Cardiovascular Adjustments to Gravitational Stress

Damgaard

Journal of Applied Physiology

et al. 2011

PetersenLG, Damgaard M, Petersen JCG, Norsk P. Mechanisms of increase in cardiac output during acute weightlessness in humans. J Appl Physiol 111: 407-411, 2011. First published June 2, 2011 doi:10.1152/japplphysiol.01188.2010.-Based on previous water immersion results, we tested the hypothesis that the acute 0-Ginduced increase in cardiac output (CO) is primarily caused by redistribution of blood from the vasculature above the legs to the cardiopulmonary circulation. In seated subjects (n ϭ 8), 20 s of 0 G induced by parabolic flight increased CO by 1.7 Ϯ 0.4 l/min (P Ͻ 0.001). This increase was diminished to 0.8 Ϯ 0.4 l/min (P ϭ 0.028), when venous return from the legs was prevented by bilateral venous thigh-cuff inflation (CI) of 60 mmHg. Because the increase in stroke volume during 0 G was unaffected by CI, the lesser increase in CO during 0 G ϩ CI was entirely caused by a lower heart rate (HR). Thus blood from vascular beds above the legs in seated subjects can alone account for some 50% of the increase in CO during acute 0 G. The remaining increase in CO is caused by a higher HR, of which the origin of blood is unresolved. In supine subjects, CO increased from 7.1 Ϯ 0.7 to 7.9 Ϯ 0.8 l/min (P ϭ 0.037) when entering 0 G, which was solely caused by an increase in HR, because stroke volume was unaffected. In conclusion, blood originating from vascular beds above the legs can alone account for one-half of the increase in CO during acute 0 G in seated humans. A Bainbridge-like reflex could be the mechanism for the HR-induced increase in CO during 0 G in particular in supine subjects. regional blood flow; blood pressure; vascular resistance; gravity GRAVITY CONSTANTLY STRESSES the cardiovascular system in upright humans by diminishing venous return. It decreases stroke volume (SV) and thus cardiac output (CO) and, through the baroreflex system, induces arteriolar constriction and an increase in heart rate (HR) to counteract a fall in blood pressure (4,21,30). Compared with the upright 1-G position, sudden entry into weightlessness (0 G) causes a redistribution of blood from the lower vascular beds to the central circulation (4, 29), leading to an increase in venous return, and thus in SV and CO, and, through stimulation of the baroreflexes, to arteriolar vasodilatation and a decrease in HR (18,21,23,29).Our laboratory has previously observed that, in upright seated humans, short-term 0 G created by parabolic airplane flights increases CO by as much as 1.8 l/min, which corresponds to some 0.6 liters over the 20-s period of 0 G (21). Bailliart et al. (2) estimated that, in standing humans, some 165 ml of fluid disappeared from each leg during 20 s of 0 G. These and our observations collectively suggest that the contribution of blood from the legs to the acute 0-G-induced increase in CO amounts to some 50%. In another study from our laboratory, we have, however, observed that water immersion in humans, which is used to simulate the effects of 0 G, increases the diameter of the left atrium to the same degree, whethe...

Section: Discussionsupporting

confidence: 72%

mentioning

confidence: 99%

Mechanisms of increase in cardiac output during acute weightlessness in humans

Damgaard

Journal of Applied Physiology

et al. 2011

“…There are several possible mechanisms by which changes in urinary sodium excretion occur in response to postural changes. Standing up from the recumbent position causes fluid to shift into the interstitial spaces in the lower limbs 22 and results in an increase in colloid osmotic pressure, which leads to a decrease in the glomerular filtration rate and an increase in the fractional tubular reabsorption of sodium. 23 Standing also alters sympathetic tone 24,25 and the production of hormones related to the sodium balance, including the renin-angiotensin system, 26 endothelin 27 and atrial natriuretic peptide, 28 resulting in changes in urinary sodium excretion.…”

Section: Discussionmentioning

confidence: 99%

“…We only examined the reliability of the SMU method in patients with a daily salt intake of 7 g, so investigation at a higher salt intake is also needed. We previously studied 35 22 patients with essential hypertension who had a salt intake of 17 g per day, followed by 7 g per day for 1 week each. The salt intake estimated at the end of each week by the SMU method was 8.0 ± 1.9 g day À1 and 16.2 ± 2.3 g day À1 , respectively, although the posture adopted until the second urine collection was not specified.…”

Section: Discussionmentioning

confidence: 99%

The influence of posture on the estimation of daily salt intake by the second morning urine method

et al. 2010

The second morning urine (SMU) method was developed to evaluate daily salt intake, but the posture that should be adopted until the SMU collection remains unclear. This study investigated the influence of posture in hypertensive patients who underwent this test. The subjects were 100 patients who could collect 24-h urine samples correctly and were on a diet containing 7 g of salt per day. Their daily salt intake was estimated for three consecutive days in the recumbent, sitting, and sitting and standing positions (one posture each day). Estimated salt intake in the recumbent position (10.9±2.4 g day À1 ) was higher than in the sitting position (7.5 ± 2.0 g day À1 ) and the sitting and standing position (6.3 ± 1.7 g day À1 ). The salt intake estimated in the sitting and standing position was similar to that obtained by 24-h urine collection (6.3 ± 1.6 g day À1 ) and was significantly (r¼0.44, Po0.05) correlated with the 24-h urine value. The actual difference in estimated salt intake between the two methods was 0.0 ± 1.7 g day À1 . There were no significant differences in estimated salt intake between the two methods in patients taking different classes of antihypertensive drugs. In conclusion, adopting the sitting and standing position until the SMU collection is important for the correct estimation of daily salt intake, and this method could replace the 24-h collection method because of its convenience, especially in outpatients.

“…1,2,5,9,15,16 POI limits an astronaut's ability to perform physical tasks during landing and the immediate postflight period.…”

Section: Introductionmentioning

confidence: 99%

Simulated Stand Tests and Centrifuge Training to Prevent Orthostatic Intolerance on Earth, Moon, and Mars

Coats

Sharp

2010

Ann Biomed Eng

Abstract-One proposed method to overcome postflight orthostatic intolerance is for astronauts to undergo inflight centrifugation. Cardiovascular responses were compared between centrifuge and gravitational conditions using a seven-compartment cardiovascular model. Vascular resistance, heart rate, and stroke volume values were adopted from literature, while compartmental volumes and compliances were derived from impedance plethysmography of subjects (n = 8) riding on a centrifuge. Three different models were developed to represent the typical male subject who completed a 10-min postflight stand test (''male finisher''), ''non-finishing male'' and ''female'' (all non-finishers). A sensitivity analysis found that both cardiac output and arterial pressure were most sensitive to total blood volume. Simulated stand tests showed that female astronauts were more susceptible to orthostatic intolerance due to lower initial blood pressure and higher pressure threshold for presyncope. Rates of blood volume loss by capillary filtration were found to be equivalent in female and male non-finishers, but four times smaller in male finishers. For equivalent times to presyncope during centrifugation as those during constant gravity, lower G forces at the level of the heart were required. Centrifuge G levels to match other cardiovascular parameters varied depending on the parameter, centrifuge arm length, and the gravity level being matched.