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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Effect of exercise training on cardiopulmonary baroreflex control of forearm vascular resistance in humans

We studied the stimulus-response characteristics of cardiopulmonary baroreflex control of forearm vascular resistance (FVR) in four groups of male volunteer subjects: i) unfit, ii) physically fit, iii) before and after 10 wk of endurance training (chronic blood volume expansion), and iv) before and after acute blood volume expansion. We assessed the relationship between reflex stimulus, i.e., changes in central venous pressure and response, i.e., FVR, during unloading of cardiopulmonary mechanoreceptors with lower body negative pressure (LBNP, 0 to -20 mm Hg). The slope of the linear relationship between FVR and CVP, the index of the responsiveness of this baroreflex, was significantly diminished (> 50%) in the fit subjects compared with the unfit. The slope of the FVR-CVP relationship was inversely correlated with the subject's total blood volume, suggesting that blood volume expansion was related to the attenuated CP baroreflex. In the exercise training study, maximal oxygen consumption and blood volume increased following 10 wk of endurance training (N = 14) but were unchanged in the time control group (N = 7). The slope of the FVR-CVP relationship was significantly reduced (32%) following 10 wk of training but was unchanged in the time control group. The reduction in slope of the FVR-CVP relationship was inversely related to the increase in blood volume associated with exercise training. Acute blood volume expansion 8 ml.kg-1 body weight with 5% human serum albumin solution) significantly reduced the slope of the FVR-CVP relationship. These data support the hypothesis that the attenuated forearm vascular reflex in physically fit individuals is related to a training-induced hypervolemia.(ABSTRACT TRUNCATED AT 250 WORDS).

Review↗
NASA NTRS1997

The hypertension of autonomic failure and its treatment

We studied the incidence and severity of supine hypertension in 117 patients with severe primary autonomic failure presenting to a referral center over a 9-year period. Patients were uniformly characterized by disabling orthostatic hypotension, lack of compensatory heart rate increase, abnormal autonomic function tests, and unresponsive plasma norepinephrine. Fifty-four patients had isolated autonomic impairment (pure autonomic failure). Sixty-three patients had central nervous system involvement in addition to autonomic impairment (multiple-system atrophy). Patients were studied off medications, in a metabolic ward, and on a controlled diet containing 150 mEq of sodium. Fifty-six percent of patients had supine diastolic blood pressure > or =90 mm Hg. The prevalence of hypertension was slightly greater in females (63%) than in males (52%). Potential mechanisms responsible for this hypertension were investigated. No correlation was found between blood volume and blood pressure. Similarly, plasma norepinephrine (92+/-15 pg/mL) and plasma renin activity (0.3+/-0.05 ng/mL per hour) were very low in the subset of patients with pure autonomic failure and supine hypertension (mean systolic/diastolic pressure, 177 +/- 6/108 +/- 2 mm Hg, range 167/97 to 219/121). Supine hypertension represents a challenge in the treatment of orthostatic hypotension. We found these patients to be particularly responsive to the hypotensive effects of transdermal nitroglycerin. Doses ranging from 0.025 to 0.1 mg/h decreased systolic blood pressure by 36+/-7 mm Hg and may effectively treat supine hypertension overnight, but the dose should be individualized and used with caution.

NASA Discipline Regulatory Physiology↗
NASA NTRS1992

Effect of hemorrhage on cardiac output, vasopressin, aldosterone, and diuresis during immersion in men

