The relationship between cardiac output and dynamic cerebral autoregulation in humans

Women at Altitude: Sex-Related Physiological Responses to Exercise in Hypoxia

Sex differences in physiological responses to various stressors, including exercise, have been well documented. However, the specific impact of these differences on exposure to hypoxia, both at rest and during exercise, has remained underexplored. Many studies on the physiological responses to hypoxia have either excluded women or included only a limited number without analyzing sex-related differences. To address this gap, this comprehensive review conducted an extensive literature search to examine changes in physiological functions related to oxygen transport and consumption in hypoxic conditions. The review encompasses various aspects, including ventilatory responses, cardiovascular adjustments, hematological alterations, muscle metabolism shifts, and autonomic function modifications. Furthermore, it delves into the influence of sex hormones, which evolve throughout life, encompassing considerations related to the menstrual cycle and menopause. Among these physiological functions, the ventilatory response to exercise emerges as one of the most sex-sensitive factors that may modify reactions to hypoxia. While no significant sex-based differences were observed in cardiac hemodynamic changes during hypoxia, there is evidence of greater vascular reactivity in women, particularly at rest or when combined with exercise. Consequently, a diffusive mechanism appears to be implicated in sex-related variations in responses to hypoxia. Despite well-established sex disparities in hematological parameters, both acute and chronic hematological responses to hypoxia do not seem to differ significantly between sexes. However, it is important to note that these responses are sensitive to fluctuations in sex hormones, and further investigation is needed to elucidate the impact of the menstrual cycle and menopause on physiological responses to hypoxia.

Measurement of non-invasive cardiac output during cycling exercise in ischemic stroke inpatients: A pilot study

BACKGROUND

Cardiac dysfunction accompanies acute ischemic stroke and affects the effective implementation of early rehabilitation interventions. There is a lack of reference hemodynamic data on cardiac function in the subacute phase of ischemic stroke.

OBJECTIVE

In this study, we aimed to identify appropriate cardiac parameters for exercise training utilizing a pilot study.

METHODS

We used a transthoracic electrical bioimpedance non-invasive cardiac output measurement (NICOM) device to monitor cardiac function in real time for two groups [i.e., subacute ischemic stroke inpatients group (n= 10) and healthy control group (n= 11)] using a cycling exercise experiment. The parameters of both groups were compared to highlight the cardiac dysfunction in the subacute phase in patients with ischemic stroke.

RESULTS

We considered stroke volume index (SVI) and systemic vascular resistance index (SVRi) as the primary outcomes, and there was significant intragroup difference (stroke group: P< 0.001; control group: P< 0.001, using one-way ANOVA) and significant intergroup difference at each individual time segment (P< 0.01, using independent t-test). Among the secondary outcomes, i.e., cardiac index (CI), ejection fraction (EF), end-diastolic volume (EDV), and cardiac contraction index (CTI), we found significant intergroup differences in CI, EF, and CTI scores (P< 0.01, using independent t-test). Significant interaction with respect to time and group were seen only in the SVRi and CI scores (P< 0.01, using two-way ANOVA). There was no significant inter- or intra-group differences in EDV scores.

CONCLUSION

SVRI, SVI, and CI values highlight cardiac dysfunction in stroke patients the most. At the same time, these parameters suggest that cardiac dysfunction in stroke patients may be closely related to the increased peripheral vascular resistance caused by infarction and the limitation of myocardial systolic function.

Impact of Intraoperative Fluctuations of Cardiac Output on Cerebrovascular Autoregulation: An Integrative Secondary Analysis of Individual-level Data

BACKGROUND

Intraoperative impairment of cerebral autoregulation (CA) has been associated with perioperative neurocognitive disorders. We investigated whether intraoperative fluctuations in cardiac index are associated with changes in CA.

METHODS

We conducted an integrative explorative secondary analysis of individual-level data from 2 prospective observational studies including patients scheduled for radical prostatectomy. We assessed cardiac index by pulse contour analysis and CA as the cerebral oxygenation index (COx) based on near-infrared spectroscopy. We analyzed (1) the cross-correlation between cardiac index and COx, (2) the correlation between the time-weighted average (TWA) of the cardiac index below 2.5 L min-1 m-2, and the TWA of COx above 0.3, and (3) the difference in areas between the cardiac index curve and the COx curve among various subgroups.

RESULTS

The final analysis included 155 patients. The median cardiac index was 3.16 [IQR: 2.65, 3.72] L min-1 m-2. Median COx was 0.23 [IQR: 0.12, 0.34]. (1) The median cross-correlation between cardiac index and COx was 0.230 [IQR: 0.186, 0.287]. (2) The correlation (Spearman ρ) between TWA of cardiac index below 2.5 L min-1 m-2 and TWA of COx above 0.3 was 0.095 (P=0.239). (3) Areas between the cardiac index curve and the COx curve did not differ significantly among subgroups (<65 vs. ≥65 y, P=0.903; 0 vs. ≥1 cardiovascular risk factors, P=0.518; arterial hypertension vs. none, P=0.822; open vs. robot-assisted radical prostatectomy, P=0.699).

CONCLUSIONS

We found no meaningful association between intraoperative fluctuations in cardiac index and CA. However, it is possible that a potential association was masked by the influence of anesthesia on CA.

Cardiac output and regional-cerebral-oxygen-saturation in preterm neonates during immediate postnatal transition: An observational study

AIM

To examine potential correlations between cardiac output (CO) with cerebral-regional-oxygen-saturation (crSO₂ ) and cerebral-fractional-tissue-oxygen-extraction (cFTOE) during immediate foetal-to-neonatal transition in term and preterm neonates with and without respiratory support.

METHODS

Post hoc analyses of secondary outcome parameters of prospective observational studies were performed. We included neonates with cerebral near-infrared-spectroscopy (NIRS) monitoring and an oscillometric blood pressure measurement at minute 15 after birth. Heart rate (HR) and arterial oxygen saturation (SpO₂ ) were monitored. CO was calculated with Liljestrand and Zander formula and correlated with crSO₂ and cFTOE.

RESULTS

Seventy-nine preterm neonates and 207 term neonates with NIRS measurements and calculated CO were included. In 59 preterm neonates (mean gestational age (GA): 29.4 ± 3.7 weeks) with respiratory support, CO correlated significantly positively with crSO₂ and significantly negatively with cFTOE. In 20 preterm neonates (GA 34.9 ± 1.3 weeks) without respiratory support and in 207 term neonates with and without respiratory support, CO correlated neither with crSO₂ nor with cFTOE.

CONCLUSION

In compromised preterm neonates with lower gestational age and in need of respiratory support, CO was associated with crSO₂ and cFTOE, whereas in stable preterm neonates with higher gestational age as well as in term neonates with and without respiratory support, no associations were observed.

Changes in cerebral autoregulation and vasoreactivity after surgical aortic valve replacement: a prospective study

NEW FINDINGS

What is the central question of this study? How are dynamic cerebral autoregulation and brain vasoreactivity influenced by severe aortic stenosis and its surgical treatment? What are the main findings and their importance? Dynamic cerebral autoregulation is preserved in the long term in patients with severe aortic stenosis and does not change after surgical aortic valve replacement. However, carbon dioxide vasoreactivity is impaired in these patients.

ABSTRACT

Surgical aortic valve replacement (SAVR) alters the natural course of severe aortic stenosis (AS). In this study, we aimed to determine the effects of the disease on dynamic cerebral autoregulation and vasoreactivity (VR) and to assess their changes after SAVR. We recruited 23 patients diagnosed with severe AS eligible for SAVR and 15 healthy matched controls. AS patients had lower mean VR to CO2 (P = 0.005) than controls, but dynamic cerebral autoregulation was preserved. Cerebral haemodynamics showed no significant change after SAVR. Patients with smaller baseline aortic valve areas presented with smaller low frequency phase changes after surgery (P = 0.016). Severe AS does not seem to impact dynamic cerebral autoregulation but does reduce VR to CO2 . SAVR does not alter cerebral autoregulation nor vasoreactivity.

