Major cardiovascular problems, secondary to cardiovascular deconditioning, may occur on extended space missions. While it is generally assumed that the microgravity state is the primary cause of cardiovascular deconditioning, sleep deprivation and disruption of diurnal rhythms may also play an important role. Factors that could be modified by either or both of these perturbations include: autonomic function and short-term cardiovascular reflexes, vasoreactivity, circadian rhythm of cardiovascular hormones (specifically the renin-angiotensin system) and renal sodium handling and hormonal influences on that process, venous compliance, cardiac mass, and cardiac conduction processes. The purpose of the Human Studies Core is to provide the infrastructure to conduct human experiments that allow the assessment of the likely role of such factors in the space travel associated cardiovascular deconditioning process, and to develop appropriate countermeasures. The Core takes advantage of the General Clinical Research Center at the Brigham and Women's Hospital, Boston, Massachusetts, to perform these studies.
The Core includes two general experimental protocols. The first protocol involves a head down tilt bed-rest study to simulate microgravity. The second protocol includes the addition of sleep deprivation to the simulated microgravity environment. Before and after each of these environmental manipulations, the subjects undergo acute stressors simulating changes in volume and/or stress, which could occur in space and on return to Earth. The subjects are maintained in a rigidly controlled environment with fixed sleep cycles, activity pattern, and dietary intake of nutrients, fluids, ions and calories. Within the Core experimental protocol framework, investigators perform specific experiments, some based on the application of new non-invasive measurement techniques, to determine the effect of the environmental modifications on the status and responsiveness of the cardiovascular, endocrine, and renal homeostatic systems. In the project led by Professor Cohen, titled Alterations in Cardiovascular Regulation and Function during Simulated Microgravity, investigators apply cardiovascular system identification (CSI) techniques to characterize important cardiovascular regulatory responses including the heart rate and peripheral resistance baroreflexes. The application of CSI involves the use of echocardiography for continuous beat to beat measurement of stroke volume. In the project led by Professor Williams, titled Renal and Cardio-Endocrine Responses in Humans to Simulated Microgravity, investigators characterize the renal and endocrine responses. Finally, in second project led by Professor Cohen, titled Non-Invasive Assessment of Susceptibility to Ventricular Arrhythmias during Simulated Microgravity, investigators apply novel techniques to quantify changes in the cardiac conduction processes and assess any increased tendency toward cardiac arrhythmias or cardiac electrical alterations. Together, these projects cover a broad spectrum of systems involved in maintaining cardiovascular homeostasis and promise to provide new insight regarding their alterations in response to the major environmental changes of microgravity and disruption of circadian rhythms. The data from these studies is being used to develop and test potential countermeasures so as to ensure the health, productivity, and safety of astronauts during and on return from extended missions.