz) was unchanged, while the TFA phase in the VLF approached zero during HUT (-0.38 ± 0.59 vs. 0.31 ± 0.78 radians, supine vs. HUT; p = 0.003), indicative of a shorter time (i.e., improved) response of CVR. These findings indicate that CVR metrics determined by TFA without a CO₂ inhalation do not track HUT-evoked reductions in CVR identified using CO₂ inhalation, suggesting that enhanced cerebral blood flow response to a change in CO₂ using CO₂ inhalation is necessary to assess CVR adequately.

Peripheral and Cerebral Vascular Reactivity in Black and White Women: Examining the Impact of Psychosocial Stress Exposure Versus Internalization and Coping

Black women have the highest rates of cardiovascular and cerebrovascular disease prevalence and mortality in part due to blunted vascular function. Psychosocial stress likely also contributes but its relationship to vascular function remains incompletely understood. Recent studies suggest that internalization and coping strategies are more important than stress exposure alone. We hypothesized that Black women have blunted peripheral and cerebral vascular function and that, among Black women, this would be inversely related with psychosocial stress internalization/coping but not stress exposures. Healthy Black ( n = 21; 20 ± 2 yr) and White ( n = 16; 25 ± 7 yr) women underwent testing for forearm reactive hyperemia (RH), brachial artery flow-mediated dilation (FMD), and cerebrovascular reactivity (CVR). Psychosocial stress exposure (adverse childhood experiences, ACEs; past week discrimination, PWD) and internalization/coping techniques (John Henryism Active Coping Scale, JHAC12; Giscombe Superwoman Schema Questionnaire, G-SWS-Q) were assessed. RH and CVR ( p > 0.05) were not different between groups whereas FMD was lower in Black women ( p = 0.007). Neither ACEs nor PWD were associated with FMD in either group ( p > 0.05 for all). JHAC12 scores were negatively associated with FMD in Black women ( p = 0.014) but positively associated with FMD in White women ( p = 0.042). SWS-Succeed was negatively associated ( p = 0.044) and SWS-Vulnerable tended to be negatively associated ( p = 0.057) with FMD in Black women. These findings indicate that blunted FMD in Black women may be due more to internalization and maladaptive coping than stress exposure alone.

Menstrual phase influences cerebrovascular responsiveness in females but may not affect sex differences

Background and aims: Sex differences in the rate and occurrence of cerebrovascular diseases (e.g., stroke) indicate a role for female sex hormones (i.e., oestrogen and progesterone) in cerebrovascular function and regulation. However, it remains unclear how cerebrovascular function differs between the sexes, and between distinct phases of the menstrual cycle. This study aimed to compare cerebrovascular-CO₂ responsiveness in 1) females during the early follicular (EF), ovulatory (O) and mid-luteal (ML) phases of their menstrual cycle; and 2) males compared to females during phases of lower oestrogen (EF) and higher oestrogen (O). Methods: Eleven females (25 ± 5 years) complete experimental sessions in the EF (n = 11), O (n = 9) and ML (n = 11) phases of the menstrual cycle. Nine males (22 ± 3 years) completed two experimental sessions, approximately 2 weeks apart for comparison to females. Middle and posterior cerebral artery velocity (MCAv, PCAv) was measured at rest, during two stages of hypercapnia (2% and 5% CO₂ inhalation) and hypocapnia (voluntary hyperventilation to an end-tidal CO₂ of 30 and 24 mmHg). The linear slope of the cerebral blood velocity response to changes in end-tidal CO₂ was calculated to measure cerebrovascular-CO₂ responsiveness.. Results: In females, MCAv-CO₂ responsiveness to hypocapnia was lower during EF (-.78 ± .45 cm/s/mmHg) when compared to the O phase (-1.17 ± .52 cm/s/mmHg; p < .05) and the ML phase (-1.30 ± .82; p < .05). MCAv-CO₂ responsiveness to hypercapnia and hypo-to-hypercapnia, and PCAv-CO₂ responsiveness across the CO₂ range were similar between menstrual phases (p ≥ .20). MCAv-CO₂ responsiveness to hypo-to hypercapnia was greater in females compared to males (3.12 ± .91 cm/s/mmHg vs. 2.31 ± .46 cm/s/mmHg; p = .03), irrespective of menstrual phase (EF or O). Conclusion: Females during O and ML phases have an enhanced vasoconstrictive capacity of the MCA compared to the EF phase. Additionally, biological sex differences can influence cerebrovascular-CO₂ responsiveness, dependent on the insonated vessel.

