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Central hypovolemia elicited by orthostasis or hemorrhage triggers sympathetically-mediated baroreflex responses to maintain organ perfusion; these reflexes are less sensitive in patients with orthostatic intolerance, and during conditions of severe blood loss, may result in cardiovascular collapse (decompensatory or circulatory shock). The ability to tolerate central hypovolemia is variable and physiological factors contributing to tolerance are emerging. We tested the hypothesis that resting muscle sympathetic nerve activity (MSNA) and sympathetic baroreflex sensitivity (BRS) are attenuated in male and female subjects who have low tolerance (LT) to central hypovolemia induced by lower body negative pressure (LBNP). MSNA and diastolic arterial pressure (DAP) were recorded in 47 human subjects who subsequently underwent LBNP to tolerance (onset of presyncopal symptoms). LT subjects experienced presyncopal symptoms prior to completing LBNP of 60mm Hg, and subjects with high tolerance (HT) experienced presyncopal symptoms after completing LBNP of 60mm Hg. Contrary to our hypothesis, resting MSNA burst incidence was not different between LT and HT subjects, and was not related to time to presyncope. BRS was assessed as the slope of the relationship between spontaneous fluctuations in DAP and MSNA during 5 min of supine rest. MSNA burst incidence/DAP correlations were greater than or equal to 0.5 in 37 subjects (LT: n = 9; HT: n = 28), and BRS was not different between LT and HT ( 1.8 0.3 vs. 2.2 0.2 bursts (100 beats) 1 mm Hg 1, p = 0.29). We conclude that tolerance to central hypovolemia is not related to either resting MSNA or sympathetic BRS., Published in Frontiers in Physiology, v5 article 241, Jun 2014. Prepared in collaboration with the University of North Texas Health Science Center, Fort Worth.

Non-freezing cold injury (NFCI) is the Cinderella of thermal injuries and is a clinical syndrome that occurs when tissues are exposed to cold temperatures close to freezing point for sustained periods. NFCI is insidious in onset, often difficult to recognize and problematic to treat, and yet the condition accounts for significant morbidity in both military and civilians who work in cold conditions. Consequently recognition of those at risk, limiting their exposure and the appropriate and timely use of suitable protective equipment are essential steps in trying to reduce the impact of the condition. This review addresses the issues surrounding NFCI., Published in the Journal of the Royal Army Medical Corps, v157 n1 p79-84, 2011.

Loss of consciousness due to central hypovolemia can occur due to sudden cardiovascular decompensation in normal individuals or hypovolemic shock in wounded patients. A variety of devices have been developed to sustain perfusion to the brain including anti-G suits worn by pilots and returning astronauts and applied to patients as shock trousers. However, all countermeasures developed to date suffer from problems that limit their utility in the field. An impedance threshold device (ITD) has recently been developed that acutely increases central blood volume by forcing the thoracic muscles to develop increased negative pressure, thus drawing venous blood from extrathoracic cavities into the heart and lungs. We review here a series of experiments that demonstrate the application of the ITD to a variety of experimental conditions, including its use to: (a) increase heart rate, stroke volume, and arterial blood pressure in normovolemia and hypovolemia; (b) increase cerebral blood flow velocity; (c) reset cardiac baroreflex function to a higher operating range for blood pressure; (d) lower intracranial pressure; and (e) reduce orthostatic symptoms. In this brief review, we present evidence that supports further consideration of using inspiratory resistance as a countermeasure against circulatory collapse associated with orthostatic instability and hemorrhagic shock., See also ADM002075., The original document contains color images.

