Quantitative electrocardiography during extended space flight: the second manned Skylab mission.

: To assess the effects of space flight on cardiac electrical properties, vectorcardiograms (VCG) were obtained on the Skylab crew at regular intervals during the 59-d flight (SL3) and during the pre- and postflight periods. The Frank lead system was used and observations were made at rest; during 25%, 50%, and 75% of maximum exercise; during a short pulse of exercise (150 W, 2 min); and after exercise. Data from 47 in-flight tests were analyzed by computer and compared to preflight and postflight values. A statistically significant increase in QRS maximum vector magnitude (all SL3 crewman); and an increase in resting PR interval (all SL3 crewmen) occurred. During exercise, the PR interval did not differ from preflight. Exercise heart rates in flight were same as preflight, but increased in the immediate postflight period. No major changes in QRS, T, or ST vector direction occurred. The scientist pilot had premature ventricular beats sporadically during the flight. This astronaut also had intermittent periods of atrioventricular junctional rhythm throughout the flight. The increase in QRS maximum vector magnitude resembles the electrocardiographic changes associated with athletic conditioning and may be related to increased ventricular volume secondary to centripetal shifts of fluid and/or the in-flight isotonic exercise program. Prolongation of the PR interval at rest with normalization of exercise suggests that there was increased vagal tone or suppressed sympathetic influence during the flight.