Heart rate dynamics during long-term space flight: report on Mir cosmonauts.
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Adult↗
Biomedical subjects
J B Charles
Publications and source records attributed to J B Charles.
Does bed rest produce changes in orthostatic function comparable to those induced by space flight?
Use of bed rest to simulate microgravity exposure is not well validated. We compared heart rate (HR) and blood pressure (BP) responses to standing in bed-rest (BR) subjects (n=11) to those of two astronaut groups. One astronaut group (n=28) fluid loaded (FL) before landing by consuming a water and salt tablet mixture, the second astronaut group (n=8) did not (NL). Bed-rest or microgravity exposure lasted approximately 7.0 days. Preexposure, the responses to standing did not differ between groups. Postexposure, all groups demonstrated an increased HR response (p<0.01), a decreased SBP response (p<0.05), no change in DBP response, and a reduced PP response (p<0.05) compared to preexposure. Change in HR response was lowest for the FL group, presumably due to increased plasma volume induced by fluid consumption. These findings generally support bed rest as a valid simulator of microgravity.
Adult↗
Spaceflight alters autonomic regulation of arterial pressure in humans.
Spaceflight is associated with decreased orthostatic tolerance after landing. Short-duration spaceflight (4-5 days) impairs one neural mechanism: the carotid baroreceptor-cardiac reflex. To understand the effects of longer-duration spaceflight on baroreflex function, we measured R-R interval power spectra, antecubital vein plasma catecholamine levels, carotid baroreceptor-cardiac reflex responses, responses to Valsalva maneuvers, and orthostatic tolerance in 16 astronauts before and after shuttle missions lasting 8-14 days. We found the following changes between preflight and landing day: 1) orthostatic tolerance decreased; 2) R-R interval spectral power in the 0.05 to 0.15-Hz band increased; 3) plasma norepinephrine and epinephrine levels increased; 4) the slope, range, and operational point of the carotid baroreceptor cardiac reflex response decreased; and 5) blood pressure and heart rate responses to Valsalva maneuvers were altered. Autonomic changes persisted for several days after landing. These results provide further evidence of functionally relevant reductions in parasympathetic and increases in sympathetic influences on arterial pressure control after spaceflight.
Adult↗
The effect of dDAVP with saline loading on fluid balance during LBNP and standing after 24-hr head-down bedrest.
Shuttle astronauts currently drink approximately a quart of water with eight salt tablets before reentry to restore lost body fluid and thereby reduce the likelihood of cardiovascular instability and syncope during reentry and after landing. However, the saline loading countermeasure is not entirely effective in restoring orthostatic tolerance to preflight levels. We tested the hypothesis that the effectiveness of this countermeasure could be improved with the use of a vasopressin analog, 1-deamino-8-D-arginine vasopressin (dDAVP). The rationale for this approach is that reducing urine formation with exogenous vasopressin should increase the magnitude and duration of the vascular volume expansion produced by the saline load, and in so doing improve orthostatic tolerance during reentry and postflight.
Adult↗
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.
Computer Simulation↗
The effect of blood volume loss on cardiovascular response to lower body negative pressure using a mathematical model.
Different mathematical models of varying complexity have been proposed in recent years to study the cardiovascular (CV) system. However, only a few of them specifically address the response to lower body negative pressure (LBNP), a stress that can be applied in weightlessness to predict changes in orthostatic tolerance. Also, the simulated results produced by these models agree only partially with experimental observations. In contrast, the model proposed by Melchior et al., and modified by Karam et al. is a simple representation of the CV system capable of accurately reproducing observed LBNP responses up to presyncopal levels. There are significant changes in LBNP response due to a loss of blood volume and other alterations that occur in weightlessness and related one-g conditions such as bedrest. A few days of bedrest can cause up to 15% blood volume loss (BVL), with consequent decreases in both stroke volume and cardiac output, and increases in heart rate, mean arterial pressure, and total peripheral resistance. These changes are more pronounced at higher levels of LBNP. This paper presents the results of a simulation study using our CV model to examine the effect of BVL on LBNP response.
