Effect on the cardiac function of repeated LBNP during a 1-month head down tilt

Cardiovascular assessment by ultrasound methods was performed during two long duration (1 month) Head Down Tilt (HDT) on 6 healthy volunteers. On a first 1 month HDT session, 3 of the 6 subjects (A, B, C) had daily several lower body negative pressure tests (LBNP), whereas the 3 subjects remaining (D, E, F) rested without LBNP. On a second 1 month HDT session subjects D, E, and F had daily LBNP tests and the A, B and C subjects did not. The cardiac function was assessed by Echocardiography, (B mode, TM mode). On all the "6 non LBNP" subjects the left ventricule diastolic volume (LVDV), the stroke volume (SV) and the cardiac output (CO) increase (+10%, -15%) after HDT then decrease and remain inferior (-5%, -5%) or equal to the basal value during the HDT. Immediately after the end of the HDT the heart rate (HR) increase (+10%, +30%) whereas the cardiac parameters decrease weakly (-5%, -10%) and normalize after 3 days of recovery. On the "6 LBNP" subjects the LVDV, SV and CO increase (+10%, 15%) after 1 h HDT as in the previous group then decrease but remain superior (+5%, +15%) or equal to the basal value. After the HDT session, the HR is markedly increased (+20%, +40%) the LVDV and SV decrease (-15%, -20%) whereas the CO increases or decreases depending on the amplitude of the HR variations. These parameters do not completely normalize after 3 day's recovery. Repeated LBNP sessions have a significant effect on the cardiovascular function as it maintains all cardiac parameters above the basal value. The LBNP manoeuvre can be considered as an efficient countermeasure to prevent cardiac disadaptation induced by HDT position and probably microgravity.     

Early renal changes in 45 degrees HDT rats

Background: Both microgravity and simulated microgravity models, such as the 45HDT (45 degrees head-down tilt), cause a redistribution of body fluids indicating a possible adaptive process to the microgravity stressor. Understanding the physiological processes that occur in microgravity is a first step to developing countermeasures to stop its harmful effects, i.e., (edema, motion sickness) during long-term space flights. Hypothesis: Because of the kidneys' functional role in the regulation of fluid volume in the body, it plays a key role in the body's adaptation to microgravity. Methods: Rats were injected intramuscularly with a radioactive tracer and then lightly anesthetized in order to facilitate their placement in the 45HDT position. They were then placed in the 45HDT position using a specially designed ramp (45HDT group) or prone position (control group) for an experimental time period of 1 h. During this period, the 99mTc-DTPA (technetium-labeled diethylenepentaacetate, MW=492 amu, physical half-life of 6.02 h) radioactive tracer clearance rate was determined by measuring gamma counts per minute. The kidneys were then fixed and sectioned for electron microscopy. A point counting method was used to quantitate intracellular spaces of the kidney proximal tubules. Results: 45HDT animals show a significantly (p=0.0001) increased area in the interstitial space of the proximal tubules. Conclusions: There are significant changes in the kidneys during a 1 h exposure to a simulated microgravity environment that consist primarily of anatomical alterations in the kidney proximal tubules. The kidneys also appear to respond differently to the initial periods of head-down tilt.     

Dynamic change of ERPs related to selective attention to signals from left and right visual field during head-down tilt

To study further the effect of head-down tilt (HDT) on slow positive potential in the event-related potentials (ERPs), the temporal and spatial features of visual ERPs changes during 2 hour HDT (-10 degrees) were compared with that during HUT (+20 degrees) in 15 normal subjects. The stimuli were consisted of two color LED flashes appeared randomly in left or right visual field (LVF or RVF) with same probability. The subjects were asked to make switch response to target signals (T) differentially: switching to left for T in LVF and to right for T in RVF, ignoring non-target signals(N). Five sets of tests were made during HUT and HDT. ERPs were obtained from 9 locations on scalp. The mean value of the ERPs in the period from 0.32-0.55 s was taken as the amplitude of slow positive potential(P400). The main results were as follows. 1) The mean amplitude of P400 decreased during HDT which was more significant at the 2nd, 3rd and 5th set of tests; 2) spatially, the reduction of mean P400 amplitude during HDT was more significant for signals from RVF and was more significant at posterior and central brain regions than that on frontal locations. As that the positive potential probably reflects the active inhibition activity in the brain during attention process, these data provide further evidence showing that the higher brain function was affected by the simulated weightlessness and that this effect was not only transient but also with interesting spatial characteristics.     

