The Skylab experiment M093 was aimed at studying changes in electrical activity of the heart during space flight. The objectives were to measure electrocardiographic changes caused by prolonged exposure to weightlessness and to assess their effects on the human cardiovascular system.
The experiment employed vectorcardiographic (VCG) techniques to provide a comprehensive three-dimensional analysis of electrocardiographic signals. A specially designed Frank lead vectorcardiograph system was used. It included eight electrodes which were applied to the crewmen in a modified Frank lead configuration, the modification being the transfer of the left leg electrode to the left sacral region to lessen muscle noise during exercise. Standardized exercise loads were incorporated into the experiment protocol to increase the sensitivity of the electrocardiogram to the affects of cardiovascular deconditioning and to detect susceptibility for cardiac arrhythmias. Vectorcardiograms were obtained in experiment M093 for 5 minutes at rest, 2 minutes during bicycle ergometer exercise at 150 watts, and 10 minutes during recovery after exercise. They were also obtained at rest and during recovery (5 minutes each) during Skylab experiment M171 on metabolic activity. The analog signals were sampled at 320 samples per second, and the digitized data were analyzed using a computer program. The program identified the baseline and end-points of P, QRS, and T waves, and computed various parameters such as duration and amplitude, which were analyzed statistically.
Parameters from 131 inflight vectorcardiograms were analyzed and compared to pre-flight and post-flight values. Maximum QRS vector magnitude showed a statistically significant increase in six of the nine Skylab astronauts. The magnitude of the spatial T vector increased in five crewmen. There was no consistent changes in QRS, T, or ST vector direction during the flights. The PR interval measured at rest increased in six astronauts, but the average PR interval for each inflight test did not exceed the upper limit of normal in any crewman. Resting heart rate decreased significantly only in the Skylab 2 crew. There was a marked increase in heart rate both at rest and at any given exercise load in the immediate postflight period.
The vectorcardiographic data were also analyzed to detect changes of clinical importance. During the three Skylab missions there were no ST segment abnormalities that suggested myocardial ischemia or other abnormalities. However, cardiac arrhythmias were occasionally observed during the three flights. Except for these arrhythmias, no adverse electrocardiographic changes were observed in the Skylab crews that could be attributed to exposure to weightlessness or other stresses of space flight.