Effect of lower body negative pressure on orthostatic tolerance and cardiac function during 21 days head-down tilt bed rest

The purpose of the present study was to investigate the changes of orthostatic tolerance and cardiac function during 21 d head-down tilt (HDT) bed rest and effect of lower body negative pressure in the first and the last week in humans. Twelve healthy male volunteers were exposed to -6 degrees HDT bed rest for 21 d. Six subjects received -30 mmHg LBNP sessions for 1 h per day from the 1st to the 7th day and from the 15th to the 21st day of the HDT, and six others served as control. Orthostatic tolerance was assessed by means of standard tilt test. Stroke volume (SV), cardiac output (CO), preejection period (PEP) and left ventricular ejection time (LVET) were measured before and during HDT. Before HDT, all the subjects in the two groups completed the tilt tests. After 10 d and 21 d of HDT, all the subjects of the control group and one subject of the LBNP group could not complete the tilt test due to presyncopal or syncopal symptoms. The mean upright time in the control group (15.0 +/- 3.2 min) was significantly shorter than those in the LBNP group (19.7 +/- 0.9 min). SV and CO decreased significantly in the control group on days 3 and 10 of HDT, but remained unchanged throughout HDT in the LBNP group. A significant increase in PEP/LVET was observed on days 3 and 14 of HDT in both groups. The PEP/LVET in the LBNP group was significantly lower on day 3 of HDT, while LVET in the LBNP group was significantly higher on days 3, 7 and 14 of HDT than those in the control group. The results of this study suggest that brief daily LBNP sessions used in the first and the last weeks of 21 d HDT bed rest were effective in diminished the effect of head-down tilt on orthostatic tolerance, and LBNP might partially improve cardiac pumping function and cardiac systole function.     

Lower body negative pressure exercise plus brief postexercise lower body negative pressure improve post-bed rest orthostatic tolerance.

Orthostatic intolerance follows actual weightlessness and weightlessness simulated by bed rest. Orthostasis immediately after acute exercise imposes greater cardiovascular stress than orthostasis without prior exercise. We hypothesized that 5 min/day of simulated orthostasis [supine lower body negative pressure (LBNP)] immediately following LBNP exercise maintains orthostatic tolerance during bed rest. Identical twins (14 women, 16 men) underwent 30 days of 6 degrees head-down tilt bed rest. One of each pair was randomly selected as a control, and their sibling performed 40 min/day of treadmill exercise while supine in 53 mmHg (SD 4) [7.05 kPa (SD 0.50)] LBNP. LBNP continued for 5 min after exercise stopped. Head-up tilt at 60 degrees plus graded LBNP assessed orthostatic tolerance before and after bed rest. Hemodynamic measurements accompanied these tests. Bed rest decreased orthostatic tolerance time to a greater extent in control [34% (SD 10)] than in countermeasure subjects [13% (SD 20); P < 0.004]. Controls exhibited cardiac stroke volume reduction and relative cardioacceleration typically seen after bed rest, yet no such changes occurred in the countermeasure group. These findings demonstrate that 40 min/day of supine LBNP treadmill exercise followed immediately by 5 min of resting LBNP attenuates, but does not fully prevent, the orthostatic intolerance associated with 30 days of bed rest. We speculate that longer postexercise LBNP may improve results. Together with our earlier related studies, these ground-based results support spaceflight evaluation of postexercise orthostatic stress as a time-efficient countermeasure against postflight orthostatic intolerance.     

