Dependency of blood flow velocity in the middle cerebral artery on end-tidal carbon dioxide partial pressure--a transcranial ultrasound Doppler study

Avalanche burial pathophysiology - a unique combination of hypoxia, hypercapnia and hypothermia

For often unclear reasons, the survival times of critically buried avalanche victims vary widely from minutes to hours. Individuals can survive and sustain organ function if they can breathe under the snow and maintain sufficient delivery of oxygen and efflux of carbon dioxide. We review the physiological responses of humans to critical avalanche burial, a model which shares similarities and differences with apnoea and accidental hypothermia. Within a few minutes of burial, an avalanche victim is exposed to hypoxaemia and hypercapnia, which have important effects on the respiratory and cardiovascular systems and pose a major threat to the central nervous system. As burial time increases, an avalanche victim also develops hypothermia. Despite progressively reduced metabolism, reduced oxygen and increased carbon dioxide tensions may exacerbate the pathophysiological consequences of hypothermia. Hypercapnia seems to be the main cause of cardiovascular instability, which, in turn, is the major reason for reduced cerebral oxygenation despite reductions in cerebral metabolic activity caused by hypothermia. 'Triple H syndrome' refers to the interaction of hypoxia, hypercapnia and hypothermia in a buried avalanche victim. Future studies should investigate how the respiratory gases entrapped in the porous snow structure influence the physiological responses of buried individuals and how haemoconcentration, blood viscosity and cell deformability affect blood flow and oxygen delivery. Attention should also be devoted to identifying strategies to prolong avalanche survival by either mitigating hypoxia and hypercapnia or reducing core temperature so that neuroprotection occurs before the onset of cerebral hypoxia.

Determining the effects of elevated partial pressure of oxygen on hypercapnia-induced cerebrovascular reactivity

Evaluation of cerebrovascular reactivity (CVR) to hypo- and hypercapnia is a valuable test for the assessment of vasodilatory reserve. While hypercapnia-induced CVR testing is usually performed at normoxia, mild hyperoxia may increase tolerability of hypercapnia by reducing the ventilatory distress. However, the effects of mild hyperoxia on CVR was unknown. We therefore recruited 21 patients with a range of steno-occlusive diseases and 12 healthy participants who underwent a standardized 13-minute step plus ramp CVR test with a carbon dioxide gas challenge at the subject's resting end-tidal partial pressure of oxygen or at mild hyperoxia (PetO2 = 150 mmHg) depending on to which group they were assigned. In 11 patients, the second CVR test was at normoxia to examine test-retest differences. CVR was defined as % Δ Signal/ΔPetCO2. We found that there was no significant difference between CVR test results conducted at normoxia and at mild hyperoxia for participants in Groups 1 and 2 for the step and ramp portion. We also found no difference between test and retest CVR at normoxia for patients with cerebrovascular pathology (Group 3) for step and ramp portion. We concluded normoxic CVR is repeatable, and that mild hyperoxia does not affect CVR.

Hypothermia impairs glymphatic drainage in traumatic brain injury as assessed by dynamic contrast-enhanced MRI with intrathecal contrast

Introduction

The impact of hypothermia on the impaired drainage function of the glymphatic system in traumatic brain injury (TBI) is not understood.

Methods

Male Sprague-Dawley rats undergoing controlled cortical impact injury (CCI) were subjected to hypothermia or normothermia treatment. The rats undergoing sham surgery without CCI were used as the control. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with intrathecal administration of low- and high-molecular-weight contrast agents (Gd-DTPA and hyaluronic acid conjugated Gd-DTPA) was performed after TBI and head temperature management. The semiquantitative kinetic parameters characterizing the contrast infusion and cleanout in the brain, including influx rate, efflux rate, and clearance duration, were calculated from the average time-intensity curves.

Results and discussion

The qualitative and semiquantitative results of DCE-MRI obtained from all examined perivascular spaces and most brain tissue regions showed a significantly increased influx rate and efflux rate and decreased clearance duration among all TBI animals, demonstrating a significant impairment of glymphatic drainage function. This glymphatic drainage dysfunction was exacerbated when additional hypothermia was applied. The early glymphatic drainage reduction induced by TBI and aggravated by hypothermia was linearly related to the late increased deposition of p-tau and beta-amyloid revealed by histopathologic and biochemical analysis and cognitive impairment assessed by the Barnes maze and novel object recognition test. The glymphatic system dysfunction induced by hypothermia may be an indirect alternative pathophysiological factor indicating injury to the brain after TBI. Longitudinal studies and targeted glymphatic dysfunction management are recommended to explore the potential effect of hypothermia in TBI.

The Influence of Carbon Dioxide on Cerebral Autoregulation During Sevoflurane-based Anesthesia in Patients With Type 2 Diabetes

BACKGROUND

Cerebral autoregulation (CA) continuously adjusts cerebrovascular resistance to maintain cerebral blood flow (CBF) constant despite changes in blood pressure. Also, CBF is proportional to changes in arterial carbon dioxide (CO 2 ) (cerebrovascular CO 2 reactivity). Hypercapnia elicits cerebral vasodilation that attenuates CA efficacy, while hypocapnia produces cerebral vasoconstriction that enhances CA efficacy. In this study, we quantified the influence of sevoflurane anesthesia on CO 2 reactivity and the CA-CO 2 relationship.

METHODS

We studied patients with type 2 diabetes mellitus (DM), prone to cerebrovascular disease, and compared them to control subjects. In 33 patients (19 DM, 14 control), end-tidal CO 2 , blood pressure, and CBF velocity were monitored awake and during sevoflurane-based anesthesia. CA, calculated with transfer function analysis assessing phase lead (degrees) between low-frequency oscillations in CBF velocity and mean arterial blood pressure, was quantified during hypocapnia, normocapnia, and hypercapnia.

RESULTS

In both control and DM patients, awake CO 2 reactivity was smaller (2.8%/mm Hg CO 2 ) than during sevoflurane anesthesia (3.9%/mm Hg; P <0.005). Hyperventilation increased CA efficacy more (3 deg./mm Hg CO 2 ) in controls than in DM patients (1.8 deg./mm Hg CO 2 ; P <0.001) in both awake and sevoflurane-anesthetized states.

CONCLUSIONS

The CA-CO 2 relationship is impaired in awake patients with type 2 DM. Sevoflurane-based anesthesia does not further impair this relationship. In patients with DM, hypocapnia induces cerebral vasoconstriction, but CA efficacy does not improve as observed in healthy subjects.

Cerebrovascular responses to somatomotor stimulation in Parkinson's disease: A multivariate analysis

Parkinson's disease (PD) is a common neurodegenerative disorder, yet little is known about cerebral haemodynamics in this patient population. Previous studies assessing dynamic cerebral autoregulation (dCA), neurovascular coupling (NVC) and vasomotor reactivity (VMR) have yielded conflicting findings. By using multi-variate modelling, we aimed to determine whether cerebral blood flow (CBF) regulation is impaired in PD patients.55 healthy controls (HC) and 49 PD patients were recruited. PD subjects underwent a second recording following a period of abstinence from their anti-Parkinsonian medication. Continuous bilateral transcranial Doppler in the middle cerebral arteries, beat-to-beat mean arterial blood pressure (MAP; Finapres), heart rate (HR; electrocardiogram), and end-tidal CO₂ (EtCO₂; capnography) were measured. After a 5-min baseline period, a passive motor paradigm comprising 60 s of elbow flexion was performed. Multi-variate modelling quantified the contributions of MAP, ETCO₂ and neural stimulation to changes in CBF velocity (CBFV). dCA, VMR and NVC were quantified to assess the integrity of CBF regulation.Neural stimulation was the dominant input. dCA, NVC and VMR were all found to be impaired in the PD population relative to HC (p < 0.01, p = 0.04, p < 0.01, respectively). Our data suggest PD may be associated with depressed CBF regulation. This warrants further assessment using different neural stimuli.