CADASIL Affects Multiple Aspects of Cerebral Small Vessel Function on 7T-MRI

OBJECTIVE

Cerebral small vessel diseases (cSVDs) are a major cause of stroke and dementia. We used cutting-edge 7T-MRI techniques in patients with Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL), to establish which aspects of cerebral small vessel function are affected by this monogenic form of cSVD.

METHODS

We recruited 23 CADASIL patients (age 51.1 ± 10.1 years, 52% women) and 13 age- and sex-matched controls (46.1 ± 12.6, 46% women). Small vessel function measures included: basal ganglia and centrum semiovale perforating artery blood flow velocity and pulsatility, vascular reactivity to a visual stimulus in the occipital cortex and reactivity to hypercapnia in the cortex, subcortical gray matter, white matter, and white matter hyperintensities.

RESULTS

Compared with controls, CADASIL patients showed lower blood flow velocity and higher pulsatility index within perforating arteries of the centrum semiovale (mean difference - 0.09 cm/s, p = 0.03 and 0.20, p = 0.009) and basal ganglia (mean difference - 0.98 cm/s, p = 0.003 and 0.17, p = 0.06). Small vessel reactivity to a short visual stimulus was decreased (blood-oxygen-level dependent [BOLD] mean difference -0.21%, p = 0.04) in patients, while reactivity to hypercapnia was preserved in the cortex, subcortical gray matter, and normal appearing white matter. Among patients, reactivity to hypercapnia was decreased in white matter hyperintensities compared to normal appearing white matter (BOLD mean difference -0.29%, p = 0.02).

INTERPRETATION

Multiple aspects of cerebral small vessel function on 7T-MRI were abnormal in CADASIL patients, indicative of increased arteriolar stiffness and regional abnormalities in reactivity, locally also in relation to white matter injury. These observations provide novel markers of cSVD for mechanistic and intervention studies. ANN NEUROL 2023;93:29-39.

The Utility of High Intensity Interval Training to Improve Cognitive Aging in Heart Disease Patients

Adults with cardiovascular disease and heart failure are at higher risk of cognitive decline. Cerebral hypoperfusion appears to be a significant contributor, which can result from vascular dysfunction and impairment of cerebral blood flow regulation. In contrast, higher cardiorespiratory fitness shows protection against brain atrophy, reductions in cerebral blood flow, and cognitive decline. Given that high intensity interval training (HIIT) has been shown to be a potent stimulus for improving cardiorespiratory fitness and peripheral vascular function, its utility for improving cognitive aging is an important area of research. This article will review the physiology related to cerebral blood flow regulation and cognitive decline in adults with cardiovascular disease and heart failure, and how HIIT may provide a more optimal stimulus for improving cognitive aging in this population.

Primum non nocere; It's time to consider altitude training as the medical intervention it actually is!

Sleep is one of the most important aspects of recovery, and is known to be severely affected by hypoxia. The present position paper focuses on sleep as a strong moderator of the altitude training-response. Indeed, the response to altitude training is highly variable, it is not a fixed and classifiable trait, rather it is a state that is determined by multiple factors (e.g., iron status, altitude dose, pre-intervention hemoglobin mass, training load, and recovery). We present an overview of evidence showing that sleep, and more specifically the prolonged negative impact of altitude on the nocturnal breathing pattern, affecting mainly deep sleep and thus the core of physiological recovery during sleep, could play an important role in intra- and interindividual variability in the altitude training-associated responses in professional and recreational athletes. We conclude our paper with a set of suggested recommendations to customize the application of altitude training to the specific needs and vulnerabilities of each athlete (i.e., primum non nocere). Several factors have been identified (e.g., sex, polymorphisms in the TASK2/KCNK5, NOTCH4 and CAT genes and pre-term birth) to predict individual vulnerabilities to hypoxia-related sleep-disordered breathing. Currently, polysomnography should be the first choice to evaluate an individual's predisposition to a decrease in deep sleep related to hypoxia. Further interventions, both pharmacological and non-pharmacological, might alleviate the effects of nocturnal hypoxia in those athletes that show most vulnerable.