The Spatiotemporal Dynamics of Cerebral Autoregulation in Functional Magnetic Resonance Imaging

The thigh-cuff release (TCR) maneuver is a physiological challenge that is widely used to assess dynamic cerebral autoregulation (dCA). It is often applied in conjunction with Transcranial Doppler ultrasound (TCD), which provides temporal information of the global flow response in the brain. This established method can only yield very limited insights into the regional variability of dCA, whereas functional MRI (fMRI) has the ability to reveal the spatial distribution of flow responses in the brain with high spatial resolution. The aim of this study was to use whole-brain blood-oxygenation-level-dependent (BOLD) fMRI to characterize the spatiotemporal dynamics of the flow response to the TCR challenge, and thus pave the way toward mapping dCA in the brain. We used a data driven approach to derive a novel basis set that was then used to provide a voxel-wise estimate of the TCR associated haemodynamic response function (HRF TCR ). We found that the HRF TCR evolves with a specific spatiotemporal pattern, with gray and white matter showing an asynchronous response, which likely reflects the anatomical structure of cerebral blood supply. Thus, we propose that TCR challenge fMRI is a promising method for mapping spatial variability in dCA, which will likely prove to be clinically advantageous.

Cerebral Hemodynamics During Exposure to Hypergravity (+Gz) or Microgravity (0 G)

BACKGROUND: Optimal human performance and health is dependent on steady blood supply to the brain. Hypergravity (+Gz) may impair cerebral blood flow (CBF), and several investigators have also reported that microgravity (0 G) may influence cerebral hemodynamics. This has led to concerns for safe performance during acceleration maneuvers in aviation or the impact long-duration spaceflights may have on astronaut health.METHODS: A systematic PEO (Population, Exposure, Outcome) search was done in PubMed and Web of Science, addressing studies on how elevated +Gz forces or absence of such may impact cerebral hemodynamics. All primary research containing anatomical or physiological data on relevant intracranial parameters were included. Quality of the evidence was analyzed using the GRADE tool.RESULTS: The search revealed 92 eligible articles. It is evident that impaired CBF during +Gz acceleration remains an important challenge in aviation, but there are significant variations in individual tolerance. The reports on cerebral hemodynamics during weightlessness are inconsistent, but published data indicate that adaptation to sustained microgravity is also characterized by significant variations among individuals.DISCUSSION: Despite a high number of publications, the quality of evidence is limited due to observational study design, too few included subjects, and methodological challenges. Clinical consequences of high +Gz exposure are well described, but there are significant gaps in knowledge regarding the intracranial pathophysiology and individual hemodynamic tolerance to both hypergravity and microgravity environments.Saehle T. Cerebral hemodynamics during exposure to hypergravity (+Gz) or microgravity (0 G). Aerosp Med Hum Perform. 2022; 93(7):581–592.

Machine Learning Models and Statistical Complexity to Analyze the Effects of Posture on Cerebral Hemodynamics

The mechanism of cerebral blood flow autoregulation can be of great importance in diagnosing and controlling a diversity of cerebrovascular pathologies such as vascular dementia, brain injury, and neurodegenerative diseases. To assess it, there are several methods that use changing postures, such as sit-stand or squat-stand maneuvers. However, the evaluation of the dynamic cerebral blood flow autoregulation (dCA) in these postures has not been adequately studied using more complex models, such as non-linear ones. Moreover, dCA can be considered part of a more complex mechanism called cerebral hemodynamics, where others (CO2 reactivity and neurovascular-coupling) that affect cerebral blood flow (BF) are included. In this work, we analyzed postural influences using non-linear machine learning models of dCA and studied characteristics of cerebral hemodynamics under statistical complexity using eighteen young adult subjects, aged 27 ± 6.29 years, who took the systemic or arterial blood pressure (BP) and cerebral blood flow velocity (BFV) for five minutes in three different postures: stand, sit, and lay. With models of a Support Vector Machine (SVM) through time, we used an AutoRegulatory Index (ARI) to compare the dCA in different postures. Using wavelet entropy, we estimated the statistical complexity of BFV for three postures. Repeated measures ANOVA showed that only the complexity of lay-sit had significant differences.

A method to evaluate dynamic cerebral pressure-flow relationships in the conscious rat

The classic dogma of cerebral autoregulation is that cerebral blood flow is steadily maintained across a wide range of perfusion pressures. This has been challenged by recent studies suggesting little to no "autoregulatory plateau" in the relationship between cerebral blood flow and blood pressure (BP). Therefore, the mechanisms underlying the cerebral pressure-flow relationship still require further understanding. Here, we present a novel approach to examine dynamic cerebral autoregulation in conscious Wistar rats (n = 16) instrumented to measure BP and internal carotid blood flow (iCBF), as an indicator of cerebral blood flow. Transient reductions in BP were induced by occluding the vena cava via inflation of a chronically implanted intravascular silicone balloon. Falls in BP were paralleled by progressive decreases in iCBF, with no evidence of a steady-state plateau. No significant changes in internal carotid vascular resistance (iCVR) were observed. In contrast, intravenous infusions of the vasoactive drug sodium nitroprusside (SNP) produced a similar fall in BP but increases in iCBF and decreases in iCVR were observed. These data suggest a considerable confounding influence of vasodilatory drugs such as SNP on cerebrovascular tone in the rat, making them unsuitable to investigate cerebral autoregulation. We demonstrate that our technique of transient vena cava occlusion produced reliable and repeatable depressor responses, highlighting the potential for our approach to permit assessment of the dynamic cerebral pressure-flow relationship over time in conscious rats.NEW & NOTEWORTHY We present a novel technique to overcome the use of vasoactive agents when studying cerebrovascular dynamics in the conscious rat. Our method of vena cava occlusion to reduce BP was associated with decreased iCBF and no change in iCVR. In contrast, comparable BP falls with intravenous SNP increased iCBF and reduced iCVR. Thus, the dynamic cerebral pressure-flow relationship shows a narrower, less level autoregulatory plateau than conventionally thought. We confirm our method allows repeatable assessment of cerebrovascular dynamics in conscious rats.

Dynamic Autoregulation is Impaired in Circulatory Shock

BACKGROUND

Circulatory shock is a life-threatening disorder that is associated with high mortality, with a state of systemic and tissue hypoperfusion that can lead to organ failure, including the brain, where altered mental state is often observed. We hypothesized that cerebral autoregulation (CA) is impaired in patients with circulatory shock.

METHODS

Adult patients with circulatory shock and healthy controls were included. Cerebral blood flow velocity (CBFV, transcranial Doppler ultrasound) and arterial blood pressure (BP, Finometer or intra-arterial line) were continuously recorded during 5 min in both groups. Autoregulation Index (ARI) was estimated from the CBFV response to a step change in BP, derived by transfer function analysis; ARI ≤ 4 was considered impaired CA. The relationship between organ dysfunction, assessed with the Sequential Organ Failure Assessment (SOFA) score and the ARI, was assessed with linear regression.

RESULTS

Twenty-five shock patients and 28 age-matched healthy volunteers were studied. The mean ± SD SOFA score was 10.8 ± 4.3. Shock patients compared with control subjects had lower ARI values (4.0 ± 2.1 vs. 5.9 ± 1.5, P = 0.001). Impaired CA was more common in shock patients (44.4% vs. 7.1%, P = 0.003). There was a significant inverse relationship between the ARI and the SOFA score (R = -0.63, P = 0.0008).

CONCLUSIONS

These results suggest that circulatory shock is often associated with impaired CA and that the severity of CA alterations is correlated with the degree of multiple organ failure, reinforcing the need to monitor cerebral hemodynamics in patients with circulatory shock.