Gender differences in organ density in a rat simulated microgravity model

Research investigating the physiological effects of microgravity on the human body has demonstrated a shift of body fluids in actual spaceflight and in simulated Earth-based microgravity models in both males and females, possibly causing many deleterious physiological effects. Twenty-five anatomically normal female (NF) and 20 ovariectomized (OE) Fischer 344 rats were randomly selected to be in an experimental (1 h of 45 degrees head-down tilt, 45HDT) or control (1 h of prone position) group. At the end of the hour experimental period, the density of the brain, lungs, heart, liver, and left and right kidneys were measured using spiral computed tomography (SCT) while the rats remained in their experimental positions. A sub-group of OE rats (N=6) was administered estrogen replacement therapy on a daily basis (5 micrograms/kg body weight, s.c.) for 4 days and then underwent 1 h of 45HDT and SCT analysis at one day, 2 days, and 5 days to determine if estrogen replacement therapy would alter organ densities. Our data demonstrate that 1 h of 45HDT produced significant increases (p<0.05) in the organ densities of the brain, liver, left kidney, and lung of the OE female group compared to their prone controls. However, only the brain density was significantly increased in the NF group. Estrogen replacement therapy caused a significant decrease in brain organ density at the 5 day time point compared to the 24 h time point. We conclude that estrogen plays a role in fluid distribution in a rat 45HDT model.     

Changes in organ perfusion and weight ratios in post-simulated microgravity recovery

Head-down tilt models have been used as ground-based simulations of microgravity. Our previous animal research has demonstrated that there are significant changes in fluid distribution within 2 h after placement in a 45 degrees head-down tilt (45HDT) position and these changes in fluid distribution were still present after 14 days of 45HDT. Consequently, we investigated changes in fluid distribution during recovery from 16 days of 45HDT. Changes in radioactive tracer distribution and organ/body weight ratio were examined in rats randomly assigned to a 45HDT or prone control group. The 45HDT rats were suspended for 16 days and then allowed to recover at the prone position 0, 77, 101, or 125 h post-suspension. Animals were injected with technetium-labeled diethylenetriamine pentaacetate (99mTcDTPA, MW=492 amu, physical half-life of 6.02 h) and then killed 30 min post-injection. Lungs, heart, liver, spleen, kidneys, and brain were harvested, weighed, and measured for radioactive counts. Statistical analyses included two-way analysis of variance (ANOVA) that compared 45HDT versus controls at the four experimental time points. The organ weight divided by the body weight ratio for the brain, heart, kidneys and liver in the 45HDT rats was significantly different than the control rats, regardless of time (treatment). There was no difference between the different time points (time). The average 99mTcDTPA count divided by the organ weight ratio values for the heart, liver, and spleen were significantly higher in the 45HDT group than the control group. The average counts for the heart and spleen were significantly higher at 77, 101, and 125 h than at time zero. We conclude that the major organs have different recovery patterns after 45HDT for 16 days in the rat.     

Early cardiovascular adaptation to zero gravity simulated by head-down tilt

The early cardiovascular adaptation to zero gravity, simulated by head-down tilt at 5 degrees, was studied in a series of 10 normal young men. The validity of the model was confirmed by comparing the results with data from Apollo and Skylab flights. Tilt produced a significant central fluid shift with a transient increase in central venous pressure, later followed by an increase in left ventricular size without changes in cardiac output, arterial pressure, or contractile state. The hemodynamic changes were transient with a nearly complete return to the control state within 6 hr. The adaptation included a diuresis and a decrease in blood volume, associated with ADH, renin and aldosterone inhibition.     

