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  • 1
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    Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight (2000)
    In:  Other Sources
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    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
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  • 2
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    Two-phase flow research using the DC-9/KC-135 apparatus (1996)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: Low-gravity gas-liquid flow research can be conducted aboard the NASA Lewis Research Center DC-9 or the Johnson Space Center KC-135. Air and water solutions serve as the test liquids in cylindrical test sections with constant or variable inner diameters of approximately 2.54 cm and lengths of up to 3.0 m. Superficial velocities range from 0.1 to 1.1 m/sec for liquids and from 0.1 to 25 m/sec for air. Flow rate, differential pressure, void fraction, film thickness, wall shear stress, and acceleration data are measured and recorded at data rates of up to 1000 Hz throughout the 20-sec duration of the experiment. Flow is visualized with a high-speed video system. In addition, the apparatus has a heat-transfer capability whereby sensible heat is transferred between the test-section wall and a subcooled liquid phase so that the heat-transfer characteristics of gas-liquid two-phase flows can be determined.
    Keywords: Fluid Mechanics and Heat Transfer
    Type: NASA-TM-107175 , NAS 1.15:107175 , E-10132
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  • 3
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    Gender affects sympathetic and hemodynamic response to postural stress (2001)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We tested the hypothesis that differences in sympathetic reflex responses to head-up tilt (HUT) between males (n = 9) and females (n = 8) were associated with decrements in postural vasomotor responses in women. Muscle sympathetic nerve activity (MSNA; microneurography), heart rate, stroke volume (SV; Doppler), and blood pressure (Finapres) were measured during a progressive HUT protocol (5 min at each of supine, 20 degrees, 40 degrees, and 60 degrees ). MSNA and hemodynamic responses were also measured during the cold pressor test (CPT) to examine nonbaroreflex neurovascular control. SV was normalized to body surface area (SV(i)) to calculate the index of cardiac output (Q(i)), and total peripheral resistance (TPR). During HUT, heart rate increased more in females versus males (P 〈 0.001) and SV(i) and Q(i) decreased similarly in both groups. Mean arterial pressure (MAP) increased to a lesser extent in females versus males in the HUT (P 〈 0.01) but increases in TPR during HUT were similar. MSNA burst frequency was lower in females versus males in supine (P 〈 0.03) but increased similarly during HUT. Average amplitude/burst increased in 60 degrees HUT for males but not females. Both males and females demonstrated an increase in MAP as well as MSNA burst frequency, mean burst amplitude, and total MSNA during the CPT. However, compared with females, males demonstrated a greater neural response (DeltaTotal MSNA) due to a larger increase in mean burst amplitude (P 〈 0.05). Therefore, these data point to gender-specific autonomic responses to cardiovascular stress. The different MSNA response to postural stress between genders may contribute importantly to decrements in blood pressure control during HUT in females.
    Keywords: Life Sciences (General)
    Type: American journal of physiology. Heart and circulatory physiology (ISSN 0363-6135); 281; 5; H2028-35
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  • 4
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    Sympathetic responses to Valsalva's manoeuvre following bed rest (2003)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The purpose of this study was to examine whether 14 days of head-down tilt bed rest (HDBR) alters autonomic regulation during Valsalva's manoeuvre (VM) and if this would predict blood pressure control during a 60 degrees head-up tilt (HUT) test. To examine autonomic control of blood pressure, we measured the changes in systolic (delta SBP) and diastolic (delta DBP) blood pressure between baseline and the early straining (Phase IIE) period of VM (20 sec straining to 40 mmHg; N = 7) in conjunction with changes in muscle sympathetic nerve activity (MSNA; microneurography) burst frequency (B/min) and total activity (% delta) from baseline over the 20-sec straining period. MSNA data were successfully recorded from 6 of the 7 individuals. The averaged responses from three repeated VMs performed in the supine position were compared between the pre- and post-HDBR tests. Compared with the pre-HDBR test, a greater reduction in SBP, DBP, and MAP was observed during Phase IIE following HDBR, p 〈 0.05. The increase in MSNA burst frequency during straining was augmented in the post- compared with the pre-HDBR test, p 〈 0.0001, as was the Phase IV blood pressure overshoot, p 〈 0.05. Although all subjects completed the 20-min pre-HDBR tilt test without evidence of hypotension or orthostatic intolerance, the post-HDBR test was stopped early in 5 of the 7 subjects due to systolic hypotension. The responses during the VM suggest that acute autonomic adjustments to rapid blood pressure changes are preserved after bed rest. Furthermore, MSNA and blood pressure responses during VM did not predict blood pressure control during orthostasis following HDBR.