In the first two weeks of high altitude (HA) exposure, blood pressure typically rises due, in part, to a large increase in sympathetic stimulation. We hypothesized that blocking al-adrenergic receptors would impair circulatory compensation to an orthostatic challenge to a greater extent at HA than at sea level (SL). Sixteen healthy women (23 +/- 2 yr) were randomly assigned to receive either 2 mg prazosin (n = 8) or placebo (n = S) t.i.d. (double-blind design) for 12 d at SL and during the first 12 d of HA residence (4300 m). Passive &)O upright tilt was performed at SL (10 d of treatment), and after 3 and 10 d at HA. Mean arterial blood pressure (MBP, auscultation) and heart rate (HR, ECG) were measured every min during 10 min each of supine rest and tilt. For the prazosin group compared to the placebo group: (1) At SL, supine and tilt MBP were lower (P < 0.05) and, at HA, MBP was lower only in the first several min of tilt on day 10 (P <0.05); (2) At SL or HA, HR was similar for either position; and (3) From supine to tilt, the drop in MBP was greater only at SL and the increase in HR was consistently greater only at HA (both P < 0.05). We conclude that al-adrenergic blockade altered MBP and HR responses to tilt at SL and HA, but that orthostatic tolerance was well maintained in both environments. Compensatory adjustments were likely in either sympathetic and parasympathetic neural discharge or in other receptor activities.

Exposure to microgravity is associated with increased leg venous compliance and reductions in cardiac output, baroreflex functions, and tolerance to orthostatism. However, the effects of chronic exposure to high-G environments are unknown. In addition, there is evidence that females have lower orthostatic tolerance than males, although the underlying mechanisms are unclear. Therefore, we tested the hypotheses that high-G training will enhance baroreflex and orthostatic functions and that females will demonstrate similar adaptations compared with males. Results: G-training increased calf compliance in both men and women. Stroke volume and cardiac output were increased during the squat-to-stand test in the males, but not in the females, following G-training and provided protection against the development of acute hypotension in the men. Conclusions: G-training caused adaptations in orthostatic functions opposite to those observed following exposure to microgravity environments. However, adaptations to G-training were limited in females, a finding that may provide a physiological basis for their lower simulated combat tracking performance during simulated aerial combat maneuvers compared with males.

Functions of baroreflex control of heart rate and vascular resistance, adrenoreceptor responsiveness, indices of baseline vagal and sympathetic tone, plasma volume, and venous compliance were compared in men and women to test the hypothesis that greater orthostatic intolerance in women would be associated with impairment of specific mechanisms of blood pressure regulation. Heart rate (HR), stroke volume (SV), cardiac output (Q), mean arterial blood pressure (MAP), forearm (FVR) and leg (LVR) vascular resistance, catecholamines (NE), and changes in leg volume (%LV) were measured during various protocols of lower body negative pressure (LBNP), carotid stimulation, and infusions of adrenoreceptor agonists in 10 females and 10 males matched for age and fitness. LBNP tolerance for women (797 +/- 63 mmHg.min) was 35% lower (P=0.0017) than for men. At presyncope, SV, Q, MAP and %LV were lower (P<0.05) in females compared to males while HR, FVR, and TPR were similar in both groups. Lower LBNP tolerance in females was associated with impairment of the heart rate response to carotid baroreceptor stimulation, lower baseline cardiac vagal activity, greater decline in Q and SV induced by LBNP, increased Beta1-adrenoreceptor responsiveness, greater vasoconstriction under equal LBNP, lower levels of NE at presyncope, and lower blood volume. Results support the hypothesis that women have significant deficiencies in mechanisms that underlie blood pressure regulation under orthostatic challenge. These findings should be considered in selection and training of women for military combat, especially in combat missions requiring high-G aerial maneuvers.

Functions of baroreflex control of heart rate and vascular resistance, adrenoreceptor responsiveness, indices of baseline vagal and sympathetic tone, plasma volume, and venous compliance were compared in men and women to test the hypothesis that greater orthostatic intolerance in women would be associated with impairment of specific mechanisms of blood pressure regulation. Heart rate (HR), stroke volume (SV), cardiac output (Q), mean arterial blood pressure (MAP), forearm (FVR) and leg (LVR) vascular resistance, catecholamines (NE), and changes in leg volume (%LV) were measured during various protocols of lower body negative pressure (LBNP), carotid stimulation, and infusions of adrenoreceptor agonists in 10 females and 10 males matched for age and fitness. LBNP tolerance for women (797 +/- 63 mmHg.min) was 35% lower (P 0.0017) than for men. At presyncope, SV, Q, MAP and %LV were tolerance in females was associated with impairment of the heart rate response to carotid baroreceptor stimulation, lower baseline cardiac vagal activity, greater decline in Q and SV induced by LBNP, increased B1-adrenoreceptor responsiveness, greater vasoconstriction under equal LBNP, lower levels of NE at presyncope, and lower blood volume. Results support the hypothesis that women have significant deficiencies in mechanisms that underlie blood pressure regulation under orthostatic challenge. These findings should be considered in selection and training of women for military combat, especially in combat missions requiring high-G aerial maneuvers.