Does mental arithmetic before head up tilt have an effect on the orthostatic cardiovascular and hormonal responses?

Passive head up tilt (HUT) and mental arithmetic (MA) are commonly used for providing mental and orthostatic challenges, respectively. In animal experiments, even a single exposure to a stressor has been shown to modify the response to subsequent stress stimulus. We investigated whether MA applied before HUT elicits synergistic responses in orthostatic heart rate (HR), cardiac output (CO), heart rate variability and arterial blood pressure. The 15 healthy young males were subjected to two randomized protocols: (a) HUT and (b) HUT preceded by MA, with sessions randomized and ≥2 weeks apart. Beat to beat continuous hemodynamic variables were measured and saliva samples taken for hormonal assay. HUT alone increased HR from 59±7 (baseline) to 80±10 bpm (mean±SD) and mean blood pressure (MBP) from 88±10 to 91±14 mmHg. HUT results after MA were not different from those with HUT alone. The activity of alpha amylase showed differences during the experiments irrespective of the protocols. We conclude that mental challenge does not affect orthostatic cardiovascular responses when applied before; the timing of mental loading seems to be critical if it is intended to alter cardiovascular responses to upright standing.     

Spacelab Life Sciences flight experiments: an integrated approach to the study of cardiovascular deconditioning and orthostatic hypotension

The microgravity environment of spaceflight produces rapid cardiovascular changes which are adaptive and appropriate in that setting, but are associated with significant deconditioning and orthostatic hypotension on return to Earth's gravity. The rapidity with which these space flight induced changes appear and disappear provides an ideal model for studying the underlying pathophysiological mechanisms of deconditioning and orthostatic hypotension, regardless of etiology. Since significant deconditioning is seen after flights of very short duration, muscle atrophy due to inactivity plays, at most, a small role. These changes in circulatory control associated with cephalad fluid shifts, rather than inactivity per se, are probably more important factors. In order to test this hypothesis in a systematic way, a multidisciplinary approach which defines and integrates inputs and responses from a wide variety of circulatory sub-systems is required. The cardiovascular experiments selected for Spacelab Life Sciences flights 1 and 2 provide such an approach. Both human and animal models will be utilized. Pre- and post-flight characterization of the payload crew includes determination of maximal exercise capacity (bicycle ergometry), orthostatic tolerance (lower body negative pressure), alpha and beta adrenergic sensitivity (isoproterenol and phenylephrine infusions), baroreflex sensitivity (ECG-gated, stepwise changes in carotid artery transmural pressure with a pneumatic neck collar), and responses to a 24 h period of 5 deg head-down tilt. Measurements of cardiac output (CO2 and C2H2 rebreathing), cardiac chamber dimensions (phased-array 2-dimensional echocardiography), direct central venous pressure, leg volume (Thornton sock), limb blood flow and venous compliance (occlusion plethysmography), blood and plasma volumes, renal plasma flow and glomerular filtration rates, and various hormonal levels including catecholamines and atrial natriuretic factor will also be obtained. The central venous catheter will be inserted immediately pre-launch and monitored with heart rate and blood pressure in-flight until cardiac output, respiratory gas exchange and quantitative 2D echocardiography measurements can be performed. In-flight hemodynamic measurements will be repeated at rest and during submaximal exercise daily and also during maximal exercise midway through the flight to document the timecourse and extent of cardiovascular changes in the payload crew. Parallel studies are planned for the animals. In addition to measurements of right atrial and aortic pressures and cardiac output, a dorsal micro-circulatory chamber will allow determinations of changes in capillary and venular architecture and function in six of the rats. The techniques and findings from many of the SLS-1 and 2 supporting studies have already yielded significant information about circulatory regulation in patients with both hypo- and hypertension. The flight experiments themselves will provide new data to test the validity of both animal and human models currently used for simulating the fluid shifts of a micro-gravity environment. The development of effective countermeasures, not only for short and long duration space travellers, but also for Earth-bound medical patients can then be physiologically based on experimental data rather than anecdote.     