Cardiovascular and hormonal response during a 4-week head-down tilt with and without exercise and LBNP countermeasures

To determine whether exercise and Lower Body Negative Pressure (LBNP) during 28 days of -6 degrees head-down tilt (HDT) would modify orthostatic tolerance and blood volume regulating hormones, twelve healthy men were assigned to either a no- countermeasure (No-CM, n=6), or a countermeasure (CM, n=6) group. LBNP sessions consisted of 15 minutes exposure to -30 mm Hg, on days 16, 18, 20 and 22-28 of HDT. Muscular exercise began on day 8 and consisted of combined graded dynamic and isometric resistance bilateral leg exercise on a specially designed supine ergometer, in two sessions of 15-20 min. each, every day, 6 days per week. A tilt test was performed before and at the end of HDT. Changes in resting plasma volume from control day (D-5) to HDT day 24 were -11.2% for No-CM and -2.2% for CM. After HDT three among the 6 subjects of the No-CM group presented presyncopal or syncopal symptoms, no tilt test was interrupted in CM group. Atrial Natriuretic Peptide (ANP) decreased at day 7 for the two groups and remained low during all the HDT period for No-CM group only. Plasma Renin Activity and Aldosterone increased at day 7 and remained elevated for the two groups. Norepinephrine and epinephrine were unchanged. Elevated diuresis and natriuresis were evident during the first day of HDT. However, renal excretory patterns were different between the two groups: indeed, a decrease of Na+, ANP and cGMP was observed only in No-CM at Day 13 during HDT. Our data showed that the subjects of the No-CM group experienced a greater increase in heart rate and a decrease in systolic blood pressure during tilt tests after HDT; nevertheless, after HDT, blood pressure was better maintained in CM group during the tilt test. The plasma volume decrease measured at the end of HDT was significantly lower in CM group, in contrast, these countermeasures were ineffective in preventing at least certain changes in blood volume regulating hormones.     

Portal vein cross-sectional area and flow and orthostatic tolerance: a 90-day bed rest study.

The objective of this study was to evaluate the changes in the portal vein cross-sectional area (PV CSA) and flow during a stand test associated with orthostatic intolerance. Eighteen subjects underwent a 90-day head-down tilt (HDT) bed rest at 6 degrees: 9 controls (Con) and 9 with flywheel exercise countermeasures (CM). At post-HDT, nine subjects (5 CM, 4 Con) were tolerant, and nine were intolerant. The PV CSA was measured by echography. We found that at HDT day 85, the PV CSA at rest had increased less in the CM subjects than in the Con (+12 vs. +27% from pre-HDT supine; P < 0.05), whereas it increased similarly in tolerant and intolerant subjects (23 and 16%, respectively). Two days after the HDT, there was a decrease in the PV CSA supine compared with the pre-HDT PV CSA supine that was similar for all groups (Con: -11%, CM: -21%; tolerant: -10%, intolerant: -16%; P < 0.05). The PV CSA decreased significantly less from supine to standing in the Con than in the CM group (-2 vs. -10% compared with the pre-HDT stand test; P < 0.05). The PV CSA also decreased significantly from supine to standing compared with the pre-HDT stand test in the tolerant group but not in the intolerant group (-20 vs. +2%; P < 0.05). From these findings, we conclude the following. 1) Because the portal vein is the only output from the splanchnic vascular area, we suggest that the lower reduction in the PV CSA and flow associated with orthostatic intolerance was related to a lower splanchnic arterial vasoconstriction. 2) The flywheel exercise CM helped to reduce the distention of the splanchnic network at rest and to maintain partially the splanchnic vasoconstriction, but it did not reduce the orthostatic intolerance.     

Insufficient flow reduction during LBNP in both splanchnic and lower limb areas is associated with orthostatic intolerance after bedrest

