Effects of room temperature and body position change on cerebral blood volume and center-of-foot pressure in healthy young adults

This study aimed to examine the effects of room temperature and body position changes on cerebral blood volume, blood pressure and center-of-foot pressure (COP). Cerebral oxygenation kinetics and blood pressure were measured by near infrared spectroscopy (NIRS) and volume-compensation, respectively, in 9 males and 9 females after rapid standing from sitting and supine positions in low (12 degrees C) or normal (22 degrees C) room temperatures. COP was also measured in a static standing posture for 90 s after rapid standing. The total hemoglobin (Hb) decreased just after standing. Blood pressure after standing at normal temperature tended to decrease immediately but at low temperature tended to decrease slightly and then to increase greatly. The decreasing ratio of total Hb and blood pressure upon standing from a supine position at normal room temperatures was the largest of any condition. Total Hb recovered to a fixed level approximately 25 sec after standing from a sitting position and approximately 35 sec after standing from a supine position. All COP parameters after standing tended to change markedly in the supine position compared to the sitting position, especially at normal temperatures. The COP parameters after standing in any condition were not significantly related to the decreasing ratio of total Hb but were related to the recovery time of total Hb after standing. In conclusion, decreasing ratios of total Hb and blood pressure after standing from a supine position at normal temperatures were large and may affect body sway.     

Assessment of Interaction Between Cardio-Respiratory Signals Using Directed Coherence on Healthy Subjects During Postural Change

PurposeTo detect and quantify the directional interaction changes between cardio-respiratory system during postural change.MethodTraditional frequency domain analysis based on power spectrum and coherence are insufficient to quantify nonlinear structures and complexity of physiological subsystems. Recently, Granger causality is found as preferable method for evaluation of causality i.e., directional interaction. Frequency domain Granger causality based on directed coherence has been used in this study to identify directional interaction between cardiac and respiratory signal during postural change from supine to standing for healthy subjects.ResultECG and respiration signal are recorded for this study. The beat-to-beat variability series from ECG provides heart rate (RR) and the respiration amplitude corresponds to RESP time series. It was observed that respiration is responsible for the changes in ECG signal during supine position as compared to standing. The outflow of information from RESP to RR increases during supine results in stronger interaction but reduces during standing result in reduction of interaction. Similarly, the effect of RR on RESP is found significant only during standing.ConclusionThe proposed directed coherence approach detects the cardio-respiratory regulation during postural change and provide information about coupling changes during this transition.     

Body Position Influences Which Neural Structures Are Recruited by Lumbar Transcutaneous Spinal Cord Stimulation

Transcutaneous stimulation of the human lumbosacral spinal cord is used to evoke spinal reflexes and to neuromodulate altered sensorimotor function following spinal cord injury. Both applications require the reliable stimulation of afferent posterior root fibers. Yet under certain circumstances, efferent anterior root fibers can be co-activated. We hypothesized that body position influences the preferential stimulation of sensory or motor fibers. Stimulus-triggered responses to transcutaneous spinal cord stimulation were recorded using surface-electromyography from quadriceps, hamstrings, tibialis anterior, and triceps surae muscles in 10 individuals with intact nervous systems in the supine, standing and prone positions. Single and paired (30-ms inter-stimulus intervals) biphasic stimulation pulses were applied through surface electrodes placed on the skin between the T11 and T12 inter-spinous processes referenced to electrodes on the abdomen. The paired stimulation was applied to evaluate the origin of the evoked electromyographic response; trans-synaptic responses would be suppressed whereas direct efferent responses would almost retain their amplitude. We found that responses to the second stimulus were decreased to 14%±5% of the amplitude of the response to the initial pulse in the supine position across muscles, to 30%±5% in the standing, and to only 80%±5% in the prone position. Response thresholds were lowest during standing and highest in the prone position and response amplitudes were largest in the supine and smallest in the prone position. The responses obtained in the supine and standing positions likely resulted from selective stimulation of sensory fibers while concomitant motor-fiber stimulation occurred in the prone position. We assume that changes of root-fiber paths within the generated electric field when in the prone position increase the stimulation thresholds of posterior above those of anterior root fibers. Thus, we recommend conducting spinal reflex or neuromodulation studies with subjects lying supine or in an upright position, as in standing or stepping.     

