Effect of the prelaunch position on the cardiovascular response to standing

Astronauts spend a minimum of 2 h in the reclined seated position (prelaunch position) prior to the launch of the Space Shuttle. This position favors a cephalad shift of blood volume and subsequent loss of body fluid volume, a physiologic situation associated with cardiovascular deconditioning and orthostatic intolerance following spaceflight. It is not known if the prelaunch position results in cardiovascular deconditioning and, therefore, impaired cardiovascular performance during standing. If so, this might hinder the ability of the crewmember to make an emergency egress from the shuttle during the launch. The purpose of this study was to evaluate the cardiovascular responses of men and women to the stand test before and after 2 h in the prelaunch position. Nine men and nine women performed the stand test before and after 2 h in the seated position (SIT), the prelaunch position (PL), or seated immersed to the neck in water (WI). Heart rate, blood pressure, and cardiac output were measured. Men had higher blood pressures than women and demonstrated increases in blood pressure with standing. There were no other gender differences. SIT did not alter the cardiovascular response to standing, while PL and WI resulted in greater increases in heart rate after 2 h in the position. This increase was only 4 to 7 beats/min greater than after SIT, and was the same for PL and WI. Thus, a small, but significant, degree of cardiovascular deconditioning occurred during the minimum time astronauts spend in the prelaunch position.