We quantified the impact of a 60-day head-down tilt bed rest (HDBR) with countermeasures on the arterial response to supine lower body negative pressure (LBNP). Twenty-four women [8 control (Con), 8 exercise + LBNP (Ex-LBNP), and 8 nutrition (Nut) subjects] were studied during LBNP (0 to -45 mmHg) before (pre) and on HDBR day 55 (HDBR-55). Left ventricle diastolic volume (LVDV) and mass, flow velocities in the middle cerebral artery (MCA flow) and femoral artery (femoral flow), portal vein cross-sectional area (portal flow), and lower limb resistance (femoral resistance index) were measured. Muscle sympathetic nerve activity (MSNA) was measured in the fibular nerve. Subjects were identified as finishers or nonfinishers of the 10-min post-HDBR tilt test. At HDBR-55, LVDV, mass, and portal flow were decreased from pre-HDBR (P < 0.05) in the Con and Nut groups only. During LBNP at HDBR-55, femoral and portal flow decreased less, whereas leg MSNA increased similarly, compared with pre-HDBR in the Con, Nut, and NF groups; 11 of 13 nonfinishers showed smaller LBNP-induced reductions in both femoral and portal flow (less vasoconstriction), whereas 10 of 11 finishers maintained vasoconstriction in either one or both regions. The relative distribution of blood flow in the cerebral versus portal and femoral beds during LBNP [MCA flow/(femoral + portal flow)] increased or reduced < 15% from pre-HDBR in 10 of 11 finishers but decreased > 15% from pre-HDBR in 11 of 13 nonfinishers. Abnormal vasoconstriction in both the portal and femoral vascular areas was associated with orthostatic intolerance. The vascular deconditioning was partially prevented by Ex-LBNP.     

Does long-term experimental antiorthostasis lead to cardiovascular deconditioning in the rat?

Microgravity or simulated microgravity induces acute and chronic cardiovascular responses, whose mechanism is pivotal for understanding of physiological adaptation and pathophysiological consequences. We investigated hemodynamic responses of conscious Wistar rats to 45? head-down tilt (HDT) for 7 days. Arterial blood pressure (BP) was recorded by telemetry. Heart rate (HR), spectral properties and the spontaneous baroreflex sensitivity (sBRS) were calculated. Head-up tilt (HUT) was applied for 2 h before and after HDT to assess the degree of any possible cardiovascular deconditioning. Horizontal control BP and HR were 112.5+/-2.8 mmHg and 344.7+/-10 bpm, respectively. HDT elicited an elevation in BP and HR by 8.3 % and 8.8 %, respectively, in less than 1 h. These elevations in BP and HR were maintained for 2 and 3 days, respectively, and then normalized. Heart rate variability was unchanged, while sBRS was permanently reduced from the beginning of HDT (1.01+/-0.08 vs. 0.74+/-0.05 ms/mmHg). HUT tests before and after HDT resulted in BP elevations (6.9 vs. 11.6 %) and sBRS reduction (0.44 vs. 0.37 ms/mmHg), respectively. The pressor response during the post-HDT HUT test was accompanied by tachycardia (13.7 %). In conclusion, chronic HDT does not lead to symptoms of cardiovascular deconditioning. However the depressed sBRS and tachycardic response seen during the post-HDT HUT test may indicate disturbances in cardiovascular control.     

Cardiac hemodynamics at rest and during LBNP after 4 days and 6 weeks in HDT

The first objective was to assess the left ventricle function at rest, at the end of a short term (4 days) HDT, and at the end of a long term (6 weeks) HDT. The second objective was to check if these short and long HDT induced any abnormal cardiac hemodynamic response to LBNP (lower body negative pressure) test that could be related to orthostatic intolerance.     

Forehead skin microcirculation during tilt table testing and lower body negative pressure

Noninvasive skin microcirculation measurements based on a new Near Infrared sensor technique (NIR/Fa. Silicon Sensor GmbH; Berlin) were embeded in a tilt table experiment for simulation of acute effects of weightlessness (HDT -6 degrees) and active standing with the Russian Tschibis-LBNP device. The phenomenon of orthostatic intolerance depends on complex interactions among functional characteristics of central and peripheral cardiovascular control. The purpose of this study was to assess the blood volume and flow motion changes as well as pulsatile spectral pattern during orthostatic and antiorthostatic stress.     