    Keywords: Aerospace Medicine
    Type: Canadian journal of applied physiology = Revue canadienne de physiologie appliquee (ISSN 1066-7814); 28; 3; 342-55
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  • 5
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    WISE-2005: Adrenergic Responses Before and After 60 Days of 6 Degree Head-Down Bed-Rest in Women (2006)
    In:  Other Sources
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    Publication Date: 2019-08-16
    Description: Sixteen women who participated in the WISE-2005 headdown bed rest (HDBR) were studied before and on day 56 of bed rest to test the hypothesis that chronic changes in circulating norepinephrine (NOR) would change the response to adrenergic receptor agonists. Five minute infusions of 2 doses of isoproterenol (ISO), and 2 doses of NOR were administered while heart rate (HR), mean arterial pressure (MAP) and total peripheral resistance (TPR) were measured. Before HDBR, the higher dose of ISO increased HR by 13 beats/min (P〈O.OOl) and decreased TPR by -50% (P〈O.OOl) with no changes in MAP, the higher dose of NOR increased MAP by 8 mmHg (P〈O.OOl) and increased TPR -20% (P=0.007) with no change in HR. On HDBR day 56, subjects who took part in daily exercise+lower body negative pressure (EX+LBNP) countermeasures had no change in HR response to ISO while the control group (CON) had greater increases in HR. MAP tended to be lower post-HDBR in CON but not in EX+LBNP, while TPR was higher in both groups. In response to NOR, MAP increased with elevated TPR with no change in HR. Post-HDBR TPR increased more in the EX+LBNP group. These studies indicate that HDBR alters the HR response to IS0 but that EX+LBNP countermeasure prevented this change. The greater TPR response to NOR in the post-HDBR for the EX+LBNP group might reflect enhanced peripheral vasoconstrictor response that could reduce the risk of orthostatic intolerance after this countermeasure.
    Keywords: Aerospace Medicine
    Type: Journal of Gravitational Physiology, Volume 13, No. 1; P-37 - P-38
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  • 6
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    WISE-2005: Integrative Cardiovascular Responses with LBNP during 60-Day Bed Rest in Women (2005)
    In:  Other Sources
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    Publication Date: 2019-08-16
    Description: During 2005, 24 women will take part in the Women International Space-simulation for Exploration (WISE). In this paper we report on the first phase that studied 4 Exercise (EX+LBNP), 4 nutrition (NUT), and 4 no countermeasure control (CON) subjects. The EX+LBNP group completed regular exercise on a treadmill inside LBNP, flywheel resistive exercise and static periods of LBNP, and had recovery days. The NUT group received daily protein supplements. Integrative cardiovascular responses were obtained and here we report data for heart rate during LBNP, blood volume and angiotensin 11. LBNP was applied at 0, -10, -20 and -30 mmHg for 2-minutes for each stage. Blood was sampled prebed rest and on HDT-60. After 60-days head down bed rest, HR in the CON group increased by 6.1+/-2.8 bpm at rest and by 20.7+/-5.0 bpm at -30 mmHg LBNP. The EX+LBNP group had increases of 3.6+/-5.6 and 11.6+/-5.4 bpm, while the NUT group HR increased 2.6+/-3.1 and 9.4+/-3.6 bpm. The EX+LBNP group had almost no change in blood volume or plasma angiotensin II from pre-bed rest to HDT60, while both the CON and NUT groups had larger increases in plasma volume and almost double concentrations of angiotensin II. These data show a positive effect in the EX+LBNP group on the heart rate response as well as an unexpected possible benefit in the NUT group. Further studies are required to confirm possible cardiovascular benefits of the protein supplement.
