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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Raised cerebrovascular resistance in idiopathic orthostatic intolerance: evidence for sympathetic vasoconstriction

Patients with idiopathic orthostatic intolerance (IOI) exhibit symptoms suggestive of cerebral hypoperfusion and an excessive decrease in cerebral blood flow associated with standing despite sustained systemic blood pressure. In 9 patients (8 women and 1 man aged 22 to 48 years) with IOI, we tested the hypothesis that volume loading (2000 cc normal saline) and alpha-adrenoreceptor agonism improve systemic hemodynamics and cerebral perfusion and that the decrease in cerebral blood flow with head-up tilt (HUT) could be attenuated by alpha-adrenoreceptor blockade with phentolamine. At 5 minutes of HUT, volume loading (-20+/-3.2 bpm) and phenylephrine (-18+/-3.4 bpm) significantly reduced upright heart rate compared with placebo; the effect was diminished at the end of HUT. Phentolamine substantially increased upright heart rate at 5 minutes (20+/-3.7 bpm) and at the end of HUT (14+/-5 bpm). With placebo, mean cerebral blood flow velocity decreased by 33+/-6% at the end of HUT. This decrease in cerebral blood flow with HUT was attenuated by all 3 interventions. We conclude that in patients with IOI, HUT causes a substantial decrease in cerebrovascular blood flow velocity. The decrease in blood flow velocity with HUT can be attenuated with interventions that improve systemic hemodynamics and therefore decrease reflex sympathetic activation. Moreover, alpha-adrenoreceptor blockade also blunts the decrease in cerebral blood flow with HUT but at the price of deteriorated systemic hemodynamics. These observations may suggest that in patients with IOI, excessive sympathetic activity contributes to the paradoxical decrease in cerebral blood flow with upright posture.

Non-NASA Center↗
NASA NTRS1993

The heart works against gravity

The circulatory systems of vertebrate animals are closed, and blood leaves and returns to the heart at the same level. It is often concluded, therefore, that the heart works only against the viscous resistance of the system, not against gravity, even in vascular loops above the heart in which the siphon principle operates. However, we argue that the siphon principle does not assist blood flow in superior vascular loops if any of the descending vasculature is collapsible. If central arterial blood pressure is insufficient to support a blood column between the heart and the head, blood flow ceases because of vascular collapse. Furthermore, the siphon principle does not assist the heart even when a continuous stream of blood is flowing in a superior loop. The potential energy gained by blood as it is pumped to the head is lost to friction in partially collapsed descending vessels and thus is not regained. Application of the Poiseuille equation to flow in collapsible vessels is limited; resistance depends on flow rate in partially collapsed vessels with no transmural pressure difference, but flow rate is independent of resistance. Thus the pressure developed by the heart to establish a given flow rate is independent of the resistance occurring in the partially collapsed vessels. The pressure depends only on the height of the blood column and the resistance in the noncollapsed parts of the system. Simple laboratory models, involving water flow in collapsible tubing, dispel the idea that the siphon principle facilitates blood flow and suggest that previously published results may have been affected by experimental artifact.

Review, Tutorial↗
NASA NTRS2000

The pressor response to water drinking in humans : a sympathetic reflex?

BACKGROUND: Water drinking increases blood pressure profoundly in patients with autonomic failure and substantially in older control subjects. The mechanism that mediates this response is not known. METHODS AND RESULTS: We studied the effect of drinking tap water on seated blood pressure in 47 patients with severe autonomic failure (28 multiple system atrophy [MSA], 19 pure autonomic failure patients [PAF]). Eleven older controls and 8 young controls served as control group. We also studied the mechanisms that could increase blood pressure with water drinking. Systolic blood pressure increased profoundly with water drinking, reaching a maximum of 33+/-5 mm Hg in MSA and 37+/-7 in PAF mm Hg after 30 to 35 minutes. The pressor response was greater in patients with more retained sympathetic function and was almost completely abolished by trimethaphan infusion. Systolic blood pressure increased by 11+/-2.4 mm Hg in elderly but not in young controls. Plasma norepinephrine increased in both groups. Plasma renin activity, vasopressin, and blood volume did not change in any group. CONCLUSIONS: Water drinking significantly and rapidly raises sympathetic activity. Indeed, it raises plasma norepinephrine as much as such classic sympathetic stimuli as caffeine and nicotine. This effect profoundly increases blood pressure in autonomic failure patients, and this effect can be exploited to improve symptoms due to orthostatic hypotension. Water drinking also acutely raises blood pressure in older normal subjects. The pressor effect of oral water is an important yet unrecognized confounding factor in clinical studies of pressor agents and antihypertensive medications.

