Heterogeneous regulation of cerebral blood flow in hypoxia; implications for dynamic cerebral autoregulation and susceptibility to acute mountain sickness

Hysteresis in the cerebral pressure-flow relationship describes the superior ability of the cerebrovasculature to buffer cerebral blood flow changes when mean arterial pressure (MAP) acutely increases compared to when it decreases. This phenomenon can be evaluated by comparing the relative change in middle cerebral artery mean blood velocity (MCAv) per relative change in MAP (%ΔMCAv/%ΔMAP) during either acute increases or decreases in MAP induced by repeated squat-stands (RSS). However, no real baseline can be employed for this particular protocol as there is no true stable reference point. Herein, we characterized the %ΔMCAv/%ΔMAP metric using the greatest MAP oscillations induced by RSS without using an independent baseline value. We also examined whether %ΔMCAv/%ΔMAP during each RSS transition were comparable between each other over the 5-min period. %ΔMCAv/%ΔMAP was calculated using the minimum to maximum MCAv and MAP for each RSS performed at 0.05 Hz and 0.10 Hz. We compared averaged %ΔMCAv/%ΔMAP during MAP increases and decreases in 74 healthy subjects [9 women; 32 ± 13 years]. %ΔMCAv/%ΔMAP was lower for MAP increases than MAP decreases (0.05 Hz: 1.25 ± 0.22 vs. 1.35 ± 0.27 %/%, p<0.0001; 0.10Hz: 1.31 ± 0.24 vs. 1.60 ± 0.50 %/%, p<0.0001). For both frequency and MAP direction, time during RSS had no effect on %ΔMCAv/%ΔMAP. This novel analytical method supports the use of the RSS model to evaluate the directional behavior of the pressure-flow relationship. These results contribute to the importance of considering the direction of MAP changes when evaluating dynamic cerebral autoregulation.

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Impaired dynamic cerebral autoregulation in trained breath-hold divers

Journal of Applied Physiology ◽

10.1152/japplphysiol.00210.2019 ◽

2019 ◽

Vol 126 (6) ◽

pp. 1694-1700 ◽

Cited By ~ 2

Author(s):

M. Erin Moir ◽

Stephen A. Klassen ◽

Baraa K. Al-Khazraji ◽

Emilie Woehrle ◽

Sydney O. Smith ◽

...

Keyword(s):

Blood Pressure ◽

Blood Flow ◽

Cerebral Blood Flow ◽

Flow Velocity ◽

Blood Flow Velocity ◽

Cerebral Autoregulation ◽

Cerebral Blood Flow Velocity ◽

Carbon Dioxide Partial Pressure ◽

Breath Hold ◽

Dynamic Cerebral Autoregulation

Breath-hold divers (BHD) experience repeated bouts of severe hypoxia and hypercapnia with large increases in blood pressure. However, the impact of long-term breath-hold diving on cerebrovascular control remains poorly understood. The ability of cerebral blood vessels to respond rapidly to changes in blood pressure represents the property of dynamic autoregulation. The current investigation tested the hypothesis that breath-hold diving impairs dynamic autoregulation to a transient hypotensive stimulus. Seventeen BHD (3 women, 11 ± 9 yr of diving) and 15 healthy controls (2 women) completed two or three repeated sit-to-stand trials during spontaneous breathing and poikilocapnic conditions. Heart rate (HR), finger arterial blood pressure (BP), and cerebral blood flow velocity (BFV) from the right middle cerebral artery were measured continuously with three-lead electrocardiography, finger photoplethysmography, and transcranial Doppler ultrasonography, respectively. End-tidal carbon dioxide partial pressure was measured with a gas analyzer. Offline, an index of cerebrovascular resistance (CVRi) was calculated as the quotient of mean BP and BFV. The rate of the drop in CVRi relative to the change in BP provided the rate of regulation [RoR; (∆CVRi/∆T)/∆BP]. The BHD demonstrated slower RoR than controls ( P ≤ 0.001, d = 1.4). Underlying the reduced RoR in BHD was a longer time to reach nadir CVRi compared with controls ( P = 0.004, d = 1.1). In concert with the longer CVRi response, the time to reach peak BFV following standing was longer in BHD than controls ( P = 0.01, d = 0.9). The data suggest impaired dynamic autoregulatory mechanisms to hypotension in BHD. NEW & NOTEWORTHY Impairments in dynamic cerebral autoregulation to hypotension are associated with breath-hold diving. Although weakened autoregulation was observed acutely in this group during apneic stress, we are the first to report on chronic adaptations in cerebral autoregulation. Impaired vasomotor responses underlie the reduced rate of regulation, wherein breath-hold divers demonstrate a prolonged dilatory response to transient hypotension. The slower cerebral vasodilation produces a longer perturbation in cerebral blood flow velocity, increasing the risk of cerebral ischemia.

