Building off of his ground-based studies in orthostatic intolerance, Dr. Richard J. Cohen is working with NASA investigators to study the effects of weightlessness on the cardiovascular system by studying astronauts pre- and post-flight. The group will use two non-invasive technologies developed in Dr. Cohen's laboratory in this study. Microvolt T-wave alternans testing will be used to determine whether space flight can make the heart susceptible to serious heart rhythm disturbances. Microvolt T-wave alternans testing equipment has been cleared by the FDA and is now in widespread clinical use to identify patients at risk of sudden cardiac death. Cardiovascular system identification will be used to assess the effects of space flight on cardiovascular regulation. The researchers also will study the drug midrodrine to determine its effectiveness as a countermeasure to the development of orthostatic intolerance following space flight.
Richard J. Cohen, M.D., Ph.D.
Massachusetts Institute of Technology
After being weightless in space, many astronauts become hypotensive and presyncopal upon assuming an upright position. This phenomenon, known as orthostatic intolerance, may interfere with astronaut function during reentry and following space flight, and may limit the ability of an astronaut to exit a landed spacecraft unaided during an emergency. Orthostatic intolerance is more pronounced following long-term space flight and is a major concern with respect to the extended flights expected aboard the International Space Station and for interplanetary exploration class missions, such as a human mission to Mars. This problem has also been observed to be more pronounced among women than among men. In addition to the problem of postflight orthostatic intolerance, a variety of heart rhythm disturbances have been observed in astronauts during and after space flight. The potential lethal arrhythmic risk for astronauts is sustained ventricular tachycardia or ventricular fibrillation, while non-sustained ventricular tachycardia could cause syncope.
In previous ground-based bed-rest studies sponsored by NSBRI, we have applied two new techniques that we have developed to study the effects of simulated microgravity on the cardiovascular system. Cardiovascular system identification (CSI) has been used a noninvasive means of measuring alterations in closed-loop cardiovascular regulation and the measurement of Microvolt level T-Wave Alternans (TWA) has been used as a noninvasive measure of susceptibility to ventricular arrhythmias. We have also successfully tested the alpha-l sympathetic agonist midodrine as a countermeasure to the development of orthostatic intolerance. We have found that 16 days of bed rest results in altered cardiovascular regulation. In particular, we have demonstrated alterations in baroreceptor sensitivity, altered electrical stability of the heart, and that midodrine is an effective countermeasure to the development of orthostatic intolerance.
In this proposal, we plan to apply the same measurement techniques of CSI and TWA to astronauts pre- and postflight and to test midodrine as a countermeasure to the development of orthostatic intolerance. This study will allow us to determine if the changes in cardiovascular regulation and cardiac electrical stability measured in a ground-based model also occur during actual space flight. In addition, we will test for the first time a potentially effective countermeasure for the development of postflight orthostatic intolerance.