The Acute Cardiorespiratory and Cerebrovascular Response to Resistance Exercise

Resistance exercise (RE) is a popular modality for the general population and athletes alike, due to the numerous benefits of regular participation. The acute response to dynamic RE is characterised by temporary and bidirectional physiological extremes, not typically seen in continuous aerobic exercise (e.g. cycling) and headlined by phasic perturbations in blood pressure that challenge cerebral blood flow (CBF) regulation. Cerebral autoregulation has been heavily scrutinised over the last decade with new data challenging the effectiveness of this intrinsic flow regulating mechanism, particularly to abrupt changes in blood pressure over the course of seconds (i.e. dynamic cerebral autoregulation), like those observed during RE. Acutely, RE can challenge CBF regulation, resulting in adverse responses (e.g. syncope). Compared with aerobic exercise, RE is relatively understudied, particularly high-intensity dynamic RE with a concurrent Valsalva manoeuvre (VM). However, the VM alone challenges CBF regulation and generates additional complexity when trying to dissociate the mechanisms underpinning the circulatory response to RE. Given the disparate circulatory response between aerobic and RE, primarily the blood pressure profiles, regulation of CBF is ostensibly different. In this review, we summarise current literature and highlight the acute physiological responses to RE, with a focus on the cerebral circulation.

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.

Cardiac Output and Cerebral Blood Flow: A Systematic Review of Cardio-Cerebral Coupling

Control of cerebral blood flow (CBF) is crucial to the management of neurocritically ill patients. Small studies which have examined the role of cardiac output (CO) as a determinant of CBF have inconsistently demonstrated evidence of cardio-cerebral coupling. Putative physiological mechanisms underpinning such coupling include changes in arterial blood pressure pulsatility, which would produce vasodilation through increased oscillatory wall-shear-stress and baroreceptor mediated reflex sympatholysis, and changes in venous backpressure which may improve cerebral perfusion pressure. We sought to summarize and contextualize the literature on the relationship between CO and CBF and discuss the implications of cardio-cerebral coupling for neurocritical care. A systematic review of the literature yielded 41 studies; all were of low-quality and at high-risk of bias. Results were heterogenous, with evidence for both corroboration and confutation of a relationship between CO and CBF in both normal and abnormal cerebrovascular states. Common limitations of studies were lack of instantaneous CBF measures with reliance on transcranial Doppler-derived blood flow velocity as a surrogate, inability to control for fluctuations in established determinants of CBF (eg, PaCO2), and direct effects on CBF by the interventions used to alter CO. Currently, the literature is insufficiently robust to confirm an independent relationship between CO and CBF. Hypothetically, the presence of cardio-cerebral coupling would have important implications for clinical practice. Manipulation of CBF could occur without the risks associated with extremes of arterial pressure, potentially improving therapy for those with cerebral ischemia of various etiologies. However, current literature is insufficiently robust to confirm an independent relationship between CO and CBF, and further studies with improved methodology are required before therapeutic interventions can be based on cardio-cerebral coupling.

Alterations in Cerebral Hemodynamics During Microgravity: A Literature Review

Most reported neurological symptoms that happen after exposure to microgravity could be originated from alterations in cerebral hemodynamics. The complicated mechanisms involved in the process of hemodynamics and the disparate experimental protocols designed to study the process may have contributed to the discrepancies in results between studies and the lack of consensus among researchers. This literature review examines spaceflight and ground-based studies of cerebral hemodynamics and aims to summarize the underlying physiological mechanisms that are altered in cerebral hemodynamics during microgravity. We reviewed studies that were published before July 2020 and sought to provide a comprehensive summary of the physiological or pathological theories of hemodynamics and to arrive at firm conclusions from incongruous results that were reported in those related articles.

We give plausible explanations of inconsistent results on factors including intracranial pressure, cerebral blood flow, and cerebrovascular autoregulation. Although there are no definitive data to confirm how cerebral hemodynamics changes during microgravity, every discrepancy in results was interpreted by existing theories, which were derived from physiological and pathological processes. We conclude that microgravity-induced alterations of hemodynamics at the brain level are multifaceted. Factors including duration, partial pressures of carbon dioxide, and individual adaptability contribute to this process and are unpredictable. With a growing understanding of this hemodynamics model, additional factors will likely be considered. Aiming for a full understanding of the physiological and/or pathological changes of hemodynamics will enable researchers to investigate its cellular and molecular mechanisms in future studies, which are desperately needed.

Sex differences in the autonomic and cerebrovascular responses to upright tilt

Sex differences in the regulation of autonomic and cerebrovascular responses to orthostatic stress remain unclear. The objectives of this study were to concurrently investigate autonomic control and cerebrovascular resistance indices, including critical closing pressure (CrCP) and resistance area product (RAP), during upright tilt in men and women. In 13 women and 14 men (18-29 years), ECG, non-invasive blood pressure, middle cerebral artery blood velocity, and end-tidal CO₂ (ETCO₂) were continuously measured during supine rest and 70° tilt. Heart rate variability (HRV), cardiovagal baroreflex sensitivity (cBRS), and transfer function parameters of dynamic cerebral autoregulation were calculated. Compared to supine, upright tilt increased the low frequency-to-high frequency ratio of HRV in men only (P = 0.044), and decreased cBRS more in women (P = 0.001). Cerebrovascular resistance index (CVRi) increased during tilt only in men (sex-by-time interaction: P = 0.004). RAP was lower in women throughout tilt (main effect of sex: P = 0.022). CrCP decreased during tilt in both sexes (main effect of time: P < 0.001). Normalizing to ETCO2 did not alter the effect of tilt on cerebrovascular resistance. Men displayed a greater increase of sympathetic indices and CVRi during tilt while women had greater parasympathetic withdrawal. We hypothesize that increased sympathetic activity in men may drive sex differences in the cerebrovascular response to upright posture.

Comparison of diurnal variation, anatomical location, and biological sex within spontaneous and driven dynamic cerebral autoregulation measures

Presently, the literature describing the influence of diurnal variation on dynamic cerebral autoregulation (dCA) metrics is sparse. Additionally, there is little data with respect to dCA comparisons between anterior/posterior circulation beds and biological sexes using squat-stand maneuvers. Eight male and eight female participants (n = 16) performed 5 min of spontaneous upright rest and squat-stand maneuvers at 0.05 and 0.10 Hz across seven time points throughout the day. All testing sessions commenced at 8:00 a.m. each day and dCA parameters were quantified across the cardiac cycle (diastole, mean, and systole) using transcranial Doppler ultrasound to insonate cerebral blood velocity within the middle and posterior cerebral arteries (MCA, PCA). No cardiac cycle alternations were seen spontaneous (all p > .207) while a trend was noted in some driven (all p > .051) dCA metrics. Driven dCA produced much lower coefficient of variances (all <21%) compared with spontaneous (all <58%). Moreover, no sex differences were found within driven metrics (all p > .096). Between vessels, PCA absolute gain was reduced within all spontaneous and driven measures (all p < .014) whereas coherence, phase, and normalized gain were unchanged (all p > .099). There appears to be little influence of diurnal variation on dCA measures across the day (8:00 a.m. to 6:00 p.m.). Absolute gain was blunted in the PCA relative to the MCA and consistent with previous literature, driven methods demonstrated vastly improved reproducibility metrics compared to spontaneous methods. Finally, no dCA differences were found between biological sexes, demonstrating that males and females regulate in a harmonious manner, when females are tested within the early follicular phase of the menstrual cycle.