The purpose of this research was to test the hypotesis that a reduction in blood volume would attenuate or eliminate immersion-induced increases in cardiac output (Q(sub co)) and urine excretion, and to investigate accompanying vasoactive and fluid-electrolyte hormonal responses. Eight men (19-23 yr) were supine during a 2-hr control period in air, and then sat for 5-hr test periods in air at 20 C (dry control, DC); water at 34.5 C (wet control, WC); and water (34.5 C) after hemorrhage (WH) of 14.8 plus or minus 0.3 percent of their blood volume. Blood volume was -11.6 plus or minus 0.6 percent at immersion (time 0). Mean (bar-X hrs 1-5) Q(sub co) was unchanged in WC (5.3 plus or minus 0.01 l/min) and in WH (4.5 plus or minus 0.1 l/min), but decreased (P less than 0.05) in DC to 3.6 plus or minus 0.1 l/min. Mean urine excretion rates were 1.0 plus or minus 0.2 ml/min for DC and 1.1 plus or minus 0.2 ml/min for WH; both were lower (P less than 0.05) than that for WC of 2.0 plus or minus 0.4 ml/min. Plasma (Na+) and (Osm) were unchanged in all experiments. Mean plasma vasopressin (PVP) (bar-X hrs 1-5) was 1.1 plus or minus 0.1 pg/ml in WC, and higher (P less than 0.05) in DC (2.1 plus or minus 0.2 pg/ml)and WH (2.1 plus or minus 0.1 pg/ml); it was unchanged during air and water test periods. Thus, hemorrhage attenuated the immersion-induced increase in Q(sub co), eliminated the WC diuresis, maintained plasma renin activity and PVP at DC levels and did not change immersion-induced aldosterone suppression; the osmotic diuresis during control immersion is apparently not due to either aldosterone suppression or vasopressin suppression.

Greenleaf, J. E.↗
NASA NTRS1996

Diminished forearm vasomotor response to central hypervolemic loading in aerobically fit individuals

The aim of this study was to test the hypothesis that cardiopulmonary baroreflex control of forearm vascular resistance (FVR) during central hypervolemic loading was less sensitive in exercise trained high fit individuals (HF) compared to untrained average fit individuals (AF). Eight AF (age: 24 +/- 1 yr and weight: 78.9 +/- 1.7 kg) and eight HF (22 +/- 1 yr 79.5 +/- 2.4 kg) voluntarily participated in the investigation. Maximal aerobic power (determined on a treadmill), plasma volume and blood volume (Evans blue dilution method) were significantly greater in the HF than AF (60.8 +/- 0.7 vs. 41.2 +/- 1.9 ml.kg-1.min-1, 3.96 +/- 0.17 vs 3.36 +/- 0.08 1, and 6.33 +/- 0.23 vs 5.28 +/- 0.13 1). Baseline heart rate (HR), central venous pressure (CVP), mean arterial pressure (MAP, measured by an intraradial catheter or a Finapres finger cuff), forearm blood flow (FBF, plethysmography), and FVR, calculated from the ratio (MAP-CVP)/FBF, were not different between the HF and the AF. Lower body negative pressure (LBNP, -5, -10, -15, and -20 torr) and passive leg elevation (LE, 50 cm) combined with lower body positive pressure (LBPP, +5, +10, and +20 torr) were utilized to elicit central hypovolemia and hypervolemia, respectively. Range of CVP (from LBNP to LE+LBPP) was similar in the AF (from -3.9 to +1.9 mm Hg) and HF (from -4.0 to +2.2 mm Hg). However, FVR/CVP was significantly less in the HF (-1.8 +/- 0.1 unit.mm Hg-1) than AF (-34 +/- 0.1 unit.mm Hg-1). The FVR decrease in response to increase in CVP was significantly diminished in the HF (-1.46 +/- 0.45 unit.mm Hg-1) compared to the AF (-4.40 +/- 0.97 unit.mm Hg-1), and during LBNP induced unloading the FVR/CVP of the HF (-2.01 +/- 0.49 unit.mm Hg-1) was less (P < 0.08) than the AF (-3.28 +/- 0.69 unit.mm Hg-1). We concluded that the cardiopulmonary baroreceptor mediated FVR reflex response was significantly less sensitive to changes in CVP in individuals who practice exercise training.

NASA Discipline Number 93-10↗
NASA NTRS2001

Effects of 17 days of head-down bed rest on hydro-electrolytic regulation in men

Prolonged periods of head-down bed rest (HDBR) are commonly used to mimic the effects of microgravity. HDBR has been shown to produce, as in space, a cephalad redistribution of circulating blood volume with an increase in central blood volume which induces the early adaptations in blood volume regulating hormones. Changes in atrial natriuretic peptide (ANP), arginine vasopressin (AVP), renin activity and aldosterone have been observed. Many reports describe these endocrine adaptations but few investigations of rhythms are in the literature. We proposed to evaluate the circadian rhythms of the hormones and electrolytes involved in the hydro-electrolytic regulation during a HDBR study which was designed to simulate a 17-day spaceflight (Life and Microgravity Spacelab experiment, LMS, NASA).