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Use of lower body negative pressure to counter symptoms of orthostatic intolerance in patients, bed rest subjects, and astronauts.
This report briefly discusses some aspects of autonomic cardiovascular dysfunction as related to changes in orthostatic function in patients, bed rest subjects, and astronauts. This relationship is described in normal individuals to provide the basis for discussion of parameters that may be altered in patients, bed rest subjects, and astronauts. The relationships between disease states, age, periods of weightlessness during space flight, and autonomic dysfunction, and their contribution to changes in orthostatic tolerance are presented. The physiologic effects of lower body negative pressure are illustrated by presenting data obtained in bed rest subjects and in astronauts. Finally, the usefulness of lower body negative pressure to counter symptoms of orthostatic intolerance in patients, bed rest subjects, and astronauts is discussed.
Autonomic Nervous System Diseases↗
Echocardiograms during six hours of bedrest at head-down and head-up tilt and during space flight.
Left ventricular end-diastolic volume increased after 4 1/2 to 6 hours of space flight, but was significantly decreased after 5 to 6 days of space flight. To determine the role of acute gravitational effects in this phenomenon, responses to a 6-hour bedrest model of 0 gravity (G; 5 degrees head-down tilt) were compared with those of fractional gravity loads of 1/6 G, 1/3 G, and 2/3 G by using head-up tilts of 10 degrees, 20 degrees, and 42 degrees, respectively. On 4 different days, six healthy male subjects were tilted at one of the four angles for 6 hours. Cardiac dimensions and volumes were determined from two-dimensional and M-mode echocardiograms in the left lateral decubitus position at control (0), 2, 4, and 6 hours. Stroke volume decreased with time (P < .05) for all tilt angles when compared with control. Ejection fraction (EF) at -5 degrees was greater than at +20 degrees and +42 degrees (not significant); EF at +10 degrees was greater than at +42 degrees (not significant). For the tilt angles of -5 degrees, +10 degrees, and +20 degrees, mean heart rate decreased during the first 2 hours, and returned to control or was slightly elevated above control (+20 degrees) by 6 hours (not significant). At the +42 degrees angle of tilt, heart rate was increased above control at hours 2, 4, and 6. There were no significant differences in cardiac output at any time point for any tilt angle.(ABSTRACT TRUNCATED AT 250 WORDS)
Bed Rest↗
Effects of lower-body negative pressure on blood flow with applications to the human cardiovascular system.
The paper reports a theoretical investigation into the effects of lower-body negative pressure on blood flow through the human cardiovascular system. The human cardiovascular system is modelled as a closed network of arteries, arterioles, capillaries, venules and veins of different lengths and cross-sections. The pumping action is provided by the contraction of the ventricles. The model has been analysed using the finite-element method. The pertinent equations incorporating the effects of lower-body negative pressure for the pressures and flow velocities have been derived, and the quantitative results have been computed. Percentage changes in flow velocities, pressure drops and conductances under the application of lower-body negative pressure in the various segments and organs of the entire cardiovascular system are obtained. The lower-body negative pressure has no effect on the flow rates in carotid, ulnar and coronary arteries, nor on the supply of blood to the upper extremities, kidneys, spleen and liver. The major effects are found in the lower extremities.
Blood Vessels↗
Mathematical modelling of the human cardiovascular system in the presence of stenosis.
This paper reports a theoretical study on the distribution of blood flow in the human cardiovascular system when one or more blood vessels are affected by stenosis. The analysis employs a mathematical model of the entire system based on the finite element method. The arterial-venous network is represented by a large number of interconnected segments in the model. Values for the model parameters are based upon the published data on the physiological and rheological properties of blood. Computational results show how blood flow through various parts of the cardiovascular system is affected by stenosis in different blood vessels. No significant changes in the flow parameters of the cardiovascular system were found to occur when the reduction in the lumen diameter of the stenosed vessels was less than 65%.
Blood Circulation↗
Cerebral blood velocity and other cardiovascular responses to 2 days of head-down tilt.