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Fundamental relationships between arterial baroreflex sensitivity and dynamic cerebral autoregulation in humans

Journal of Applied Physiology ◽

10.1152/japplphysiol.01390.2009 ◽

2010 ◽

Vol 108 (5) ◽

pp. 1162-1168 ◽

Cited By ~ 67

Author(s):

Yu-Chieh Tzeng ◽

Samuel J. E. Lucas ◽

Greg Atkinson ◽

Chris K. Willie ◽

Philip N. Ainslie

Keyword(s):

Blood Pressure ◽

Blood Flow ◽

Cerebral Blood Flow ◽

Transfer Function ◽

Blood Flow Velocity ◽

Cerebral Autoregulation ◽

Baroreflex Sensitivity ◽

Cerebral Blood Flow Velocity ◽

Arterial Baroreflex ◽

Dynamic Cerebral Autoregulation

The functional relationship between dynamic cerebral autoregulation (CA) and arterial baroreflex sensitivity (BRS) in humans is unknown. Given that adequate cerebral perfusion during normal physiological challenges requires the integrated control of CA and the arterial baroreflex, we hypothesized that between-individual variability in dynamic CA would be related to BRS in humans. We measured R-R interval, blood pressure, and cerebral blood flow velocity (transcranial Doppler) in 19 volunteers. BRS was estimated with the modified Oxford method (nitroprusside-phenylephrine injections) and spontaneous low-frequency (0.04–0.15) α-index. Dynamic CA was quantified using the rate of regulation (RoR) and autoregulatory index (ARI) derived from the thigh-cuff release technique and transfer function analysis of spontaneous oscillations in blood pressure and mean cerebral blood flow velocity. Results show that RoR and ARI were inversely related to nitroprusside BRS [ R = −0.72, confidence interval (CI) −0.89 to −0.40, P = 0.0005 vs. RoR; R = −0.69, CI −0.88 to −0.35, P = 0.001 vs. ARI], phenylephrine BRS ( R = −0.66, CI −0.86 to −0.29, P = 0.0002 vs. RoR; R = −0.71, CI −0.89 to −0.38, P = 0.0001 vs. ARI), and α-index ( R = −0.70, CI −0.89 to −0.40, P = 0.0008 vs. RoR; R = −0.62, CI −0.84 to −0.24, P = 0.005 vs. ARI). Transfer function gain was positively related to nitroprusside BRS ( R = 0.62, CI 0.24–0.84, P = 0.0042), phenylephrine BRS ( R = 0.52, CI 0.10–0.79, P = 0.021), and α-index ( R = 0.69, CI 0.35–0.88, P = 0.001). These findings indicate that individuals with an attenuated dynamic CA have greater BRS (and vice versa), suggesting the presence of possible compensatory interactions between blood pressure and mechanisms of cerebral blood flow control in humans. Such compensatory adjustments may account for the divergent changes in dynamic CA and BRS seen, for example, in chronic hypotension and spontaneous hypertension.

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Reliability, Asymmetry, and Age Influence on Dynamic Cerebral Autoregulation Measured by Spontaneous Fluctuations of Blood Pressure and Cerebral Blood Flow Velocities in Healthy Individuals

Journal of Neuroimaging ◽

10.1111/jon.12019 ◽

2013 ◽

Vol 24 (4) ◽

pp. 379-386 ◽

Cited By ~ 12

Author(s):

Santiago Ortega-Gutierrez ◽

Nils Petersen ◽

Arjun Masurkar ◽

Andres Reccius ◽

Amy Huang ◽

...

Keyword(s):

Blood Pressure ◽

Blood Flow ◽

Cerebral Blood Flow ◽

Cerebral Autoregulation ◽

Healthy Individuals ◽

Blood Flow Velocities ◽

Spontaneous Fluctuations ◽

Dynamic Cerebral Autoregulation ◽

Flow Velocities

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