Dynamics of the cerebral autoregulatory response to paced hyperventilation assessed using sub-component and time-varying analyses

Cerebral blood flow (CBF) can be altered by a change in partial pressure of arterial CO₂ (pCO₂), being reduced during hyperventilation (HPV). Critical closing pressure (CrCP) and resistance area product (RAP) are parameters which can be studied to understand this change, but their dynamic response has not been investigated during paced HPV (PHPV). Seventy five participants had recordings at rest and during PHPV. Blood pressure (BP) (Finometer), bilateral CBF velocity (CBFV) (transcranial Doppler), end-tidal CO₂ (capnography) and heart rate (HR) were recorded continuously. Subcomponent analysis (SCA) and time-varying CrCP, RAP and dynamic cerebral autoregulation (Autoregulation Index, ARI) were estimated comparing PHPV to poikilocapnia. PHPV caused a change in CBFV (p<0.01), EtCO₂, (p<0.01), HR (p<0.001) and RAP (p<0.01). SCA demonstrated RAP was the main parameter explaining the changes in CBFV due to PHPV. The time-varying step responses for CBFV and RAP during PHPV demonstrated considerable non-stationarity compared to poikilocapnia (p<0.00001). Although time-varying ARI was temporarily depressed, after 60 s of PHPV it was significantly higher (6.81 ± 1.88) (p<0.0001) than in poikilocapnia (5.08 ± 1.86). The mean plateau of the RAP step response was -98.3 ± 58.8 % 60 s after the onset of PHPV but -71.7 ± 45.0 % for poikilocapnia (p=0.0026), with no corresponding changes in CrCP (p=0.6). Further work is needed to assess the role of sex and aging in our findings, and the potential for using RAP and CrCP to improve the sensitivity and specificity of CO₂ reactivity studies in cerebrovascular conditions.

Transcranial Doppler ultrasonography detects the elevation of cerebral blood flow during ictal-phase of pentetrazol-induced seizures in dogs

OBJECTIVE

To investigate the association between changes in cerebral blood flow and electrographic epileptic seizure in dogs using transcranial Doppler ultrasonography (TCD).

ANIMALS

6 healthy Beagle dogs.

PROCEDURES

Each dog was administered pentetrazol (1.5 mg/kg/min) or saline (0.9% NaCl) solution under general anesthesia with continuous infusion of propofol. Both pentetrazol and saline solution were administered to all 6 dogs, with at least 28 days interval between the experiments. Blood flow waveforms in the middle cerebral artery and the basilar artery were obtained using TCD at baseline, after pentetrazol administration, and after diazepam administration. TCD velocities, including peak systolic velocity, end-diastolic velocity, and mean velocity and resistance variables, were determined from the Doppler waveforms.

RESULTS

During ictal-phase of pentetrazol-induced seizures, the TCD velocities significantly increased in the basilar and middle cerebral arteries while TCD vascular resistance variables did not change in either artery. The TCD velocities significantly decreased after diazepam administration. Systemic parameters, such as the heart rate, mean arterial pressure, systemic vascular resistance, cardiac index, end-tidal carbon dioxide, oxygen saturation, and body temperature, did not change significantly during seizures.

CLINICAL RELEVANCE

This study showed that cerebral blood flow, as obtained from TCD velocities, increased by 130% during ictal-phase of pentetrazol-induced seizures in dogs. The elevated velocities returned to baseline after seizure suppression. Thus, TCD may be used to detect electrographic seizures during the treatment of status epilepticus in dogs, and further clinical studies clarifying the association between changes in cerebral blood flow and non-convulsive seizure cases are needed.

Association of Hemodynamic and Cerebrovascular Responses to Exercise With Symptom Severity in Adolescents and Young Adults With Concussion

BACKGROUND AND OBJECTIVES

Aerobic exercise has become a useful method to assist with post-concussion management. Exercise can exacerbate concussion symptoms even when symptoms are not apparent at rest. Few studies have examined the reasons for symptom exacerbation during exercise following a concussion. We had two primary objectives. 1) To delineate cardiopulmonary and cerebrovascular responses to exercise in adolescents and young adults with a concussion and healthy controls. 2) To determine the association between cerebrovascular responses and symptom burden.

METHODS

We recruited participants with a recent concussion from a sport concussion clinic between 9/1/2018-2/22/2020. They were included if their concussion occurred <3 weeks before initial testing and if they were symptomatic at rest. Participants were excluded if they sustained a concussion in the past year (excluding index injury), reported history of neurological disorders, or were using medications/devices that may alter neurological function. Participants completed a progressive, symptom-limited, sub-maximal exercise protocol on a stationary bike. We assessed heart rate, blood pressure, fraction of end tidal CO2 (FETCO₂) and middle cerebral artery blood flow velocity (CBF) and cerebrovascular function (vasoreactivity and autoregulation) at seated rest and during exercise.

RESULTS

We conducted 107 exercise tests (40 concussed, 37 healthy participants initially; 30 concussed at follow-up). Concussed participants were tested initially (mean=17.6±2.2 [SD] years old; 55% female; mean=12.5±4.7 days post-concussion) and again 8 weeks later (mean=73.3±9.5 days post-concussion). Control participants (mean=18.3±2.4 years; 62% female) were tested once. FETCO₂ increased throughout the exercise protocol as heart rate increased, reached a plateau, and declined at higher exercise intensities. CO₂ explained >25% of the variation in resting CBF (R²>0.25; p<0.01) in most (73% individuals). Within the concussion group, resting symptom severity and the heart rate at which FETCO₂ reached a plateau explained ∼two-thirds of variation in exercise-induced symptom exacerbation (R ² =0.65; FETCO₂ β=-1.210±0.517[S.E.], p<0.05). There was a moderate, statistically significant relationship between cerebrovascular responses to CO₂ at rest (cerebral vasoreactivity) and cerebrovascular responses to exercise-induced changes in FETCO₂ (R²=0.13, p=0.01).

DISCUSSION

The arterial CO₂ response and symptom exacerbation relationship during post-concussion aerobic exercise may be mediated by increased sensitivity of cerebral vasculature to exercise-related increase in CO₂.

MEDEX 2015: Prophylactic Effects of Positive Expiratory Pressure in Trekkers at Very High Altitude

Purpose: Positive expiratory pressure (PEP) breathing has been shown to increase arterial oxygenation during acute hypoxic exposure but the underlying mechanisms and consequences on symptoms during prolonged high-altitude exposure remain to be elucidated. Methods: Twenty-four males (41 ± 16 years) were investigated, at sea level and at 5,085 m after 18 days of trekking from 570 m. Participants breathed through a face-mask with PEP = 0 cmH₂O (PEP₀, 0-45^th min) and with PEP = 10 cmH₂O (PEP₁₀, 46-90^th min). Arterial (SpO₂), quadriceps and prefrontal (near infrared spectroscopy) oxygenation was measured continuously. Middle cerebral artery blood velocity (MCAv, transcranial Doppler), cardiac function (2D-echocardiography), extravascular lung water accumulation (UsLC, thoracic ultrasound lung comets) and acute mountain sickness (Lake Louise score, LLS) were assessed during PEP₀ and PEP₁₀. Results: At 5,085 m with PEP₀, SpO₂ was 78 ± 4%, UsLC was 8 ± 5 (a.u.) and the LLS was 2.3 ± 1.7 (all P < 0.05 versus sea level). At 5,085 m, PEP₁₀ increased significantly SpO₂ (+9 ± 5%), quadriceps (+2 ± 2%) and prefrontal cortex (+2 ± 2%) oxygenation (P < 0.05), and decreased significantly MCAv (-16 ± 14 cm.s^-1) and cardiac output (-0.7 ± 1.2 L.min^-1) together with a reduced stroke volume (-9 ± 15 mL, all P < 0.05) and no systemic hypotension. PEP₁₀ decreased slightly the number of UsLC (-1.4 ± 2.7, P = 0.04) while the incidence of acute mountain sickness (LLS ≥ 3) fell from 42% with PEP₀ to 25% after PEP₁₀ (P = 0.043). Conclusion: PEP₁₀ breathing improved arterial and tissue oxygenation and symptoms of acute mountain sickness after trekking to very high altitude, despite reduced cerebral perfusion and cardiac output. Further studies are required to establish whether PEP-breathing prophylactic mechanisms also occur in participants with more severe acute mountain sickness.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Effects of GV14 Acupuncture on Cerebral Blood Flow Velocity in the Basilar and Middle Cerebral Arteries and CO2 Reactivity during Hypercapnia in Normal Individuals

The Governing Vessel 14 (GV14) (Dazhui) is one of the acupuncture points referred to as "seven acupoints for stroke." Nevertheless, there is a scarcity of research on the effects of acupuncture treatment at GV14. This study investigated the effects of acupuncture at GV14 on cerebral blood flow (CBF), especially that in the basilar artery (BA) and the middle cerebral arteries (MCA). Sixteen healthy men aged 20 to 29 years were enrolled in this study. CBF velocity and cerebrovascular reactivity (CVR) were measured using transcranial Doppler sonography (TCD). The following were assessed: closed circuit rebreathing- (CCR-) induced carbon dioxide (CO₂) reactivity, modified blood flow velocity at 40 mmHg (CV40) on BA and MCAs, blood pressure (BP), and heart rate (HR). Observed results were obtained after comparison with the baseline evaluation. Statistically significant elevations in CO₂ reactivity were recorded in the BA (3.28 to 4.70, p < 0.001) and MCAs (right: 3.81 to 5.25, p=0.001; left: 3.84 to 5.12, p=0.005) after acupuncture at GV14. The CV40 increased statistically significantly only in the BA (45.49 to 50.41, p=0.003). No change was observed in BP (106.83 to 107.08 (mmHg), p=0.335) and HR (77 to 75 (bpm), p=0.431). Acupuncture at GV14 improved CBF velocity. These results could be explained by the regulation of endothelium-dependent vessel dilation effected by acupuncture. This trial is registered with Korean Clinical Trial Registry (http://cris.nih.go.kr; registration number: KCT0004787).