Interaction of mental and orthostatic stressors

We assessed hemodynamic responses induced by orthostatic and mental stressors, using passive head up tilt (HUT) and mental arithmetic (MA), respectively. The 15 healthy males underwent three protocols: (1) HUT alone, (2) MA in supine position and (3) MA+HUT, with sessions randomized and ≥2 weeks apart. In relation to baseline, HUT increased heart rate (HR) (+20.4±7.1 bpm; p<0.001), mean blood pressure (MBP) (+4.7±11.3 mmHg; p<0.05), diastolic blood pressure (DBP) (+6.1±11.6 mmHg; p<0.05) and total peripheral resistance (TPR) (+155±232 dyne*s/cm⁵; p<0.001) but decreased stroke volume (SV) (?33.1±13.4 ml; p<0.001) and cardiac output (CO) (?0.6±1.0 l/min; p<0.01). MA increased HR (+8.0±6.0 bpm; p<0.001), systolic blood pressure (SBP) (+9.0±7.7 mmHg; p<0.001), MBP (+10.0±6.5 mmHg; p<0.001), DBP (+9.5±7.2 mmHg; p<0.001) and CO (+0.6±0.8 l/min; p<0.01). MA+HUT increased HR (+28.8±8.4 bpm; p<0.001), SBP (+4.6±14.3 mmHg; p<0.05), MBP (+11.2±11.6 mmHg; p<0.001), DBP (+13.5±10.1 mmHg; p<0.001) and TPR (+160±199 dyne*s/cm⁵; p<0.001) but SV (?34.5±14.6 ml; p<0.001) decreased. Mental challenge during orthostatic challenge elicited greater increases in heart rate, despite similar reductions in stroke volume such as those during orthostatic stress alone. Overall, cardiac output decreases were less with combinations of mental and orthostatic challenges in comparison to orthostasis alone. This would suggest that carefully chosen mental stressors might affect orthostatic responses of people on standing up. Therefore, additional mental loading could be a useful countermeasure to alleviate the orthostatic responses of persons, particularly in those with histories of dizziness on standing up or on return to earth from the spaceflight environment of microgravity.     

Central and regional hemodynamics in prolonged space flights.

This paper presents the results of measuring central and regional (head, forearm, calf) hemodynamics at rest and during provocative tests by the method of tetrapolar rheography in the course of Salyut-6-Soyuz and Salyut-7-Soyuz missions. The measurements were carried out during short-term (19 man-flights of 7 days in duration) and long-term (21 man-flights of 65-237 days in duration) manned missions. At rest, stroke volume (SV) and cardiac output (CO) as well as heart rate (HR) decreased insignificantly (in short-term flights) or remained essentially unchanged (in long-term flights). In prolonged flights CO increased significantly in response to exercise tests due to an increase in HR and the lack of changes in SV. After exercise tests SV and CO decreased as compared to the preflight level. During lower body negative pressure (LBNP) tests HR and CO were slightly higher than preflight. Changes in regional hemodynamics included a distinct decrease of pulse blood filling (PBF) of the calf, a reduction of the tone of large vessels of the calf and small vessels of the forearm. Head examination (in the region of the internal carotid artery) showed a decrease of PBF of the left hemisphere (during flight months 2-8) and a distinct decline of the tone of small vessels, mainly, in the right hemisphere. During LBNP tests the tone of pre- and postcapillary vessels of the brain returned to normal while PBF of the right and left hemisphere vessels declined. It has been shown that regional circulation variations depend on the area examined and are induced by a rearrangement of total hemodynamics of the human body in microgravity. This paper reviews the data concerning changes in central and regional circulation of men in space flights of different duration.     

