Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight

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.

Cardiovascular autonomic nervous system responses and orthostatic intolerance in astronauts and their relevance in daily medicine

Background

The harsh environmental conditions during space travel, particularly weightlessness, impose a major burden on the human body including the cardiovascular system. Given its importance in adjusting the cardiovascular system to environmental challenges, the autonomic nervous system has been in the focus of scientists and clinicians involved in human space flight. This review provides an overview on human autonomic research under real and simulated space conditions with a focus on orthostatic intolerance.

Methods

The authors conducted a targeted literature search using Pubmed.

Results

Overall, 120 articles were identified and included in the review.

Conclusions

Postflight orthostatic intolerance is commonly observed in astronauts and could pose major risks when landing on another celestial body. The phenomenon likely results from changes in volume status and adaptation of the autonomic nervous system to weightlessness. Over the years, various non-pharmacological and pharmacological countermeasures have been investigated. In addition to enabling safe human space flight, this research may have implications for patients with disorders affecting cardiovascular autonomic control on Earth.

Spaceflight Associated Neuro-Ocular Syndrome (SANS): A Systematic Review and Future Directions

Purpose

To present a systematic review of the current body of literature surrounding spaceflight associated neuro-ocular syndrome (SANS) and highlight priorities for future research.

Methods

Three major biomedical databases were searched with the following terms: ((neuro ocular) OR ((brain) AND (eye))) AND ((spaceflight) OR (astronaut) OR (microgravity)) AND (ENGLISH[Language]). Once duplicates were removed, 283 papers were left. Articles were excluded if they were not written in English or conference abstracts only. We avoided including review papers which did not provide any new information; however, two reviews on the pathophysiology of SANS were included for completeness. No limitations on date of publication were used. All included entries were then summarized for their contribution to knowledge about SANS.

Results

Four main themes among the publications emerged: papers defining the clinical entity of SANS, its pathophysiology, technology used to study SANS, and publications on possible prevention of SANS. The key clinical features of SANS include optic nerve head elevation, hyperopic shifts, globe flattening, choroidal folds, and increased cerebrospinal fluid (CSF) volume in optic nerve sheaths. Two main hypotheses are proposed for the pathophysiology of SANS. The first being elevated intracranial pressure and the second compartmentalization of CSF to the globe. These hypotheses are not mutually exclusive, and our understanding of the pathophysiology of SANS is still evolving. The use of optical coherence tomography (OCT) has greatly furthered our knowledge about SANS, and with the deployment of OCT to the International Space Station, we now have ability to collect intraflight data. No effective prevention for SANS has been found, although fortunately, even with persistent anatomic and physiologic neuro-ocular changes, any functional impact has been correctable with spectacles.

Conclusion

This is the first systematic review of SANS. Despite the limitations of studying a syndrome that can only occur in a small, discrete population, we present a thorough overview of the literature surrounding SANS and several key areas important for future research are identified.

Monocular 3D Sway Tracking for Assessing Postural Instability in Cerebral Hypoperfusion During Quiet Standing

Postural instability is prevalent in aging and neurodegenerative disease, decreasing quality of life and independence. Quantitatively monitoring balance control is important for assessing treatment efficacy and rehabilitation progress. However, existing technologies for assessing postural sway are complex and expensive, limiting their widespread utility. Here, we propose a monocular imaging system capable of assessing sub-millimeter 3D sway dynamics during quiet standing. Two anatomical targets with known feature geometries were placed on the lumbar and shoulder. Upper and lower trunk 3D kinematic motion were automatically assessed from a set of 2D frames through geometric feature tracking and an inverse motion model. Sway was tracked in 3D and compared between control and hypoperfusion conditions in 14 healthy young adults. The proposed system demonstrated high agreement with a commercial motion capture system (error [Formula: see text], [-0.52, 0.52]). Between-condition differences in sway dynamics were observed in anterior-posterior sway during early and mid stance, and medial-lateral sway during mid stance commensurate with decreased cerebral perfusion, followed by recovered sway dynamics during late stance with cerebral perfusion recovery. This inexpensive single-camera system enables quantitative 3D sway monitoring for assessing neuromuscular balance control in weakly constrained environments.

Impact of Hanging Motionless in Harness on Respiratory and Blood Pressure Reflex Modulation in Mountain Climbers

Harness hang syncope (HHS) is a risk that specifically affects safety of harness users in mountain climbing.