Transcranial color-coded duplex sonography assessment of cerebrovascular reactivity to carbon dioxide: an interventional study

Background

The investigation of CO₂ reactivity (CO₂-CVR) is used in the setting of, e.g., traumatic brain injury (TBI). Transcranial color-coded duplex sonography (TCCD) is a promising bedside tool for monitoring cerebral hemodynamics. This study used TCCD to investigate CO₂-CVR in volunteers, in sedated and mechanically ventilated patients without TBI and in sedated and mechanically ventilated patients in the acute phase after TBI.

Methods

This interventional investigation was performed between March 2013 and February 2016 at the surgical ICU of the University Hospital of Zurich. Ten volunteers (group 1), ten sedated and mechanically ventilated patients (group 2), and ten patients in the acute phase (12–36 h) after severe TBI (group 3) were included. CO₂-CVR to moderate hyperventilation (∆ CO₂ -5.5 mmHg) was assessed by TCCD.

Results

CO₂-CVR was 2.14 (1.20–2.70) %/mmHg in group 1, 2.03 (0.15–3.98) %/mmHg in group 2, and 3.32 (1.18–4.48)%/mmHg in group 3, without significant differences among groups.

Conclusion

Our data did not yield evidence for altered CO₂-CVR in the early phase after TBI examined by TCCD.

Trial registration

Part of this trial was performed as preparation for the interventional trial in TBI patients (clinicaltrials.gov NCT03822026, 30.01.2019, retrospectively registered).

Vascular endothelial dysfunction associated with severity in multiple sclerosis

BACKGROUND

Impairment of cerebrovascular reactivity (CVR) has been reported in patients with multiple sclerosis (MS). Chronic inflammation and endothelial dysfunction are possible mechanisms underlying this hemodynamic impairment. This study aimed to evaluate CVR and endothelial function in patients with MS and explore their relationships with disease progression using functional sonographic procedures.

METHODS

Patients with MS and age-/sex-matched healthy controls were assessed for endothelial function, determined by flow-mediated dilation (FMD), and CVR, measured using the breath-holding index (BHI).

RESULTS

Twenty-seven patients with MS and 24 healthy controls were enrolled. FMD was significantly lower in MS subjects than in control subjects (6.0 ± 0.6 vs. 8.6 ± 0.7, p = 0.006); furthermore, BHI was similarly lower in MS than in controls, but insignificant. Remarkably, FMD was significantly lower in secondary progressive MS subjects than in relapse-remitting MS subjects (3.7 ± 1.3 vs. 6.7 ± 0.7, p = 0.045). In addition, FMD was inversely correlated with the disability score as per the expanded disability status scale (R² = 0.170, p = 0.033) and modified Rankin scale (R² = 0.187, p = 0.027).

CONCLUSION

In patients with MS, endothelial dysfunction was more noticeable than CVR impairment, correlating with the severity and progression of MS.

Alveolar Recruitment Maneuver Reduces Cerebral Oxygen Saturation and Cerebral Blood Flow Velocity in Patients During Carotid Endarterectomy

BACKGROUND This study aimed to determine the effects of alveolar recruitment maneuver (RM) on cerebral oxygen saturation and cerebral blood velocity in patients undergoing carotid endarterectomy (CEA) before clamping of the carotid artery. MATERIAL AND METHODS In this crossover exploratory study, all patients were randomized to undergo an RM (30 cmH₂O of continuous airway pressure for 30 s) and a "sham" maneuver (SM; 5 cmH₂O for 30 s), followed by an alternative intervention after a 5-min equilibration period. Near-infrared spectroscopy (NIRS) was used to monitor regional cerebral oxygen saturation (rSO₂), and transcranial Doppler ultrasonography (TCD) to evaluate blood velocity of the middle cerebral artery (V-MCA). Changes in rSO₂, V-MCA, mean arterial pressure (MAP), and heart rate (HR) in response to the 2 interventions were compared. RESULTS A total of 59 patients underwent the study procedure. RM reduced rSO₂, V-MCA, MAP, and HR, but these variables slightly changed during SM. A significant drop in rSO₂ was observed immediately after RM compared with the baseline value (68.51±4.4% vs 64.12±5.15%; P<0.001). The decrease in rSO₂ was higher during the RM than during the SM (-6±4% vs 1±2%; P<0.001). Similarly, change in V-MCA was more significant in response to RM than SM (-26±19% vs 19±16%; P<0.001). The V-MCA value changed from 39 cm/s to 29 cm/s after RM. In addition, V-MCA of the ipsilateral to the surgical side decreased more obviously than the contralateral side (-26±19% vs -20±17%; P=0.001). CONCLUSIONS An RM at 30 cmH₂O of continuous airway pressure for 30 s decreased rSO₂ and V-MCA. In addition, MAP and HR were affected.

Cerebrovascular Neuroprotection after Acute Concussion in Adolescents

OBJECTIVE

To assess acute cerebrovascular function in concussed adolescents (14-21 years of age), whether it is related to resting cerebral hemodynamics, and whether it recovers chronically.

METHODS

Cerebral vasoreactivity and autoregulation, based on middle cerebral artery blood flow velocity, was assessed in 28 concussed participants (≤14 days of injury) and 29 matched controls. The participants in the concussion group returned for an 8-week follow-up assessment. Over the course of those 8-weeks, participants recorded aerobic exercise frequency and duration.

RESULTS

Between groups, demographic, clinical, and hemodynamic variables were not significantly different. Vasoreactivity was significantly higher in the concussed group (p = 0.02). Within the concussed group, 60% of the variability in resting cerebral blood flow velocity was explained by vasoreactivity and two components of autoregulation - falling slope and effectiveness of autoregulation (adjusted R²  = 0.60, p < 0.001). Moreover, lower mean arterial pressure, lower responses to increases in arterial pressure, and lower vasoreactivity were significantly associated with larger symptom burden (adjusted R²  = 0.72, p < 0.01). By the 8-week timepoint, symptom burden, but not vasoreactivity, improved in all but four concussed participants (p < 0.01). 8-week change in vasoreactivity was positively associated with aerobic exercise volume (adjusted R²  = 0.19, p = 0.02).

INTERPRETATION

Concussion resulted in changes in cerebrovascular regulatory mechanisms, which in turn explained the variability in resting cerebral blood flow velocity and acute symptom burden. Furthermore, these alterations persisted chronically despite symptom resolution, but was positively modified by aerobic exercise volume. These findings provide a mechanistic framework for further investigation into underlying cerebrovascular related symptomatology. ANN NEUROL 2021;90:43-51.

Determinants of cerebral blood flow velocity change during squat-stand maneuvers

The large changes in mean arterial blood pressure (MABP) and cerebral blood flow velocity (CBFV) induced by squat-stand maneuvers (SSM) make this approach particularly suited for studying dynamic cerebral autoregulation (CA). However, the role of other systemic determinants of CBFV has not been described and could provide alternative physiological interpretations of SSM results. In 32 healthy subjects (16 female), continuous recordings of MABP (Finometer), bilateral CBFV (transcranial Doppler, MCA), end-tidal CO₂ (EtCO₂; capnography), and heart rate (HR; electrocardiogram) were performed for 5 min standing at rest, and during 15 SSM at the frequency of 0.05 Hz. A time-domain, multivariate dynamic model estimated the CBFV variance explained by different inputs, corresponding to significant contributions from MABP (P < 0.00001), EtCO₂ (P < 0.0001), and HR (P = 0.041). The autoregulation index (ARI; range 0-9) was estimated from the CBFV response to a step change in MABP. At rest, ARI values (typically 5.7) were independent of the number of model inputs, but during SSM, ARI was reduced compared with baseline (P < 0.0001), and the three input model yielded lower values for the right and left MCA (3.4 ± 1.2, 3.1 ± 1.3) when compared with the single-input MABP-CBFV model (4.1 ± 1.1, 3.9 ± 1.0; P < 0.0001). The high coherence of the MABP-CBFV transfer function at 0.05 Hz (typically 0.98) was considerably reduced (around 0.71-0.73; P < 0.0001) when the contribution of CBFV covariates was taken into account. Not taking into consideration other determinants of CBFV, in addition to MABP, could be misleading and introduce biases in physiological and clinical studies.