Non-NASA Center↗
NASA NTRS1998

Age alters the cardiovascular response to direct passive heating

During direct passive heating in young men, a dramatic increase in skin blood flow is achieved by a rise in cardiac output (Qc) and redistribution of flow from the splanchnic and renal vascular beds. To examine the effect of age on these responses, seven young (Y; 23 +/- 1 yr) and seven older (O; 70 +/- 3 yr) men were passively heated with water-perfused suits to their individual limit of thermal tolerance. Measurements included heart rate (HR), Qc (by acetylene rebreathing), central venous pressure (via peripherally inserted central catheter), blood pressures (by brachial auscultation), skin blood flow (from increases in forearm blood flow by venous occlusion plethysmography), splanchnic blood flow (by indocyanine green clearance), renal blood flow (by p-aminohippurate clearance), and esophageal and mean skin temperatures. Qc was significantly lower in the older than in the young men (11.1 +/- 0.7 and 7.4 +/- 0.2 l/min in Y and O, respectively, at the limit of thermal tolerance; P < 0. 05), despite similar increases in esophageal and mean skin temperatures and time to reach the limit of thermal tolerance. A lower stroke volume (99 +/- 7 and 68 +/- 4 ml/beat in Y and O, respectively, P < 0.05), most likely due to an attenuated increase in inotropic function during heating, was the primary factor for the lower Qc observed in the older men. Increases in HR were similar in the young and older men; however, when expressed as a percentage of maximal HR, the older men relied on a greater proportion of their chronotropic reserve to obtain the same HR response (62 +/- 3 and 75 +/- 4% maximal HR in Y and O, respectively, P < 0.05). Furthermore, the older men redistributed less blood flow from the combined splanchnic and renal circulations at the limit of thermal tolerance (960 +/- 80 and 720 +/- 100 ml/min in Y and O, respectively, P < 0. 05). As a result of these combined attenuated responses, the older men had a significantly lower increase in total blood flow directed to the skin.

Clinical Trial↗
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 NTRS1995

Rheoencephalographic (REG) Assessment of Head and Neck Cooling for use with Multiple Sclerosis Patients

We have prepared a computer program (RHEOSYS:RHEOencephalographic impedance trace scanning SyStem) that can be used to automate the analysis of segmental impedance blood flow waveforms. This program was developed to assist in the post test analysis of recorded impedance traces from multiple segments of the body. It incorporates many of the blood flow, segmental volume, and vascular state indices reported in the world literature. As it is currently programmed, seven points are selected from each blood flow pulse and associated ECG waveforrn: 1. peak of the first ECG QRS complex, 2. start of systolic slope on the blood flow trace, 3. maximum amplitude of the impedance pulse, 4. position of the dicrotic notch, 5. maximum amplitude of the postdicrotic segment, 6. peak of the second ECG QRS complex, and 7. start of the next blood flow pulse. These points we used to calculate various geometric, area, and time-related values associated with the impedance pulse morphology. RHEOSYS then calculates a series of 34 impedance and cardiac cycle parameters which include pulse amplitudes; areas; pulse propagation times; cardiac cycle times; and various measures of arterial and various tone, contractility, and pulse volume. We used this program to calculate the scalp and intracranial blood flow responses to head and neck cooling as it may be applied to lower the body temperatures of multiple sclerosis patients. Twelve women and twelve men were tested using a commercially available head and neck cooling system operated at its maximum cooling capacity for a period of 30 minutes. Head and neck cooling produced a transient change in scalp blood flow and a significant, (P<0.05) decrease of approx. 30% in intracranial blood flow. Results of this experiment will illustrate how REG and RHEOSYS can be used in biomedical applications.