Short-duration spaceflight impairs human carotid baroreceptor-cardiac reflex responses.

Orthostatic intolerance is a predictable but poorly understood consequence of space travel. Because arterial baroreceptors modulate abrupt pressure transients, we tested the hypothesis that spaceflight impairs baroreflex mechanisms. We studied vagally mediated carotid baroreceptor-cardiac reflex responses (provoked by neck pressure changes) in the supine position and heart rate and blood pressure in the supine and standing positions in 16 astronauts before and after 4- to 5-day Space Shuttle missions. On landing day, resting R-R intervals and standard deviations, and the slope, range, and position of operational points on the carotid transmural pressure-sinus node response relation were all reduced relative to preflight. Stand tests on landing day revealed two separate groups (one maintained standing arterial pressure better) that were separated by preflight slopes, operational points, and supine and standing R-R intervals and by preflight-to-postflight changes in standing pressures, body weights, and operational points. Our results suggest that short-duration spaceflight leads to significant reductions in vagal control of the sinus node that may contribute to, but do not account completely for, orthostatic intolerance.     

Influence of high ambient temperatures on the physiological responses and body sway in healthy young adults after quickly standing

This study was aimed to compare the variations in cerebral oxygenation, blood pressure and center-of-foot pressure after standing from sitting and supine positions at normal (22 degrees C) and high (32 degrees C) room temperatures. Thirty young adults stood up from a resting posture (sitting or supine position) and kept the static standing posture for 90 sec. Meanwhile, their center-of-foot pressure (COP), blood pressure, and cerebral oxygenation kinetics were measured in continuity. The change of the frequency domain low-to-high frequency (LF/HF) ratio of the R-R interval before and after standing from a supine position was significantly higher than that from a sitting position under both temperature conditions. Blood pressure as well as total and oxygenated hemoglobin levels decreased immediately after standing up and the ratio of blood pressure change when moving from a supine position to standing at high room temperature was the largest as compared with the other conditions. Total hemoglobin (Hb) volume was found to temporarily decrease after standing and required 22-24 sec to recover when the subject started from the sitting position and 33-36 sec when the subject started from the supine position. Cerebral oxygenation kinetics tended to be larger under high, rather than normal, temperature conditions. All COP parameters after standing were significantly larger in the high temperature condition than in the normal temperature condition. Body sway after standing was larger in the high temperature condition than in the normal temperature condition and after standing from a supine position than from a sitting position. In conclusion, cerebral oxygenation kinetics and blood pressure measured after the subject moved to the standing position changed dramatically under high temperature conditions, and variations in this parameter may influence body sway.     

Continuous monitoring of blood volume changes in humans

The mass density of antecubital venous blood was measured continuously for 80 min/session with 0.1 g/l precision at a flow rate of 1.5 ml/min in six male subjects. Each person participated in two different sessions with the same protocol. To induce transvascular fluid shifts, the subjects changed from sitting to standing and from standing to supine positions. There was transient blood density shifts immediately after postural changes, followed by an asymptotic approach to a new steady-state blood density level. Additional deviations from a simple time course were regularly observed. Blood density increased by 3.5 +/- 1.4 (SD) g/l when standing after sitting and decreased by 5.0 +/- 1.2 g/l while supine after standing. The corresponding half time of the blood density increase was 5.6 +/- 1.4 min (standing after sitting) and 6.9 +/- 3.1 min (supine after standing) of the blood density decrease. Erythrocyte density was calculated and did not change with body position. Whole-body blood density was calculated from plasma density, hematocrit, and erythrocyte density, assuming an F-cell ratio of 0.91. Volume shifts were computed from the density data; the subject's blood volume density decreased by 6.2 +/- 1.2% from sitting to standing and increased by 8.5 +/- 2.1% from standing to supine. Additional discrete plasma density and hematocrit measurements gave linear relations (P less than 0.001) between all possible combinations of blood density, plasma density, and hematocrit.(ABSTRACT TRUNCATED AT 250 WORDS)     

Blood flow to contracting human muscles: influence of increased sympathetic activity