Relationship of common hemodynamic and respiratory target parameters with brain tissue oxygen tension in the absence of hypoxemia or hypotension after cardiac arrest: A post-hoc analysis of an experimental study using a pig model

Brain tissue oxygen tension (PbtO2)-guided care, a therapeutic strategy to treat or prevent cerebral hypoxia through modifying determinants of cerebral oxygen delivery, including arterial oxygen tension (PaO2), end-tidal carbon dioxide (ETCO2), and mean arterial pressure (MAP), has recently been introduced. Studies have reported that cerebral hypoxia occurs after cardiac arrest in the absence of hypoxemia or hypotension. To obtain preliminary information on the degree to which PbtO2 is responsive to changes in the common target variables for PbtO2-guided care in conditions without hypoxemia or hypotension, we investigated the relationships between the common target variables for PbtO2-guided care and PbtO2 using data from an experimental study in which the animals did not experience hypoxemia or hypotension after resuscitation. We retrospectively analyzed 170 sets of MAP, ETCO2, PaO2, PbtO2, and cerebral microcirculation parameters obtained during the 60-min post-resuscitation period in 10 pigs resuscitated from ventricular fibrillation cardiac arrest. PbtO2 and cerebral microcirculation parameters were measured on parietal cortices exposed through burr holes. Multiple linear mixed effect models were used to test the independent effects of each variable on PbtO2. Despite the absence of arterial hypoxemia or hypotension, seven (70%) animals experienced cerebral hypoxia (defined as PbtO2 <20 mmHg). Linear mixed effect models revealed that neither MAP nor ETCO2 were related to PbtO2. PaO2 had a significant linear relationship with PbtO2 after adjusting for significant covariates (P = 0.030), but it could explain only 17.5% of the total PbtO2 variance (semi-partial R2 = 0.175; 95% confidence interval, 0.086-0.282). In conclusion, MAP and ETCO2 were not significantly related to PbtO2 in animals without hypoxemia or hypotension during the early post-resuscitation period. PaO2 had a significant linear association with PbtO2, but its ability to explain PbtO2 variance was small.

Specific effect of hypobaria on cerebrovascular hypercapnic responses in hypoxia

It remains unknown whether hypobaria plays a role on cerebrovascular reactivity to CO₂ (CVR). The present study evaluated the putative effect of hypobaria on CVR and its influence on cerebral oxygen delivery (cDO₂) in five randomized conditions (i.e., normobaric normoxia, NN, altitude level of 440 m; hypobaric hypoxia, HH at altitude levels of 3,000 m and 5,500 m; normobaric hypoxia, NH, altitude simulation of 5,500 m; and hypobaric normoxia, HN). CVR was assessed in nine healthy participants (either students in aviation or pilots) during a hypercapnic test (i.e., 5% CO₂). We obtained CVR by plotting middle cerebral artery velocity versus end‐tidal CO₂ pressure (P_ETCO₂) using a sigmoid model. Hypobaria induced an increased slope in HH (0.66 ± 0.33) compared to NH (0.35 ± 0.19) with a trend in HN (0.46 ± 0.12) compared to NN (0.23 ± 0.12, p = .069). P_ETCO₂ was decreased (22.3 ± 2.4 vs. 34.5 ± 2.8 mmHg and 19.9 ± 1.3 vs. 30.8 ± 2.2 mmHg, for HN vs. NN and HH vs. NH, respectively, p < .05) in hypobaric conditions when compared to normobaric conditions with comparable inspired oxygen pressure (141 ± 1 vs. 133 ± 3 mmHg and 74 ± 1 vs. 70 ± 2 mmHg, for NN vs. HN and NH vs. HH, respectively) During hypercapnia, cDO₂ was decreased in 5,500 m HH (p = .046), but maintained in NH when compared to NN. To conclude, CVR seems more sensitive (i.e., slope increase) in hypobaric than in normobaric conditions. Moreover, hypobaria potentially affected vasodilation reserve (i.e., MCAv autoregulation) and brain oxygen delivery during hypercapnia. These results are relevant for populations (i.e., aviation pilots; high‐altitude residents as miners; mountaineers) occasionally exposed to hypobaric normoxia.

Vasomotor influences on glymphatic-lymphatic coupling and solute trafficking in the central nervous system

Despite the recent description of meningeal lymphatic vessels draining solutes from the brain interstitium and cerebrospinal fluid (CSF), the physiological factors governing cranial lymphatic efflux remain largely unexplored. In agreement with recent findings, cervical lymphatic drainage of 70 kD and 2000 kD fluorescent tracers injected into the adult mouse cortex was significantly impaired in the anesthetized compared to waking animals (tracer distribution across 2.1 ± 4.5% and 23.7 ± 15.8% of deep cervical lymph nodes, respectively); however, free-breathing anesthetized mice were markedly hypercapnic and acidemic (paCO₂ = 64 ± 8 mmHg; pH = 7.22 ± 0.05). Mechanical ventilation normalized arterial blood gases in anesthetized animals, and rescued lymphatic efflux of interstitial solutes in anesthetized mice. Experimental hypercapnia blocked cervical lymphatic efflux of intraparenchymal tracers. When tracers were injected into the subarachnoid CSF compartment, glymphatic influx into brain tissue was virtually abolished by hypercapnia, while lymphatic drainage was not appreciably altered. These findings demonstrate that cervical lymphatic drainage of interstitial solutes is, in part, regulated by upstream changes in glymphatic CSF-interstitial fluid exchange. Further, they suggest that maintaining physiological blood gas values in studies of glymphatic exchange and meningeal lymphatic drainage may be critical to defining the physiological regulation of these processes.

MRI evaluation of cerebrovascular reactivity in obstructive sleep apnea

Obstructive sleep apnea (OSA) is characterized by intermittent obstruction of the airways during sleep. Cerebrovascular reactivity (CVR) is an index of cerebral vessels' ability to respond to a vasoactive stimulus, such as increased CO₂. We hypothesized that OSA alters CVR, expressed as a breath-hold index (BHI) defined as the rate of change in CBF or BOLD signal during a controlled breath-hold stimulus mimicking spontaneous apneas by being both hypercapnic and hypoxic. In 37 OSA and 23 matched non sleep apnea (NSA) subjects, we obtained high temporal resolution CBF and BOLD MRI data before, during, and between five consecutive BH stimuli of 24 s, each averaged to yield a single BHI value. Greater BHI was observed in OSA relative to NSA as derived from whole-brain CBF (78.6 ± 29.6 vs. 60.0 ± 20.0 mL/min²/100 g, P = 0.010) as well as from flow velocity in the superior sagittal sinus (0.48 ± 0.18 vs. 0.36 ± 0.10 cm/s², P = 0.014). Similarly, BOLD-based BHI was greater in OSA in whole brain (0.19 ± 0.08 vs. 0.15 ± 0.03%/s, P = 0.009), gray matter (0.22 ± 0.09 vs. 0.17 ± 0.03%/s, P = 0.011), and white matter (0.14 ± 0.06 vs. 0.10 ± 0.02%/s, P = 0.010). The greater CVR is not currently understood but may represent a compensatory mechanism of the brain to maintain oxygen supply during intermittent apneas.

Two-photon microscopic imaging of capillary red blood cell flux in mouse brain reveals vulnerability of cerebral white matter to hypoperfusion

Despite the importance of understanding the regulation of microvascular blood flow in white matter, no data on subcortical capillary blood flow parameters are available, largely due to the lack of appropriate imaging methods. To address this knowledge gap, we employed two-photon microscopy using a far-red fluorophore Alexa680 and photon-counting detection to measure capillary red blood cell (RBC) flux in both cerebral gray and white matter, in isoflurane-anesthetized mice. We have found that in control animals, baseline capillary RBC flux in the white matter was significantly higher than in the adjacent cerebral gray matter. In response to mild hypercapnia, RBC flux in the white matter exhibited significantly smaller fractional increase than in the gray matter. Finally, during global cerebral hypoperfusion, RBC flux in the white matter was reduced significantly in comparison to the controls, while RBC flux in the gray matter was preserved. Our results suggest that blood flow in the white matter may be less efficiently regulated when challenged by physiological perturbations as compared to the gray matter. Importantly, the blood flow in the white matter may be more susceptible to hypoperfusion than in the gray matter, potentially exacerbating the white matter deterioration in brain conditions involving global cerebral hypoperfusion.