Spaceflight induces a cephalad redistribution of fluid volume and blood flow within the human body, and space motion sickness, which is a problem during the first few days of spaceflight, could be related to these changes in fluid status and in blood flow of the cerebrum and vestibular system. To evaluate possible changes in cerebral blood flow during simulated weightlessness, we measured blood velocity in the middle cerebral artery (MCA) along with retinal vascular diameters, intraocular pressure, impedance cardiography, and sphygmomanometry on nine men (26.2 +/- 6.6 yr) morning and evening for 2 days during continuous 10 degrees head-down tilt (HDT). When subjects went from seated to head-down bed rest, their heart rate and retinal diameters decreased, and intraocular pressures increased. After 48 h of HDT, blood flow velocity in the MCA was decreased and thoracic impedance was increased, indicating less fluid in the thorax. Percent changes in blood flow velocities in the MCA after 48 h of HDT were inversely correlated with percent changes in retinal vascular diameters. Blood flow velocities in the MCA were inversely correlated (intersubject) with arterial pressures and retinal vascular diameters. Heart rate, stroke volume, cardiac output, systolic arterial pressure, and at times pulse pressure and blood flow velocities in the MCA were greater in the evening. Total peripheral resistance was higher in the morning. Although cerebral blood velocity is reduced after subjects are head down for 2 days, the inverse relationship with retinal vessel diameters, which have control analogous to that of cerebral vessels, indicates cerebral blood flow is not reduced.
Adult↗
Physiologic mechanisms effecting circulatory and body fluid losses in weightlessness as shown by mathematical modeling.
The mechanisms causing large body water losses in weightlessness are not clear. It has long been considered that a central volume expansion drives the physiologic adaptation to a reduced total blood volume, with normal blood composition eventually regained. However, inflight venous pressure measures suggest that central volume expansion in weightlessness may be very transient, or that considerable cardiovascular adaptation to fluid shifts occurs on the ground while astronauts wait in the semi-supine pre-launch position. If a central volume stimulus does not persist, other mechanisms must drive the adaptation of circulation to a reduced blood volume and account for body fluid losses. Recent results from the SLS-1 mission suggest that body fluid volumes do not simply decline to new equilibria but that they decrease to a low point, then undergo some recovery. Similar "under-shoots" of body fluid volumes have also been shown in computer simulations, providing confidence in the validity of the model. The purpose of this study was to examine the mechanisms which could explain the loss of body fluids in weightlessness and how a cardiovascular preadaptation countermeasure we previously tested ameliorated body fluid losses. It is assumed that the physiology of head down tilt (HDT) provides a reasonably accurate analog of weightless exposure.
Blood Viscosity↗
Simulation of the fluid retention effects of a vasopressin analog using the Guyton model of circulation.
Fluid-loading (FL) consisting of water and salt tablets equivalent of 32 oz of isotonic saline is a countermeasure currently in use by NASA to improve the orthostatic tolerance of astronauts during Shuttle reentry. However, the effectiveness of this countermeasure has been observed to decrease with the duration of space flight. Possible ways to improve fluid retention and thus the effectiveness of FL include use of analogs of vasopressin such as lypressin (LVP). This study used a computer simulation approach to analyze the potential benefits on fluid retention with LVP administered before FL.
Blood Volume↗
A comparison of overall mathematical models of the cardiovascular system for simulating response to orthostatic stresses.
Although numerous mathematical models of the cardiovascular system (CVS) have appeared in the literature only a few of them are models of the entire system with detailed representation of the heart, the vasculature, and the control elements. Like all models of biological systems, these models vary in complexity, and most of them are stimulus- specific. Their ability to simulate with acceptable accuracy either responses over a wide range of the stimulus or responses to stimuli of similar kind has not been reported. In this paper, three mathematical models of the CVS are examined in terms of their response to different orthostatic stresses, namely, lower body negative pressure (LBNP), head-up tilt, and blood loss. The short-term orthostatic responses of the models are compared to available experimental data. The models are: (i) Croston and Fitzjerrell's for study of LBNP and head-up tilt response, (ii) Jaron et al.'s for study of +Gz response, and (iii) Pullen's for simulation of response to blood loss. We will henceforth refer to these models by the letters C, J, and P, respectively.