The purpose of this study was to examine the effects of the increased sympathetic activity elicited by the upright posture on blood flow to exercising human forearm muscles. Six subjects performed light and heavy rhythmic forearm exercise. Trials were conducted with the subjects supine and standing. Forearm blood flow (FBF, plethysmography) and skin blood flow (laser Doppler) were measured during brief pauses in the contractions. Arterial blood pressure and heart rate were also measured. During the first 6 min of light exercise, blood flow was similar in the supine and standing positions (approximately 15 ml.min-1.100 ml-1); from minutes 7 to 20 FBF was approximately 3-7 ml.min-1.100 ml-1 less in the standing position (P less than 0.05). When 5 min of heavy exercise immediately followed the light exercise, FBF was approximately 30-35 ml.min-1.100 ml-1 in the supine position. These values were approximately 8-12 ml.min-1.100 ml-1 greater than those observed in the upright position (P less than 0.05). When light exercise did not precede 8 min of heavy exercise, the blood flow at the end of minute 1 was similar in the supine and standing positions but was approximately 6-9 ml.min-1.100 ml-1 lower in the standing position during minutes 2-8. Heart rate was always approximately 10-20 beats higher in the upright position (P less than 0.05). Forearm skin blood flow and mean arterial pressure were similar in the two positions, indicating that the changes in FBF resulted from differences in the caliber of the resistance vessels in the forearm muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Accelerometer-based body-position sensing for ambulatory electrocardiographic monitoring

Our objective is to validate the ability of 3 appropriately placed accelerometers to determine body position during ambulatory electrocardiographic (ECG) monitoring and to demonstrate the clinical applicability of this method. During ambulatory (Holter) monitoring, the ability to know a patient's position (lying down, sitting, standing, or changing from one position to another) is important in the evaluation of common symptoms such as dizziness, palpitations, and syncope. Changes in body position are also known to alter the electrical axis of the heart, resulting in artifactual changes in QRS amplitude and ST-segment morphology. We have developed an ambulatory patient-monitoring instrument that, through the use of microfabricated accelerometers, can simultaneously record body-position information and 2 channels of ECG data. The accelerometers measure the effects of gravity and dynamic acceleration, allowing determination of a patient's orientation and movements. The accelerometer and ECG signals are input to a portable recorder and are filtered and digitized. Algorithms were developed to automatically determine body position. Ten healthy volunteers wore the device for 1 hour and followed a protocol of standing, sitting, walking, lying supine, and lying in the left and right lateral decubitus positions. An observer manually recorded times of position changes. Data were recorded and analyzed using software designed with MATLAB. The ability of the accelerometers and computer algorithms to determine body position was analyzed in terms of the sensitivity and specificity for each body position. The sensitivities for sitting, standing, walking, lying supine, lying right, and lying left were 98.8%, 99.2%, 95.5%, 99.1%, 98.9%, and 94.8%, respectively. The specificities were 99.7%, 99.4%, 99.6%, 99.0%, 99.8%, and 99.9%, respectively. The use of microfabricated accelerometers is a clinically feasible method to determine body position and can be applied to future studies correlating body position with ECG or other physiologic data.     

G-suit inflation to 50 mmHg alters the cardiovascular transients when entering micro-G in parabolic flight

In the present experiments it was decided to have each test-subject serve as his own control by fitting the test-subjects with a G-suit and comparing the condition of inflated G-suit to the normal situation. G-suit inflation was intended to only displace blood on the venous side of the circulation, not to increase total peripheral resistance. Therefore, a very modest inflation of 50 mmHg was applied. This was considered sufficient to expel most of the blood from the venous pool in abdomen and legs, even under the condition of increased G-loading in the pull-up phase. The parabolas were to be undergone in three body positions: standing upright, sitting and supine. The prediction of the experimental outcome was that we would find no difference between transients with and without G-suit inflation in the supine position, that an initial overshoot in pressure and stroke volume in the upright position would be very much damped by the G-suit, even more in the standing than in the sitting position. Studies were performed in 5 flights of NASA's KC-135, in January 1993. Per flight 40 parabolas were flown in an adapted 'roller coaster profile', i.e. 0-G phases were followed by a 2-G pull-out phase, after a very brief 1-G phase again followed by the next 2-G pull-up phase. This sequence was flown for 10 parabolas, then a 1-G horizontal flight period was inserted. The first 3 parabolas of each set of 10 the subjects were sitting upright, seat belt fastened. The next three they were standing, feet stuck under a load strap on the floor, stabilizing themselves by a grip on the ceiling. Then three parabolas were flown with the test-subject supine, loosely attached to the floor by a load strap and further aided by a grip to another strap on the floor. The last parabola of a set was used as 'spare' to repeat any failed maneuver.     