Cerebrovascular reactivity and cerebral autoregulation are improved in the supine posture compared to upright in healthy men and women

Cerebrovascular reactivity and cerebral autoregulation are two major mechanisms that regulate cerebral blood flow. Both mechanisms are typically assessed in either supine or seated postures, but the effects of body position and sex differences remain unclear. This study examined the effects of body posture (supine vs. seated vs. standing) on cerebrovascular reactivity during hyper and hypocapnia and on cerebral autoregulation during spontaneous and slow-paced breathing in healthy men and women using transcranial Doppler ultrasonography of the middle cerebral artery. Results indicated significantly improved cerebrovascular reactivity in the supine compared with seated and standing postures (supine = 3.45±0.67, seated = 2.72±0.53, standing = 2.91±0.62%/mmHg, P<0.0167). Similarly, cerebral autoregulatory measures showed significant improvement in the supine posture during slow-paced breathing. Transfer function measures of gain significantly decreased and phase significantly increased in the supine posture compared with seated and standing postures (gain: supine = 1.98±0.56, seated = 2.37±0.53, standing = 2.36±0.71%/mmHg; phase: supine = 59.3±21.7, seated = 39.8±12.5, standing = 36.5±9.7°; all P<0.0167). In contrast, body posture had no effect on cerebral autoregulatory measures during spontaneous breathing. Men and women had similar cerebrovascular reactivity and similar cerebral autoregulation during both spontaneous and slow-paced breathing. These data highlight the importance of making comparisons within the same body position to ensure there is not a confounding effect of posture.

Haemodynamic and cerebrovascular effects of intermittent lower-leg compression as countermeasure to orthostatic stress

NEW FINDINGS

What is the central question of this study? Does smartly timed intermittent compression of the lower legs alter cerebral blood velocity and oxygenation during acute orthostatic challenges? What is the main finding and its importance? Intermittent compression timed to the local diastolic phase increased the blood flux through the legs and heart after two different orthostatic stress tests. Cerebral blood velocity improved during the first minute of recovery, and indices of cerebral tissue oxygenation remained elevated for 2 min. These results provide promise for the use of lower-leg active compression as a therapeutic tool for individuals vulnerable to initial orthostatic hypotension and orthostatic stress.

ABSTRACT

Intermittent compression of the lower legs provides the possibility of improving orthostatic tolerance by actively promoting venous return and improving central haemodynamics. We tested the hypothesis that intermittent compression of 65 mmHg timed to occur only within the local diastolic phase of each cardiac cycle would attenuate the decrease in blood pressure and improve cerebral haemodynamics during the first minute of recovery from two different orthostatic stress tests. Fourteen subjects (seven female) performed four squat-to-stand transitions and four repeats of standing bilateral thigh-cuff occlusion and release (TCR), with intermittent compression of the lower legs applied in half of the trials. Blood flow in the superficial femoral artery, mean arterial pressure, Doppler ultrasound cardiac output, total peripheral resistance, middle cerebral artery blood velocity (MCAv) and cerebral tissue saturation index (TSI%) were monitored. With both orthostatic stress tests, there was a significant compression × time interaction for superficial femoral artery flow (P < 0.001). The hypotensive state was attenuated with intermittent compression despite decreased total peripheral resistance (squat-to-stand, compression × time interaction, P < 0.001; TCR, compression × time interaction, P = 0.002) as a consequence of elevated cardiac output in both tests (P < 0.001). Intermittent compression also increased MCAv (P = 0.001) and TSI% (P < 0.001) during the squat-to-stand transition and during TCR (MCAv and TSI%, compression × time interaction, P < 0.001). Intermittent compression of the lower legs during quiet standing after an active orthostatic challenge augmented local, central and cerebral haemodynamics, providing potential as a therapeutic tool for individuals vulnerable to orthostatic stress.

Sex differences in cerebral autoregulation are unaffected by menstrual cycle phase in young, healthy women

Sex is known to affect the prevalence of conditions such as stroke. However, effects of sex on cerebral blood flow regulation are still not well understood. Critical to this understanding is how fluctuations in hormones across the menstrual cycle affect cerebral autoregulation. We measured autoregulation in the early follicular, late follicular, and midluteal phases during spontaneous and induced blood pressure oscillations in 26 young, healthy individuals (13 women and 13 men, age: 26 ± 4 yr). Men participated three times, ~1–3 wk apart. Beat-by-beat blood pressure, heart rate, end-tidal CO2, and transcranial Doppler ultrasonography of the middle (MCA) and anterior (ACA) cerebral arteries were obtained. We did not find a difference in cerebral autoregulation across the menstrual cycle in women but found significantly improved autoregulation in the MCA and ACA of women compared with men. Women demonstrated significantly lower MCA gain (0.97 ± 0.13 vs. 1.17 ± 0.14%/mmHg, P = 0.001), higher MCA phase (46.1 ± 12.6 vs. 35.8 ± 7.9°, P = 0.019), and higher ACA phase (40.5 ± 10.8 vs 31.5 ± 8.5°, P = 0.040) during repeated squat-to-stand maneuvers. Women also had lower MCA gain (1.50 ± 0.11 vs. 1.72 ± 0.30%/mmHg, P = 0.029) during spontaneous fluctuations in pressure while standing and less of a decrease in MCA flow velocity (−18.7 ± 2.7 vs. −23.2 ± 6.0%, P = 0.014) during sit-to-stand maneuvers. Our results suggest that young women have improved cerebral autoregulation compared with young men regardless of menstrual cycle phase and that autoregulation is relatively robust to acute fluctuations in female sex hormones.

NEW & NOTEWORTHY This is the first study to investigate thoroughly the effects of menstrual cycle phase and sex differences in cerebral autoregulation in young, healthy individuals. Cerebral autoregulation was unaffected by menstrual cycle phase during both repeated squat-to-stand and sit-to-stand maneuvers. However, women demonstrated significantly improved cerebral autoregulation in the middle and anterior cerebral arteries, suggesting women were able to maintain cerebral blood flow during changes in blood pressure more efficiently than men.

The Trendelenburg Position and Cognitive Decline: A Case-Control Interventional Study Involving Healthy Volunteers

Background

Postoperative cognitive decline (POCD) is defined as a new cognitive impairment arising after a surgical intervention. Aspects of cognitive function can be assessed using various validated cognitive function tests including the N-back task, the Stroop task, and the lexical decision-making task (LDT). There is some concern that prolonged Trendelenburg positioning during laparoscopic colorectal surgery may cause POCD.

Objective

The objective of this study was to assess the effect of time spent in the Trendelenburg position on cognitive function.

Methods

Volunteers were placed in the Trendelenburg position for 3 hours and, then, supine for 30 minutes. Validated cognitive function tests including 1-, 2-, and 3-back tasks, Stroop test, and LDT were performed at baseline and every 30 minutes after Trendelenburg positioning. Cognitive decline was defined per the International Study of Postoperative Cognitive Dysfunction trial: a decrease in accuracy from the volunteers’ baseline or an increase in response time from the volunteers’ baseline by >2 control group SDs.

Results

We recruited 15 healthy volunteers (8 males, 7 females) with an average age of 69 years (range 57-81) and average body mass index of 27.7 kg/m² (range 20.9-33). Accuracy remained within 2 SDs at all time points. An increase in response time did occur, and of 15 participants, 3 (20%) showed cognitive decline in the Trendelenburg position after 30 minutes, 4 (27%) after 1 hour, 5 (33%) after 90 minutes, 4 (27%) after 120 and 150 minutes, and 6 (40%) after 180 minutes. On moving to a supine position, 33% (5/15) participants showed cognitive decline.

Conclusions

The results of this study indicate that Trendelenburg positioning leads to cognitive decline. This may have implications for patients undergoing prolonged Trendelenburg positioning during laparoscopic colorectal surgery.