Montogomery, Leslie D.↗
NASA NTRS1998

Contribution of the vertebral artery to cerebral circulation in the rat snake Elaphe obsoleta

Blood supplying the brain in vertebrates is carried primarily by the carotid vasculature. In most mammals, cerebral blood flow is supplemented by the vertebral arteries, which anastomose with the carotids at the base of the brain. In other tetrapods, cerebral blood is generally believed to be supplied exclusively by the carotid vasculature, and the vertebral arteries are usually described as disappearing into the dorsal musculature between the heart and head. There have been several reports of a vertebral artery connection with the cephalic vasculature in snakes. We measured regional blood flows using fluorescently labeled microspheres and demonstrated that the vertebral artery contributes a small but significant fraction of cerebral blood flow (approximately 13% of total) in the rat snake Elaphe obsoleta. Vascular casts of the anterior vessels revealed that the vertebral artery connection is indirect, through multiple anastomoses with the inferior spinal artery, which connects with the carotid vasculature near the base of the skull. Using digital subtraction angiography, fluoroscopy, and direct observations of flow in isolated vessels, we confirmed that blood in the inferior spinal artery flows craniad from a point anterior to the vertebral artery connections. Such collateral blood supply could potentially contribute to the maintenance of cerebral circulation during circumstances when craniad blood flow is compromised, e.g., during the gravitational stress of climbing.

NASA Center ARC↗
NASA NTRS2011

A Data Mining Project to Identify Cardiovascular Related Factors That May Contribute to Changes in Visual Acuity Within the US Astronaut Corps

Many of the cardiovascular-related adaptations that occur in the microgravity environment are due, in part, to a well-characterized cephalad-fluid shift that is evidenced by facial edema and decreased lower limb circumference. It is believed that most of these alterations occur as a compensatory response necessary to maintain a "normal" blood pressure and cardiac output while in space. However, data from both flight and analog research suggest that in some instances these microgravity-induced alterations may contribute to cardiovascular-related pathologies. Most concerning is the potential relation between the vision disturbances experienced by some long duration crewmembers and changes in cerebral blood flow and intra-ocular pressure. The purpose of this project was to identify cardiovascular measures that may potentially distinguish individuals at risk for visual disturbances after long duration space flight. Toward this goal, we constructed a dataset from Medical Operation tilt/stand test evaluations pre- (days L-15-L-5) and immediate post-flight (day R+0) on 20 (3 females, 17 males). We restricted our evaluation to only crewmembers who participated in both shuttle and space station missions. Data analysis was performed using both descriptive and analytical methods (Stata 11.2, College Station, TX) and are presented as means +/- 95% CI. Crewmembers averaged 5207 (3447 - 8934) flight hours across both long (MIR-23 through Expedition16) and short (STS-27 through STS-101) duration missions between 1988 and 2008. The mean age of the crew at the time of their most recent shuttle flight was 41 (34-44) compared to 47 (40-54) years during their time on station. In order to focus our analysis (we did not have codes to separate out subjects by symptomotology) , we performed a visual inspection of each cardiovascular measures captured during testing and plotted them against stand time, pre- to post-flight, and between mission duration. It was found that pulse pressure most clearly differentiated the two mission types. Statistical analysis confirmed that pulse pressure was significantly higher before [45.6; (42.1 to 49.1)] and after [50.7; (46.9 to 54.6)] time on station compared with their most recent shuttle flight [31.6 (27.8 to 35.4), and 32.2 (28.3 to 36.0) respectively] even after correcting differences in age and cumulative number of mission hours. Without knowing the identity of which long duration crewmembers demonstrated visual changes, we were limited to examining whether certain crew regulate components of pulse pressure, systolic and diastolic blood pressure, differently due to microgravity exposure. To that end, we stratified crew into tertiles based on either their pre-flight measure of systolic or diastolic blood pressure. Those crew in the highest tertile for both systolic (lower tertile (n=8; 103-111), middle tertile (n=7; 113-121), and upper tertile (n=5; 125-136) and diastolic blood pressure (lower tertile (n=8; 58-64), middle tertile (n=7; 67-73), and upper tertile (n=5; 75-81) demonstrated less variability in pulse pressure between R+0 and L-10 (Figure 2). Interestingly, those crewmembers with the highest resting systolic blood pressure demonstrated either no change or in some instances an increase in total peripheral resistance, where those in the lower tertiles had lower values of total peripheral resistance compared to pre-flight levels. In this study, it was found that crewmembers in the highest tertile for both systolic and diastolic blood pressure demonstrated less variability in pulse pressure and that the decrease in variability was due in part to lower levels of compliance as indicated by similar or higher levels of total peripheral resistance after compared with before flight levels. Whether there is a relation between blood pressure regulation and total peripheral resistance in crew presenting with negative changes in visual acuity remains unknown.