Effect of posture on vital capacity

The influence of some extreme body postures on vital capacity (VC) was examined in young adult humans. Two postures required full support of body weight by the arms: arms up, hanging from a bar, and arms down with hands gripping parallel bars. Three involved muscles that flex and extend the trunk: a partial sit-up position while supine and nearly maximal spinal extension and flexion while standing. Changes at the inspiratory and expiratory volume extremes were recognized by having the subjects do two VC efforts: the first standing and the second in the posture in question while continuing to breathe on the spirometer. Control observations in which the second of a VC pair was performed in an unstressed posture allowed correction for the influence of rebreathing. The changes in corrected VC were small, the greatest being an average reduction of approximately 8% in the partial sit-up position. During full support of body weight by the arms, the VC was slightly increased due to a significant increase in the inspiratory extreme and no change in the expiratory extreme. Spinal extension produced small increases in lung volume at both extremes with no significant change in VC, whereas spinal flexion did not influence the upper extreme but did increase lung volume at the lower extreme. The changes are discussed in terms of trunk muscle action.     

Norepinephrine transporter inhibition alters the hemodynamic response to hypergravitation.

Sympathetically mediated tachycardia and vasoconstriction maintain blood pressure during hypergravitational stress, thereby preventing gravitation-induced loss of consciousness. Norepinephrine transporter (NET) inhibition prevents neurally mediated (pre)syncope during gravitational stress imposed by head-up tilt testing. Thus it seems reasonable that NET inhibition could increase tolerance to hypergravitational stress. We performed a double-blind, randomized, placebo-controlled crossover study in 11 healthy men (26 +/- 1 yr, body mass index 24 +/- 1 kg/m2), who ingested the selective NET inhibitor reboxetine (4 mg) or matching placebo 25, 13, and 1 h before testing on separate days. We monitored heart rate, blood pressure, and thoracic impedance in three different body positions (supine, seated, standing) and during a graded centrifuge run (incremental steps of 0.5 g for 3 min each, up to a maximal vertical acceleration load of 3 g). NET inhibition increased supine blood pressure and heart rate. With placebo, blood pressure increased in the seated position and was well maintained during standing. However, with NET inhibition, blood pressure decreased in the seated and standing position. During hypergravitation, blood pressure increased in a graded fashion with placebo. With NET inhibition, the increase in blood pressure during hypergravitation was profoundly diminished. Conversely, the tachycardic responses to sitting, standing, and hypergravitation all were greatly increased with NET inhibition. In contrast to our expectation, short-term NET inhibition did not improve tolerance to hypergravitation. Redistribution of sympathetic activity to the heart or changes in baroreflex responses could explain the excessive tachycardia that we observed.     

The effect of posture change on blood volume, serum potassium and whole body electrical impedance

After standing for 1h, ten subjects (7 male, 3 female) assumed a supine position for a further hour. Whole body bioelectrical impedance increased progressively during the hour spent in the supine position: after 60 min supine the increase was 13(6 to 32). Blood and plasma volumes, estimated from haematocrit and haemoglobin concentration, increased by 8.0(6.7 to 12.4)% and 16.7(12.3 to 20.8)% (median(range)) respectively after 60 min supine. Serum potassium concentration had fallen after 10 min supine (4.1(0.1)mmol 1^–1; mean(SEM)) relative to the standing value (4.6(0.1)mmol 1^–1) and was unchanged thereafter. Serum osmolality (P=0.991) and sodium (P=1.000) and chloride (P=0.998) concentrations remained unchanged throughout the study. The fall in serum potassium concentration in the supine position does not appear to be a simple dilutional effect consequent upon increases in blood and plasma volume as there was no effect of postural change on serum sodium or chloride concentrations.     

