Engineering topics

Stein, F.

Publications and source records attributed to Stein, F..

4 records

NASA NTRS2001

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.

NASA Discipline Neuroscience↗

NASA NTRS2001

Orthostatic intolerance and motion sickness after parabolic flight

Because it is not clear that the induction of orthostatic intolerance in returning astronauts always requires prolonged exposure to microgravity, we investigated orthostatic tolerance and autonomic cardiovascular function in 16 healthy subjects before and after the brief micro- and hypergravity of parabolic flight. Concomitantly, we investigated the effect of parabolic flight-induced vomiting on orthostatic tolerance, R-wave-R-wave interval and arterial pressure power spectra, and carotid-cardiac baroreflex and Valsalva responses. After parabolic flight 1) 8 of 16 subjects could not tolerate 30 min of upright tilt (compared to 2 of 16 before flight); 2) 6 of 16 subjects vomited; 3) new intolerance to upright tilt was associated with exaggerated falls in total peripheral resistance, whereas vomiting was associated with increased R-wave-R-wave interval variability and carotid-cardiac baroreflex responsiveness; and 4) the proximate mode of new orthostatic failure differed in subjects who did and did not vomit, with vomiters experiencing comparatively isolated upright hypocapnia and cerebral vasoconstriction and nonvomiters experiencing signs and symptoms reminiscent of the clinical postural tachycardia syndrome. Results suggest, first, that syndromes of orthostatic intolerance resembling those developing after space flight can develop after a brief (i.e., 2-h) parabolic flight and, second, that recent vomiting can influence the results of tests of autonomic cardiovascular function commonly utilized in returning astronauts.

NASA Center JSC↗

NASA NTRS2001

Effect of Acute Exposure to Hypergravity (Gx vs. Gz) on Dynamic Cerebral Autoregulation

We examined the effects of 30 min of exposure to either +3G(sub x) or +3G(sub z) centrifugation on cerebrovascular responses to 800 head-up tilt (HUT) in 14 healthy individuals. Both before and after +3G(sub x) or +3G(sub z) centrifugation, eye-level blood pressure (BP(sub eye)), end tidal CO2 (P(sub ET)CO2), mean cerebral flow velocity (CFV) in the middle cerebral artery (trans cranial Doppler ultrasound), cerebral vascular resistance (CVR) and dynamic cerebral autoregulatory gain (GAIN) were measured with subjects in the supine position and during subsequent 800 HUT for 30 min. Mean BP(sub eye) decreased with HUT in both the G(sub x) (n= 7) and G(sub z) (n=7) groups (P less than 0.00l), with the decrease being greater after centrifugation only in the G(sub z) group (P less than 0.05). P(sub ET)CO2 also decreased with HUT in both groups (P less than 0.0l), but the absolute level of decrease was unaffected by centrifugation. CFV decreased during HUT more significantly after than before centrifugation in both groups (P less than 0.02). However, these greater decreases were not associated with greater increases in CVR. In the supine position after compared to before centrifugation, GAIN increased in both groups (P less than 0.05, suggesting an autoregulatory deficit), with the change being correlated to a measure of otolith function (the linear vestibulo-ocular reflex) in the G(sub x) group (R=0.76, P less than 0.05) but not in the G(sub z) group (R=0.24, P=0.60). However, GAIN was subsequently restored to pre-centrifugation levels during post-centrifugation HUT (i.e., as BP(sub 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 +G(sub x)) or potentially to an adaptation to reduced cerebral perfusion pressure during +G(sub z).

Serrador, Jorge M.↗

NASA NTRS1995

Simultaneous cerebrovascular and cardiovascular responses during presyncope

BACKGROUND AND PURPOSE: Presyncope, characterized by symptoms and signs indicative of imminent syncope, can be aborted in many situations before loss of consciousness occurs. The plasticity of cerebral autoregulation in healthy humans and its behavior during this syncopal prodrome are unclear, although systemic hemodynamic instability has been suggested as a key factor in the precipitation of syncope. Using lower body negative pressure (LBNP) to simulate central hypovolemia, we previously observed falling mean flow velocities (MFVs) with maintained mean arterial blood pressure (MABP). These findings, and recent reports suggesting increased vascular tone within the cerebral vasculature at presyncope, cannot be explained by the classic static cerebral autoregulation curve; neither can they be totally explained by a recent suggestion of a rightward shift in this curve. METHODS: Four male and five female healthy volunteers were exposed to presyncopal LBNP to evaluate their cerebrovascular and cardiovascular responses by use of continuous acquisition of MFV from the right middle cerebral artery with transcranial Doppler sonography, MABP (Finapres), and heart rate (ECG). RESULTS: At presyncope, MFV dropped on average by 27.3 +/- 14% of its baseline value (P < .05), while MABP remained at 2.0 +/- 27% above its baseline level. Estimated cerebrovascular resistance increased during LBNP. The percentage change from baseline to presyncope in MFV and MABP revealed consistent decreases in MFV before MABP. CONCLUSIONS: Increased estimated cerebrovascular resistance, falling MFV, and constant MABP are evidence of an increase in cerebral vascular tone with falling flow, suggesting a downward shift in the cerebral autoregulation curve. Cerebral vessels may have a differential sensitivity to sympathetic drive or more than one type of sympathetic innervation. Future work to induce dynamic changes in MABP during LBNP may help in assessing the plasticity of the cerebral autoregulation mechanism.

Non-NASA Center↗