Strategies to improve respiratory chemoreflex characterization by Duffin's rebreathing

NEW FINDINGS

What is the central question of this study? We assessed the test-retest variability of respiratory chemoreflex characterization by Duffin's modified rebreathing method and explored whether signal averaging of repeated trials improves confidence in parameter estimation. What is the main finding and its importance? Modified rebreathing is a reproducible method to characterize responses of central and peripheral respiratory chemoreflexes. Signal averaging of multiple repeated tests minimizes within- and between-test variability, improves the confidence of chemoreflex characterization and reduces the minimal change in parameters required to establish an effect. Future experiments that apply this method might benefit from signal averaging to improve its discriminatory effect.

ABSTRACT

We assessed the test-retest variability of central and peripheral respiratory chemoreflex characterization by Duffin's modified rebreathing method and explored whether signal averaging of repeated trials improves confidence in parameter estimation. Over four laboratory visits, 13 participants (mean ± SD age, 25 ± 5 years) performed six repetitions of modified rebreathing in isoxic-hypoxic conditions [end-tidal P O 2 ${P_{{{\rm{O}}_{\rm{2}}}}}$ ( P ET , O 2 ${P_{{\rm{ET,}}{{\rm{O}}_{\rm{2}}}}}$ )  = 50 mmHg] and isoxic-hyperoxic conditions ( P ET , O 2 ${P_{{\rm{ET,}}{{\rm{O}}_{\rm{2}}}}}$   = 150 mmHg). End-tidal P C O 2 ${P_{{\rm{C}}{{\rm{O}}_{\rm{2}}}}}$ ( P ET , C O 2 ${P_{{\rm{ET,C}}{{\rm{O}}_{\rm{2}}}}}$ ), P ET , O 2 ${P_{{\rm{ET,}}{{\rm{O}}_{\rm{2}}}}}$ and minute ventilation ( V ̇ $\dot {\rm V}$ _E ) were measured breath-by-breath, by gas analyser and pneumotachograph. The V ̇ $\dot {\rm V}$ _E versus P ET , C O 2 ${P_{{\rm{ET,C}}{{\rm{O}}_{\rm{2}}}}}$ relationships were fitted with a piecewise model to estimate the ventilatory recruitment threshold (VRT) and the slope above the VRT ( V ̇ $\dot {\rm V}$ _E S). Breath-by-breath data from the three within- and between-day trials were averaged using two approaches [simple average (fit then average) and ensemble average (average then fit)] and compared with a single-trial fit. Variability was assessed by intraclass correlation (ICC) and coefficient of variance (CV), and the minimal detectable change was computed for each approach using two independent sets of three trials. Within days, the VRT and V ̇ $\dot {\rm V}$ _E S exhibited excellent test-retest variability in both hyperoxic conditions (VRT: ICC = 0.965, CV = 2.3%; V ̇ $\dot {\rm V}$ _E S: ICC = 0.932, CV = 15.5%) and hypoxic conditions (VRT: ICC = 0.970, CV = 2.9%; V ̇ $\dot {\rm V}$ _E S: ICC = 0.891, CV = 17.2%). Between-day reproducibility was also excellent (hyperoxia, VRT: ICC = 0.930, CV = 2.2%; V ̇ $\dot {\rm V}$ _E S: ICC = 0.918, CV = 14.2%; and hypoxia, VRT: ICC = 0.940, CV = 3.0%; V ̇ $\dot {\rm V}$ _E S: ICC = 0.880, CV = 18.1%). Compared with a single-trial fit, there were no differences in VRT or V ̇ $\dot {\rm V}$ _E S using the simple average or ensemble average approaches; however, ensemble averaging reduced the minimal detectable change for V ̇ $\dot {\rm V}$ _E S from 2.95 to 1.39 L min^-1  mmHg^-1 (hyperoxia) and from 3.64 to 1.82 L min^-1  mmHg^-1 (hypoxia). Single trials of modified rebreathing are reproducible; however, signal averaging of repeated trials improves confidence in parameter estimation.