Afferent control mechanisms involved in the development of soleus fiber alterations in simulated hypogravity

It was recently established that support withdrawal (withdrawal of support reaction force) in microgravity provokes a sequence of functional shifts in the activity of motor units (inactivation of slow ones) and peripheral muscle apparatus which lead to the decline of postural muscle contractility and alterations in fiber characteristics. However, mechanisms involved in inactivation of the slow motor units and appropriate slow-twitch muscle fiber disuse under the supportless conditions remained unknown. We show here that artificial inactivation of muscles-antagonists (which are known to be hyperactive during unloading) counteracts some of the unloading-induced events in the rat soleus (fiber size reduction, slow-to-fast fiber-type transition and decline of titin and nebulin content). It was also demonstrated that direct activation of the muscarinic receptors of the neostriatum neurons prevented slow-to-fast fiber-type transformation in soleus of hindlimb suspended rats.     

Efficacy of periodic centrifugation of primates during 4-week head-down tilt.

Creation of artificial force of gravity (AFG) to counteract the negative consequences of microgravity in manned space missions of extended duration is one of the high-priority problems of space biology and medicine. However, there are a number of especial effects of AFG (namely, structural changes in muscles and bones, and some other system) which need implantation of electrodes and sensors and are possible only with animals. That is why it is of particular interest to make studies with monkeys whose reactions to changed gravity bear much resemblance with human. The purpose of the investigation was development of a protocol of periodic gravity loads as a counter-measure against the hypokinetic syndrome in Macaca mulatta. Two series of experiments were performed. In the series, animals were split into two groups of 6 species each who were motor restrained with the head end tilted downward at 5 degrees (HDT) for 28 days. Monkeys of group-2 were periodically subjected to centrifugation (HDT+G). During the first series of experiments rotation was conducted in the +Gz direction at g-loads from 1.2 to 1.6 units for 30-40 minutes 4-5 times a week. In the second series, g-load was equal to 1.2 units and the animals were rotated 30 min. 2-3 time a week. The criterion of Y-training protocol efficacy was a test +Gz run at 3 units for 30 s. during which functioning of the cardiovascular systems and its controls was evaluated. The test run was performed prior to and after HDT. Following HDT the animals of group HDT+G were more resistant to the test than their counterparts who had not been trained on the centrifuge. Data of the investigation imply that following HDT and HDT+G alike reduced the amount of total bodily fluids (by approximately 5%), the intracellular component (approximately 4%), and plasma volume (by 6-7%). Yet, there are radical differences between the groups in the levels of reduction in extracellular fluids (by 11% and 6.5%, respectively, P<0.05) and the interstitial component (by 11.5% and 6.5, respectively, P<0.05). Prophylactic centrifugation during HDT was also positive to the muscular blood flow in lower extremities.     

Impaired T-wave amplitude adaptation to heart-rate induced by cardiac deconditioning after 5-days of head-down bed-rest

The study of QT/RR relationship is important for the clinical evaluation of possible risk of acquired or congenital ventricular tachyarrhythmias. In the hypothesis that microgravity exposure could induce changes in the repolarization mechanisms, our aim was to test if a short 5-days strict 6° head-down bed-rest (HDBR) could induce alterations in the QT/RR relationship and spatial repolarization heterogeneity. Twenty-two healthy men (mean age 31±6) were enrolled as part of the European Space Agency HDBR studies. High fidelity (1000 Hz) 24 h Holter ECG (12-leads, Mortara Instrument) was acquired before (PRE), the last day of HDBR (HDT5), and four days after its conclusion (POST). The night period (23:00–06:30) was selected for analysis. X, Y, Z leads were derived and the vectorcardiogram computed. Selective beat averaging was used to obtain averages of P–QRS–T complexes preceded by the same RR (10 ms bin amplitude, in the range 900–1200 ms). For each averaged waveform (i.e., one for each bin), T-wave maximum amplitude (Tmax), T-wave area (Tarea), RTapex, RTend, ventricular gradient (VG) magnitude and spatial QRS-T angle were computed. Non-parametric Friedman test was applied. Compared to PRE, at HDT5 both RTapex and RTend resulted shortened (−4%), with a decrease in T-wave amplitude (−8%) and area (−13%). VG was diminished by 10%, and QRS-T angle increased by 14°. At POST, QT duration and area parameters, as well as QRS-T angle were restored while Tmax resulted larger than PRE (+5%) and VG was still decreased by 3%. Also, a marked loss in strength of the linear regression with RR was found at HDT5 in Tmax and Tarea, that could represent a new dynamic marker of increased risk for life-threatening arrhythmias. Despite the short-term HDBR, ventricular repolarization during the night period was affected. This should be taken into account in astronauts for risk assessment during space flight.     