Cerebral blood flow regulation and cognitive function in women with posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is associated with structural and functional alterations in a number of interacting brain regions, but the physiological mechanism for the high risk of cerebrovascular disease or impairment in brain function remains unknown. Women are more likely to develop PTSD after a trauma than men. We hypothesized that cerebral blood flow (CBF) regulation is impaired in women with PTSD, and it is associated with impairment in cognitive function. To test our hypothesis, we examined dynamic cerebral autoregulation (CA) and cognitive function by using a transfer function analysis between arterial pressure and middle cerebral artery blood velocity and the Stroop Color and Word test (SCWT), respectively. We did not observe any different responses in these hemodynamic variables between women with PTSD ( n = 15) and healthy counterparts (all women; n = 8). Cognitive function was impaired in women with PTSD; specifically, reaction time for the neutral task of SCWT was longer in women with PTSD compared with healthy counterparts ( P = 0.011), but this cognitive dysfunction was not affected by orthostatic stress. On the other hand, transfer function phase, gain, and coherence were not different between groups in either the supine or head-up tilt (60°) position, or even during the cognitive challenge, indicating that dynamic CA was well maintained in women with PTSD. In addition, there was no relationship between cognitive function and dynamic CA. These findings suggest that PTSD-related cognitive dysfunction may not be due to compromised CBF regulation.

NEW & NOTEWORTHY Cognitive function was impaired; however, dynamic cerebral autoregulation (CA) as an index of cerebral blood flow regulation was not impaired during supine and 60° head-up tilt in women with PTSD compared with healthy females. In addition, there was no relationship between cognitive function and dynamic CA. These findings suggest that the mechanism of PTSD-related cognitive dysfunction may not be due to CBF regulation.

Dynamic cerebral autoregulation is impaired during submaximal isometric handgrip in patients with heart failure

The incidence of neurological complications, including stroke and cognitive dysfunction, is elevated in patients with heart failure (HF) with reduced ejection fraction. We hypothesized that the cerebrovascular response to isometric handgrip (iHG) is altered in patients with HF. Adults with HF and healthy volunteers were included. Cerebral blood velocity (CBV; transcranial Doppler, middle cerebral artery) and arterial blood pressure (BP; Finometer) were continuously recorded supine for 6 min, corresponding to 1 min of baseline and 3 min of iHG exercise, at 30% maximum voluntary contraction, followed by 2 min of recovery. The resistance-area product was calculated from the instantaneous BP-CBV relationship. Dynamic cerebral autoregulation (dCA) was assessed with the time-varying autoregulation index estimated from the CBV step response derived by an autoregressive moving-average time-domain model. Forty patients with HF and 23 BP-matched healthy volunteers were studied. Median left ventricular ejection fraction was 38.5% (interquartile range: 0.075%) in the HF group. Compared with control subjects, patients with HF exhibited lower time-varying autoregulation index during iHG, indicating impaired dCA ( P < 0.025). During iHG, there were steep rises in CBV, BP, and heart rate in control subjects but with different temporal patterns in HF, which, together with the temporal evolution of resistance-area product, confirmed the disturbance in dCA in HF. Patients with HF were more likely to have impaired dCA during iHG compared with age-matched control subjects. Our results also suggest an impairment of myogenic, neurogenic, and metabolic control mechanisms in HF. The relationship between impaired dCA and neurological complications in patients with HF during exercise deserves further investigation. NEW & NOTEWORTHY Our findings provide the first direct evidence that cerebral blood flow regulatory mechanisms can be affected in patients with heart failure during isometric handgrip exercise. As a consequence, eventual blood pressure modulations are buffered less efficiently and metabolic demands may not be met during common daily activities. These deficits in cerebral autoregulation are compounded by limitations of the systemic response to isometric exercise, suggesting that patients with heart failure may be at greater risk for cerebral events during exercise.

Dynamic cerebral autoregulation is preserved during isometric handgrip and head-down tilt in healthy volunteers

In healthy humans, cerebral blood flow (CBF) is autoregulated against changes in arterial blood pressure. Spontaneous fluctuations in mean arterial pressure (MAP) and CBF can be used to assess cerebral autoregulation. We hypothesized that dynamic cerebral autoregulation is affected by changes in autonomic activity, MAP, and cardiac output (CO) induced by handgrip (HG), head‐down tilt (HDT), and their combination. In thirteen healthy volunteers, we recorded blood velocity by ultrasound in the internal carotid artery (ICA), HR, MAP and CO‐estimates from continuous finger blood pressure, and end‐tidal CO₂. Instantaneous ICA beat volume (ICABV, mL) and ICA blood flow (ICABF, mL/min) were calculated. Wavelet synchronization index γ (0–1) was calculated for the pairs: MAP–ICABF, CO–ICABF and HR–ICABV in the low (0.05–0.15 Hz; LF) and high (0.15–0.4 Hz; HF) frequency bands. ICABF did not change between experimental states. MAP and CO were increased during HG (+16% and +15%, respectively, P < 0.001) and during HDT + HG (+12% and +23%, respectively, P < 0.001). In the LF interval, median γ for the MAP–ICABF pair (baseline: 0.23 [0.12–0.28]) and the CO–ICABF pair (baseline: 0.22 [0.15–0.28]) did not change with HG, HDT, or their combination. High γ was observed for the HR–ICABV pair at the respiratory frequency, the oscillations in these variables being in inverse phase. The unaltered ICABF and the low synchronization between MAP and ICABF in the LF interval suggest intact dynamic cerebral autoregulation during HG, HDT, and their combination.

Effect of acute hypoxemia on cerebral blood flow velocity control during lower body negative pressure

The ability to maintain adequate cerebral blood flow and oxygenation determines tolerance to central hypovolemia. We tested the hypothesis that acute hypoxemia during simulated blood loss in humans would cause impairments in cerebral blood flow control. Ten healthy subjects (32 ± 6 years, BMI 27 ± 2 kg·m-2 ) were exposed to stepwise lower body negative pressure (LBNP, 5 min at 0, -15, -30, and -45 mmHg) during both normoxia and hypoxia (Fi O2  = 0.12-0.15 O2 titrated to an SaO2 of ~85%). Physiological responses during both protocols were expressed as absolute changes from baseline, one subject was excluded from analysis due to presyncope during the first stage of LBNP during hypoxia. LBNP induced greater reductions in mean arterial pressure during hypoxia versus normoxia (MAP, at -45 mmHg: -20 ± 3 vs. -5 ± 3 mmHg, P < 0.01). Despite differences in MAP, middle cerebral artery velocity responses (MCAv) were similar between protocols (P = 0.41) due to increased cerebrovascular conductance index (CVCi) during hypoxia (main effect, P = 0.04). Low frequency MAP (at -45 mmHg: 17 ± 5 vs. 0 ± 5 mmHg2 , P = 0.01) and MCAv (at -45 mmHg: 4 ± 2 vs. -1 ± 1 cm·s-2 , P = 0.04) spectral power density, as well as low frequency MAP-mean MCAv transfer function gain (at -30 mmHg: 0.09 ± 0.06 vs. -0.07 ± 0.06 cm·s-1 ·mmHg-1 , P = 0.04) increased more during hypoxia versus normoxia. Contrary to our hypothesis, these findings support the notion that cerebral blood flow control is not impaired during exposure to acute hypoxia and progressive central hypovolemia despite lower MAP as a result of compensated increases in cerebral conductance and flow variability.

Cerebral blood flow, frontal lobe oxygenation and intra-arterial blood pressure during sprint exercise in normoxia and severe acute hypoxia in humans

Cerebral blood flow (CBF) is regulated to secure brain O₂ delivery while simultaneously avoiding hyperperfusion; however, both requisites may conflict during sprint exercise. To determine whether brain O₂ delivery or CBF is prioritized, young men performed sprint exercise in normoxia and hypoxia (P_IO₂ = 73 mmHg). During the sprints, cardiac output increased to ∼22 L min^-1, mean arterial pressure to ∼131 mmHg and peak systolic blood pressure ranged between 200 and 304 mmHg. Middle-cerebral artery velocity (MCAv) increased to peak values (∼16%) after 7.5 s and decreased to pre-exercise values towards the end of the sprint. When the sprints in normoxia were preceded by a reduced P_ETCO₂, CBF and frontal lobe oxygenation decreased in parallel ( r = 0.93, P < 0.01). In hypoxia, MCAv was increased by 25%, due to a 26% greater vascular conductance, despite 4-6 mmHg lower PaCO₂ in hypoxia than normoxia. This vasodilation fully accounted for the 22 % lower CaO₂ in hypoxia, leading to a similar brain O₂ delivery during the sprints regardless of P_IO₂. In conclusion, when a conflict exists between preserving brain O₂ delivery or restraining CBF to avoid potential damage by an elevated perfusion pressure, the priority is given to brain O₂ delivery.

