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  • 1
    Electronic Resource
    Electronic Resource
    Development of methods to analyse transcranial Doppler ultrasound signals recorded in microgravity (1990)
    Springer
    Medical & biological engineering & computing 28 (1990), S. 306-311 
    Details
    ISSN: 1741-0444
    Keywords: Microgravity ; TCD ; Transcranial Doppler ultrasound
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Abstract During space flights, several clinical syndromes may be the result of changes in cerebral circulation. The purpose of the paper is to describe the development and initial evaluation of a system for recording, processing and displaying transcranial Doppler ultrasound (TCD) waveforms from the middle cerebral artery (MCA) in microgravity. Volunteers were repeatedly subjected to 15–20 s intervals of microgravity (‘near zero gravity’) during flights on the KC-135 military aircraft. Continuous TCD recordings from the MCA were stored on magnetic tape. The paper describes the system that was developed to digitise the Doppler ultrasound data and markers that corresponded to the various levels of microgravity, obtain the maximum and mean Doppler waveforms, identify the waveforms and quantify them. The results demonstrate the feasibility of making TCD recordings in a microgravity environment and illustrate excellent performance of the system and its ease of operation. Quantitative waveform analysis of the recordings from the first subject studied in the supine position showed statistically significant changes in MCA velocity waveforms during microgravity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02446147
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    Paper (German National Licenses)
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  • 2
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    Simultaneous cerebrovascular and cardiovascular responses during presyncope (1995)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: 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.
    Keywords: Aerospace Medicine
    Type: Stroke; a journal of cerebral circulation (ISSN 0039-2499); Volume 26; 10; 1794-800
    Format: text
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  • 3
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    Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight (2000)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The effects of brief but repeated bouts of micro- and hypergravity on cerebrovascular responses to head-up tilt (HUT) were examined in 13 individuals after (compared to before) parabolic flight. Middle cerebral artery mean flow velocity (MCA MFV; transcranial Doppler ultrasound), eye level blood pressure (BP) and end tidal CO(2) (P(ET)CO(2)) were measured while supine and during 80 degrees HUT for 30 min or until presyncope. In the postflight tests subjects were classified as being orthostatically tolerant (OT) (n = 7) or intolerant (OI) (n = 6). BP was diminished with HUT in the OT group in both tests (p 〈 0.05) whereas postflight BP was not different from supine in the OI group. Postflight compared to preflight, the reduction in P(ET)CO(2) with HUT (p 〈 0.05) increased in both groups, although significantly so only in the OI group (p 〈 0.05). The OI group also had a significant decrease in supine MCA MFV postflight (p 〈 0.05) that was unaccompanied by a change in supine P(ET)CO(2). The decrease in MCA MFV that occurred during HUT in both groups preflight (p 〈 0.05) was accentuated only in the OI group postflight, particularly during the final 30 s of HUT (p 〈 0.05). However, this accentuated decrease in MCA MFV was not correlated to the greater decrease in P(ET)CO(2) during the same period (R = 0.20, p = 0.42). Although cerebral vascular resistance (CVR) also increased in the OI group during the last 30 s of HUT postflight (p 〈 0.05), the dynamic autoregulatory gain was not simultaneously changed. Therefore, we conclude that in the OI individuals, parabolic flight was associated with cerebral hypoperfusion following a paradoxical augmentation of CVR by a mechanism that was not related to changes in autoregulation nor strictly to changes in P(ET)CO(2).
    Keywords: Aerospace Medicine
    Type: Brain research bulletin (ISSN 0361-9230); Volume 53; 1; 113-20
    Format: text
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  • 4
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    Orthostatic intolerance and motion sickness after parabolic flight (2001)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: 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.
    Keywords: Aerospace Medicine
    Type: Journal of applied physiology (Bethesda, Md. : 1985) (ISSN 8750-7587); Volume 90; 1; 67-82
    Format: text
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  • 5
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    Effect of acute exposure to hypergravity (GX vs. GZ) on dynamic cerebral autoregulation (2001)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: 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.
    Keywords: Aerospace Medicine
    Type: Journal of applied physiology (Bethesda, Md. : 1985) (ISSN 8750-7587); Volume 91; 5; 1986-94
    Format: text
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  • 6
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    Canadian medical experiments on Shuttle Flight 41-G (1985)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: During the 41-G mission, two payload specialist astronauts took part in six Canadian medical experiments designed to measure how the human nervous system adapts to weightlessness, and how this might contribute to space motion sickness. Similar tests conducted pre-flight provided base-line data, and post-flight experiments examined re-adaptation to the ground. No changes were detected in the vestibulo-ocular reflex during this 8-day mission. Pronounced proprioceptive illusions were experienced, especially immediately post-flight. Tactile acuity was normal in the fingers and toes, but the ability to judge limb position was degraded. Estimates of the locations of familiar targets were grossly distorted in the absence of vision. There were no differences in taste thresholds or olfaction. Despite pre-flight tests showing unusual susceptibility to motion sickness, the Canadian payload specialist turned out to be less susceptible than normal on-orbit. Re-adaptation to the normal gravity environment occurred within the first day after landing.
    Keywords: AEROSPACE MEDICINE
    Type: Canadian Aeronautics and Space Journal (ISSN 0008-2821); 31; 215-226
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  • 7
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    Effect of Acute Exposure to Hypergravity (Gx vs. Gz) on Dynamic Cerebral Autoregulation (2001)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: 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).
    Keywords: Life Sciences (General)
    Type: JSC-CN-6756
    Format: application/pdf
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