A multiscale in silico model of endothelial to mesenchymal transformation in a tumor microenvironment

Endothelial to mesenchymal transformation (EndMT) is a process in which endothelial cells gain a mesenchymal-like phenotype in response to mechanobiological signals that results in the remodeling or repair of underlying tissue. While initially associated with embryonic development, this process has since been shown to occur in adult tissue remodeling including wound healing, fibrosis, and cancer. In an attempt to understand the role of EndMT in cancer progression and metastasis, we present a multiscale, three-dimensional, in silico model. The model couples tissue level phenomena such as extracellular matrix remodeling, cellular level phenomena such as migration and proliferation, and chemical transport in the tumor microenvironment to mimic in vitro tissue models of the cancer microenvironment. The model is used to study the presence of EndMT-derived activated fibroblasts (EDAFs) and varying substrate stiffness on tumor cell migration and proliferation. The simulations accurately model the behavior of tumor cells under given conditions. The presence of EDAFs and/or an increase in substrate stiffness resulted in an increase in tumor cell activity. This model lays the foundation of further studies of EDAFs in a tumor microenvironment on a cellular and subcellular physiological level.

Interaction between the respiratory system and cerebral blood flow regulation

This review summarizes the interaction between the regulatory system of respiration and cerebral vasculature. Some clinical reports provide evidence for the association between these two physiological regulatory systems. Physiologically, arterial carbon dioxide concentration is mainly regulated by two feedback control systems: respiration and cerebral blood flow. In other words, both of these systems are sensitive to the same mediator, i.e., carbon dioxide, at a set point. In addition, respiratory dysfunction alters various physiological factors that affect the cerebral vasculature. Therefore, it is physiologically plausible that these systems are closely linked. The regulation of arterial carbon dioxide concentration affected by respiration and cerebral blood flow may be a key factor for a rise in the risk of brain disease in the patients with respiratory dysfunction. For example, the management of respiratory disease (e.g., patients with chronic obstructive pulmonary disease) and the use of prophylactic therapy are essential to reduce the risk of stroke.

Positive expiratory pressure improves arterial and cerebral oxygenation in acute normobaric and hypobaric hypoxia

Positive expiratory pressure (PEP) has been shown to limit hypoxia-induced reduction in arterial oxygen saturation, but its effectiveness on systemic and cerebral adaptations, depending on the type of hypoxic exposure [normobaric (NH) versus hypobaric (HH)], remains unknown. Thirteen healthy volunteers completed three randomized sessions consisting of 24-h exposure to either normobaric normoxia (NN), NH (inspiratory oxygen fraction, FiO2 = 13.6%; barometric pressure, BP = 716 mmHg; inspired oxygen partial pressure, PiO2 = 90.9 ± 1.0 mmHg), or HH (3,450 m, FiO2 = 20.9%, BP = 482 mmHg, PiO2 = 91.0 ± 0.6 mmHg). After the 6th and the 22nd hours, participants breathed quietly through a facemask with a 10-cmH₂O PEP for 2 × 5 min interspaced with 5 min of free breathing. Arterial (SpO2, pulse oximetry), quadriceps, and cerebral (near-infrared spectroscopy) oxygenation, middle cerebral artery blood velocity (MCAv; transcranial Doppler), ventilation, and cardiovascular responses were recorded continuously. SpO2without PEP was significantly lower in HH (87 ± 4% on average for both time points, P < 0.001) compared with NH (91 ± 3%) and NN (97 ± 1%). PEP breathing did not change SpO2 in NN but increased it similarly in NH and HH (+4.3 ± 2.5 and +4.7 ± 4.1% after 6h; +3.5 ± 2.2 and +4.1 ± 2.9% after 22h, both P < 0.001). Although MCAv was reduced by PEP (in all sessions and at all time points, -6.0 ± 4.2 cm/s on average, P < 0.001), the cerebral oxygenation was significantly improved (P < 0.05) with PEP in both NH and HH, with no difference between conditions. These data indicate that PEP could be an attractive nonpharmacological means to improve arterial and cerebral oxygenation under both normobaric and hypobaric mild hypoxic conditions in healthy participants.

Perfusion Enhancement with Respiratory Impedance After Stroke (PERI-Stroke)

Intrathoracic pressure influences cardiac output and may affect cerebral blood flow (CBF). We aimed to quantify the cerebral hemodynamic response to intrathoracic pressure reduction in patients with acute ischemic stroke using a noninvasive respiratory impedance (RI) device. We assessed low-level (6 cm H2O) and high-level (12 cm H2O) RI in 17 spontaneously breathing patients within 72 h of anterior circulation acute ischemic stroke. Average age was 65 years, and 35% were female. Frontal lobe tissue perfusion and middle cerebral artery velocity (MCAv) were continuously monitored with optical diffuse correlation spectroscopy (DCS) and transcranial Doppler ultrasound, respectively. High-level RI resulted in a 7% increase in MCAv (p = 0.004). MCAv varied across all studied levels (baseline vs low-level vs high-level, p = 0.006), with a significant test of trend (p = 0.002). Changes were not seen in DCS measured tissue perfusion by nonparametric pairwise comparison. Mixed effects regression analysis identified a small increase in both MCAv (low-level RI: β 2.1, p < 0.001; high-level RI: β 5.0, p < 0.001) and tissue-level flow (low-level RI: β 5.4, p < 0.001; high-level RI: β 5.9, p < 0.001). There was a small increase in mean arterial pressure during low-level and high-level RI, 4% (p = 0.013) and 4% (p = 0.017), respectively. End-tidal CO2 remained stable throughout the protocol. RI was well tolerated. Manipulating intrathoracic pressure via noninvasive RI was safe and produced a small but measurable increase in cerebral perfusion in acute ischemic stroke patients. Future studies are warranted to assess whether RI is feasible and tolerable for prolonged use in hyperacute stroke management.

Effects of GB34 acupuncture on hyperventilation-induced carbon dioxide reactivity and cerebral blood flow velocity in the anterior and middle cerebral arteries of normal subjects

OBJECTIVES

To determine whether acupuncture at GB34 affects cerebral blood flow (CBF) via the anterior cerebral arteries (ACAs) and middle cerebral arteries (MCAs).

METHODS

This study included 10 healthy young male volunteers. CBF velocity and cerebrovascular reactivity (CVR) were measured using transcranial Doppler sonography (TCD). The changes in hyperventilation-induced carbon dioxide (CO₂) reactivity and modified blood flow velocity at 40 mm Hg (CV40) were observed for both ACAs and MCAs before and after GB34 acupuncture treatment. Blood pressure and heart rate were also measured before and after GB34 acupuncture treatment.

RESULTS

The CO₂ reactivity of the ipsilateral MCA significantly increased after GB34 acupuncture treatment, compared with that at baseline (P=0.007). In contrast, the CO₂ reactivity of both ACAs and the contralateral MCA remained unchanged. The CV40 of both ACAs and MCAs did not change after GB34 acupuncture treatment and neither did the mean arterial blood pressure and heart rate.

CONCLUSIONS

GB34 acupuncture treatment increased CO₂ reactivity specifically in the ipsilateral MCA, but had no effect on either the ACAs or the contralateral MCA. These data suggest that GB34 acupuncture treatment improves the vasodilatory potential of the cerebral vasculature to compensate for fluctuations caused by changes in external conditions and could potentially be useful for the treatment of disorders of the ipsilateral MCA circulation.

Does respiratory drive modify the cerebral vascular response to changes in end-tidal carbon dioxide?

NEW FINDINGS

What is the central question of this study? There is an interaction between the regulatory systems of respiration and cerebral blood flow, because the mediator (CO₂ ) is the same for both physiological systems. We examined whether the traditional method for determining cerebrovascular reactivity to CO₂ is modified by changes in respiration. What is the main finding and its importance? Cerebrovascular reactivity was modified by voluntary changes in respiration during hypercapnia. This finding suggests that an alteration in the respiratory system may result in under- or overestimation of cerebrovascular reactivity determined by traditional methods in healthy adults.