Cardiovascular response to lower body negative pressure before and after 20 days horizontal bed rest

To evaluate the effects of 20 days bed rest (BR) on cardiovascular system in normal subjects, left ventricular (LV) echocardiography and vascular ultrasound of the common carotid artery and abdominal aorta were performed during rest and a supine lower body negative pressure (LBNP) test in 14 healthy volunteers (mean age: 22 years) before and after BR. After BR, heart rates (HR) at rest and during LBNP (-40 mmHg) increased. In contrast, LV dimensions, stroke volume, and blood pressures decreased both at rest and during LBNP. Also LBNP tolerance time decreased after BR. Although resting cardiac output (CO) and abdominal aortic flow decreased after bed rest, CO and abdominal aortic flow were unchanged during LBNP comparing before and after BR. Common carotid artery flows both at rest and during LBNP showed no change after BR. LBNP did not increase HR before BR, but increased HR prominently after BR. In conclusion, LBNP tolerance time and LV size during LBNP decreased after BR, suggesting orthostatic intolerance due to a decreased blood volume. However, CO and flow in the abdominal aorta and common carotid artery during LBNP were similar before and after BR due to a compensatory increase after BR.     

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. Accepted: 17 January 1997     

Influences of thigh cuffs on the cardiovascular system during 7-day head-down bed rest.

Thigh cuffs, presently named "bracelets," consist of two straps fixed to the upper part of each thigh, applying a pressure of 30 mmHg. The objective was to evaluate the cardiac, arterial, and venous changes in a group of subjects in head-down tilt (HDT) for 7 days by using thigh cuffs during the daytime, and in a control group not using cuffs. The cardiovascular parameters were measured by echography and Doppler. Seven days in HDT reduced stroke volume in both groups (-10%; P < 0.05). Lower limb vascular resistance decreased more in the cuff group than in the control group (-29 vs. -4%; P < 0.05). Cerebral resistance increased in the control group only (+6%; P < 0.05). The jugular vein increased (+45%; P < 0.05) and femoral and popliteal veins decreased in cross-sectional area in both groups (-45 and -8%, respectively; P < 0.05). Carotid diameter tended to decrease (-5%; not significant) in both groups. Heart rate, blood pressure, cardiac output, and total resistance did not change significantly. After 8 h with thigh cuffs, the cardiac and arterial parameters had recovered their pre-HDT level except for blood pressure (+6%; P < 0.05). Jugular vein size decreased from the pre-HDT level (-21%; P < 0.05), and femoral and popliteal vein size increased (+110 and +136%, respectively; P < 0.05). The thigh cuffs had no effect on the development of orthostatic intolerance during the 7 days in HDT.     

Exercise plus volume loading prevents orthostatic intolerance but not reduction in cerebral blood flow velocity after bed rest

This study tested the hypothesis that reduction in cerebral blood flow (CBF) during orthostatic stress after bed rest can be ameliorated with volume loading, exercise, or both. Transcranial Doppler was used to measure changes in CBF velocity during lower body negative pressure (LBNP) before and after an 18-day bed rest in 33 healthy subjects. Subjects were assigned into four groups with similar age and sex: 1) supine cycling during bed rest (Exercise group; n = 7), 2) volume loading with Dextran infusion after bed rest to restore reduced left ventricular filling pressure (Dextran group; n = 7), 3) exercise combined with volume loading to prevent orthostatic intolerance (Ex-Dex group; n = 7), and 4) a control group (n = 12). LBNP tolerance was measured using a cumulative stress index (CSI). After bed rest, CBF velocity was reduced at a lower level of LBNP in the Control group, and the magnitude of reduction was greater in the Ex-Dex group. However, reduction in orthostatic tolerance was prevented in the Ex-Dex group. Notably, volume loading alone prevented greater reductions in CBF velocity after bed rest, but CSI was reduced still by 25%. Finally, decreases in CBF velocity during LBNP were correlated with reduction in cardiac output under all conditions (r(2) = 0.86; P = < 0.001). Taken together, these findings demonstrate that volume loading alone can ameliorate reductions in CBF during LBNP. However, the lack of associations between changes in CBF velocity and orthostatic tolerance suggests that reductions in CBF during LBNP under steady-state conditions by itself are unlikely to be a primary factor leading to orthostatic intolerance.     