    Keywords: Life Sciences (General)
    Type: Journal of Gravitational Physiology, Volume 12, Number 1; P61-P62
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  • 7
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    Microgravity Fluid Separation Physics: Experimental and Analytical Results (1997)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Effective, low power, two-phase separation systems are vital for the cost-effective study and utilization of two-phase flow systems and flow physics of two-phase flows. The study of microgravity flows have the potential to reveal significant insight into the controlling mechanisms for the behavior of flows in both normal and reduced gravity environments. The microgravity environment results in a reduction in gravity induced buoyancy forces acting on the discrete phases. Thus, surface tension, viscous, and inertial forces exert an increased influence on the behavior of the flow as demonstrated by the axisymmetric flow patterns. Several space technology and operations groups have studied the flow behavior in reduced gravity since gas-liquid flows are encountered in several systems such as cabin humidity control, wastewater treatment, thermal management, and Rankine power systems.
    Keywords: Materials Processing
    Type: Aerospace Sciences; Jan 06, 1997 - Jan 09, 1997; Reno, NV; United States
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  • 8
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    Blood flow dynamics in heart failure (1999)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: BACKGROUND: Exercise intolerance in heart failure (HF) may be due to inadequate vasodilation, augmented vasoconstriction, and/or altered muscle metabolic responses that lead to fatigue. METHODS AND RESULTS: Vascular and metabolic responses to rhythmic forearm exercise were tested in 9 HF patients and 9 control subjects (CTL) during 2 protocols designed to examine the effect of HF on the time course of oxygen delivery versus uptake (protocol 1) and on vasoconstriction during exercise with 50 mm Hg pressure about the forearm to evoke a metaboreflex (protocol 2). In protocol 1, venous lactate and H+ were greater at 4 minutes of exercise in HF versus CTL (P〈0.05) despite similar blood flow and oxygen uptake responses. In protocol 2, mean arterial pressure increased similarly in each group during ischemic exercise. In CTL, forearm blood flow and vascular conductance were similar at the end of ischemic and ambient exercise. In HF, forearm blood flow and vascular conductance were reduced during ischemic exercise compared with the ambient trial. CONCLUSIONS: Intrinsic differences in skeletal muscle metabolism, not vasodilatory dynamics, must account for the augmented glycolytic metabolic responses to moderate-intensity exercise in class II and III HF. The inability to increase forearm vascular conductance during ischemic handgrip exercise, despite a normal pressor response, suggests that enhanced vasoconstriction of strenuously exercising skeletal muscle contributes to exertional fatigue in HF.
    Keywords: Life Sciences (General)
    Type: Circulation (ISSN 0009-7322); 99; 23; 3002-8
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  • 9
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    Contributions of MSNA and stroke volume to orthostatic intolerance following bed rest (1999)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We examined whether the altered orthostatic tolerance following 14 days of head-down tilt bed rest (HDBR) was related to inadequate sympathetic outflow or to excessive reductions in cardiac output during a 10- to 15-min head-up tilt (HUT) test. Heart rate, blood pressure (BP, Finapres), muscle sympathetic nerve activity (MSNA, microneurography), and stroke volume blood velocity (SVV, Doppler ultrasound) were assessed during supine 30 degrees (5 min) and 60 degrees (5-10 min) HUT positions in 15 individuals who successfully completed the pre-HDBR test without evidence of orthostatic intolerance. Subjects were classified as being orthostatically tolerant (OT, n = 9) or intolerant (OI, n = 6) following the post-HDBR test. MSNA, BP, and SVV during supine and HUT postures were not altered in the OT group. Hypotension during 60 degrees HUT in the post-bed rest test for the OI group (P 〈 0.05) was associated with a blunted increase in MSNA (P 〈 0.05). SVV was reduced following HDBR in the OI group (main effect of HDBR, P 〈 0.02). The data support the hypothesis that bed rest-induced orthostatic intolerance is related to an inadequate increase in sympathetic discharge that cannot compensate for a greater postural reduction in stroke volume.
    Keywords: Aerospace Medicine
    Type: The American journal of physiology (ISSN 0002-9513); 277; 4 Pt 2; R1084-90
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  • 10
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    Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip (2000)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and (31)P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P 〈 0.03 and P 〈 0.02, respectively). However, despite this increase in flow, venous K(+) release and H(+) release were both increased during exercise (P 〈 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 +/- 0.08 vs. 6.20 +/- 0.06; P 〈 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H(+) release, presumably through a preferential recruitment of oxidative fibers.
    Keywords: Life Sciences (General)
    Type: American journal of physiology. Heart and circulatory physiology (ISSN 0363-6135); 279; 2; H586-93
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