Westby, Christian M.↗
NASA NTRS2000

Pyruvate ingestion for 7 days does not improve aerobic performance in well-trained individuals

The purposes of the present studies were to test the hypotheses that lower dosages of oral pyruvate ingestion would increase blood pyruvate concentration and that the ingestion of a commonly recommended dosage of pyruvate (7 g) for 7 days would enhance performance during intense aerobic exercise in well-trained individuals. Nine recreationally active subjects (8 women, 1 man) consumed 7, 15, and 25 g of pyruvate and were monitored for a 4-h period to determine whether blood metabolites were altered. Pyruvate consumption failed to significantly elevate blood pyruvate, and it had no effect on indexes of carbohydrate (blood glucose, lactate) or lipid metabolism (blood glycerol, plasma free fatty acids). As a follow-up, we administered 7 g/day of either placebo or pyruvate, for a 1-wk period to seven, well-trained male cyclists (maximal oxygen consumption, 62.3 +/- 3.0 ml. kg(-1). min(-1)) in a randomized, double-blind, crossover trial. Subjects cycled at 74-80% of their maximal oxygen consumption until exhaustion. There was no difference in performance times between the two trials (placebo, 91 +/- 9 min; pyruvate, 88 +/- 8 min). Measured blood parameters (insulin, peptide C, glucose, lactate, glycerol, free fatty acids) were also unaffected. Our results indicate that oral pyruvate supplementation does not increase blood pyruvate content and does not enhance performance during intense exercise in well-trained cyclists.

