Swanson_2001abstr

A MODEL FOR MINIMIZING THE RISK OF SUDDEN DEATH DURING EXERCISE IN LONG-TERM SPACE FLIGHT

George D. Swanson PhD

Department of Physical Education and Exercise Science

California State University

Chico, CA

dswanson@csuchico.edu

Crew members in long-term space flight may be relatively sedentary for months. An exercise program will be necessary to maintain health and physical capacity. However, vigorous exercise stresses the heart and can trigger sudden death. This risk increases for those relatively sedentary individuals who occasionally attempt vigorous exercise. Alternatively, the risk is also high for those who exercise regularly for longer periods of time. The purpose of this paper is to introduce a model that characterizes the risk of sudden death during exercise and to use that model to determine an optimal amount of regular vigorous exercise that minimizes that risk. The Physicians Health Study (Albert et al., N Engl J Med 343: 1353-1361, 2000) followed 21,481 physicians for 12 years during which 122 died of sudden death. These physicians were free of cardiovascular disease at the beginning of the study and will form the reference group for our analysis (Swanson, N Engl J Med 344: 854-855, 2001). Nested case-crossover methods were used to lay out contingency tables in terms of sudden death and person hours of exercise or sedentary time. Combining these tables yields an odds ratio for sudden death during exercise compared to rest: OR = {Fx / (1-Fx)] [(1-Px) / Px] where Fx is the fraction of sudden deaths that occur during exercise and is Px is the proportion of time that exercise could trigger sudden death. As in the Physicians Health Study, Fx is linearly related to Px: Fx = a + b Px. Combining these two equations yields a quadratic equation for the minimum risk: b(b-1) Px2 + 2ab Px + a(a-1) = 0. The solution characterizes the optimal amount of exercise. For the Physicians Health Study, this was about 30 min per day six days a week. The implications of this model will be explored for a Mars mission.