Abdominal pressure transmission in humans during slow breathing maneuvers

Pressure transmission within the abdomen was studied in four subjects standing and supine, breathing slowly and performing slow breathing maneuvers. Pressures were measured in the stomach and rectum with air-containing balloon-catheter systems Pga(air) and Prec and in the stomach with a water-filled catheter system Pga(liq). Changes in Pga(air), Pga(liq), and Prec were nearly in phase and linearly related. The changes in Pga(liq) and Prec were nearly equal in all maneuvers, whereas the changes in Pga(air) were systematically greater than those of Pga(liq) and Prec during quiet breathing and relaxation maneuvers in the upright position. During expulsive maneuvers in either posture the abdominal shape and stomach position were relatively constant, and the changes in Pga(air) were nearly equal to those of Pga(liq) and Prec. Discrepancies between changes in Pga(air) and the other pressures were consistent with presumed changes in the height of the gastric air bubble. We conclude that abdominal pressure in humans is essentially hydrostatic during quiet breathing and slow breathing maneuvers.     

Abdominal muscle activity during speech production

Abdominal muscle activity was investigated during resting tidal breathing and speech production in upright and supine body positions in five male and five female young adult subjects. Results showed that patterns of abdominal electromyographic (EMG) activity were highly dependent on body position. Data for resting tidal breathing resembled those of previous investigations and revealed one sex-related finding. Data for speech production indicated that the lateral region of the abdomen was highly active in the upright position and occasionally active in the supine position. In the upright position, lateral EMG levels during speech production were characterized by generally higher levels in the lower than upper lateral sites and were almost always higher than during resting tidal breathing. In the supine position, EMG levels during speech production occasionally exceeded those associated with resting tidal breathing but were substantially lower than those associated with upright speech production. Abdominal EMG activity was most prevalent during loud speech production and during speech produced at low lung volumes. Findings are discussed in relation to current knowledge of respiratory mechanics and neural control.     

Assessment of neuromuscular activity during maximal isometric contraction in supine vs standing body positions

BackgroundWhen comparing neuromuscular activity between different individuals or different conditions by use of surface electromyography (sEMG) it is necessary to apply standardized assessment protocol. Most frequently used method is the maximum voluntary isometric contraction (MVIC). However, the influence of body posture on sEMG activity during MVIC testing remains largely unknown.AimTo evaluate the MVIC method for sEMG normalization in supine versus standing positions for selected muscles of the lower extremity and trunk.MethodsTwelve healthy individuals participated; five females and seven males (age 22–51 yrs). sEMG signals were recorded bilaterally from mm tibialis anterior, gluteus medius, adductor longus, rectus abdominus, external oblique and internal oblique/transversus abdominus according to standardized test protocol. Two different body positions were used: supine and standing position.ResultsMVIC peak sEMG signal amplitudes did not differ systematically between supine and standing test positions. Pronounced inter-subject variability in MVIC reference sEMG activity were observed between participants, during both supine and standing test positions.ConclusionPresent data demonstrate that MVIC EMG normalization is a biomechanically stable procedure that can be performed in a reproducible manner for the major leg and trunk muscles when comparing supine vs. standing test positions.     

Measurement of inferior vena cava diameter for evaluation of venous return in subjects on day 10 of a bed-rest experiment.

We evaluated the usefulness of measurements of the inferior vena cava (IVC) diameters on abdominal echograms as an indicator of changes of venous return in subjects with orthostatic intolerance (OI) induced by simulated microgravity. We performed a standing test and recorded the IVC diameters on abdominal echograms in 12 subjects placed on a 20-day 6 degrees head-down-tilt bed-rest experiment. We found that different patterns of changes in IVC diameter occurred in the standing test on day 10 of the experiment; in five subjects with a marginal decrease in pulse pressure, IVC diameters in the upright position were markedly decreased compared with those in the supine position. In five subjects with feelings of discomfort, the IVC diameters in the upright position distended or did not decrease from those in the supine position. These results suggested that the changes in IVC diameter on the standing test indicated the presence of various types of hemodynamic responses of OI caused by simulated microgravity. In this study, we also evaluated changes in body-water compartments by conducting multifrequency bioelectrical impedance analysis. Longitudinal data analysis showed that the total body-water-to-fat-free mass and extracellular fluid-to-fat-free mass ratios decreased during the experimental period and recovered thereafter, and that the ratio of intracellular fluid to fat-free mass decreased during the experiment. No significant difference in changes in body-water compartments was seen among subjects with different patterns of changes in IVC diameters. Measurement of IVC diameter was useful to estimate hemodynamic changes in subjects with OI.     