NASA Center ARC↗
NASA NTRS1991

Elevated central venous pressure: a consequence of exercise training-induced hypervolemia?

Resting blood volumes and arterial and central venous pressures (CVP) were measured in 14 men before and after exercise training to determine whether training-induced hypervolemia is accompanied by a change in total vascular capacitance. In addition, resting levels of plasma arginine vasopressin (AVP), atrial natriuretic peptide (ANP), aldosterone (Ald), and norepinephrine (NE) were measured. The same measurements were conducted in seven subjects who did not undergo exercise and acted as controls. Exercise training consisted of 10 wk of controlled cycle exercise for 30 min/day, 4 days/wk at 75-80% of maximal O2 uptake (VO2max). A training effect was verified by a 20% increase in VO2max, a resting bradycardia, and a 9% increase in blood volume. Mean arterial blood pressure was unaltered by exercise training, but resting CVP increased by 16% (P less than 0.05). The percent change in blood volume from before to after training was linearly related to the percent change in CVP (r = 0.903, P less than 0.05). As a consequence of elevations in both blood volume and CVP, the volume-to-pressure ratio was unchanged after exercise training. Plasma AVP, ANP, Ald, and NE were unaltered. Our results indicate that elevated CVP is a consequence of training-induced hypervolemia without alteration in total effective venous capacitance.

NASA Program Space Physiology and Countermeasures↗
NASA NTRS1990

Elevated central venous pressure: A consequence of exercise training-induced hypervolemia

Resting plasma volumes, and arterial and central venous pressures (CVP) were measured in 16 men before and after exercise training to determine if training-induced hypervolemia could be explained by a change in total vascular capacitance. In addition, resting levels of plasma vasopressin (AVP), atrial natriuretic peptide (ANP), aldosterone (ALD), and norepinephrine (NE) were measured before and after training. The same measurements of vacular volume, pressures, and plasma hormones were measured in 8 subjects who did not undergo exercise and acted as controls. The exercise training program consisted of 10 weeks of controlled cycle exercise for 30 min/d, 4 d/wk at 75 to 80 percent of maximal oxygen uptake (VO2max). A training effect was verified by a 20 percent increase in VO2max, a resting bradycardia, and a 370 ml (9 percent) increase in blood volume. Mean arterial blood pressure was unaltered by exercise training, but resting CVP increased. The percent change in blood volume from before to after training was linearly related to the percent change in CVP. As a consequence of elevations in both blood volume and CVP, the volume-to-pressure ratio was essentially unchanged following exercise training. Plasma AVP, ANP, ALD, and NE were unaltered. Results indicate that elevated CVP is a consequence of training-induced hypervolemia without alteration in total effective venous capacitance. This may represent a resetting of the pressure-volume stimulus-response relation for regulation of blood volume.

Convertino, Victor A.↗
NASA NTRS1989

Potentiation of Hormonal Responses to Hemorrhage and Fasting, but not Hypoglycemia in Conscious Adrenalectomized Rats

