Carotid-cardiac baroreflex: relation with orthostatic hypotension following simulated microgravity and implications for development of countermeasures

Resting sympathetic baroreflex sensitivity in subjects with low and high tolerance to central hypovolemia induced by lower body negative pressure

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 −60 mm Hg, and subjects with high tolerance (HT) experienced presyncopal symptoms after completing LBNP of −60 mm 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.

Sex differences in blood pressure control during 6° head-down tilt bed rest

Spaceflight-induced orthostatic intolerance has been studied for decades. Although ∼22% of the astronaut corps are women, most mechanistic studies use mostly male subjects, despite known sex differences in autonomic control and postflight orthostatic intolerance. We studied adrenergic, baroreflex, and autonomic indexes during continuous infusions of vasoactive drugs in men and women during a 60-day head-down bed rest. Volunteers were tested before bed rest (20 men and 10 women) and around day 30 (20 men and 10 women) and day 60 (16 men and 8 women) of bed rest. Three increasing doses of phenylephrine (PE) and sodium nitroprusside were infused for 10 min after an infusion of normal saline. A 20-min rest period separated the phenylephrine and sodium nitroprusside infusions. Autonomic activity was approximated by spectral indexes of heart rate and blood pressure variability, and baroreflex sensitivity was measured by the spontaneous baroreflex slope. Parasympathetic modulation and baroreflex sensitivity decreased with bed rest, with women experiencing a larger decrease in baroreflex sensitivity by day 30 than men. The sympathetic activation of men and parasympathetic responsiveness of women in blood pressure control during physiological stress were preserved throughout bed rest. During PE infusions, women experienced saturation of the R-R interval at high frequency, whereas men did not, revealing a sex difference in the parabolic relationship between high-frequency R-R interval, a measurement of respiratory sinus arrhythmia, and R-R interval. These sex differences in blood pressure control during simulated microgravity reveal the need to study sex differences in long-duration spaceflight to ensure the health and safety of the entire astronaut corps.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Effects of inspiratory impedance on the carotid-cardiac baroreflex response in humans

We were interested in a therapeutic device designed to increase carotid-cardiac baroreflex sensitivity (BRS) since high BRS is associated with a lower risk for development of hypotension in humans with experimentally-induced central hypovolemia. We hypothesized that spontaneous breathing through an impedance threshold device (ITD) designed to increase negative intrathoracic pressure during inspiration and elevate arterial blood pressure would acutely increase BRS in humans. We tested this hypothesis by measuring heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressures, and carotid-cardiac BRS in 10 female and 10 male subjects breathing through a face mask at three separate ITD conditions: (a) 6 cm H(2)O; (b) 12 cm H(2)O; and (c) a control (0 cm H(2)O). HR was increased (P = 0. 013) from 64 +/- 3 bpm during control to 68 +/- 3 bpm at 6 cm H(2)O ITD and 71 +/- 4 bpm at 12 cm H(2)O ITD breathing conditions. During ITD breathing, BRS was not altered but responses were shifted to higher arterial pressures. However, SBP and DBP were elevated for both the 6 and 12 cm H(2)O conditions compared to the 0 cm H(2)O condition, but returned to control (sham) levels by 30 minutes after cessation of ITD breathing. There were no gender effects for BRS or any hemodynamic responses to breathing through the ITD. We conclude that breathing with inspiratory impedance at relatively low pressures can increase baseline arterial blood pressure, i. e., reset the operational point for SBP on the baroreflex stimulus-response relationship, in healthy subjects. This resetting of the cardiac baroreflex may represent a mechanism that allows blood pressure to increase without a reflex-mediated reduction in HR.

Calf venous volume during stand-test after a 90-day bed-rest study with or without exercise countermeasure