Randomized Controlled Trial↗
NASA NTRS1994

Cerebral versus systemic hemodynamics during graded orthostatic stress in humans

BACKGROUND: Orthostatic syncope is usually attributed to cerebral hypoperfusion secondary to systemic hemodynamic collapse. Recent research in patients with neurocardiogenic syncope has suggested that cerebral vasoconstriction may occur during orthostatic hypotension, compromising cerebral autoregulation and possibly contributing to the loss of consciousness. However, the regulation of cerebral blood flow (CBF) in such patients may be quite different from that of healthy individuals, particularly when assessed during the rapidly changing hemodynamic conditions associated with neurocardiogenic syncope. To be able to interpret the pathophysiological significance of these observations, a clear understanding of the normal responses of the cerebral circulation to orthostatic stress must be obtained, particularly in the context of the known changes in systemic and regional distributions of blood flow and vascular resistance during orthostasis. Therefore, the specific aim of this study was to examine the changes that occur in the cerebral circulation during graded reductions in central blood volume in the absence of systemic hypotension in healthy humans. We hypothesized that cerebral vasoconstriction would occur and CBF would decrease due to activation of the sympathetic nervous system. We further hypothesized, however, that the magnitude of this change would be small compared with changes in systemic or skeletal muscle vascular resistance in healthy subjects with intact autoregulation and would be unlikely to cause syncope without concomitant hypotension. METHODS AND RESULTS: To test this hypothesis, we studied 13 healthy men (age, 27 +/- 7 years) during progressive lower body negative pressure (LBNP). We measured systemic flow (Qc is cardiac output; C2H2 rebreathing), regional forearm flow (FBF; venous occlusion plethysmography), and blood pressure (BP; Finapres) and calculated systemic (SVR) and forearm (FVR) vascular resistances. Changes in brain blood flow were estimated from changes in the blood flow velocity in the middle cerebral artery (VMCA) using transcranial Doppler. Pulsatility (systolic minus diastolic/mean velocity) normalized for systemic arterial pressure pulsatility was used as an index of distal cerebral vascular resistance. End-tidal PACO2 was closely monitored during LBNP. From rest to maximal LBNP before the onset of symptoms or systemic hypotension, Qc and FBF decreased by 29.9% and 34.4%, respectively. VMCA decreased less, by 15.5% consistent with a smaller decrease in CBF. Similarly, SVR and FVR increased by 62.8% and 69.8%, respectively, whereas pulsatility increased by 17.2%, suggestive of a mild degree of small-vessel cerebral vasoconstriction. Seven of 13 subjects had presyncope during LBNP, all associated with a sudden drop in BP (29 +/- 9%). By comparison, hyperventilation alone caused greater changes in VMCA (42 +/- 2%) and pulsatility but never caused presyncope. In a separate group of 3 subjects, superimposition of hyperventilation during highlevel LBNP caused a further decrease in VMCA (31 +/- 7%) but no change in BP or level of consciousness. CONCLUSIONS: We conclude that cerebral vasoconstriction occurs in healthy humans during graded reductions in central blood volume caused by LBNP. However, the magnitude of this response is small compared with changes in SVR or FVR during LBNP or other stimuli known to induce cerebral vasoconstriction (hypocapnia). We speculate that this degree of cerebral vasoconstriction is not by itself sufficient to cause syncope during orthostatic stress. However, it may exacerbate the decrease in CBF associated with hypotension if hemodynamic instability develops.

NASA Discipline Cardiopulmonary↗
NASA NTRS2003

Sympathetic responses to Valsalva's manoeuvre following bed rest

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.

NASA Discipline Musculoskeletal↗
NASA NTRS2002

Limb neurovascular control during altered otolithic input in humans

Head-down rotation (HDR), which activates the vestibulosympathetic reflex, increases leg muscle sympathetic nerve activity (MSNA) and produces calf vasoconstriction with no change in either cardiac output or arterial blood pressure. Based on animal studies, it was hypothesized that differential control of arm and leg MSNA explains why HDR does not alter arterial blood pressure. Fifteen healthy subjects were studied. Heart rate, arterial blood pressure, forearm and calf blood flow, and leg MSNA responses were measured during HDR in these subjects. Simultaneous recordings of arm and leg MSNA were obtained from five of the subjects. Forearm and calf blood flow, vascular conductances, and vascular resistances were similar before HDR, as were arm and leg MSNA. HDR elicited similar significant increases in leg (Delta 6 +/- 1 bursts min(-1); 59 +/- 16 % from baseline) and arm MSNA (Delta 5 +/- 1 bursts min(-1); 80 +/- 28 % from baseline). HDR significantly decreased calf (-19 +/- 2 %) and forearm vascular conductance (-12 +/- 2 %) and significantly increased calf (25 +/- 4 %) and forearm vascular resistance (15 +/- 2 %), with ~60 % greater vasoconstriction in the calf than in the forearm. Arterial blood pressure and heart rate were not altered by HDR. These results indicate that there is no differential control of MSNA in the arm and leg during altered feedback from the otolith organs in humans, but that greater vasoconstriction occurs in the calf than in the forearm. These findings indicate that vasodilatation occurs in other vascular bed(s) to account for the lack of increase in arterial blood pressure during HDR.