Bilateral adrenalectomy (ADRX) in rats removes the source of two major stress-responsive hormones, corticosterone and epinephrine. To test how ADRX rats with-stand stress, we performed the following experiments in adult male rats provided with indwelling femoral arterial and venous cannulae and either ADRX or sham-adrenalectomized (Sham) 3 days later and given 0.5% NaCl to drink. Five to 6 days after adrenal surgery the rats were studied after either a 15 ml/kg.5 min hemorrhage or after an overnight fast followed by insulin-induced hypoglycemia. In fed unstressed ADRX rats, basal mean arterial blood pressure was slightly decreased; heart rate was increased; blood volume, vasopressin, and oxytocin concentrations were not different from sham values; and renin and norepinephrine were significantly elevated. The recovery of arterial pressure after hemorrhage in the ADRX rats was similar to that in the sham group over a 5-h period; however, the responses of vasopressin and oxytocin were significantly greater, and those of renin and norepinephrine were markedly potentiated in the ADRX group. Heart rate recovered faster in the ADRX group and was elevated, compared to the sham value, for most of the 5-h period. Restitution of blood volume was attenuated in the ADRX group, although the restitution of plasma protein was not different between the groups. A significant difference in the change in plasma osmolality between groups after hemorrhage may account for the attenuated restitution of blood volume. After an overnight fast, which reduced blood volume in both groups of rats, the plasma renin concentration rose still further in ADRX rats; the differences in other measured variables observed between fed ADRX and sham groups remained the same. The insulin-induced 50% decrease in glucose caused minor effects on arterial blood pressure and heart rate and occasioned responses in renin and norepinephrine of similar magnitudes in the two groups. We conclude that in the absence of the adrenals, rats restore arterial pressure after hemorrhage remarkably well through potentiation of the responses of other vasoactive neural and hormonal systems. In these studies the marked potentiation of the renin response suggests that the renin-angiotensin system may be important in the maintenance of arterial blood pressure after reductions in blood volume.

Darlington, Daniel N.↗
NASA NTRS1991

Cardiopulmonary responses to acute hypoxia, head-down tilt and fluid loading in anesthetized dogs

Cardiopulmonary responses to acute hypoxia (HY), fluid loading by saline infusion (FL), and head-down tilt (HD) of mechanically ventilated anesthetized dogs were investigated by measuring thermodynamics and pulmonary gas exchange. It was found that HD decreased the total respiratory compliance both during HY and normoxia (NO) and that the reduction in compliance by FL was twice as large as by HD. Superimposing HD on HY doubled the increase in vascular resistance due to HY alone. In the systemic circulation, HD lowered the resistance to below NO levels. There was a significant positive correlation between the changes in blood volume and in pulmonary artery pressure for experimental transitions, suggesting that a shift in blood volume from systemic to pulmonary circulations and changes in the total blood volume may contribute substantially to these apparent changes in resistance.

Loeppky, J. A.↗
NASA NTRS1993

Physiologic mechanisms of circulatory and body fluid losses in weightlessness identified by mathematical modeling

Central volume expansion due to fluid shifts in weightlessness is believed to activate adaptive reflexes which ultimately result in a reduction of the total circulating blood volume. However, the flight data suggests that a central volume overdistention does not persist, in which case some other factor or factors must be responsible for body fluid losses. We used a computer simulation to test the hypothesis that factors other than central volume overdistention are involved in the loss of blood volume and other body fluid volumes observed in weightlessness and in weightless simulations. Additionally, the simulation was used to identify these factors. The results predict that atrial volumes and pressures return to their prebedrest baseline values within the first day of exposure to head down tilt (HDT) as the blood volume is reduced by an elevated urine formation. They indicate that the mechanisms for large and prolonged body fluid losses in weightlessness is red cell hemoconcentration that elevates blood viscosity and peripheral resistance, thereby lowering capillary pressure. This causes a prolonged alteration of the balance of Starling forces, depressing the extracellular fluid volume until the hematocrit is returned to normal through a reduction of the red cell mass, which also allows some restoration of the plasma volume. We conclude that the red cell mass becomes the physiologic driver for a large 'undershoot' of the body fluid volumes after the normalization of atrial volumes and pressures.