Orthostatically- induced syncope is accompanied by venous pooling and vasodilatation. Loss of consciousness during head-to-foot acceleration (G-LOC) in aviators may be caused by a different mechanism, as venous pooling should be prevented through the use of an anti-G suit. This research was conducted to test the hypothesis that in individuals wearing a well-fitted anti-G garment, there are no important changes in volume of dependent regions during loss of consciousness resulting from rapid onset acceleration stress. Furthermore, this work compares venous pooling patterns in G-LOC subjects to patterns seen during syncope in volunteers and patients subjected to orthostatic stress. The tilt/LBNP tests were conducted to establish what level of venous pooling was required to induce syncope in the absence of a hydrostatic component (other than 1 G) and to confirm that our equipment was sensitive enough to detect volume changes large enough to cause syncope.

To evaluate the effects of orthostatic, attitude, and pharmacologic stresses upon civil aviation-specific performance, a double-blind, randomized, crossover trial of atenolol, 100mg, was designed and executed. Hypertensive mates and females qualifying for the FAA class 3 certificate with mean age of 34 were studied during simulated attitude exposure to 12,500 ft, orthostatic stress, and moderate exercise. Seated Lower body negative pressure to -40 mmHg supplied orthostatic stress simulating +2G vertical acceleration. A total of 160 Lower body negative pressure tests were performed, 80 at ground and 80 at altitude. Beta-blockade caused a modest impairment in orthostatic tolerance. Five of the 80 Lower body negative pressure runs at ground level were marked by intolerance, and all of those responses were in beta-blocked subjects. of the 80 attitude runs, 30 were terminated for intolerance, of which 18 included beta- blockade. These findings had a Chi-square significance value of P < .05. The effect of attitude was significant at P < .01. In a modest exercise protocol (100 watts for 3 minutes) meant to be no more stressful than the exertional requirements of piloting an aircraft during adverse conditions, neither beta- blockade or altitude appeared to Limit performance. Quantitative performance on a computerized cognitive battery clearly demonstrated impaired performance during Lower body negative pressure stress at altitude. The degree of impairment was significant compared to a learning curve response at the P < .001 level. The degree of impairment was similar for placebo treated and beta-blocked subjects.

Data indicate that for sinusoidal acceleration of 1, 2, and 3 cycles per minute at amplitudes of 0 to +2g(z) and +1 to +2g(z), the cardiovascular system of chronically instrumented dogs was capable of following with peak to peak responses equal to those obtained for sustained tests with mean responses correlatable with the mean g level of the test. The usefulness of the awake, chronically instrumented canine as a surrogate for man under conditions of orthostatic loading along the spinal axis was studied and the cardiovascular responses were found to be similar. It was further suggested the canine might also be used as a surrogate for man in non-extreme states of cardiovascular deconditioning. Other data suggests that in these subjects the combined stress of +g(z) orthostasis and g(z) vibration produces a more sensitive cardiovascular system capable of responding with larger changes in mean aortic flow than those seen with g(z) vibration alone.

Inferences can be drawn as to the stability of the circulation in an experimental subject by observing the values of such circulatory factors as pulse rate(1), systolic and diastolic blood pressure (RR2 and RRd) in the horizontal and in the standing positions respectively. The height of the haemostatic pressure was determined; its mean value for 500 subjects was 10 cm. Another possibility of determining the relative stroke volume, comparing the lying with the upright position, is based on the equivalent bodily performance in either position. It is also attempted to establish a practical numerical index (OJ), which would indicate the degree of orthostatic stability or lability according to the numerical value. Similar to the performance index, a higher number is indicative of more unfavourable circulatory conditions. The final part of the study consisted in establishing correlations of the O-indices with body weight, height and age. In not a single case was there any significant correlation., Trans. of Wehrmedizin (West Germany) v4 n5 p89-99 May 1966.