Comparison of cardiovascular responses between lower body negative pressure and head-up tilt.

To investigate local blood-flow regulation during orthostatic maneuvers, 10 healthy subjects were exposed to -20 and -40 mmHg lower body negative pressure (LBNP; each for 3 min) and to 60 degrees head-up tilt (HUT; for 5 min). Measurements were made of blood flow in the brachial (BF(brachial)) and femoral arteries (BF(femoral)) (both by the ultrasound Doppler method), heart rate (HR), mean arterial pressure (MAP), cardiac stroke volume (SV; by echocardiography), and left ventricular end-diastolic volume (LVEDV; by echocardiography). Comparable central cardiovascular responses (changes in LVEDV, SV, and MAP) were seen during LBNP and HUT. During -20 mmHg LBNP, -40 mmHg LBNP, and HUT, the following results were observed: 1) BF(brachial) decreased by 51, 57, and 41%, and BF(femoral) decreased by 40, 53, and 62%, respectively, 2) vascular resistance increased in the upper limb by 110, 147, and 85%, and in the lower limb by 76, 153, and 250%, respectively. The increases in vascular resistance were not different between the upper and lower limbs during LBNP. However, during HUT, the increase in the lower limb was much greater than that in the upper limb. These results suggest that, during orthostatic stimulation, the vascular responses in the limbs due to the cardiopulmonary and arterial baroreflexes can be strongly modulated by local mechanisms (presumably induced by gravitational effects).     

'Neutral point titration': cardiovascular regulation during combined (LBNP/HDT vs. LBPP/HUT) stimulation

This study was undertaken to identify combinations ('neutral points', NP) of orthostatic (tilt: head-down = HDT, head-up = HUT) and pseudo-orthostatic (lower body pressure: positive = LBPP, negative = LBNP) stimuli able to compensate one another in their effect on hemodynamic variables, electrical thoracic impedance (TI), hematocrit and plasma mass density (PD), and blood hormone concentrations. We asked if NP's exist for tested variables (hypothesis 1), if NP's differ with variables (hypothesis 2), and if NP's change as a function of time (hypothesis 3). For the blood volume sensitive variables (PD, plasma total protein concentration, and hematocrit) we found a NP at > or = 30 degrees HDT at LBNP-35 and -15 degrees HUT with LBPP+35. There was no clear PD / total plasma protein concentration effect with various degrees of LBNP-15 / HDT. NP's could be derived for some hemodynamic variables: With LBNP-35, a NP for heart rate was derived at -25 degrees HDT and for MAP at -30 degrees HDT. Heart rate intersected at > or = 30 degrees HDT with LBNP-15 (extrapolated), stroke volume index (SVI) at -20 degrees HDT. With LBPP+35, SVI had its NP at 11 degrees HUT. The hormonal responses displayed a pattern where plasma renin activity (PRA) NP's were logically scattered with LBNP intensity, whereas aldosterone displayed similar NP's with both LBNP intensities.     

Computer simulation of the effect of dDAVP with saline loading on fluid balance after 24-hour head-down tilt