Aims: To evaluate individual patterns of breathing resulting from deranged cardiovascular reflexes triggering a syncopal event when a mismatch between cerebral O₂ demand and supply is present.

Results: Forty healthy participants [aged 39.1 (8.2) years] were enrolled in a motionless suspension test while hanging in harness. Respiratory gas exchange values were analyzed to assess the pattern of breathing (EpInW_el, respiratory elastic power) and cardiovascular parameters were monitored (BP, blood pressure). Four participants experienced HHS after 30.0 (7.6) minutes, with an early manifestation of loss of control of both a sustainable EpInW_el and BP, starting after 10–12 minutes. Among the other participants, two different reactions were observed during suspension: (1) group G1 tolerated 32.7 (11.4) minutes of suspension by a favorable adaptation of the EpInW_el and BP parameters and (2) group G2 showed significantly shorter time of suspension 24.0 (10.4) minutes with unfavorable increase in EpInW_el and BP.

Conclusions: Greater resistance to HHS occurs in people developing less marked fluctuations of both respiratory and cardiovascular reflex responses. Conversely, wider fluctuations both in control of EpInW_el and BP were observed in the event of decreased suspension tolerance or in syncopal events.

Relative contributions of systemic hemodynamic variables to cerebral autoregulation during orthostatic stress

Postural changes impair the ability of the cerebrovasculature to buffer against dynamic pressure fluctuations, but the mechanisms underlying this impairment have not been elucidated. We hypothesized that autoregulatory impairment may reflect the impact of static central volume shifts on hemodynamic factors other than arterial pressure (AP). In 14 young volunteers, we assessed the relation of fluctuations in cerebral blood flow (CBF) to those in AP, cardiac output, and CO₂, during oscillatory lower body pressure (LBP) (±20 mmHg at 0.01 and 0.06 Hz) at three static levels (-20, 0, and +20 mmHg). Static and dynamic changes in AP, cardiac output, and CO₂ explained over 70% of the variation in CBF fluctuations. However, their contributions were different across frequencies and levels: dynamic AP changes explained a substantial proportion of the variation in faster CBF fluctuations (partial R² = 0.75, standardized β = 0.83, P < 0.01), whereas those in CO₂ explained the largest portion of the variation in slow fluctuations (partial R² = 0.43, β = 0.51, P < 0.01). There was, however, a major contribution of slow dynamic AP changes during negative (β = 0.43) but not neutral (β = 0.05) or positive (β = -0.07) LBP. This highlights the differences in contributions of systemic variables to dynamic and static autoregulation and has important implications for understanding orthostatic intolerance. NEW & NOTEWORTHY While fluctuations in blood pressure drive faster fluctuations in cerebral blood flow, overall level of CO₂ and the magnitude of its fluctuations, along with cardiac output, determine the magnitude of slow ones. The effect of slow blood pressure fluctuations on cerebrovascular responses becomes apparent only during pronounced central volume shifts (such as when standing). This underlines distinct but interacting contributions of static and dynamic changes in systemic hemodynamic variables to the cerebrovascular regulation.

The Elusive Path of Brain Tissue Oxygenation and Cerebral Perfusion in Harness Hang Syncope in Mountain Climbers

Lanfranconi, Francesca, Luca Pollastri, Giovanni Corna, Manuela Bartesaghi, Massimiliano Novarina, Alessandra Ferri, and Giuseppe Andrea Miserocchi. The elusive path of brain tissue oxygenation and cerebral perfusion in harness hang syncope in mountain climbers. High Alt Med Biol. 18:363-371, 2017.

AIM

Harness hang syncope (HHS) is a risk that specifically affects wide ranges of situations requiring safety harnesses in mountains. An irreversible orthostatic stasis could lead to death if a prompt rescue is not performed. We aimed at evaluating the risk of developing HHS and at identifying the characteristics related to the pathogenesis of HHS.

RESULTS

Forty adults (aged 39.1 [8.2] years) were enrolled in a suspension test lasting about 28.7 (11.4) minutes. We measured cardiovascular parameters, and near infrared spectroscopy (NIRS) was used to assess cerebral hypoxia by changes in the concentration of oxyhemoglobin (Δ[HbO₂]) and de-oxyhemoglobin (Δ[HHb]). In the four participants who developed HHS: (1) systolic and diastolic blood pressure showed ample oscillations with a final abrupt drop (∼30 mmHg); (2) Δ[HbO₂] increased after 8-12 minutes of suspension and reached a plateau before HHS; and (3) Δ[HHb] decreased with a final abrupt increase before syncope.