Paradigm Change? Cardiac Output Better Associates with Cerebral Perfusion than Blood Pressure in Ischemic Stroke

Introduction

In patients with acute ischemic stroke, penumbral perfusion is maintained by collateral flow and so far is maintained by normal mean arterial pressure (MAP) levels. Since MAP is dependent on cardiac function, optimization of cardiac output might be a valuable hemodynamic goal in order to optimize cerebral perfusion (CP).

Methods

Cerebral perfusion was assessed by transcranial color-coded duplex and transcranial perfusion sonography in 10 patients with acute large hemispheric stroke. Time-to-peak (TTP) values of defined regions of interest (ROI) within the middle cerebral artery (MCA) territory were assessed bilaterally in addition to mean flow velocities of the MCA. Via semi-invasive advanced hemodynamic monitoring systemic hemodynamic parameters were assessed, including MAP and cardiac index (CI). Patients received sonographic follow-up after optimizing CI.

Results

TTP values of the deeply located ROIs of the non-affected as well as the affected hemisphere correlated highly significantly with CI (in affected side r = −0.827, p = 0.002; and in non-affected side r = −0.908, p < 0.0001). This demonstrates dependence of CP on CI, while correlation with MAP was not detected. Neither CI nor MAP revealed significant correlation with MCA velocity.

Changes in Muscle and Cerebral Deoxygenation and Perfusion during Repeated Sprints in Hypoxia to Exhaustion

During supramaximal exercise, exacerbated at exhaustion and in hypoxia, the circulatory system is challenged to facilitate oxygen delivery to working tissues through cerebral autoregulation which influences fatigue development and muscle performance. The aim of the study was to evaluate the effects of different levels of normobaric hypoxia on the changes in peripheral and cerebral oxygenation and performance during repeated sprints to exhaustion. Eleven recreationally active participants (six men and five women; 26.7 ± 4.2 years, 68.0 ± 14.0 kg, 172 ± 12 cm, 14.1 ± 4.7% body fat) completed three randomized testing visits in conditions of simulated altitude near sea-level (~380 m, F_IO₂ 20.9%), ~2000 m (F_IO₂ 16.5 ± 0.4%), and ~3800 m (F_IO₂ 13.3 ± 0.4%). Each session began with a 12-min warm-up followed by two 10-s sprints and the repeated cycling sprint (10-s sprint: 20-s recovery) test to exhaustion. Measurements included power output, vastus lateralis, and prefrontal deoxygenation [near-infrared spectroscopy, delta (Δ) corresponds to the difference between maximal and minimal values], oxygen uptake, femoral artery blood flow (Doppler ultrasound), hemodynamic variables (transthoracic impedance), blood lactate concentration, and rating of perceived exertion. Performance (total work, kJ; −27.1 ± 25.8% at 2000 m, p < 0.01 and −49.4 ± 19.3% at 3800 m, p < 0.001) and pulse oxygen saturation (−7.5 ± 6.0%, p < 0.05 and −18.4 ± 5.3%, p < 0.001, respectively) decreased with hypoxia, when compared to 400 m. Muscle Δ hemoglobin difference ([Hbdiff]) and Δ tissue saturation index (TSI) were lower (p < 0.01) at 3800 m than at 2000 and 400 m, and lower Δ deoxyhemoglobin resulted at 3800 m compared with 2000 m. There were reduced changes in peripheral [Δ[Hbdiff], ΔTSI, Δ total hemoglobin ([tHb])] and greater changes in cerebral (Δ[Hbdiff], Δ[tHb]) oxygenation throughout the test to exhaustion (p < 0.05). Changes in cerebral deoxygenation were greater at 3800 m than at 2000 and 400 m (p < 0.01). This study confirms that performance in hypoxia is limited by continually decreasing oxygen saturation, even though exercise can be sustained despite maximal peripheral deoxygenation. There may be a cerebral autoregulation of increased perfusion accounting for the decreased arterial oxygen content and allowing for task continuation, as shown by the continued cerebral deoxygenation.

Postural effects on cerebral blood flow and autoregulation

Cerebral autoregulation (CA) is thought to maintain relatively constant cerebral blood flow (CBF) across normal blood pressures. To determine if postural changes alter CA, we measured cerebral blood flow velocity (CBFv) in the middle cerebral arteries, mean arterial blood pressure (MABP), cardiac output (Q), and end‐tidal carbon dioxide (PETCO₂) in 18 healthy individuals (11 female and seven male; 26 ± 9 years) during repeated periods of supine and seated rest. Multiple regression was used to evaluate the influence of PETCO₂, MABP, Q, and hydrostatic pressure on CBFv. Static CA was assessed by evaluating absolute changes in steady‐state CBFv. Dynamic CA was assessed by transfer function analysis of the CBFv response to spontaneous oscillations in MABP. In the seated versus supine posture, MABP (67.2 ± 7.2 vs. 84.2 ± 12.1 mmHg; P < 0.001), CBFv (55.2 ± 9.1 vs. 63.6 ± 10.6 cm/sec; P < 0.001) and PETCO₂ (29.1 ± 2.6 vs. 30.9 ± 2.3 mmHg; P < 0.001) were reduced. Changes in CBFv were not explained by variance in PETCO₂, MABP, Q, or hydrostatic pressure. A reduction in MABP to CBFv transfer function gain while seated (P < 0.01) was explained by changes in the power spectrum of MABP, not CBFv. Our findings suggest that changes in steady‐state cerebral hemodynamics between postures do not appear to have a large functional consequence on the dynamic regulation of CBF.

Cerebral blood flow autoregulation in ischemic heart failure

Patients with ischemic heart failure (iHF) have a high risk of neurological complications such as cognitive impairment and stroke. We hypothesized that iHF patients have a higher incidence of impaired dynamic cerebral autoregulation (dCA). Adult patients with iHF and healthy volunteers were included. Cerebral blood flow velocity (CBFV, transcranial Doppler, middle cerebral artery), end-tidal CO₂ (capnography), and arterial blood pressure (Finometer) were continuously recorded supine for 5 min at rest. Autoregulation index (ARI) was estimated from the CBFV step response derived by transfer function analysis using standard template curves. Fifty-two iHF patients and 54 age-, gender-, and BP-matched healthy volunteers were studied. Echocardiogram ejection fraction was 40 (20-45) % in iHF group. iHF patients compared with control subjects had reduced end-tidal CO₂ (34.1 ± 3.7 vs. 38.3 ± 4.0 mmHg, P < 0.001) and lower ARI values (5.1 ± 1.6 vs. 5.9 ± 1.0, P = 0.012). ARI <4, suggestive of impaired CA, was more common in iHF patients (28.8 vs. 7.4%, P = 0.004). These results confirm that iHF patients are more likely to have impaired dCA compared with age-matched controls. The relationship between impaired dCA and neurological complications in iHF patients deserves further investigation.