Fluid loading (FL) before Shuttle reentry is a countermeasure currently in use by NASA to improve the orthostatic tolerance of astronauts during reentry and postflight. The fluid load consists of water and salt tablets equivalent to 32 oz (946 ml) of isotonic saline. However, the effectiveness of this countermeasure has been observed to decrease with the duration of spaceflight. The countermeasure's effectiveness may be improved by enhancing fluid retention using analogs of vasopressin such as lypressin (LVP) and desmopressin (dDAVP). In a computer simulation study reported previously, we attempted to assess the improvement in fluid retention obtained by the use of LVP administered before FL. The present study is concerned with the use of dDAVP. In a recent 24-hour, 6 degree head-down tilt (HDT) study involving seven men, dDAVP was found to improve orthostatic tolerance as assessed by both lower body negative pressure (LBNP) and stand tests. The treatment restored Luft's cumulative stress index (cumulative product of magnitude and duration of LBNP) to nearly pre-bedrest level. The heart rate was lower and stroke volume was marginally higher at the same LBNP levels with administration of dDAVP compared to placebo. Lower heart rates were also observed with dDAVP during stand test, despite the lower level of cardiovascular stress. These improvements were seen with only a small but significant increase in plasma volume of approximately 3 percent. This paper presents a computer simulation analysis of some of the results of this HDT study.     

Skin surface cooling improves orthostatic tolerance in normothermic individuals

Previous studies suggest that skin surface cooling (SSC) preserves orthostatic tolerance; however, this hypothesis has not been experimentally tested. Thus the purpose of this project was to identify whether SSC improves orthostatic tolerance in otherwise normothermic individuals. Eight subjects underwent two presyncope limited graded lower-body negative pressure (LBNP) tolerance tests. On different days, and randomly assigned, LBNP tolerance was assessed under control conditions and during SSC (perfused 16 degrees C water through tube-lined suit worn by each subject). Orthostatic tolerance was significantly elevated in each individual due to SSC, as evidenced by a significant increase in a standardized cumulative stress index (normothermia 564 +/- 58 mmHg.min; SSC 752 +/- 58 mmHg.min; P < 0.05). At most levels of LBNP, blood pressure during the SSC tolerance test was significantly greater than during the control test. Furthermore, the reduction in cerebral blood flow velocity was attenuated during some of the early stages of LBNP for the SSC trial. Plasma norepinephrine concentrations were significantly higher during LBNP with SSC, suggesting that SSC may improve orthostatic tolerance through increased sympathetic activity. These data demonstrate that SSC is effective in improving orthostatic tolerance in otherwise normothermic individuals.     

Mechanisms of changes in human hemodynamics under the conditions of microgravity and prognosis of postflight orthostatic stability

The mechanisms of hemodynamic responses to orthostatic stresses and orthostatic stability (OS) of cosmonauts were studied before and after short-and long-term spaceflights (SFs) using orthostatic tests, as well as before, during, and after SFs using ultrasonic methods in tests with exposure to lower body negative pressure (LBNP). The capacitance and distensibility of the veins of the lower extremities were studied using occlusive air plethysmography before, during, and after SFs of different durations. A stay in microgravity has been proved to result in detraining of, mainly, the vascular mechanisms of compensating orthostatic perturbations. It has been established that the decrease in OS under the influence of microgravity is determined by a reduction of the vasoconstrictive ability of large blood vessels of the lower extremities; an increase in venous distensibility and capacitance of the legs; and an impairment of blood flow regulation, which leads to a cerebral blood flow deficit in orthostatic stresses, and of the initial individual OS before the flight. The results of preflight studies of hemodynamics by ultrasonic methods at LBNP and the data of orthostatic tests before SFs make it possible to predict the degree of decrease of OS after an SF proceeding in the normal mode. At the same time, the data of ultrasonic blood flow examination provide more a accurate estimation of OS and make it possible to assess the physiological reserves of hemodynamic regulation and to reveal the loss of regulation capacity even in cases where integrated indices (heart rate and blood pressure) are within the normal ranges.     

Hemodynamic response to LBNP during the 14 month MIR spaceflight (94-95)

Lower body negative pressure (LBNP) was used during the Mir spaceflight in a study of orthostatic tolerance. Hemodynamic responses were measured including heart rate, blood pressure, cerebral artery blood flow, and lower limb vascular resistance. Results showed that femoral flow volume decreased, which may be due to hypovolemia and reduced cardiac output. Additional changes in femoral vascular response and cerebral to femoral blood flow are discussed.     