Vascular smooth muscle cell dysfunction in neurodegeneration

Vascular smooth muscle cells (VSMCs) are the key moderators of cerebrovascular dynamics in response to the brain's oxygen and nutrient demands. Crucially, VSMCs may provide a sensitive biomarker for neurodegenerative pathologies where vasculature is compromised. An increasing body of research suggests that VSMCs have remarkable plasticity and their pathophysiology may play a key role in the complex process of neurodegeneration. Furthermore, extrinsic risk factors, including environmental conditions and traumatic events can impact vascular function through changes in VSMC morphology. VSMC dysfunction can be characterised at the molecular level both preclinically, and clinically ex vivo. However the identification of VSMC dysfunction in living individuals is important to understand changes in vascular function at the onset and progression of neurological disorders such as dementia, Alzheimer's disease, and Parkinson's disease. A promising technique to identify changes in the state of cerebral smooth muscle is cerebrovascular reactivity (CVR) which reflects the intrinsic dynamic response of blood vessels in the brain to vasoactive stimuli in order to modulate regional cerebral blood flow (CBF). In this work, we review the role of VSMCs in the most common neurodegenerative disorders and identify physiological systems that may contribute to VSMC dysfunction. The evidence collected here identifies VSMC dysfunction as a strong candidate for novel therapeutics to combat the development and progression of neurodegeneration, and highlights the need for more research on the role of VSMCs and cerebrovascular dynamics in healthy and diseased states.

Efficacy and safety of remifentanil dose titration to correct the spontaneous hyperventilation in aneurysmal subarachnoid haemorrhage: protocol and statistical analysis for a prospective physiological study

INTRODUCTION

Spontaneous hyperventilation (SHV) is common in aneurysmal subarachnoid haemorrhage (aSAH). The reduction in arterial partial pressure of carbon dioxide (PaCO₂) may change the brain physiology, such as haemodynamics, oxygenation, metabolism and may lead to secondary brain injury. However, how to correct SHV safely and effectively in patients with aSAH has not been well investigated. The aim of this study is to investigate the efficacy and safety of remifentanil dose titration to correct hyperventilation in aSAH, as well as the effect of changes in PaCO₂ on cerebral blood flow (CBF).

METHODS AND ANALYSIS

This study is a prospective, single-centre, physiological study in patients with aSAH. The patients who were mechanically ventilated and who meet with SHV (tachypnoea combined with PaCO₂ <35 mm Hg and pH >7.45) will be enrolled. The remifentanil will be titrated to correct the SHV. The predetermined initial dose of remifentanil is 0.02 μg/kg/min and will be maintained for 30 min, and PaCO₂ and CBF will be measured. After that, the dose of remifentanil will be sequentially increased to 0.04, 0.06, and 0.08 μg/kg/min, and the measurements for PaCO₂ and CBF will be repeated 30 min after each dose adjustment and will be compared with their baseline values.

ETHICS AND DISSEMINATION

This study has been approved by the Institutional Review Board of Beijing Tiantan Hospital, Capital Medical University (KY 2021-006-02) and has been registered at ClinicalTrials.gov. The results of this study will be disseminated through peer-reviewed publications and conference presentations.

TRIAL REGISTRATION NUMBER

NCT04940273.

Efficacy and safety of permissive hypercapnia in preterm infants: A systematic review

CONTEXT

In adults, permissive hypercapnia reduces mortality and ventilation duration. However, in preterm infants, the findings from past research regarding the efficacy and safety of permissive hypercapnia are controversial.