ABSTRACT

The cerebral vasculature is sensitive to changes in the arterial partial pressure of CO₂ . This physiological mechanism has been well established as a cerebrovascular reactivity to CO₂ (CVR). However, arterial CO₂ may not be an independent variable in the traditional method for assessment of CVR, because the cerebral blood flow response is also affected by the activation of respiratory drive or higher centres in the brain. We hypothesized that CVR is modified by changes in respiration. To test our hypothesis, in the present study, 10 young, healthy subjects performed hyper- or hypoventilation to change end-tidal CO₂ ( P ET , C O 2 ) with different concentrations of CO₂ in the inhaled gas (0, 2.0 and 3.5%). We measured middle cerebral artery mean blood flow velocity by transcranial Doppler ultrasonography to identify the cerebral blood flow response to change in P ET , C O 2 during each set of conditions. In each set of conditions, P ET , C O 2 was significantly altered by changes in ventilation, and middle cerebral artery mean blood flow velocity changed accordingly. However, the relationship between changes in middle cerebral artery mean blood flow velocity and P ET , C O 2 as a response curve of CVR was reset upwards and downwards by hypo- and hyperventilation, respectively, compared with CVR during normal ventilation. The findings of the present study suggest the possibility that an alteration in respiration might lead to under- or overestimation of CVR determined by the traditional methods.

Non-invasive methods for measuring vascular changes in neurovascular headaches

Vascular changes during spontaneous headache attacks have been studied over the last 30 years. The interest in cerebral vessels in headache research was initially due to the hypothesis of cerebral vessels as the pain source. Here, we review the knowledge gained by measuring the cerebral vasculature during spontaneous primary headache attacks with the use of single photon emission tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRA) and transcranial Doppler (TCD). Furthermore, the use of near-infrared spectroscopy in headache research is reviewed. Existing TCD studies of migraine and other headache disorders do not provide solid evidence for cerebral blood flow velocity changes during spontaneous attacks of migraine headache. SPECT studies have clearly shown cortical vascular changes following migraine aura and the differences between migraine with aura compared to migraine without aura. PET studies have shown focal activation in brain structures related to headache, but whether the changes are specific to different primary headaches have yet to be demonstrated. MR angiography has shown precise changes in large cerebral vessels during spontaneous migraine without aura attacks. Future development in more precise imaging methods may further elucidate the pathophysiological mechanisms in primary headaches.

Cerebrovascular Reactivity Impairment in Preclinical Alzheimer's Disease

BACKGROUND AND PURPOSE

A substantial overlap exists between declines in cerebral vasoreactivity (CVR) and symptomatic Alzheimer's disease (AD). CVR can be quantified using transcranial Doppler (TCD) measurement of cerebral blood flow velocities (CBFV) in the middle cerebral artery (MCA) with CO₂ as a vasodilatory stimulus. The breath-hold acceleration index (BHAI) is a new, more reliable measure of CVR developed recently in our laboratory. Our primary goal is to explore the possibility of using TCD for asymptomatic AD screening.

METHODS

A pilot study population was divided into three groups: 9 healthy control subjects, 8 subjects identified as preclinical AD, and 10 patients diagnosed with prodromal or mild AD. Control subjects had a Clinical Dementia Rating (CDR) score of 0 without elevated amyloid-β (Aβ) on amyloid positron emission tomography (PET) imaging, preclinical AD subjects had CDR = 0 with elevated Aβ, and prodromal to mild AD subjects had CDR scores ≥.5 and elevated Aβ. CVR was calculated using two indices: the conventional breath-holding index (BHI) and the new BHAI. TCD parameters between the three groups were compared.

RESULTS

BHAI was able to distinguish between 9 normal control subjects and 8 preclinical-AD subjects with high statistical significance (P < .001). BHI and pulsatility index were able only to distinguish AD from healthy and preclinical subjects (P < .001).

CONCLUSIONS

In this exploratory pilot study, CVR was significantly decreased in preclinical, prodromal, and mild AD subjects as compared to the healthy group. Lower CVR in the preclinical AD group was detected using the new BHAI index but not the conventional BHI index.

Cerebral Blood Flow, Oxygen Delivery, and Pulsatility Responses to Oxygen Inhalation at High Altitude: Highlanders vs. Lowlanders

Objective: To determine whether the acute cerebral hemodynamic responses to oxygen inhalation are impacted by race or acclimation to high altitude.

Methods: Three groups of young healthy males, who were Tibetans (highlanders, n = 15) with lifelong exposure to high altitude, and Han Chinese (lowlanders) with five-year (Han-5 yr, n = 15) and three-day (Han-3 d, n = 16) exposures, participated in the study at an altitude of 3658 m. Cerebral blood flow velocity (CBFV) was recorded for three minutes prior to and during pure oxygen inhalation (2 L/min), respectively, using a transcranial color-coded duplex (TCCD) sonography at the middle cerebral artery (MCA). The blood draw and simultaneous monitoring of blood pressure (BP), heart rate (HR), and finger arterial oxygen saturation (SaO₂) were also performed.

Results: Values are Mean ± SEM. The three groups had similar demographic characteristics and HR responses, with the group differences (P < 0.05) found in hemoglobin concentration (16.9 ± 0.9, 18.4 ± 1.3, and 15.5 ± 1.0 gm/dL), baseline BPs and HR as expected. Both the Tibetans and Han-5yr groups presented blunted BP responses to O₂-inhalation when compared to the Han-3d group; more interestingly, the Tibetans showed significantly reduced responses compared with Han-5yr and Han-3d in CBFV, cerebral oxygen delivery (COD), and pulsatility index (PI) as assessed by Δ%CBFV/ΔSaO₂ (-1.50 ± 0.25 vs. -2.24 ± 0.25 and -2.23 ± 0.27, P = 0.049 and 0.048), Δ%COD/ΔSaO₂ (-0.52 ± 0.27 vs. -1.33 ± 0.26 and -1.38 ± 0.28, P = 0.044 and 0.031), and Δ%PI (7 ± 2 vs. 16 ± 3 and 16 ± 3 %, P = 0.036 and 0.023), respectively.

Conclusion: These findings provide evidence on the Tibetans trait of a distinct cerebral hemodynamic regulatory pattern to keep more stable cerebral blood flow (CBF), oxygen delivery, and pulsatility in response to oxygen inhalation as compared with Han Chinese, which is likely due to a genetic adaptation to altitude.

Sex differences in cerebral haemodynamics across the physiological range of PaCO2

OBJECTIVE

Cerebral blood flow (CBF) is influenced by changes in arterial CO₂ (PaCO₂). Recently, cerebral haemodynamic parameters were demonstrated to follow a four parameter logistic curve offering simultaneous assessment of dCA and CO₂ vasoreactivity. However, the effects of sex on cerebral haemodynamics have yet to be described over a wide range of PaCO₂.

APPROACH

CBF velocity (CBFV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO₂ (EtCO₂, capnography) were measured in healthy volunteers at baseline, and in response to hypo- (-5 mmHg and  -10 mmHg below baseline) and hypercapnia (5% and 8% CO₂), applied in random order.

MAIN RESULTS

Forty-five subjects (19 male, 26 female, mean age 37.5 years) showed significant differences between males and females in CBFV (50.9  ±  10.4 versus 61.5  ±  12.3 cm · s^-1, p  =  0.004), EtCO₂ (39.2  ±  2.8 versus 36.9  ±  3.0 mmHg, p  =  0.005), RAP (1.16  ±  0.23 versus 0.94  ±  0.40 mmHg cm · s^-1, p  =  0.005) and systolic BP (125.2  ±  8.0 versus 114.6  ±  12.4 mmHg, p  =  0.0372), respectively. Significant differences between sexes were observed in the four logistic parameters: y _min, y _max, k (exponential coefficient) and x (EtCO₂ level) across the haemodynamic variables. Significant differences included the CBFV-EtCO₂ and ARI-EtCO₂ relationship; ARI_min (p  =  0.036) and CBFV_max (p  =  0.001), respectively. Furthermore, significant differences were observed for both CrCP_min (p  =  0.045) and CrCP_max (p  =  0.005) and RAP_min (p  <  0.001) and RAP_max (p  <  0.001).

SIGNIFICANCE

This is the first study to examine sex individually within the context of a multi-level CO₂ protocol. The demonstration that the logistic curve parameters are influenced by sex, highlights the need to take into account sex differences between participants in both physiological and clinical studies.