Internal carotid artery blood flow in healthy awake subjects is reduced by simulated hypovolemia and noninvasive mechanical ventilation

Intact cerebral blood flow (CBF) is essential for cerebral metabolism and function, whereas hypoperfusion in relation to hypovolemia and hypocapnia can lead to severe cerebral damage. This study was designed to assess internal carotid artery blood flow (ICA-BF) during simulated hypovolemia and noninvasive positive pressure ventilation (PPV) in young healthy humans. Beat-by-beat blood velocity (ICA and aorta) were measured by Doppler ultrasound during normovolemia and simulated hypovolemia (lower body negative pressure), with or without PPV in 15 awake subjects. Heart rate, plethysmographic finger arterial pressure, respiratory frequency, and end-tidal CO2 (ETCO2) were also recorded. Cardiac index (CI) and ICA-BF were calculated beat-by-beat. Medians and 95% confidence intervals and Wilcoxon signed rank test for paired samples were used to test the difference between conditions. Effects on ICA-BF were modeled by linear mixed-effects regression analysis. During spontaneous breathing, ICA-BF was reduced from normovolemia (247, 202-284 mL/min) to hypovolemia (218, 194-271 mL/min). During combined PPV and hypovolemia, ICA-BF decreased by 15% (200, 152-231 mL/min, P = 0.001). Regression analysis attributed this fall to concurrent reductions in CI (β: 43.2, SE: 17.1, P = 0.013) and ETCO2 (β: 32.8, SE: 9.3, P = 0.001). Mean arterial pressure was maintained and did not contribute to ICA-BF variance. In healthy awake subjects, ICA-BF was significantly reduced during simulated hypovolemia combined with noninvasive PPV Reductions in CI and ETCO2 had additive effects on ICA-BF reduction. In hypovolemic patients, even low-pressure noninvasive ventilation may cause clinically relevant reductions in CBF, despite maintained arterial blood pressure.

Transcranial doppler and near infrared spectroscopy in the perioperative period

PURPOSE OF REVIEW

Maintenance of adequate blood flow and oxygen to the brain is one of the principal endpoints of all surgery and anesthesia. During operations in general anesthesia, however, the brain is at particular risk for silent ischemia. Despite this risk, the brain still remains one of the last monitored organs in clincial anesthesiology.

RECENT FINDINGS

Transcranial Doppler (TCD) sonography and near-infrared spectroscopy (NIRS) experience a revival as these noninvasive technologies help to detect silent cerebral ischemia. TCD allows for quantification of blood flow velocities in basal intracranial arteries. TCD-derived variables such as the pulsatility index might hint toward diminished cognitive reserve or raised intracranial pressure. NIRS allows for assessment of regional cerebral oxygenation. Monitoring should be performed during high-risk surgery for silent cerebral ischemia and special circumstances during critical care medicine. Both techniques allow for the assessment of cerebrovascular autoregulation and individualized management of cerebral hemodynamics.

SUMMARY

TCD and NIRS are noninvasive monitors that anesthesiologists apply to tailor cerebral oxygen delivery, aiming to safeguard brain function in the perioperative period.

Is mean arterial pressure the best parameter in ischemic stroke?

This case series of 27 patients with large stroke challenges the current state of the art guiding hemodynamic management by blood pressure levels. The results show that assumed correlations of blood pressure, cardiac output, and systemic vascular resistance do not exist. Therefore, hemodynamic therapy may better be guided by cardiac output.

Cardiac Output and Cerebral Blood Flow

Cerebral blood flow (CBF) is rigorously regulated by various powerful mechanisms to safeguard the match between cerebral metabolic demand and supply. The question of how a change in cardiac output (CO) affects CBF is fundamental, because CBF is dependent on constantly receiving a significant proportion of CO. The authors reviewed the studies that investigated the association between CO and CBF in healthy volunteers and patients with chronic heart failure. The overall evidence shows that an alteration in CO, either acutely or chronically, leads to a change in CBF that is independent of other CBF-regulating parameters including blood pressure and carbon dioxide. However, studies on the association between CO and CBF in patients with varying neurologic, medical, and surgical conditions were confounded by methodologic limitations. Given that CBF regulation is multifactorial but the various processes must exert their effects on the cerebral circulation simultaneously, the authors propose a conceptual framework that integrates the various CBF-regulating processes at the level of cerebral arteries/arterioles while still maintaining autoregulation. The clinical implications pertinent to the effect of CO on CBF are discussed. Outcome research relating to the management of CO and CBF in high-risk patients or during high-risk surgeries is needed.

Effects of Steroid Hormones on Sex Differences in Cerebral Perfusion

Sex differences in the brain appear to play an important role in the prevalence and progression of various neuropsychiatric disorders, but to date little is known about the cerebral mechanisms underlying these differences. One widely reported finding is that women demonstrate higher cerebral perfusion than men, but the underlying cause of this difference in perfusion is not known. This study investigated the putative role of steroid hormones such as oestradiol, testosterone, and dehydroepiandrosterone sulphate (DHEAS) as underlying factors influencing cerebral perfusion. We acquired arterial spin labelling perfusion images of 36 healthy adult subjects (16 men, 20 women). Analyses on average whole brain perfusion levels included a multiple regression analysis to test for the relative impact of each hormone on the global perfusion. Additionally, voxel-based analyses were performed to investigate the sex difference in regional perfusion as well as the correlations between local perfusion and serum oestradiol, testosterone, and DHEAS concentrations. Our results replicated the known sex difference in perfusion, with women showing significantly higher global and regional perfusion. For the global perfusion, DHEAS was the only significant predictor amongst the steroid hormones, showing a strong negative correlation with cerebral perfusion. The voxel-based analyses revealed modest sex-dependent correlations between local perfusion and testosterone, in addition to a strong modulatory effect of DHEAS in cortical, subcortical, and cerebellar regions. We conclude that DHEAS in particular may play an important role as an underlying factor driving the difference in cerebral perfusion between men and women.

Hemodilution Combined With Hypercapnia Impairs Cerebral Autoregulation During Normothermic Cardiopulmonary Bypass

OBJECTIVE

To investigate the influence of hemodilution and arterial pCO2 on cerebral autoregulation and cerebral vascular CO2 reactivity.

DESIGN

Prospective interventional study.

SETTING

University hospital-based single-center study.

PARTICIPANTS

Forty adult patients undergoing elective cardiac surgery using normothermic cardiopulmonary bypass.

INTERVENTIONS

Blood pressure variations induced by 6/minute metronome-triggered breathing (baseline) and cyclic 6/min changes of indexed pump flow at 3 levels of arterial pCO2.

MEASUREMENTS AND MAIN RESULTS

Based on median hematocrit on bypass, patients were assigned to either a group of a hematocrit ≥28% or<28%. The autoregulation index was calculated from cerebral blood flow velocity and mean arterial blood pressure using transfer function analysis. Cerebral vascular CO2 reactivity was calculated using cerebral tissue oximetry data. Cerebral autoregulation as reflected by autoregulation index (baseline 7.5) was significantly affected by arterial pCO2 (median autoregulation index amounted to 5.7, 4.8, and 2.8 for arterial pCO2 of 4.0, 5.3, and 6.6 kPa, p≤0.002) respectively. Hemodilution resulted in a decreased autoregulation index; however, during hypocapnia and normocapnia, there were no significant differences between the two hematocrit groups. Moreover, the autoregulation index was lowest during hypercapnia when hematocrit was<28% (autoregulation index 3.3 versus 2.6 for hematocrit ≥28% and<28%, respectively, p = 0.014). Cerebral vascular CO2 reactivity during hypocapnia was significantly lower when perioperative hematocrit was<28% (p = 0.018).

CONCLUSIONS

Hemodilution down to a hematocrit of<28% combined with hypercapnia negatively affects dynamic cerebral autoregulation, which underlines the importance of tight control of both hematocrit and paCO2 during CPB.