OBJECTIVE

To evaluate the efficacy and safety of permissive hypercapnia versus normocapnia in preterm infants on mechanical ventilation.

DATA SOURCES

MEDLINE, EMBASE, CENTRAL, and CINAHL STUDY SELECTION: Published randomized controlled trials (RCTs), non-RCTs, interrupted time series, cohort studies, case-control studies, and controlled before-and-after studies were included.

DATA EXTRACTION

Two reviewers independently screened the title, abstract, and full text, extracted data, assessed the risk of bias, and evaluated certainty of evidence (CoE) according to the Grading of Recommendations Assessment and Development and Evaluation approach. A meta-analysis of RCTs was performed using the random-effects model.

RESULTS

Four RCTs (693 infants) and one cohort study (371 infants) were included. No significant differences existed between the permissive hypercapnia and normocapnia groups for bronchopulmonary dysplasia (BPD) (risk ratio [RR], 0.94; 95% confidence interval [CI], 0.74-1.18; very low CoE) and a composite outcome of death or BPD (RR, 1.05; 95% CI, 0.90-1.23; very low CoE). Permissive hypercapnia may increase necrotizing enterocolitis (RR, 1.69; 95% CI, 0.98-2.91; very low CoE), but the null or trivial effect cannot be excluded. No significant differences existed between the two groups for any other outcome assessed (very low-to-low CoE).

LIMITATIONS

The sample sizes were less than the optimal sizes for all outcomes assessed, indicating the need for further trials.

CONCLUSIONS

Permissive hypercapnia did not have any significant benefit or harm in preterm infants.

Co-administration of Ketamine in Pediatric Patients with Neurologic Conditions at Risk for Intracranial Hypertension

BACKGROUND

Ketamine has traditionally been avoided as an induction agent for tracheal intubation in patients with neurologic conditions at risk for intracranial hypertension due to conflicting data in the literature. The objective of this study was to evaluate and compare the effects of ketamine versus other medications as the primary induction agent on peri-intubation neurologic, hemodynamic and respiratory associated events in pediatric patients with neurologic conditions at risk for intracranial hypertension.

METHODS

This retrospective observational study enrolled patients < 18 years of age at risk for intracranial hypertension who were admitted to a quaternary children's hospital between 2015 and 2020. Associated events included neurologic, hemodynamic and respiratory outcomes comparing primary induction agents of ketamine versus non-ketamine for tracheal intubation.

RESULTS

Of 143 children, 70 received ketamine as the primary induction agent prior to tracheal intubation. Subsequently after tracheal intubation, all the patients received adjunct analgesic and sedative medications (fentanyl, midazolam, and/or propofol) at doses that were inadequate to induce general anesthesia but would keep them comfortable for further diagnostic workup. There were no significant differences between associated neurologic events in the ketamine versus non-ketamine groups (p = 0.42). This included obtaining an emergent computed tomography scan (p = 0.28), an emergent trip to the operating room within 5 h of tracheal intubation (p = 0.6), and the need for hypertonic saline administration within 15 min of induction drug administration for tracheal intubation (p = 0.51). There were two patients who had clinical and imaging evidence of herniation, which was not more adversely affected by ketamine compared with other medications (p = 0.49). Of the 143 patients, 23 had pre-intubation and post-intubation intracranial pressure values recorded; 11 received ketamine, and 3 of these patients had intracranial hypertension that resolved or improved, whereas the remaining 8 children had intracranial pressure within the normal range that was not exacerbated by ketamine. There were no significant differences in overall associated hemodynamic or respiratory events during tracheal intubation and no 24-h mortality in either group.

CONCLUSIONS

The administration of ketamine as the primary induction agent prior to tracheal intubation in combination with other agents after tracheal intubation in children at risk for intracranial hypertension was not associated with an increased risk of peri-intubation associated neurologic, hemodynamic or respiratory events compared with those who received other induction agents.