Fast-food meal reduces peripheral artery endothelial function but not cerebral vascular hypercapnic reactivity in healthy young men

Consumption of a representative fast-food meal (FFMeal) acutely impairs peripheral conduit artery vascular function; however, the effect on cerebral vascular function remains unknown. This study tested the hypothesis that a FFMeal would impair cerebral vascular function as indexed by an attenuated increase in cerebral vascular conductance (CVCI) in the middle cerebral artery (MCA) during a hypercapnic challenge. Ten healthy men (age: 24 ± 3 years, BMI: 24.3 ± 3.8 kg/m2 ) were studied under two conditions; a standardized FFMeal (990 kcals, 50% fat, 36% carbohydrate, 14% protein, and 2120 mg sodium) and a fasting control condition. Basal hemodynamics, cerebral vasomotor reactivity (CVMR), and brachial artery flow-mediated dilation (BA FMD) were completed after an overnight fast (Pre) and again 2 h and 4 h later both days. To assess CVMR, subjects rebreathed from a 5-L bag while MCA velocity (MCAVmean ) was measured using transcranial Doppler (TCD) ultrasound and converted into CVCI (MCAVmean /mean arterial pressure). Peripheral artery endothelial function was assessed via BA FMD following a standard 5-min occlusion protocol. As expected, BA FMD was reduced at 2 h (Pre: 6.6 ± 1.7% vs. 5.2 ± 1.8%, P = 0.01). However, despite significant impairment in BA FMD, neither peak CVCI%baseline nor CVMR was affected by the FFMeal (Control-Pre: 1.9 ± 1.1, 2 h: 2.1 ± 1.1, 4 h: 1.7 ± 1.1 ∆CVCI%·∆PET CO2-1 vs. FFMeal-Pre: 2.1 ± 1.1, 2 h: 2.2 ± 0.7, 4 h: 1.9 ± 0.9 ∆CVCI%·∆PET CO2-1 , time × condition P = 0.88). These results suggest that cerebral vascular reactivity to hypercapnia in healthy young men is not altered by an acute FFMeal.

Neurovascular coupling response to cognitive examination in healthy controls: a multivariate analysis

Cognitive testing with transcranial Doppler ultrasonography (TCD) has been used to assess neurovascular coupling (NVC), but few studies address its multiple contributions. Subcomponent analysis considers the relative myogenic (resistance area product, RAP) and metabolic (critical closing pressure (CrCP)) contributors. The aim of this study was to investigate the changes in subcomponents that occur with cognitive stimulation with the Addenbrooke's Cognitive Examination (ACE-III) in healthy controls. Healthy volunteers underwent continuous recording of bilateral TCD, heart rate (HR, three-lead ECG), end-tidal CO2 (ETCO2 , capnography), and mean arterial pressure (MAP, Finometer). The study comprised a 5-min baseline recording, followed by all 20 paradigms from the ACE-III. The cerebral blood flow velocity (CBFv) response was decomposed into the relative contributions (subcomponents); VBP (MAP), VCrCP (CrCP), and VRAP (RAP). Data are presented as peak population normalized mean changes from baseline, and median area under the curve (AUC). Forty bilateral datasets were obtained (27 female, 37 right hand dominant). VBP increased at task initiation in all paradigms but differed between tasks (range (SD): 4.06 (8.92)-16.04 (12.23) %, P < 0.05). HR, but not ETCO2 , also differed significantly (P < 0.05). Changes in VRAP reflected changes in MAP, but in some paradigms atypical responses were seen. VCrCP AUC varied significantly within paradigm sections (range [SD]: 18.4 [24.17] to 244.21 [243.21] %*s, P < 0.05). All paradigms demonstrated changes in subcomponents with cognitive stimulation, and can be ranked based on their relative presumed metabolic demand. The integrity of NVC requires further investigation in patient populations.

Triptans and CGRP blockade - impact on the cranial vasculature

The trigeminovascular system plays a key role in the pathophysiology of migraine. The activation of the trigeminovascular system causes release of various neurotransmitters and neuropeptides, including serotonin and calcitonin gene-related peptide (CGRP), which modulate pain transmission and vascular tone. Thirty years after discovery of agonists for serotonin 5-HT1B and 5-HT1D receptors (triptans) and less than fifteen after the proof of concept of the gepant class of CGRP receptor antagonists, we are still a long way from understanding their precise site and mode of action in migraine. The effect on cranial vasculature is relevant, because all specific anti-migraine drugs and migraine pharmacological triggers may act in perivascular space. This review reports the effects of triptans and CGRP blocking molecules on cranial vasculature in humans, focusing on their specific relevance to migraine treatment.

Evidence from high-altitude acclimatization for an integrated cerebrovascular and ventilatory hypercapnic response but different responses to hypoxia

Ventilation and cerebral blood flow (CBF) are both sensitive to hypoxia and hypercapnia. To compare chemosensitivity in these two systems, we made simultaneous measurements of ventilatory and cerebrovascular responses to hypoxia and hypercapnia in 35 normal human subjects before and after acclimatization to hypoxia. Ventilation and CBF were measured during stepwise changes in isocapnic hypoxia and iso-oxic hypercapnia. We used MRI to quantify actual cerebral perfusion. Measurements were repeated after 2 days of acclimatization to hypoxia at 3,800 m altitude (partial pressure of inspired O₂ = 90 Torr) to compare plasticity in the chemosensitivity of these two systems. Potential effects of hypoxic and hypercapnic responses on acute mountain sickness (AMS) were assessed also. The pattern of CBF and ventilatory responses to hypercapnia were almost identical. CO₂ responses were augmented to a similar degree in both systems by concomitant acute hypoxia or acclimatization to sustained hypoxia. Conversely, the pattern of CBF and ventilatory responses to hypoxia were markedly different. Ventilation showed the well-known increase with acute hypoxia and a progressive decline in absolute value over 25 min of sustained hypoxia. With acclimatization to hypoxia for 2 days, the absolute values of ventilation and O₂ sensitivity increased. By contrast, O₂ sensitivity of CBF or its absolute value did not change during sustained hypoxia for up to 2 days. The results suggest a common or integrated control mechanism for CBF and ventilation by CO₂ but different mechanisms of O₂ sensitivity and plasticity between the systems. Ventilatory and cerebrovascular responses were the same for all subjects irrespective of AMS symptoms. NEW & NOTEWORTHY Ventilatory and cerebrovascular hypercapnic response patterns show similar plasticity in CO₂ sensitivity following hypoxic acclimatization, suggesting an integrated control mechanism. Conversely, ventilatory and cerebrovascular hypoxic responses differ. Ventilation initially increases but adapts with prolonged hypoxia (hypoxic ventilatory decline), and ventilatory sensitivity increases following acclimatization. In contrast, cerebral blood flow hypoxic sensitivity remains constant over a range of hypoxic stimuli, with no cerebrovascular acclimatization to sustained hypoxia, suggesting different mechanisms for O₂ sensitivity in the two systems.

Non-Invasive Respiratory Impedance Enhances Cerebral Perfusion in Healthy Adults

Optimization of cerebral blood flow (CBF) is the cornerstone of clinical management in a number of neurologic diseases, most notably ischemic stroke. Intrathoracic pressure influences cardiac output and has the potential to impact CBF. Here, we aim to quantify cerebral hemodynamic changes in response to increased respiratory impedance (RI) using a non-invasive respiratory device. We measured cerebral perfusion under varying levels of RI (6 cm H₂O, 9 cm H₂O, and 12 cm H₂O) in 20 healthy volunteers. Simultaneous measurements of microvascular CBF and middle cerebral artery mean flow velocity (MFV), respectively, were performed with optical diffuse correlation spectroscopy and transcranial Doppler ultrasound. At a high level of RI, MFV increased by 6.4% compared to baseline (p = 0.004), but changes in cortical CBF were non-significant. In a multivariable linear regression model accounting for end-tidal CO₂, RI was associated with increases in both MFV (coefficient: 0.49, p < 0.001) and cortical CBF (coefficient: 0.13, p < 0.001), although the magnitude of the effect was small. Manipulating intrathoracic pressure via non-invasive RI was well tolerated and produced a small but measurable increase in cerebral perfusion in healthy individuals. Future studies in acute ischemic stroke patients with impaired cerebral autoregulation are warranted in order to assess whether RI is feasible as a novel non-invasive therapy for stroke.

The Effect of Chunghyul-Dan on Hyperventilation-Induced Carbon Dioxide Reactivity of the Middle Cerebral Artery in Normal Subjects: A Dose-Dependent Study

Background. This study was conducted to show the prompt effect of chunghyul-dan (CHD) on cerebral hemodynamics in order to provide evidence for its use in stroke prevention. Methods. Hyperventilation-induced CO₂ reactivity of the middle cerebral artery was measured in 12 healthy male volunteers (mean age: 26.3 ± 1.1 years) using transcranial Doppler sonography. All subjects were examined before and for 3 hours after administration, with an interval of 1 week between measurements. Results. Compared to baseline, the CO₂ reactivity of the middle cerebral artery increased significantly at 2 and 3 hours after the administration of CHD (600 mg and 1200 mg). The mean blood pressure and heart rate did not vary from the baseline values in all groups. Conclusion. These data suggest that CHD administration (especially 600 mg) immediately improves cerebral blood flow.