Exercise throughout 6 degrees head-down tilt bed rest preserves thermoregulatory responses.

Spaceflight and its bed rest analog [6 degrees head-down tilt (HDT)] decrease plasma and blood volume and aerobic capacity. These responses may be associated with impaired thermoregulatory responses observed during exercise and passive heating after HDT exposure. This project tested the hypothesis that dynamic exercise during 13 days of HDT bed rest preserves thermoregulatory responses. Throughout HDT bed rest, 10 subjects exercised for 90 min/day (75% of pre-HDT maximum heart rate; supine). Before and after HDT bed rest, each subject exercised in the supine position at the same workload in a 28 degrees C room. The internal temperature (Tcore) threshold for the onset of sweating and cutaneous vasodilation, as well as the slope of the relationship between the elevation in Tcore relative to the elevation in sweat rate (SR) and cutaneous vascular conductance (CVC; normalized to local heating maximum), were quantified pre- and post-HDT. Tcore thresholds for the onset of cutaneous vasodilation on the chest and forearm (chest: 36.79 +/- 0.12 to 36.94 +/- 0.13 degrees C, P = 0.28; forearm: 36.76 +/- 0.12 to 36.91 +/- 0.11 degrees C, P = 0.16) and slope of the elevation in CVC relative to Tcore (chest: 77.9 +/- 14.2 to 80.6 +/- 17.2%max/ degrees C; P = 0.75; forearm: 76.3 +/- 11.8 to 67.5 +/- 14.3%max/ degrees C, P = 0.39) were preserved post-HDT. Moreover, the Tcore threshold for the onset of SR (36.66 +/- 0.12 to 36.74 +/- 0.10 degrees C; P = 0.36) and the slope of the relationship between the elevation in SR and the elevation in Tcore (1.23 +/- 0.19 to 1.01 +/- 0.14 mg x cm(-2) x min(-1) x degrees C(-1); P = 0.16) were also maintained. Finally, after HDT bed rest, peak oxygen uptake and plasma and blood volumes were not different relative to pre-HDT bed rest values. These data suggest that dynamic exercise during this short period of HDT bed rest preserves thermoregulatory responses.     

Caloric Restriction Decreases Orthostatic Tolerance Independently from 6° Head-Down Bedrest

Astronauts consume fewer calories during spaceflight and return to earth with an increased risk of orthostatic intolerance. Whether a caloric deficiency modifies orthostatic responses is not understood. Thus, we determined the effects of a hypocaloric diet (25% caloric restriction) during 6° head down bedrest (an analog of spaceflight) on autonomic neural control during lower body negative pressure (LBNP). Nine healthy young men completed a randomized crossover bedrest study, consisting of four (2 weeks each) interventions (normocaloric bedrest, normocaloric ambulatory, hypocaloric bedrest, hypocaloric ambulatory), each separated by 5 months. Muscle sympathetic nerve activity (MSNA) was recorded at baseline following normocaloric and hypocaloric interventions. Heart rate (HR) and arterial pressure were recorded before, during, and after 3 consecutive stages (7 min each) of LBNP (-15, -30, -45 mmHg). Caloric and posture effects during LBNP were compared using two-way ANOVA with repeated measures. There was a strong trend toward reduced basal MSNA following caloric restriction alone (normcaloric vs. hypocaloric: 22±3 vs. 14±4 burst/min, p = 0.06). Compared to the normocaloric ambulatory, both bedrest and caloric restriction were associated with lower systolic blood pressure during LBNP (p<0.01); however, HR responses were directionally opposite (i.e., increase with bedrest, decrease with caloric restriction). Survival analysis revealed a significant reduction in orthostatic tolerance following caloric restriction (normocaloric finishers: 12/16; hypocaloric finishers: 6/16; χ2, p = 0.03). Caloric restriction modifies autonomic responses to LBNP, which may decrease orthostatic tolerance after spaceflight.