Submitted for publication December 1995.
Accepted for publication May 1996.
Session 1: Doing Research in Space
Session 2: Cardiovascular Systems Responses and Cardiopulmonary InteractionsThe following recommendations for future research emerged from the discussions of 1 and 2.
Recommendations for future research:
I. Determine a practical and feasible countermeasure to post-flight orthostatic hypotension.
II. Develop an understanding of the changes in the relationships between intrapleural pericardial and intrathoracic pressures that occur in space and define the effects on measures of central venous pressure, cardiac filling pressures, and volumes.
III. Determine the role of gravity in lung function and cardiac function and gravity's relationship to pericardial and intrathoracic pressures by using the 0 g environment.
IV. Investigate the loss of control of peripheral vasoconstriction.
V. Investigate further the role of the arterial and cardiopulmonary baroreflexes in post-flight regulation of blood pressure during orthostasis.
VI. Define the loading conditions on the heart during and following space flight.
VII. Investigate the basic mechanisms underlying changes in cardiac mass and integrate with the changes in cardiac function.
VIII. Establish a reference baseline body position and orientation in position 1 g and body position in 0 g measurements for heart rate, blood pressure, hydration, and neurohumoral changes.
IX. Investigate further the mechanism of post-flight orthostatic hypotension and the lack of relationship between plasma volume changes and the production of orthostatic hypotension.
X. Define the changes in neurohumoral regulation of the cardiovascular system when the human moves from the 1 g to the 0 g environment and then returns to the 1 g environment.
XI. Establish the role of thermal environment of the orbiter in the production of changes in plasma volume and determine whether manipulation of the orbiter environment, together with the use of exercise would be an effective expander of plasma volume.
Session 3: Body Fluid Metabolism and Control of Intravascular Volume
Session 4: Mechanisms Mediating Cardiovascular Adaptation to Actual and Simulated MicrogravityIn developing this set of recommendations, it was emphasized that control(ground-based) experiments have the following goals: the development of specific reference positions and protocols for each of the measurements, as well as the prioritizing of the in-flight human experiments, a practical necessity. Indeed, all investigators need to be aware of and able to account for operational variations and compromises in protocol of the experiments that occurd during a specific orbital flight. Because payload crew time is utilized fully in performing experiments and doing operational work, it was suggested that perhaps the orbiter flight crew be considered as potential subjects whenever feasible. Specifically, the work load of the payload crew is such that they rarely are able to maintain hydration or a proper sleep cycle. Therefore, the attainment of an adequate baseline measure for fluid balance or nutritional energy expenditures studies has been impossible on most flights. Another complication of in-flight experimentation that needs to be addressed in ground-based studies is the time sequencing of one investigation with another investigation on the same subject such that the initial investigation may compromise data collection of the second experiment which in turn may compromise the data on the experiment, etc. Having identified these problems, the following recommendations were made:
I. Establish the true nature of fluid and energy homeostasis immediately prior to launch and during flight and account for insensible losses.
II. Investigate the neurohumoral regulation of fluid balance and sodium ion regulation during space flight.
III. Determine the mechanism of gravity's action on erythropoietin metabolism and red blood cell production.
IV. Use ground-based models of actual in-flight time-lines of experimental protocols to determine potential mechanisms of fluid and energy balance.
V. Investigate the interaction between fluid and energy balance and cardiovascular and pulmonary function within the experimental paradigms of confinement and isolation.
VI. Investigate the shift of fluids into the different body segments and compartments, e.g., some ground-based models could be the giraffe or the bat.
VII. Investigate the mechanisms of neural control of the cardiovascular system's response to exercise and thermal challenges during simulated microgravity and actual space flight.
VIII. Investigate neural control mechanisms requiring sympathetic nerve recordings to skin and muscle and the relationship of the neural response, to the calculation of norepinephrine and epinephrine spillover data.
IX. Investigate the mechanisms of baroreceptor function, autonomic nervous system dysfunction, and orthostatic hypotension, with special emphasis on peripheral vascular dysfunction.
X. Develop and validate measurements of peripheral blood flow to quantify regional distribution of blood flow during microgravity, e.g., using Doppler techniques.
Session 5: Recapitulation of Completed Sessions During Sessions 1-4
Session 6: Integrated Physiology of μGIn developing these recommendations, it was realized that answering questions related to basic molecular and cellular mechanisms requires rigorous use of animal models. However, the problems to be solved also involve those integrative physiologic questions which addressed issues of molecular and cellular change and the resultant outcome with regard to organ system functions and biological behavior. It was further emphasized that, historically, specific species, such as the cat, have a large body of ground-based information on neural control of the cardiovascular and respiratory system and central nervous system function and yet the cat has never been used in space. Other animals that have been used in space include mice, rats, and monkeys, and it is important that in the future, the animal species used in space be selected for their applicability to human functional changes and that there is a significant ground-based literature on this species, ensuring that one does re-invent the wheel.Finally, in developing these recommendations, it was emphasized that we should focus on the use of the unique ability of a space laboratory to address complex, inter-related issues concerned with the human's ability to function in space and return to earth after increasingly lengthy exposures to space flight.
Recommendations for future research:
I. Investigate by using animal models and pharmacologic interventions direct cellular and molecular mechanisms of cardiovascular conditioning and deconditioning and the changes in maximal exercise capacity.
II. Investigate the mechanisms of red blood cell mass regulation, splenic function, and muscle atrophy during 0 g environments and specific activity paradigms.
III. Develop and test, using ground based models, and validate, using space flight, an exercise training protocol which optimizes the recruitment of motor units, activates sympathetic vasomotor output and blood pressure control and, at the same time, counteracts cardiac, blood vessel and skeletal muscle atrophy and bone demineralization.