Microgravity alters respiratory sinus arrhythmia and short-term heart rate variability in humans

We studied heart rate (HR), heart rate variability (HRV), and respiratory sinus arrhythmia (RSA) in four male subjects before, during, and after 16 days of spaceflight. The electrocardiogram and respiration were recorded during two periods of 4 min controlled breathing at 7.5 and 15 breaths/min in standing and supine postures on the ground and in microgravity. Low (LF)- and high (HF)-frequency components of the short-term HRV (< or =3 min) were computed through Fourier spectral analysis of the R-R intervals. Early in microgravity, HR was decreased compared with both standing and supine positions and had returned to the supine value by the end of the flight. In microgravity, overall variability, the LF-to-HF ratio, and RSA amplitude and phase were similar to preflight supine values. Immediately postflight, HR increased by approximately 15% and remained elevated 15 days after landing. LF/HF was increased, suggesting an increased sympathetic control of HR standing. The overall variability and RSA amplitude in supine decreased postflight, suggesting that vagal tone decreased, which coupled with the decrease in RSA phase shift suggests that this was the result of an adaptation of autonomic control of HR to microgravity. In addition, these alterations persisted for at least 15 days after return to normal gravity (1G).     

Sustained microgravity reduces the human ventilatory response to hypoxia but not to hypercapnia.

We measured the isocapnic hypoxic ventilatory response and the hypercapnic ventilatory response by using rebreathing techniques in five normal subjects (ages 37-47 yr) before, during, and after 16 days of exposure to microgravity (microG). Control measurements were performed with the subjects in the standing and supine postures. In both microG and in the supine position, the hypoxic ventilatory response, as measured from the slope of ventilation against arterial O(2) saturation, was greatly reduced, being only 46 +/- 10% (microG) and 52 +/- 11% (supine) of that measured standing (P < 0.01). During the hypercapnic ventilatory response test, the ventilation at a PCO(2) of 60 Torr was not significantly different in microG (101 +/- 5%) and the supine position (89 +/- 3%) from that measured standing. Inspiratory occlusion pressures agreed with these results. The findings can be explained by inhibition of the hypoxic but not hypercapnic drive, possibly as a result of an increase in blood pressure in carotid baroreceptors in microG and the supine position.     

Phase-averaged characterization of respiratory sinus arrhythmia pattern

A method for the accurate time-domain characterization of respiratory sinus arrhythmia (RSA) pattern is presented and applied to two groups of healthy subjects to lay the baseline of RSA patterns and to underlay their features: response to standing, stability in successive recordings, and individuality of the shape of RSA pattern. RSA pattern is evaluated by selective averaging of heart rate (HR) changes from multiple respiratory cycles over the respiratory phase and represents the complete modulating function of HR by respiration. The RSA pattern is evaluated with free respiration and even in cases of severe arrhythmia. Estimation error is 6-8% in magnitude, phase resolution is 0.2 rad, and sensitivity margin for respiratory-related HR variability (HRV) components is 1%. RSA magnitude, phase lag, and expiration-to-inspiration time ratio are derived in addition to the entire pattern. In a group of 10 healthy young adults, a phase lag difference of 11.4 +/- 8.5% (mean +/- SD, P < 0.004) was observed between supine and standing postures, possibly ascribed to breathing mechanics. A second group of 15 healthy young adults at supine rest showed stability of the RSA pattern in successive recordings (several weeks apart) as well as individuality among subjects. This may suggest a nonscalar individual long-term index for cardiorespiratory coupling. The method is complementary to the existing statistical and spectral methods. It allows the complete characterization of the primary RSA components and may provide new insight into the effects of vagal activity and changes in clinical conditions.     

Heart rate variability during head down tilt and lower body negative pressure in the Russian Tschibis

The cardiovascular function in space seems to be normal. However, abnormalities of cardiovascular responses have been found during lower body negative pressure suction in space. The etiology of the cardiovascular deconditioning in space is still unknown. A previous study showed, that short periods of head down tilt (HDT-6 degrees) induce changes in the spectral pattern of heart rate variabilty (HRV) and an increase in the sympathethic activation caused by orthostatic stress. The aim of this study was to test following hypotheses: 1. The dynamic of heart rate variability is different in the head down tilt and supine positions. 2. The application of lower body negative pressure (LBNP) during head down tilt induces similar heart rate variability patterns like the standing position. 3. After short term head down tilt the cardiovascular response to lower body negative pressure stressor is altered.