CONCLUSIONS

Participants who developed HHS failed to activate cardiovascular reflexes that usually safeguard O₂ availability to match the metabolic needs of the brain tissue. Since cerebral hypoxia was detected as an early phenomenon by Δ[HbO₂] and Δ[HHb] changes, NIRS measurement appears to be the most important parameter to monitor the onset of HHS.

Enhanced Cholinergic Activity Improves Cerebral Blood Flow during Orthostatic Stress

Cerebral blood flow (CBF) and consequently orthostatic tolerance when upright depends on dilation of the cerebral vasculature in the face of reduced perfusion pressure associated with the hydrostatic gradient. However, it is still unclear if cholinergic activation plays a role in this dilation. To determine if enhancing central cholinergic activity with the centrally acting acetylcholinesterase inhibitor, physostigmine would increase CBF when upright compared to the peripherally acting acetylcholinesterase inhibitor, neostigmine, or saline. We performed a randomized double-blind dose-ranging study that took place over 3 days in a hospital-based research lab. Eight healthy controls (six women and two men, mean age, 26 years; range 21–33) were given infusions of physostigmine, neostigmine, or saline on three different days. Five-minute tilts were repeated at baseline (no infusion), Dose 1 (0.2 μg/kg/min physostigmine; 0.1 μg/kg/min neostigmine) and Dose 2 (0.6 μg/kg/min physostigmine or 0.3 μg/kg/min neostigmine), and placebo (0.9% NaCl). Cerebral blood velocity, beat-to-beat blood pressure, and end-tidal CO₂ were continuously measured during tilts. Physostigmine (0.6 μg/kg/min) resulted in higher cerebral blood velocity during tilt (90.5 ± 1.5%) than the equivalent neostigmine (85.5 ± 2.6%) or saline (84.8 ± 1.7%) trials (P < 0.05). This increase occurred despite a greater postural hypocapnia, suggesting physostigmine had a direct vasodilatory effect on the cerebral vasculature. Cerebral hypoperfusion induced by repeated tilts was eliminated by infusion of physostigmine not neostigmine. In conclusion, this study provides the first evidence that enhancement of central, not peripheral, cholinergic activity attenuates the physiological decrease in CBF seen during upright tilt. These data support the need for further research to determine if enhancing central cholinergic activity may improve symptoms in patients with symptomatic orthostatic intolerance.

Increased phase synchronization and decreased cerebral autoregulation during fainting in the young

Vasovagal syncope may be due to a transient cerebral hypoperfusion that accompanies frequency entrainment between arterial pressure (AP) and cerebral blood flow velocity (CBFV). We hypothesized that cerebral autoregulation fails during fainting; a phase synchronization index (PhSI) between AP and CBFV was used as a nonlinear, nonstationary, time-dependent measurement of cerebral autoregulation. Twelve healthy control subjects and twelve subjects with a history of vasovagal syncope underwent 10-min tilt table testing with the continuous measurement of AP, CBFV, heart rate (HR), end-tidal CO2 (ETCO2), and respiratory frequency. Time intervals were defined to compare physiologically equivalent periods in fainters and control subjects. A PhSI value of 0 corresponds to an absence of phase synchronization and efficient cerebral autoregulation, whereas a PhSI value of 1 corresponds to complete phase synchronization and inefficient cerebral autoregulation. During supine baseline conditions, both control and syncope groups demonstrated similar oscillatory changes in phase, with mean PhSI values of 0.58+/-0.04 and 0.54+/-0.02, respectively. Throughout tilt, control subjects demonstrated similar PhSI values compared with supine conditions. Approximately 2 min before fainting, syncopal subjects demonstrated a sharp decrease in PhSI (0.23+/-0.06), representing efficient cerebral autoregulation. Immediately after this period, PhSI increased sharply, suggesting inefficient cerebral autoregulation, and remained elevated at the time of faint (0.92+/-0.02) and during the early recovery period (0.79+/-0.04) immediately after the return to the supine position. Our data demonstrate rapid, biphasic changes in cerebral autoregulation, which are temporally related to vasovagal syncope. Thus, a sudden period of highly efficient cerebral autoregulation precedes the virtual loss of autoregulation, which continued during and after the faint.