Cardiovascular reflexes during rest and exercise modified by gravitational stresses.

The hypotheses tested were whether variations in central venous pressure via the low pressure baroreceptors would take over or modify the arterial baroreceptor function, and further to which extent local and "whole body" hydrostatic stresses influence blood flow distribution. We investigated total forearm and skin blood flow (venous occlusion plethysmography and 133-Xe clearance) and cardiac output (rebreathing method) among other parameters. Hypo- and hyper-gravitational stresses were simulated by LBNP, LBPP, water immersion and lowering of the arm. The changes in flow distribution in the arm were ascribed to arterial baroreceptor function and not to low pressure baroreceptor activity. The enhancement of venous return during water immersion increased exercise tolerance during heat stress presumably due both to increased stroke volume and decreased venous pooling. The response to sustained handgrip exercise during LBNP and LBPP was not different from control measurements and the effects explained by arterial baroreceptor function. Application of exercise and local hydrostatic stresses in combination with gravitational stresses represent an interesting model for further study of the mechanisms behind the distribution of cardiac output to the peripheral organs.     

Correlation of macro and micro cardiovascular function during weightlessness and simulated weightlessness

The investigation of cardiovascular function necessarily involves a consideration of the exchange of substances at the capillary. If cardiovascular function is compromised or in any way altered during exposure to zero gravity in space, then it stands to reason that microvascular function is also modified. We have shown that an increase in cardiac output similar to that reported during simulated weightlessness is associated with a doubling of the number of post-capillary venules and a reduction in the number of arterioles by 35%. If the weightlessness of space travel produces similar changes in cardiopulmonary volume and cardiac output, a reasonable expectation is that astronauts will undergo venous neovascularization. We have developed an animal model in which to correlate microvascular and systemic cardiovascular function. The microcirculatory preparation consists of a lightweight, thermo-neutral chamber implanted around intact skeletal muscle on the back of a rat. Using this technique, the performed microvasculature of the cutaneous maximus muscle may be observed in the conscious, unanesthetized animal. Microcirculatory variables which may be obtained include venular and arteriolar numbers, lengths and diameters, single vessel flow velocities, vasomotion, capillary hematocrit anastomoses and orders of branching. Systemic hemodynamic monitoring of cardiac output by electromagnetic flowmetry, and arterial and venous pressures allows correlation of macro- and microcirculatory changes at the same time, in the same animal. Observed and calculated hemodynamic variables also include pulse pressure, heart rate, stroke volume, total peripheral resistance, aortic compliance, minute work, peak aortic flow velocity and systolic time interval. In this manner, an integrated assessment of total cardiovascular function may be obtained in the same animal without the complicating influence of anesthetics.     

Exercise response to simulated weightlessness

Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult.     

Biochemical and hemodynamic changes in normal subjects during acute and rigorous bed rest and ambulation