Simanonok, K. E.↗
NASA NTRS2002

Exploring Heart and Lung Function in Space: ARMS Experiments

The Advanced Respiratory Monitoring System (ARMS) is a suite of monitoring instruments and supplies used to study the heart, lungs, and metabolism. Many experiments sponsored by the European Space Agency (ESA) will be conducted using ARMS during STS-107. The near-weightless environment of space causes the body to undergo many physiological adaptations, and the regulation of blood pressure is no exception. Astronauts also experience a decrease in blood volume as an adaptation to microgravity. Reduced blood volume may not provide enough blood pressure to the head during entry or landing. As a result, astronauts often experience light-headedness, and sometimes even fainting, when they stand shortly after returning to Earth. To help regulate blood pressure and heart rate, baroreceptors, sensors located in artery walls in the neck and near the heart, control blood pressure by sending information to the brain and ensuring blood flow to organs. These mechanisms work properly in Earth's gravity but must adapt in the microgravity environment of space. However, upon return to Earth during entry and landing, the cardiovascular system must readjust itself to gravity, which can cause fluctuation in the control of blood pressure and heart rate. Although the system recovers in hours or days, these occurrences are not easily predicted or understood - a puzzle investigators will study with the ARMS equipment. In space, researchers can focus on aspects of the cardiovascular system normally masked by gravity. The STS-107 experiments using ARMS will provide data on how the heart and lungs function in space, as well as how the nervous system controls them. Exercise will also be combined with breath holding and straining (the Valsalva maneuver) to test how heart rate and blood pressure react to different stresses. This understanding will improve astronauts' cardiopulmonary function after return to Earth, and may well help Earthbound patients who experience similar effects after long-term bed rest.

Andre Kuipers↗
NASA NTRS1997

Cardiovascular consequences of bed rest: effect on maximal oxygen uptake

Maximal oxygen uptake (VO2max) is reduced in healthy individuals confined to bed rest, suggesting it is independent of any disease state. The magnitude of reduction in VO2max is dependent on duration of bed rest and the initial level of aerobic fitness (VO2max), but it appears to be independent of age or gender. Bed rest induces an elevated maximal heart rate which, in turn, is associated with decreased cardiac vagal tone, increased sympathetic catecholamine secretion, and greater cardiac beta-receptor sensitivity. Despite the elevation in heart rate, VO2max is reduced primarily from decreased maximal stroke volume and cardiac output. An elevated ejection fraction during exercise following bed rest suggests that the lower stroke volume is not caused by ventricular dysfunction but is primarily the result of decreased venous return associated with lower circulating blood volume, reduced central venous pressure, and higher venous compliance in the lower extremities. VO2max, stroke volume, and cardiac output are further compromised by exercise in the upright posture. The contribution of hypovolemia to reduced cardiac output during exercise following bed rest is supported by the close relationship between the relative magnitude (% delta) and time course of change in blood volume and VO2max during bed rest, and also by the fact that retention of plasma volume is associated with maintenance of VO2max after bed rest. Arteriovenous oxygen difference during maximal exercise is not altered by bed rest, suggesting that peripheral mechanisms may not contribute significantly to the decreased VO2max. However reduction in baseline and maximal muscle blood flow, red blood cell volume, and capillarization in working muscles represent peripheral mechanisms that may contribute to limited oxygen delivery and, subsequently, lowered VO2max. Thus, alterations in cardiac and vascular functions induced by prolonged confinement to bed rest contribute to diminution of maximal oxygen uptake and reserve capacity to perform physical work.

Non-NASA Center↗
NASA NTRS1997

Resistance exercise training and the orthostatic response

Resistance exercise has been suggested to increase blood volume, increase the sensitivity of the carotid baroreceptor cardiac reflex response (BARO), and decrease leg compliance, all factors that are expected to improve orthostatic tolerance. To further test these hypotheses, cardiovascular responses to standing and to pre-syncopal limited lower body negative pressure (LBNP) were measured in two groups of sedentary men before and after a 12-week period of either exercise (n = 10) or no exercise (control, n = 9). Resistance exercise training consisted of nine isotonic exercises, four sets of each, 3 days per week, stressing all major muscle groups. After exercise training, leg muscle volumes increased (P < 0.05) by 4-14%, lean body mass increased (P = 0.00) by 2.0 (0.5) kg, leg compliance and BARO were not significantly altered, and the maximal LBNP tolerated without pre-syncope was not significantly different. Supine resting heart rate was reduced (P = 0.03) without attenuating the heart rate or blood pressure responses during the stand test or LBNP. Also, blood volume (125I and 51Cr) and red cell mass were increased (P < 0.02) by 2.8% and 3.9%, respectively. These findings indicate that intense resistance exercise increases blood volume but does not consistently improve orthostatic tolerance.