Human cerebral autoregulation before, during and after spaceflight

Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts approximately 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02-0.07 Hz), low- (0.07-0.20 Hz) and high-frequency (0.20-0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (+/- s.e.m.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, -4 +/- 3 cm s(-1) after versus -14 +/- 3 cm s(-1) before, P = 0.001; and percentage, -8.0 +/- 4.8% after versus -24.8 +/- 4.4% before, P < 0.05), consistent with improved rather than impaired cerebral blood flow regulation. Low-frequency gain decreased significantly (P < 0.05) by 26, 23 and 27% after 1 and 2 weeks in space and on landing day, respectively, compared with preflight values, which is also consistent with improved autoregulation. We conclude that human cerebral autoregulation is preserved, and possibly even improved, by short-duration spaceflight.

Cerebral blood flow during orthostasis: role of arterial CO2

Reductions in end-tidal Pco(2) (Pet(CO(2))) during upright posture have been suggested to be the result of hyperventilation and the cause of decreases in cerebral blood flow (CBF). The goal of this study was to determine whether decreases in Pet(CO(2)) reflected decreases in arterial Pco(2) (Pa(CO(2))) and their relation to increases in alveolar ventilation (Va) and decreases in CBF. Fifteen healthy subjects (10 women and 5 men) were subjected to a 10-min head-up tilt (HUT) protocol. Pa(CO(2)), Va, and cerebral flow velocity (CFV) in the middle and anterior cerebral arteries were examined. In 12 subjects who completed the protocol, reductions in Pet(CO(2)) and Pa(CO(2)) (-1.7 +/- 0.5 and -1.1 +/- 0.4 mmHg, P < 0.05) during minute 1 of HUT were associated with a significant increase in Va (+0.7 +/- 0.3 l/min, P < 0.05). However, further decreases in Pa(CO(2)) (-0.5 +/- 0.5 mmHg, P < 0.05), from minute 1 to the last minute of HUT, occurred even though Va did not change significantly (-0.2 +/- 0.3 l/min, P = not significant). Similarly, CFV in the middle and anterior cerebral arteries decreased (-7 +/- 2 and -8 +/- 2%, P < 0.05) from minute 1 to the last minute of HUT, despite minimal changes in Pa(CO(2)). These data suggest that decreases in Pet(CO(2)) and Pa(CO(2)) during upright posture are not solely due to increased Va but could be due to ventilation-perfusion mismatch or a redistribution of CO(2) stores. Furthermore, the reduction in Pa(CO(2)) did not fully explain the decrease in CFV throughout HUT. These data suggest that factors in addition to a reduction in Pa(CO(2)) play a role in the CBF response to orthostatic stress.

Cerebral oxygenation declines despite maintained orthostatic tolerance after brief exposure to gravitational stress

We examined the effect of a single 120 s of exposure to +3Gz (head-to-foot inertial forces) centrifugation as orthostatic stress on cerebral oxygenation (oxy-Hb) and cerebral blood volume (CBV) changes in response to stand test, in order to relate the occurrence of altered cerebral oxygenation control to any increase in sympathetic activity. Frontal near-infrared spectroscopy and mean arterial blood pressure at brain level (MAPbrain) were recorded in 14 subjects in supine and then in standing (10 min) position, before and after +3Gz centrifugation. The decrease in oxy-Hb (-7 +/- 5 a.u. versus -27 +/- 4 a.u., P<0.001) and in CBV (-6 +/- 10 a.u. versus -15 +/- 8 a.u., P<0.05) upon standing was more important after +3Gz centrifugation, with unchanged MAPbrain (-8 +/- 8 mmHg versus -3 +/- 11 mmHg). Upon standing, the high-frequency component of heart rate was lower (1090 +/- 460 ms2 versus 827 +/- 412 ms2, P<0.05) after +3Gz centrifugation. These findings suggest a downward shift in the static cerebral autoregulatory curve. We conclude that cerebral vasoconstriction might have occurred without centrally mediated increase in the entire peripheral sympathetic activity of the body.