The purpose of this study was to examine the relationship between fitness level and cardiovascular response to orthostatic stress in healthy women between the ages of 20 and 35. Subjects were divided into three groups on the basis of VOxmax: High-Fit (n = 4; mean VO2max = 57.8 ml/kg/min); Med-Fit (n =8; mean VO2max = 46.4 ml/kg/min); and Low-Fit (n = 4; mean VO2max = 34.7 ml/kg/min) . Subjects were exposed to three trials of 70 deg. head up tilt each followed by a ten minute supine rest period. Heart rate and blood pressure were recorded at 1, 3 and 5 minute intervals during head up tilt and at 5 and 10 minute intervals during the supine rest period. The mean heart rate and blood pressure changes from supine to one minute upright were calculated for each group. The change in heart rate per change in systolic blood pressure from supine to 70 deg. upright was used as an index of baroreflex responsiveness. There was a strong inverse correlation between fitness level and heart rate both resting and at 1 minute upright (r=.83;r=.84 p < .001). An analysis of baroreflex indicies between groups showed no fitness related differences in heart rate and blood pressure response to orthostasis.

Ten healthy men, 20-35 years old, were tested for tolerance to hypoxia, orthostatic stress, and physical fatigue after a period of rest, and, on another occasion, after a period of physical exertion. Exertion consisted of four 10-min periods of pedal ergometry; each period consisted of a 30-watt (W) load imposed for 2 min, 60 W for 4 min, and 100 W for 4 min. Testing included a 100-min exposure to an oxygen/nitrogen gas mixture equivalent to 3658 m of altitude, 2 min of lower body negative pressure (LBNP) at -40 torr differential pressure, and 6 min of 50 W pedal ergometry. Psychomotor testing was conducted during hypoxic exposure. Although some statistically significant (p < or = 0.05) physiological decrements were associated with prior physical exertion, psychomotor performance and mentation were not significantly affected. In this study, prior physical exertion produced no adverse effects on physiological tolerances, mentation, or psychomotor performance.

The final report addresses advances in anthropometric and physical conditioning that will improve physical fitness and orthostatic tolerance related to improvement in handling high sustained G (HGS) stress. Topics include: (1) Man, exercise and orthostasis, (2) Animal model response to HGS; and Man, thermal stress and physical performance. Five years of work are condensed in the report.

After World War II an increasing number of aeromedical investigators became interested in the medical problems of Space Flight, particularly its most challenging aspect: Weightlessness. Until 1950 their efforts remained limited to theoretical deliberations and predictions. Beginning in the early fifties, however, researchers began actively to experiment with weightlessness aboard aircraft in vertical diving flights and later by flying Keplerian parabolic trajectories. H.J. von Beckh made the first weightlessness experiments in aircraft with humans and test animals (water turtles) which were published at the 4th International Astronautical Congress in Zurich and in the Journal of Aviation Medicine. Von Beckh's experiments showed that the subjects during weightlessness had by no means a strong fall reflex, as predicted by Haber and Gerathewohl in their theoretical apaper, in which they evoked the Weber-Fechner Law. The disorientation and lack of neuromuscular coordination occurs only in the first seconds of weightlessness. Later, the control of the vision sense makes aiming movement possible. The incidence of motion sickness was moderate in the weightlessness flights with the Fighter aircraft Fiat G55 and the F 94 C, because the subjects were tied down in the seat and avoided head movements. Later, in the flights with the cargo aircraft C 131, the incidence was considerably higher, because the subjects could move freely and were even allowed to make somersaults, In a later paper (1959) von Beckh showed that weightlessness decreases the acceleration tolerance. This was reconfirmed many years later when the astronauts suffered orthostatic hypotension after their return to the earth.