NASA Discipline Musculoskeletal↗
NASA NTRS1991

Diminished Baroreflex Control of Forearm Vascular Resistance Following Training

The stimulus-response characteristics of cardiopulmonary baroreflex control of forearm vascular resistance (FVR units in mm Hg x min x I00 ml/ml) were studied in 14 volunteers before and after 10 wk of endurance training. We assessed the relationship betaleen reflex stimulus (changes in central venous pressure, CVP) and response (FVR) during unloading of cardiopulmonary baroreceptors with lower body negative pressure (LBNP, 0 to - 2O mm Hg). Changes in CVP during LBNP were estimated from pressure changes in a large peripheral vein in the dependent arm of the subject in the right lateral decubitus position. Maximal oxygen uptake (VO(sub 2max)) and total blood volume increased with endurance training from 37.8 +/- 1.4 ml/min x kg and 63.6 +/- 2.1 ml/kg to 45.3 +/- 1.4 ml/ min x kg and 69.3 +/- 2.8 ml/kg respectively (P less than 0.05). Reflex forearm vasoconstriction occurred in response to a reduction in estimated CVP, and the absolute change in FVR per unit of CVP was reduced from -5.96 +/- 0.79 to -4.06 +/- 0.52 units x mm/ Hg (P less than 0.05) following exercise training but was unchanged from -6.10 to 0.57 to -6.22 +/- 0.94 units x mm/ Hg for the time control group (N = 7). Resting values for FVR were similar before and after exercise training; however, resting estimated CVP was elevated from 9.5 +/- 0.5 mm x Hg before training to 11.3 +/- 0.6 mm x Hg after training. The reduction in sensitivity of the cardiopulmonary baroreflex control of FVR was linearly related to the increase in blood volume (r = 0.65, P less than 0.05). suggesting that diminished cardiopulmonary baroreflex control of FVR in physically fit individuals is related, in part, to a training-induced blood volume expansion.

Mack, G. W.↗
NASA NTRS1978

Specialized physiological studies in support of manned space flight

The effects of a diuretic (Lasix) induced dehydration on the cardiovascular and hematological responses to lower body negative pressure (LBNP) were analyzed and compared to previous observations on dehydration following exercise in the heat. During LBNP runs the subjects were monitored for changes in blood volume, heart rate, blood pressure, and variations in the volume of the left calf. It was concluded that Lasix dehydration produced a depletion of the body electrolytes at the expense of both the plasma and extravascular compartments. Striking differences were found between those subjects who were physically active (Runners: R) and those who did not engage in any regular physical activity (Non-runners: NR). Tolerance to LBNP (Torr x min) was significantly lower in the R's than the NR's before and after dehydration, however the R's lost more of their tolerance after dehydration with Lasix than after exercise in the heat for about the same fluid loss. The opposite was true for the NR's. Two factors appear to be responsible for the lower LBNP tolerance in R's: parasympathetic inhibition of cardiac activity during LBNP and a greater propensity to pool blood in the lower extremities.

Luft, U. C.↗
NASA NTRS1996

Determining blood and plasma volumes using bioelectrical response spectroscopy

We hypothesized that an electric field (inductance) produced by charged blood components passing through the many branches of arteries and veins could assess total blood volume (TBV) or plasma volume (PV). Individual (N = 29) electrical circuits (inductors, two resistors, and a capacitor) were determined from bioelectrical response spectroscopy (BERS) using a Hewlett Packard 4284A Precision LCR Meter. Inductance, capacitance, and resistance from the circuits of 19 subjects modeled TBV (sum of PV and computed red cell volume) and PV (based on 125I-albumin). Each model (N = 10, cross validation group) had good validity based on 1) mean differences (-2.3 to 1.5%) between the methods that were not significant and less than the propagated errors (+/- 5.2% for TBV and PV), 2) high correlations (r > 0.92) with low SEE (< 7.7%) between dilution and BERS assessments, and 3) Bland-Altman pairwise comparisons that indicated "clinical equivalency" between the methods. Given the limitation of this study (10 validity subjects), we concluded that BERS models accurately assessed TBV and PV. Further evaluations of the models' validities are needed before they are used in clinical or research settings.

