Rossum et al. used 24-hour Holter recordings acquired during and after high altitude chamber activity, 30 days before launch, during and after each extravehicular activity performed, and on return to Earth. The investigators observed no change in the number of premature ventricular contractions of premature atrial contractions per hour during flight compared to preflight or postflight (Figure 2). Likewise, arrhythmias were not observed by Fritsch-Yelle et al. in 12 astronauts studied before, during and after 6 Space Shuttle missions. Given the fact that these data disagreed with previous reports, the investigators suggested that further study was required. It is unknown whether long-duration exposure to microgravity itself may precipitate cardiac arrhythmias. Based on observations and clinical judgement, medical operations personnel have suggested that some of these incidents have been related to pre-existing, undiagnosed coronary artery disease. Additional pre-selection crew screening tests, including calcium scoring, have been added to reduce such occurrences in the future. Recent evidence suggests that the development of apoptosis, or “programmed cell death” in response to pathological, physiologic, and/or genetic signals, may be a key developmental factor in causing cardiac arrhythmias. For example, apoptosis associated with atrophy and fibrofatty replacement of right ventricular tissue has been identified as the likely mechanism forarrhythmia development in arrhythmogenic right ventricular dysplasia, a condition that may lead to sudden death in otherwise healthy young individuals. Two publications have reported decreases in left ventricular mass after short-duration space flight. In one of these publications, cardiac MRI was used and showed a reduction in left ventricular mass on landing day; however, extended recovery data were not obtained (Figure 3). In the other publication, echocardiography was used and showed a similar decrease in mass on landing day with full recovery 3 days after landing. Unpublished data (also measured with ultrasound) show decreases in left ventricular mass after 6-month missions aboard the ISS. These decreases are double those observed after short flights and do not fully recover by the third day after landing (Figure 5). There is some disagreement over the mechanism of the decrease in mass, especially after short-duration missions. While there is evidence to support the idea that tissue dehydration contributes to the loss in mass after short-duration space flights, there are data from bed rest studies showing that the decrease in mass can be prevented with exercise and/or nutritional countermeasures.