Dynamic cardiorespiratory interaction during head-up tilt-mediated presyncope

In 28 healthy adults, we compared the dynamic interaction between respiration and cerebral autoregulation in 2 groups of subjects: those who did and did not develop presyncopal symptoms during 70 degrees passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, CO2, cerebral blood flow velocity (CBF), ECG, and blood pressure (BP) were recorded. To determine whether influences of mean BP (MBP) and systolic SP (SBP) on CBF were altered in presyncopal subjects, coherencies and transfer functions between these variables and mean and peak CBF (CBFm and CBFp) were estimated. To determine the influence of end-tidal CO2 (ETco2) on CBF, the relative CO2 reactivity (%change in CBFm per mmHg change in ETco2) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CBFp was higher (P=0.02) and gains of transfer functions between BP (MBP and SBP) and CBFm were larger (MBP, P=0.01; SBP, P=0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals had a reduced relative CO2 reactivity (P=0.005), likely a consequence of the larger decrease in ETco2. We hypothesize that the CO2-mediated increase in resistance attenuates autoregulation such that the relationship between systemic and cerebral hemodynamics is enhanced. Our results suggest that an altered cardiorespiratory interaction involving cerebral hemodynamics may contribute in the cascade of events during tilt that culminate in unexplained syncope.

Circulating norepinephrine and cerebrovascular control in conscious humans

BACKGROUND

Cerebral vasoconstriction without concurrent changes in systemic arterial blood pressure has been observed in both normal individuals and those with idiopathic orthostatic intolerance following several minutes of postural stress when circulating catecholamines are elevated. Therefore, we tested the hypothesis that alpha-adrenergic activation with and without elevated circulating norepinephrine (NE) directly affects cerebrovascular tone in healthy individuals.

METHODS

Mean arterial pressure (MAP; tonometry) and cerebral blood flow velocity (MFV) in the middle cerebral artery (transcranial Doppler) were measured in seven healthy individuals during 15 min periods of saline and of 50 (low NE) and 100 (high NE) ng kg(-1) min(-1) infusions of NE. Following this, phentolamine (PHO) was administered to return MAP back to baseline while high NE infusion continued (high NE+PHO). Finally, NE infusion was stopped allowing the persistent effects of PHO to dominate.

RESULTS

Circulating NE caused a dose-dependent increase in MAP (P<0.05). During combined high NE+PHO, blood pressure was initially reduced to baseline levels but then increased a second time (P<0.05) during the final approximately 5 min of this phase. MFV remained constant during both low NE and high NE. In contrast, the secondary increase in BP during the late high NE+PHO phase was associated with elevated MFV. Cerebral vascular resistance (CVR) increased during high NE but was reduced to baseline during both early and late portions of the combined high NE+PHO phase (i.e. despite the late-phase increase in blood pressure).

CONCLUSIONS

The increase in CVR during NE infusion was explained by an autoregulatory response to the increased blood pressure and not an alpha-mediated constriction. However, PHO appeared to interfere with the normal autoregulatory response to increasing blood pressure.

Cerebral autoregulation is compromised during simulated fluctuations in gravitational stress

Gravity places considerable stress on the cardiovascular system but cerebral autoregulation usually protects the cerebral blood vessels from fluctuations in blood pressure. However, in conditions such as those encountered on board a high-performance aircraft, the gravitational stress is constantly changing and might compromise cerebral autoregulation. In this study we assessed the effect of oscillating orthostatic stress on cerebral autoregulation. Sixteen (eight male) healthy subjects [aged 27 (1) years] were exposed to steady-state lower body negative pressure (LBNP) at -15 and -40 mmHg and then to oscillating LBNP at the same pressures. The oscillatory LBNP was applied at 0.1 and 0.2 Hz. We made continuous recordings of RR-interval, blood pressure, cerebral blood flow velocity (CBFV), respiratory frequency and end-tidal CO(2). Oscillations in mean arterial pressure (MAP) and CBFV were assessed by autoregressive spectral analysis. Respiration was paced at 0.25 Hz to avoid interference from breathing. Steady-state LBNP at -40 mmHg significantly increased low-frequency (LF, 0.03-0.14 Hz) powers of MAP ( P<0.01) but not of CBFV. Oscillatory 0.1 Hz LBNP (0 to -40 mmHg) significantly increased the LF power of MAP to a similar level as steady-state LBNP but also resulted in a significant increase in the LF power of CBFV ( P<0.01). Oscillatory LBNP at 0.2 Hz induced oscillations in MAP and CBFV at 0.2 Hz. Cross-spectral analysis showed that the transfer of LBNP-induced oscillations in MAP onto the CBFV was significantly greater at 0.2 Hz than at 0.1 Hz ( P<0.01). These results show that the ability of the cerebral vessels to modulate fluctuations in blood pressure is compromised during oscillatory compared with constant gravitational stress. Furthermore, this effect seems to be more pronounced at higher frequencies of oscillatory stress.