Hemorrhage↗
The relationship between space sickness and preflight diet.
The nine astronauts who flew three Skylab missions during 1973 and 1974 were on carefully planned diets. Each astronaut selected his preferences from a list of approximately 70 food items, from which dietitians developed a rotating sequence of six daily menus. In addition to strict control of the inflight diet, each crewman consumed his planned diet for 21 days preflight and for 18 days postflight. While the amount consumed at each particular meal was largely discretionary, all deviations from the planned meals (which were few) were fully documented. Diets were controlled to provide adequate calcium, phosphorus, and protein because of bone and muscle losses observed on previous flights. Each day's menu had a similar elemental composition; if any particular item was not consumed, supplements in the form of capsules or tablets were prescribed the next day to make up for the shortfall. However, the dieticians did not so carefully control the relative proportions of carbohydrate, fat, and protein (though meticulous records were kept), which were instead selected somewhat incidentally as menus were constructed around each astronaut's food preferences. There are several possible physiologic determinants for food preferences and the regulation of dietary macro-nutrients. Monozygotic twins chose more similar intakes of total energy, protein, fat, and carbohydrate than did dizygotic twins, suggesting a genetic component of dietary regulation. The biosynthesis of neurotransmitters is directly influenced by the availability of their precursor nutrients, such as tryptophan for serotonin and choline for acetylcholine. Diet affects behavior, and it has been suggested that dietary self-selection by rats is regulated by effects of the diet on brain neurotransmitters. Some preflight variables relating to fluid, electrolyte and cardiovascular status, and to environmental exposures, have previously been found to have significant relationships to space sickness. The purpose of this study was to determine if diet could be another factor contributing to space sickness, since diet determines so much of physiology and behavior.
Diet Records↗
Mathematical modelling of flow distribution in the human cardiovascular system.
The paper presents a detailed model of the entire human cardiovascular system which aims to study the changes in flow distribution caused by external stimuli, changes in internal parameters, or other factors. The arterial-venous network is represented by 325 interconnected elastic segments. The mathematical description of each segment is based on equations of hydrodynamics and those of stress/strain relationships in elastic materials. Appropriate input functions provide for the pumping of blood by the heart through the system. The analysis employs the finite-element technique which can accommodate any prescribed boundary conditions. Values of model parameters are from available data on physical and rheological properties of blood and blood vessels. As a representative example, simulation results on changes in flow distribution with changes in the elastic properties of blood vessels are discussed. They indicate that the errors in the calculated overall flow rates are not significant even in the extreme case of arteries and veins behaving as rigid tubes.
Blood Flow Velocity↗
The effects of exercise on blood flow with reference to the human cardiovascular system: a finite element study.
This paper reports on a theoretical investigation into the effects of vasomotion on blood through the human cardiovascular system. The finite element method has been used to analyse the model. Vasoconstriction and vasodilation may be effected either through the action of the central nervous system or autoregulation. One of the conditions responsible for vasomotion is exercise. The proposed model has been solved and quantitative results of flows and pressures due to changing the conductances of specific networks of arterioles, capillaries and venules comprising the arms, legs, stomach and their combinations have been obtained.
Blood Circulation↗
Mathematical modeling of human cardiovascular system for simulation of orthostatic response.
This paper deals with the short-term response of the human cardiovascular system to orthostatic stresses in the context of developing a mathematical model of the overall system. It discusses the physiological issues involved and how these issues have been handled in published cardiovascular models for simulation of orthostatic response. Most of the models are stimulus specific with no demonstrated capability for simulating the responses to orthostatic stimuli of different types. A comprehensive model incorporating all known phenomena related to cardiovascular regulation would greatly help to interpret the various orthostatic responses of the system in a consistent manner and to understand the interactions among its elements. This paper provides a framework for future efforts in mathematical modeling of the entire cardiovascular system.
Blood Pressure↗