Rigorous bed rest (RBR) induces significant biochemical and circulatory changes. However, little is known about acute rigorous bed rest (ARBR). Measuring biochemical and circulatory variables during ARBR and RBR the aim of this study was to establish the significance of ARBR effect. Studies were done during 3 days of a pre-bed rest (BR) period and during 7 days of ARBR and RBR period. Thirty normal male individuals aged, 24.1 +/- 6.3 years were chosen as subjects. They were divided equally into three groups: 10 subjects placed under active control conditions served as unrestricted ambulatory control subjects (UACS), 10 subjects submitted to an acute rigorous bed rest served as acute rigorous bed rested subjects (ARBRS) and 10 subjects submitted to a rigorous bed rest served as rigorous bed rested subjects (RBRS). The UACS were maintained under an average running distance of 9.7 km day-1. For the ARBR effect simulation, ARBRS were submitted abruptly to BR for 7 days. They did not have any prior knowledge of the exact date and time when they would be asked to confine to RBR. For the RBR effect simulation, RBRS were subjected to BR for 7 days on a predetermined date and time known to them right away from the start of the study. Plasma renin activity (PRA), plasma cortisol (PC), plasma aldosterone (PA), plasma and urinary sodium (Na) and potassium (K) levels, heart rate (HR), cardiac output (CO), and arterial blood pressure (ABP) increased significantly, and urinary aldosterone (UA), stroke volume (SV) and plasma volume (PV) decreased significantly (p<0.05) in ARBRS and RBRS as compared with their pre-BR values and the values in UACS. Electrolyte, hormonal and hemodynamic responses were significantly (p<0.05) greater and occurred significantly faster (p<0.05) during ARBR than RBR. Parameters change insignificantly (p>0.05) in UACS compared with pre-BR control values. It was concluded that, the more abruptly muscular activity is restricted in experimental subjects while they are very active, the greater hemodynamic and biochemical change there is and probably in individuals whose muscular activity is abruptly terminated after an accident or sudden illness.     

Effect of the thigh-cuffs on the carotid artery diameter jugular vein section and facial skin edema: HDT study.

Objective: To evaluate the distal arterial, venous and skin changes in a group using thigh cuffs during daytime and in a control group. Method: Cardiac, arterial, venous parameters were measured by echography and Doppler. Skin thickness was measured by high frequency echography. Results & discussion: Head down position induced plasma volume reduction, increased cerebral resistance, reduced lower limb resistance. The jugular vein increased whereas the femoral and popliteal veins decreased. All these changes were already observed in previous HDT. Common carotid diameter decreased, Front head skin thickness increased and Tibial skin thickness decreased. Eight hours with thigh cuffs increased the cardiac and carotid sizes which is in agreement with the plasma volume increase. Conversely they reduced the cerebral vascular resistance, jugular section and front head edema which may explain the sensation of comfort reported by the subjects. At the lower limb level the thigh cuffs restored the skin thickness to pre-HDT level but enlarged markedly the femoral and popliteal veins. HR, BP, CO, TPR did not change.     

Carotid artery pulsatility during parabolic flights

In cardio-vascular hemodynamic, the arterial pulsatility, represented by the arterial pulse pressure (PP= systolic blood pressure-diastolic blood pressure), is different from one site to another, in opposite with the mean blood pressure almost identical in the whole body in supine position (or in microgravity). This is due to the arterial tree geometry and regional differences in the distensibility properties of the arterial wall. As the level of blood pressure opposed to the cardiac left ventricle work is the central pressure, on one hand and as the arterial pulsatility at the site of arterial baro-receptors (located on aortic arch and carotid arteries' bifurcation) regulates the sympathetic and vagal control of heart and peripheral resistances on the other hand, to determine the evolution of this central pulse pressure is of major importance in the knowledge of cardio-vascular hemodynamic during hyper or hypogravity as observed during parabolic flights. The aim of this study was to evaluate noninvasively the carotid artery pulsatility and mechanic properties during parabolic flights.     

Recent advances in the physiology of whole body immersion

Recent investigations have furnished a complete analysis of the hemodynamic events accompanying whole-body immersion. About 700 ml of blood are translocated into the intrathoracic circulation, and heart volume increases by 180 +/- 62 ml. These changes are followed by an increase in stroke volume and cardiac output of over 30%. At the same time a reflex reduction of total peripheral resistance and venous tone occurs. Renin and aldosterone activity are reduced while the 17-hydroxycorticosteroid is not affected. Treatment of the subject with DOCA attenuates but does not extinguish the excess sodium excretion of immersion. This finding strengthens the arguments in favor of an unknown factor enhancing sodium excretion. Finally, the relative activation of the three factors that serve volume control, the excretory function of the kidney, capillary filtration pressure, and the thirst mechanism, is discussed.