Non-NASA Center↗
NASA NTRS1994

Manipulation of norepinephrine metabolism with yohimbine in the treatment of autonomic failure

It has been postulated that alpha 2-adrenergic receptors play a modulatory role in the regulation of blood pressure. Activation of alpha 2-receptors located in the central nervous system results in inhibition of sympathetic tone and decrease of blood pressure. This indeed may be the mechanism of action of central sympatholytic antihypertensives such as alpha-methyldopa. Presynaptic alpha 2-receptors also are found in adrenergic nerve terminals. These receptors act as a negative feedback mechanism by inhibiting the release of norepinephrine. The relevance of alpha 2-adrenergic receptors for blood pressure regulation can be explored with yohimbine, a selective antagonist of these receptors. Yohimbine increases blood pressure in resting normal volunteers. This effect is associated with an increase in both sympathetic nerve activity, reflecting an increase in central sympathetic outflow, and in norepinephrine spillover, reflecting potentiation of the release of norepinephrine from adrenergic nerve terminals. These actions, therefore, underscore the importance of alpha 2-adrenergic receptors for blood pressure regulation even under resting conditions. Patients with autonomic failure, even those with severe sympathetic deprivation, are hypersensitive to the pressor effects of yohimbine. This increased responsiveness can be explained by sensitization of adrenergic receptors, analogous to denervation supersensitivity, and by the lack of autonomic reflexes that would normally buffer any increase in blood pressure. Preliminary studies suggest that the effectiveness of yohimbine in autonomic failure can be enhanced with monoamine oxidase inhibitors. Used in combination, yohimbine increases norepinephrine release, whereas monoamine oxidase inhibitors inhibit its degradation. Therefore, yohimbine is not only a useful tool in the study of blood pressure regulation, but may offer a therapeutic option in autonomic dysfunction.

Review, Tutorial↗
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 NTRS1983

Non-invasive method and apparatus for measuring pressure within a pliable vessel

A non-invasive method and apparatus is disclosed for measuring pressure within a pliable vessel such as a blood vessel. The blood vessel is clamped by means of a clamping structure having a first portion housing a pressure sensor and a second portion extending over the remote side of the blood vessel for pressing the blood vessel into engagement with the pressure sensing device. The pressure sensing device includes a flat deflectable diaphragm portion arranged to engage a portion of the blood vessel flattened against the diaphragm by means of the clamp structure. In one embodiment, the clamp structure includes first and second semicylindrical members held together by retaining rings. In a second embodiment the clamp structure is of one piece construction having a solid semicylindrical portion and a hollow semicylindrical portion with a longitudinal slot in the follow semicylindrical portion through which a slip the blood vessel. In a third embodiment, an elastic strap is employed for clamping the blood vessel against the pressure sensing device.

Shimizu, M.↗
NASA NTRS1993

Development of a model to assess orthostatic responses

A major change for crewmembers during weightlessness in microgravity is the redistribution of body fluids from the legs into the abdomen, thorax, and head. The fluids continue to be sequestered in these areas throughout the flight. Upon reentry into gravity on landing, these same body fluids are displaced again to their normal locations, however, not without hazardous incidence to the crewmembers. The problem remains that upon landing, crewmembers are subject to orthostasis, that is, the blood flowing into the legs reduces the blood supply to the brain and may result in the crewmember fainting. The purpose of this study was to develop a model of testing orthostatic responses of blood pressure regulating mechanisms of the cardiovascular system, when challenged, to maintain blood pressure to the brain. To accomplish this, subjects' responses were assessed as they proceeded from the supine position of progressive head-up tilt positions of 30 deg, 60 deg, and 90 deg angles. A convenience sample consisted of 21 subjects, females (N=11) and males (N=10), selected from a list of potential subjects available through the NASA subject screening office. The methodology included all non-invasive measurements of blood pressure, heart rate, echocardiograms, cardiac output, cardiac stroke volume, fluid shifts in the thorax, ventricular ejection and velocity times, and skin blood perfusion. The Fischer statistical analysis was done of all data with the significance level at .05. Significant differences were demonstrated in many instances of changes of posture for all variables. Based on the significance of the findings of this study, this model for assessing orthostatic responses does provide an adequate challenge to the blood pressure regulatory systems. While individuals may use different adaptations to incremental changes in gravity, the subjects, in aggregate, demonstrated significant adaptive cardiovascular changes to orthostatic challenges which were presented to them.