NASA Discipline Number 00-00↗
NASA NTRS2007

Role for Lower Extremity Interstitial Fluid Volume Changes in the Development of Orthostasis after Simulated Microgravity

Reentry orthostasis after exposure to the conditions of spaceflight is a persistent problem among astronauts. In a previous study, a computer model systems analysis was used to examine the physiologic mechanisms involved in this phenomenon. In this analysis, it was determined that an augmented capacitance of lower extremity veins due to a fluid volume contracture of the surrounding interstitial spaces during spaceflight results in an increase in sequestered blood volume upon standing and appears to be the initiating mechanism responsible for reentry orthostasis. In this study, we attempt to validate the central premise of this hypothesis using a ground-based spaceflight analog. 10 healthy subjects were placed at bed rest in a 6 head down tilt position for 60 days of bed rest. The impact of adaptations in interstitial fluid volume and venous capacitance in the lower extremities were then observed during a standard tilt test protocol performed before and after the confinement period. The interstitial thickness superficial to the calcaneous immediately below the lateral malleolus was measured using ultrasound with a 17-5 MHz linear array transducer. Measurements of the changes in anterior tibial vein diameter during tilt were obtained by similar methods. The measurements were taken while the subjects were supine and then during upright tilt (80') for thirty minutes, or until the subject had signs of presyncope. Additional measurements of the superficial left tibia interstitial thickness and stroke volume by standard echocardiographic methods were also recorded. In addition, calf compliance was measured over a pressure range of 10-60 mmHg, using plethysmography, in a subset of these subjects (n = 5). There was a average of 6% diminution in the size of the lower extremity interstitial space as compared to measurements acquired prior to bed rest. This contracture of the interstitial space coincided with a subsequent relative increase in the percentage change in tibial vein diameter and stroke volume upon tilting in contrast to the observations made before bed rest (54 vs 23% respectively). Compliance in the calf increased by an average of 36% by day 27 of bedrest. A systems analysis using a computer model of cardiovascular physiology suggests that microgravity induced interstitial volume depletion results in an accentuation of venous blood volume sequestration and is the initiating event in reentry orthostasis. This hypothesis was tested in volunteer subjects using a ground-based spaceflight analog model that simulated the body fluid redistribution induced by microgravity exposure. Measurements of changes in the interstitial spaces and observed responses of the anterior tibial vein with tilt, together with the increase in calf compliance, were consistent with our proposed mechanism for the initiation of postflight orthostasis often seen in astronauts.

Platts, Steven H.↗
NASA NTRS1997

Hypovolemia in syncope and orthostatic intolerance role of the renin-angiotensin system

PURPOSE: Orthostatic intolerance is the cause of significant disability in otherwise normal patients. Orthostatic tachycardia is usually the dominant hemodynamic abnormality, but symptoms may include dizziness, visual changes, discomfort in the head or neck, poor concentration, fatigue, palpitations, tremulousness, anxiety and, in some cases, syncope. It is the most common disorder of blood pressure regulation after essential hypertension. There is a predilection for younger rather than older adults and for women more than men. Its cause is unknown; partial sympathetic denervation or hypovolemia has been proposed. METHODS AND MATERIALS: We tested the hypothesis that reduced plasma renin activity, perhaps from defects in sympathetic innervation of the kidney, could underlie a hypovolemia, giving rise to these clinical symptoms. Sixteen patients (14 female, 2 male) ranging in age from 16 to 44 years were studied. Patients were enrolled in the study if they had orthostatic intolerance, together with a raised upright plasma norepinephrine (> or = 600 pg/mL). Patients underwent a battery of autonomic tests and biochemical determinations. RESULTS: There was a strong positive correlation between the blood volume and plasma renin activity (r = 0.84, P = 0.001). The tachycardic response to upright posture correlated with the severity of the hypovolemia. There was also a correlation between the plasma renin activity measured in these patients and their concomitant plasma aldosterone level. CONCLUSIONS: Hypovolemia occurs commonly in orthostatic intolerance. It is accompanied by an inappropriately low level of plasma renin activity. The degree of abnormality of blood volume correlates closely with the degree of abnormality in plasma renin activity. Taken together, these observations suggest that reduced plasma renin activity may be an important pathophysiologic component of the syndrome of orthostatic intolerance.

Non-NASA Center↗