Brain H+ /CO2 sensing and control by glial cells

Maintenance of constant brain pH is critically important to support the activity of individual neurons, effective communication within the neuronal circuits, and, thus, efficient processing of information by the brain. This review article focuses on how glial cells detect and respond to changes in brain tissue pH and concentration of CO2 , and then trigger systemic and local adaptive mechanisms that ensure a stable milieu for the operation of brain circuits. We give a detailed account of the cellular and molecular mechanisms underlying sensitivity of glial cells to H+ and CO2 and discuss the role of glial chemosensitivity and signaling in operation of three key mechanisms that work in concert to keep the brain pH constant. We discuss evidence suggesting that astrocytes and marginal glial cells of the brainstem are critically important for central respiratory CO2 chemoreception-a fundamental physiological mechanism that regulates breathing in accord with changes in blood and brain pH and partial pressure of CO2 in order to maintain systemic pH homeostasis. We review evidence suggesting that astrocytes are also responsible for the maintenance of local brain tissue extracellular pH in conditions of variable acid loads associated with changes in the neuronal activity and metabolism, and discuss potential role of these glial cells in mediating the effects of CO2 on cerebral vasculature.

Dynamic onset response of the internal carotid artery to hypercapnia is blunted in children compared with adults

Intracranial blood velocity reactivity to a steady‐state hypercapnic stimulus has been shown to be similar in children and adults, but the onset response to hypercapnia is slower in the child. Given the vasodilatory effect of hypercapnia on the cerebrovasculature, assessment of vessel diameter, and blood flow are vital to fully elucidate whether the temporal hypercapnic response differs in children versus adults. Assessment of internal carotid artery (ICA) vessel diameter (ICAd), blood velocity (ICAv), volumetric blood flow (Q_ICA), and shear rate (ICA_SR) in response to a 4 min hypercapnic challenge was completed in children (n = 14, 8 girls; 9.8 ± 0.7 years) and adults (n = 17, 7 females; 24.7 ± 1.8 years). The dynamic onset responses of partial pressure of end‐tidal CO₂ (P_ETCO₂), Q_ICA, ICAv, and ICA_SR to hypercapnia were modeled, and mean response time (MRT) was computed. Following 4 min of hypercapnia, ICA reactivity and ICAd were comparable between the groups. Despite a similar MRT in P_ETCO₂ in children and adults, children had slower Q_ICA (children 108 ± 60 s vs. adults 66 ± 37 s; p = 0.023), ICAv (children 120 ± 52 s vs. adults 52 ± 31 s; p = 0.001), and ICA_SR (children 90 ± 27 s vs. adults 47 ± 36 s; p = 0.001) MRTs compared with adults. This is the first study to show slower hypercapnic hyperemic kinetic responses of the ICA in children. The mechanisms determining these differences and the need to consider the duration of hypercapnic exposure when assessing CVR in children should be considered in future studies.

On the use and misuse of cerebral hemodynamics terminology using Transcranial Doppler ultrasound: a call for standardization

Cerebral hemodynamics (e.g., cerebral blood flow) can be measured and quantified using many different methods, with Transcranial Doppler ultrasound (TCD) being one of the most commonly utilized approaches. In human physiology, the terminology used to describe metrics of cerebral hemodynamics are inconsistent, and in some instances technically inaccurate; this is especially true when evaluating, reporting, and interpreting measures from TCD. Therefore, this perspectives article presents recommended terminology when reporting cerebral hemodynamic data. We discuss the current use and misuse of the terminology in the context of using TCD to measure and quantify cerebral hemodynamics and present our rationale and consensus on the terminology that we recommend moving forward. For example, one recommendation is to discontinue use of the term "cerebral blood flow velocity" in favor of "cerebral blood velocity" with precise indication of the vessel of interest. We also recommend clarity when differentiating between discrete cerebrovascular regulatory mechanisms, namely cerebral autoregulation, neurovascular coupling, and cerebrovascular reactivity. This will be a useful guide for investigators in the field of cerebral hemodynamics research.