Cerebral Circulation during Acute Microgravity Induced by Free Drop in Anesthetized Rats

To evaluate changes in the cerebral circulation during acute microgravity (microG), we measured intracranial pressure (ICP), aortic pressure at the diaphragm level, and cerebral flow velocity (CFV) in anesthetized rats (n = 5) during 4.5 s of microG induced by free drop, then calculated arterial pressure at the eye level (AP(eye)) and cerebral perfusion pressure (CPP = AP(eye)-ICP), and estimated CPP-CFV relationship. The rats were placed in the flat and the 30 degrees head-up positions. In the head-up position, ICP, AP(eye), and CPP were significantly increased by 2.2 +/- 0.4, 12.3 +/- 2.0, and 10.1 +/- 1.7 mmHg respectively during microG, whereas the CFV did not change significantly. In the flat position, none of these variables were significantly affected by microG. The slope of the CPP-CFV relationship was decreased only in the head-up position, suggesting that the cerebrovascular resistance was increased by microG. These findings indicate that the change in gravitational (hydrostatic) pressure is a key factor in understanding the changes in cerebral circulation during acute microG.

Effect of acute exposure to hypergravity (GX vs. GZ) on dynamic cerebral autoregulation

We examined the effects of 30 min of exposure to either +3GX (front-to-back) or +GZ (head-to-foot) centrifugation on cerebrovascular responses to 80 degrees head-up tilt (HUT) in 14 healthy individuals. Both before and after +3 GX or +3 GZ centrifugation, eye-level blood pressure (BP(eye)), end tidal PCO2 (PET(CO2)), mean cerebral flow velocity (CFV) in the middle cerebral artery (transcranial Doppler ultrasound), cerebral vascular resistance (CVR), and dynamic cerebral autoregulatory gain (GAIN) were measured with subjects in the supine position and during subsequent 80 degrees HUT for 30 min. Mean BP(eye) decreased with HUT in both the GX (n = 7) and GZ (n = 7) groups (P < 0.001), with the decrease being greater after centrifugation only in the GZ group (P < 0.05). PET(CO2) also decreased with HUT in both groups (P < 0.01), but the absolute level of decrease was unaffected by centrifugation. CFV decreased during HUT more significantly after centrifugation than before centrifugation in both groups (P < 0.02). However, these greater decreases were not associated with greater increases in CVR. In the supine position after centrifugation compared with before centrifugation, GAIN increased in both groups (P < 0.05, suggesting an autoregulatory deficit), with the change being correlated to a measure of otolith function (the linear vestibulo-ocular reflex) in the GX group (r = 0.76, P < 0.05) but not in the GZ group (r = 0.24, P = 0.60). However, GAIN was subsequently restored to precentrifugation levels during postcentrifugation HUT (i.e., as BP(eye) decreased), suggesting that both types of centrifugation resulted in a leftward shift of the cerebral autoregulation curve. We speculate that this leftward shift may have been due to vestibular activation (especially during +GX) or potentially to an adaptation to reduced cerebral perfusion pressure during +GZ.

PET(CO(2)) inversely affects MSNA response to orthostatic stress

Arterial hypocapnia has been associated with orthostatic intolerance. Therefore, we tested the hypothesis that hypocapnia may be detrimental to increases in muscle sympathetic nerve activity (MSNA) and total peripheral resistance (TPR) during head-up tilt (HUT). Ventilation was increased approximately 1.5 times above baseline for each of three conditions, whereas end-tidal PCO(2) (PET(CO(2))) was clamped at normocapnic (Normo), hypercapnic (Hyper; +5 mmHg relative to Normo), and hypocapnic (Hypo; -5 mmHg relative to Normo) conditions. MSNA (microneurography), heart rate, blood pressure (BP, Finapres), and cardiac output (Q, Doppler) were measured continuously during supine rest and 45 degrees HUT. The increase in heart rate when changing from supine to HUT (P < 0.001) was not different across PET(CO(2)) conditions. MSNA burst frequency increased similarly with HUT in all conditions (P < 0.05). However, total MSNA and the increase in total amplitude relative to baseline (%DeltaMSNA) increased more when changing to HUT during Hypo compared with Hyper (P < 0.05). Both BP and Q were higher during Hyper than both Normo and Hypo (main effect; P < 0.05). Therefore, the MSNA response to HUT varied inversely with levels of PET(CO(2)). The combined data suggest that augmented cardiac output with hypercapnia sustained blood pressure during HUT leading to a diminished sympathetic response.