The objectives to determine both the contribution to orthostatic intolerance (OI) of calf venous volume during a stand-test, and the effects of a combined eccentric-concentric resistance exercise countermeasure on both vein response to orthostatic test and OI, after 90-day head-down tilt bed-rest (HDT). The subjects consisted of a control group (Co-gr, n = 9) and an exercise countermeasure group (CM-gr, n = 9). Calf volume and vein cross-sectional area (CSA) were assessed by plethysmography and echography during pre- and post-HDT stand-tests. From supine to standing (post-HDT), the tibial and gastrocnemius vein CSA increased significantly in intolerant subjects (tibial vein, +122% from pre-HDT; gastrocnemius veins, +145%; P < 0.05) whereas it did not in tolerant subjects. Intolerant subjects tended to have a higher increase in calf filling volume than tolerant subjects, in both sitting and standing positions. The countermeasure did not reduce OI. Absolute calf volume decreased similarly in both groups. Tibial and gastrocnemius vein CSA at rest did not change during HDT in either group. During the post-HDT stand-test, the calf filling volume increased more in the CM-gr than in the Co-gr both in the sitting (+1.3 +/- 5.1%, vs. -7.3 +/- 4.3%; P < 0.05) and the standing positions (+56.1 +/- 23.7% vs. +1.6 +/- 9.6%; P < 0.05). The volume ejected by the muscle venous pump increased only in the CM-gr (+38.3 +/- 21.8%). This study showed that intolerant subjects had a higher increase in vein CSA in the standing position and a tendency to present a higher calf filling volume in the sitting and standing positions. It also showed that a combined eccentric-concentric resistance exercise countermeasure had no effects on either post-HDT OI or on the venous parameters related to it.

Planning strategies for development of effective exercise and nutrition countermeasures for long-duration space flight

Exercise and nutrition represent primary countermeasures used during space flight to maintain or restore maximal aerobic capacity, musculoskeletal structure, and orthostatic function. However, no single exercise, dietary regimen, or combination of prescriptions has proven entirely effective in maintaining or restoring cardiovascular and musculoskeletal functions to preflight levels after prolonged space flight. As human space flight exposures increase in duration, identification, assessment, and development of various effective exercise- and nutrition-based protective procedures will become paramount. The application of adequate dietary intake in combination with effective exercise prescription will be based on identification of basic physiologic stimuli that maintain normal function in terrestrial gravity, and understanding how specific combinations of exercise characteristics (e.g., duration, frequency, intensity, and mode) can be combined with minimal nutritional requirements that mimic the stimuli normally produced by living in Earth's gravity environment. This can be accomplished only with greater emphasis of research on ground-based experiments targeted at understanding the interactions between caloric intake and expenditure during space flight. Future strategies for application of nutrition and exercise countermeasures for long-duration space missions must be directed to minimizing crew time and the impact on life-support resources.

Exercise as a countermeasure for physiological adaptation to prolonged spaceflight

Exercise represents the primary countermeasure used during spaceflight to maintain or restore maximal aerobic capacity (VO2max), musculoskeletal structure, and orthostatic function. However, no single exercise or combination of prescriptions has proven entirely effective in restoring cardiovascular and musculoskeletal functions to preflight levels following prolonged spaceflight. As human spaceflight exposures increase in duration, assessment and development of various effective exercise-based protective procedures become paramount. This must involve improvement in specific countermeasure prescription as well as development of additional approaches that will allow space travelers greater flexibility and medical safety during long flights. Effective exercise prescription will be based on identification of basic physiological stimuli that maintain normal function in terrestrial gravity and understanding of how specific combinations of exercise characteristics e.g., duration, frequency, intensity, mode) can mimic these stimuli and affect the overall process of adaptation to microgravity. This can be accomplished only with greater emphasis of research on ground-based experiments. Future attention must be directed to improving exercise compliance while minimizing both crew time and the impact of the exercise on life-support resources.

Effects of exercise and inactivity on intravascular volume and cardiovascular control mechanisms

Exercise, inactivity and confinement have been used as effective tools to assess the contributions of vascular volume and baroreflexes to orthostatic hypotension associated with exposure to microgravity. Prolonged exposure to bedrest, physical inactivity, or wheelchair confinement removes baroreceptor unloading caused by regular upright standing and induces attenuation of cardiovascular baroreflex responses. The magnitude of reduced baroreflex sensitivity following bedrest or wheelchair confinement is related to the degree of orthostatic hypotension. Reduction in vascular volume caused by bedrest or progressive hypovolemia does not affect carotid-cardiac baroreflex function. In contrast, intense exercise that increases arterial baroreceptor loading causes an acute increase in carotid baroreceptor sensitivity and has been associated with enhanced orthostatic stability following exposure to simulated microgravity. Endurance exercise training designed to enhance orthostatic stability was associated with increased blood volume and vasoconstrictive reserve, but no change in the carotid baroreflex response. Therefore, using models of exercise, inactivity and confinement, integrated and redundant roles for vascular volume and cardiovascular baroreflexes have been demonstrated as probable underlying mechanisms that contribute independently to the development of orthostatic hypotension following spaceflight. These data suggest that loading of arterial baroreceptors may be necessary to maintain baroreflex function.