Rubin, Marilyn↗
NASA NTRS1994

Mechanism of Headward Fluid Shift During Exposure To Microgravity

A prominent feature of early cardiovascular adaptation to the microgravity of space flight is a shift of blood and tissue fluid from the lower body to the upper body. Symptoms of this fluid shift include facial edema, nasal congestion, and headache. Normally on Earth, the human body is exposed to hydrostatic (gravitational) blood pressure gradients during upright posture. In this posture, mean arterial pressures at head, heart, and foot levels are approximately 70, 100, and 200 mm Hg, respectively. Theoretically, all hydrostatic pressures within arteries and veins are lost during exposure to microgravity so that mean arterial pressure in all regions of the body is uniform and approximately equal to that at heart level (100 mm Hg). Acute studies of 60 head-down tilt (simulated microgravity on Earth) indicate that facial edema is caused by: 1) elevation of capillary blood pressure from 28 to 34 mm Hg, 2) reduction of blood colloid osmotic pressure 22 to 18 mm Hg, and 3) 50% increase of blood perfusion in tissues of the head. Furthermore, as compared to microvasculature in the feet, microvessels of the head have a low capacity to constrict and diminish local perfusion. Elevation of blood and tissue fluid pressures/flow in the head may also explain the higher headward bone density associated with long-term head-down tilt. These mechanistic studies of head-down tilt, along with a better understanding of the relative stresses involved with upright posture and lower body negative pressure, have facilitated development of physiologic countermeasures to maintain astronaut health during microgravity. Presently no exercise hardware is available to provide a blood pressure gradient from head to feet in space. However, recent studies in our laboratory suggest that treadmill exercise within lower body negative pressure provides equivalent or greater physiologic stress as compared to similar upright exercise on Earth.

Hargens, Alan R.↗
NASA NTRS1994

Tolerance of Snakes to Hypergravity

Sensitivity of carotid blood flow to +Gz (head-to-tail) acceleration was studied in six species of snakes hypothesized to show varied adaptive cardiovascular responses to gravity. Blood flow in the proximal carotid artery was measured in 15 snakes before, during and following stepwise increments of +0.25Gz force produced on a 2.4 m diameter centrifuge. During centrifugation each snake was confined to a straight position within an individually- fitted acrylic tube with the head facing the center of rotation. We measured the centrifugal force at the tail of the snake in order to quantify the maximum intensity of force gradient promoting antero-posterior pooling of blood. Tolerance to increased gravity was quantified as the acceleration force at which carotid blood flow ceased. This parameter varied according to the gravitational adaptation of species defined by their ecology and behavior. At the extremes, carotid blood flow decreased in response to increasing gravity and approached zero near +1Gz in aquatic and ground-dwelling species, whereas in climbing species carotid flow was maintained at forces in excess of +2Gz. Surprisingly, tolerant (arboreal) species withstood hypergravic forces of +2 to +3 G. for periods up to 1 h without cessation of carotid blood flow or apparent loss of consciousness. Data suggest that relatively tight skin of the tolerant species provides a natural antigravity suit which is of prime importance in counteracting Gz stress on blood circulation.

Lillywhite, H. B.↗