Influence of Human Jaw Movement on Cerebral Blood Flow

Temporal changes in cerebral blood flow induced by jaw movement have yet to be investigated. To assess the influence of pattern and intensity of muscle contraction during jaw movement on task-induced change in cerebral blood flow, we performed bilateral transcranial Doppler ultrasound examination during clenching, gum chewing, and tooth tapping in healthy volunteers. A random-effects model analysis revealed a significant increase in middle cerebral artery blood flow velocity during clenching (high muscle activity) and gum chewing (moderate muscle activity), compared with the preceding rest period; however, such an increase was not detected during tooth tapping (low muscle activity). Cerebral blood flow was greater on the working side during the intensive isometric contraction of the masseter muscle in clenching. These results suggest that task-induced change in cerebral blood flow during jaw movement is influenced by the change in peripheral circulation evoked by muscle contraction.

A study of apnea induced covariations of cerebral blood flow and exhaled CO2

Sleep apnea is identified by repetitive reduction or complete cessation of breathing during sleep. Sleep apnea affects cerebral hemodynamics and it is important to study this effect. Measuring cerebral blood flow during sleep is challenging due to the need to maintain a contact between the flow probe and the skull. It is hypothesized that there exists a relationship between the variations in the exhaled CO2 and Cerebral Blood Flow during sleep apnea. To test this hypothesis, the present study was conducted in two parts: simulated and nocturnal sleep study. 9 volunteer subjects (6 Male and 3 Female Age: 23.11±1.59 years BMI: 21.9±2.409kg/m2) participated in the simulated study and 10 volunteer subjects (9 Male and 1 Female Age: 50.2±7.48 years BMI: 31.541±4.56 kg/m2 AHI: 62.84±20.44) participated in a nocturnal sleep study. Analyzing full waveforms of cerebral blood flow velocity (CBFV) and exhaled CO2, the relationship between 4 metrics from CBFV and 2 metrics from exhaled CO2 were investigated. Although one metric pair showed statistically significant and relatively high correlation (ρ= 0.68 p=7.96×10-7) during the simulated study, the same was not observed during the nocturnal study. Therefore, the proposed hypothesis could not be proven.

The CGRP-Antagonist, BIBN4096BS Does not Affect Cerebral or Systemic Haemodynamics in Healthy Volunteers

BIBN4096BS is a CGRP-antagonist effective in the treatment of migraine. Blocking the receptor of a strong vasodilator involves a theoretical risk of causing cerebral vasoconstriction, a probability not previously investigated with BIBN4096BS. Seven healthy volunteers completed this double-blinded placebo-controlled crossover study. The volunteers received randomly 10 min infusions of either placebo, 2.5 mg or 10 mg of BIBN4096BS on 3 separate days. Transcranial Doppler was used to measure the middle cerebral artery blood flow velocity (VMCA); global and regional cerebral blood flow (rCBFMCA) was measured by 133-Xenon inhalation SPECT. The diameter of the temporal and radial artery was measured by highresolution ultrasound. Systemic haemodynamics and partial pressure of CO2 (PetCO2), and adverse events were monitored regularly. BIBN4096BS had no influence on global or regional cerebral blood flow, or on the blood flow velocity in the middle cerebral artery. There was no effect on systemic haemodynamics and adverse events were minor. We conclude that there is no effect of CGRP-receptor blockade on the cerebral or systemic circulation in humans. Circulating CGRP is therefore not likely to exert a vasodilatory activity in the resting state and the use of BIBN4096BS for acute migraine seems to be without risk of cerebral vasoactivity. These data suggest that BIBN4096BS is the first specific antimigraine drug without vasoactive effect.

Vasoactive Intestinal Polypeptide Evokes Only a Minimal Headache in Healthy Volunteers

The role of the parasympathetic nervous system in the pathogenesis of migraine is disputed. The headache-eliciting effect of the parasympathetic neurotransmitter, vasoactive intestinal polypeptide (VIP), and its effect on cerebral arteries and brain haemodynamics has not been systematically studied in man. We hypothesized that infusion of VIP might induce headache in healthy subjects and cause changes in cerebral haemodynamics. VIP (8 pmol/kg per min) or placebo (0.9± saline) was infused for 25 min into 12 healthy young volunteers in a crossover, double-blind design. Headache was scored on a verbal rating scale from 0 to 10, regional cerebral blood flow (rCBF) was measured with single-photon emission computed tomography and 133Xe inhalation and mean flow velocity in the middle cerebral artery (VmeanMCA) was measured with transcranial Doppler ultrasonography. The headache was very mild with a maximum score of 2 and described as a pressing or throbbing sensation. Five participants developed headache during VIP and one during placebo. During the infusion, a significant drop in VmeanMCA was seen for VIP compared with placebo ( P < 0.001), but the effect quickly waned and no difference was found when comparing the time between 30 and 120 min. In addition, no significant difference in the diameter of the MCA could be found during the infusion. No significant differences in rCBF ( P = 0.10) were found between VIP and placebo. A marked dilation of the superficial temporal artery was seen ( P = 0.04) after VIP in the first 30 min but no difference was found when comparing the time between 30 and 120 min. We found no difference in mean arterial blood pressure between VIP and placebo days but the heart rate increased significantly on a VIP day compared with a placebo day (AUC0–30min, P < 0.001). Plasma VIP was significantly higher on a VIP day compared with placebo (AUC0–80min, P < 0.001). These results show that VIP causes a decrease in VmeanMCA without affecting rCBF. In spite of a marked vasodilator effect in the extracranial vessels and increased plasma VIP, healthy subjects developed only a very mild headache.

Lack of correlation between cerebral vasomotor reactivity and dynamic cerebral autoregulation during stepwise increases in inspired CO2 concentration

Cerebral vasomotor reactivity (CVMR) and dynamic cerebral autoregulation (CA) are measured extensively in clinical and research studies. However, the relationship between these measurements of cerebrovascular function is not well understood. In this study, we measured changes in cerebral blood flow velocity (CBFV) and arterial blood pressure (BP) in response to stepwise increases in inspired CO2 concentrations of 3 and 6% to assess CVMR and dynamic CA in 13 healthy young adults [2 women, 32 ± 9 (SD) yr]. CVMR was assessed as percentage changes in CBFV (CVMRCBFV) or cerebrovascular conductance index (CVCi, CVMRCVCi) in response to hypercapnia. Dynamic CA was estimated by performing transfer function analysis between spontaneous oscillations in BP and CBFV. Steady-state CBFV and CVCi both increased exponentially during hypercapnia; CVMRCBFV and CVMRCVCi were greater at 6% (3.85 ± 0.90 and 2.45 ± 0.79%/mmHg) than at 3% CO2 (2.09 ± 1.47 and 0.21 ± 1.56%/mmHg, P = 0.009 and 0.005, respectively). Furthermore, CVMRCBFV was greater than CVMRCVCi during either 3 or 6% CO2 (P = 0.017 and P < 0.001, respectively). Transfer function gain and coherence increased in the very low frequency range (0.02-0.07 Hz), and phase decreased in the low-frequency range (0.07-0.20 Hz) when breathing 6%, but not 3% CO2 There were no correlations between the measurements of CVMR and dynamic CA. These findings demonstrated influences of inspired CO2 concentrations on assessment of CVMR and dynamic CA. The lack of correlation between CVMR and dynamic CA suggests that cerebrovascular responses to changes in arterial CO2 and BP are mediated by distinct regulatory mechanisms.

Effect of natriuretic peptides on cerebral artery blood flow in healthy volunteers

The natriuretic peptides (NPs), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP), have vasoactive functions that concern humans and most animals, but their specific effects on cerebral circulation are poorly understood. We therefore examined the responsiveness of cerebral arteries to different doses of the natriuretic peptides in animals and humans. We conducted a dose-response experiment in guinea pigs (in vitro) and a double-blind, three-way cross-over study in healthy volunteers (in vivo). In the animal experiment, we administered cumulative doses of NPs to pre-contracted segments of cerebral arteries. In the main study, six healthy volunteers were randomly allocated to receive two intravenous doses of ANP, BNP or CNP, respectively, over 20 min on three separate study days. We recorded blood flow velocity in the middle cerebral artery (VMCA) by transcranial Doppler. In addition, we measured temporal and radial artery diameters, headache response and plasma concentrations of the NPs. In guinea pigs, ANP and BNP but not CNP showed significant dose-dependent relaxation of cerebral arteries. In healthy humans, NP infusion had no effect on mean VMCA, and we found no difference in hemodynamic responses between the NPs. Furthermore, natriuretic peptides did not affect temporal and radial artery diameters or induce headache. In conclusion, natriuretic peptides in physiological and pharmacological doses do not affect blood flow velocity in the middle cerebral artery or dilate extracerebral arteries in healthy volunteers.