Cerebrovascular effects of the thigh cuff maneuver

Arterial hypotension can be induced by sudden release of inflated thigh cuffs (THC), but its effects on the cerebral circulation have not been fully described. In nine healthy subjects [aged 59 (9) yr], bilateral cerebral blood flow velocity (CBFV) was recorded in the middle cerebral artery (MCA), noninvasive arterial blood pressure (BP) in the finger, and end-tidal CO2 (ETCO2) with nasal capnography. Three THC maneuvers were performed in each subject with cuff inflation 20 mmHg above systolic BP for 3 min before release. Beat-to-beat values were extracted for mean CBFV, BP, ETCO2 , critical closing pressure (CrCP), resistance-area product (RAP), and heart rate (HR). Time-varying estimates of the autoregulation index [ARI(t)] were also obtained using an autoregressive-moving average model. Coherent averages synchronized by the instant of cuff release showed significant drops in mean BP, CBFV, and RAP with rapid return of CBFV to baseline. HR, ETCO2 , and ARI(t) were transiently increased, but CrCP remained relatively constant. Mean values of ARI(t) for the 30 s following cuff release were not significantly different from the classical ARI [right MCA 5.9 (1.1) vs. 5.1 (1.6); left MCA 5.5 (1.4) vs. 4.9 (1.7)]. HR was strongly correlated with the ARI(t) peak after THC release (in 17/22 and 21/24 recordings), and ETCO2 was correlated with the subsequent drop in ARI(t) (19/22 and 20/24 recordings). These results suggest a complex cerebral autoregulatory response to the THC maneuver, dominated by myogenic mechanisms and influenced by concurrent changes in ETCO2 and possible involvement of the autonomic nervous system and baroreflex.

Prior head-down tilt does not impair the cerebrovascular response to head-up tilt

The hypothesis that cerebrovascular autoregulation was not impaired during head-up tilt (HUT) that followed brief exposures to varying degrees of prior head-down tilt (HDT) was tested in 10 healthy young men and women. Cerebral mean flow velocity (MFV) and cardiovascular responses were measured in transitions to a 60-s period of 75° HUT that followed supine rest (control) or 15 s HDT at -10°, -25°, and -55°. During HDT, heart rate (HR) was reduced for -25° and -55°, and cardiac output was lower at -55° HDT. MFV increased during -10° HDT, but not in the other conditions even though blood pressure at the middle cerebral artery (BPMCA) increased. On the transition to HUT, HR increased only for -55° condition, but stroke volume and cardiac output transiently increased for -25° and -55°. Total peripheral resistance index decreased in proportion to the magnitude of HDT and recovered over the first 20 s of HUT. MFV was significantly less in all HDT conditions compared with the control in the first 5-s period of HUT, but it recovered quickly. An autoregulation correction index derived from MFV recovery relative to BPMCA decline revealed a delay in the first 5 s for prior HDT compared with control but then a rapid increase to briefly exceed control after -55° HDT. This study showed that cerebrovascular autoregulation is modified by but not impaired by brief HDT prior to HUT and that cerebral MFV recovered quickly and more rapidly than arterial blood pressure to protect against cerebral hypoperfusion and potential syncope.

Heart rate passivity of cerebral tissue oxygenation is associated with predictors of poor outcome in preterm infants

AIM

Near-infrared spectroscopy (NIRS) and transcranial Doppler (TCD) allow non-invasive assessment of cerebral haemodynamics. We assessed cerebrovascular reactivity in preterm infants by investigating the relationship between NIRS- and TCD-derived indices and correlating them with severity of clinical illness.

METHODS

We recorded the NIRS-derived cerebral tissue oxygenation index (TOI) and TCD-derived flow velocity (Fv), along with other physiological variables. Moving correlation coefficients between measurements of cerebral perfusion (TOI, Fv) and heart rate were calculated. We presumed that positivity of these correlation coefficients - tissue oxygenation heart rate reactivity index (TOHRx) and flow velocity heart rate reactivity index (FvHRx) - would indicate a direct relationship between cerebral perfusion and cardiac output representing impaired cerebrovascular autoregulation.

RESULTS

We studied 31 preterm infants at a median age of 2 days, born at a median gestational age of 26 + 1 weeks. TOHRx was significantly correlated with gestational age (R = -0.57, p = 0.007), birth weight (R = -0.58, p = 0.006) and the Clinical Risk Index for Babies II (R = 0.55, p = 0.0014). TOHRx and FvHRx were significantly correlated (R = 0.39, p = 0.028).

CONCLUSION

Heart rate has a key influence on cerebral haemodynamics in preterm infants, and TOHRx may be of diagnostic value in identifying impaired cerebrovascular reactivity leading to adverse clinical outcome.

Autonomic dysfunction affects dynamic cerebral autoregulation during Valsalva maneuver: comparison between healthy and autonomic dysfunction subjects

The role of autonomic nervous system (ANS) in adapting cerebral blood flow (CBF) to arterial blood pressure (ABP) fluctuations [cerebral autoregulation (CA)] is still controversial. We aimed to study the repercussion of autonomic failure (AF) on dynamic CA during the Valsalva maneuver (VM). Eight AF subjects with familial amyloidotic polineuropahty (FAP) were compared with eight healthy controls. ABP and CBF velocity (CBFV) were measured continuously with Finapres and transcranial Doppler, respectively. Cerebrovascular response was evaluated by cerebrovascular resistance index (CVRi), critical closing pressure (CrCP), and resistance-area product (RAP) changes. Dynamic CA was derived from continuous estimates of autoregulatory index (ARI) [ARI(t)]. During phase II of VM, FAP subjects showed a more pronounced decrease in normalized CBFV (78 ± 19 and 111 ± 16%; P = 0.002), ABP (78 ± 19 and 124 ± 12%; P = 0.0003), and RAP (67 ± 17 and 89 ± 17%; P = 0.019) compared with controls. CrCP and CVRi increased similarly in both groups during strain. ARI(t) showed a biphasic variation in controls with initial increase followed by a decrease during phase II but in FAP this response was blunted (5.4 ± 3.0 and 2.0 ± 2.9; P = 0.033). Our data suggest that dynamic cerebral autoregulatory response is a time-varying phenomena during VM and that it is disturbed by autonomic dysfunction. This study also emphasizes the fact that RAP + CrCP model allowed additional insights into understanding of cerebral hemodynamics, showing a higher vasodilatory response expressed by RAP in AF and an equal CrCP response in both groups during the increased intracranial and intrathoracic pressure, while classical CVRi paradoxically suggests a cerebral vasoconstriction.

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

Cerebral autoregulation (CA) is integral to the delicate process of maintaining stable cerebral perfusion and brain tissue oxygenation against changes in arterial blood pressure. The last four decades has seen dramatic advances in understanding CA physiology, and the role that CA might play in the causation and progression of disease processes that affect the cerebral circulation such as stroke. However, the translation of these basic scientific advances into clinical practice has been limited by the maintenance of old constructs and because there are persistent gaps in our understanding of how this vital vascular mechanism should be quantified. In this review, we re-evaluate relevant studies that challenge established paradigms about how the cerebral perfusion pressure and blood flow are related. In the context of blood pressure being a major haemodynamic challenge to the cerebral circulation, we conclude that: (1) the physiological properties of CA remain inconclusive, (2) many extant methods for CA characterisation are based on simplistic assumptions that can give rise to misleading interpretations, and (3) robust evaluation of CA requires thorough consideration not only of active vasomotor function, but also the unique properties of the intracranial environment.

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

Cerebral autoregulation (CA) is integral to the delicate process of maintaining stable cerebral perfusion and brain tissue oxygenation against changes in arterial blood pressure. The last four decades has seen dramatic advances in understanding CA physiology, and the role that CA might play in the causation and progression of disease processes that affect the cerebral circulation such as stroke. However, the translation of these basic scientific advances into clinical practice has been limited by the maintenance of old constructs and because there are persistent gaps in our understanding of how this vital vascular mechanism should be quantified. In this review, we re-evaluate relevant studies that challenge established paradigms about how the cerebral perfusion pressure and blood flow are related. In the context of blood pressure being a major haemodynamic challenge to the cerebral circulation, we conclude that: (1) the physiological properties of CA remain inconclusive, (2) many extant methods for CA characterisation are based on simplistic assumptions that can give rise to misleading interpretations, and (3) robust evaluation of CA requires thorough consideration not only of active vasomotor function, but also the unique properties of the intracranial environment.