Cardiac rhythm problems during spaceflight
Heart rhythm disturbances have been seen among astronauts. Most of these have been related to cardiovascular disease, but it is not clear whether this was due to pre-existing conditions or effects of space flight. It is hoped that advanced screening for coronary disease has greatly mitigated this risk. Other heart rhythm problems, such as atrial fibrillation, can develop over time, necessitating periodic screening of crewmembers’ heart rhythms. Beyond these terrestrial heart risks, some concern exists that prolonged exposure to microgravity may lead to heart rhythm disturbances. Although this has not been observed to date, further surveillance is warranted.
The incidence and clinical significance of cardiac arrhythmias during long-term exposure to microgravity experienced on the International Space Station (ISS) or during a prolonged (that is, up to 3 years) sojourn to Mars or on the Moon are a concern for the National Aeronautics and Space Administration (NASA). At present, there are only anecdotal reports of cardiac arrhythmias in space, including one documented episode of non-sustained ventricular tachycardia. However, the potential catastrophic nature of a sudden cardiac death in the remote, but highly public, environment of space flight has led to continued concern since the early days of the space program over the possibility that space flight might be arrhythmogenic. Indeed, there are known and well-defined changes in the cardiovascular system with space flight: a) plasma volume is reduced; b) left ventricular mass in decreased; and c) the autonomic nervous system adapts to the microgravity environment. Combined, these physiologic adaptations suggest that changes in cardiac structure and neurohumoral environment during space flight could alter electrical conduction, although the evidence supporting this contention consists mostly of minor changes in QT interval in a small number of astronauts after long-duration space flight. Concurrent with efforts by Flight Medicine to improve screening techniques, as NASA enters the era of exploration class missions, it will be critical to determine with the highest degree of certainty whether space flight by itself alters cardiac structure and function sufficiently to increase the risk for arrhythmias. This undertaking must be done in a highly systematic way.
At present, there is little evidence suggesting that cardiovascular adaptation to microgravity or space flight increases susceptibility to life threatening arrhythmias in astronauts. From a clinical perspective, according to the “biological model” of sudden cardiac death,1 both the substrate and the trigger for arrhythmias should be considered to determine whether long-term space flight could lead to an increased risk of sudden death. In this model, structural abnormalities interact with functional alterations, such as exercise, electrolyte disturbances, or neurohumoral modulation, to create an environment in which arrhythmias can be initiated and/or sustained. In patients with coronary artery disease, the substrate is clear: a myocardial infarction (MI) and/or scar leading to focal areas of slowed conduction, a necessary condition for re-entry. For patients with apparently normal ventricular function, the potential substrate is less certain. In fact, reentry often is not the mechanism of arrhythmia development in these clinical cases: the arrhythmias may be caused by delayed after-depolarizations, and the triggered activity may be mediated via catecholamines.2 The published report of non-sustained ventricular tachycardia during prolonged space flight3 supports this hypothesis, in that initiation of tachycardia by a late diastolic premature ventricular contraction (PVC) is more consistent with triggered activity than it is with re-entry.
While there are no definitive data showing that long-duration space flight is associated with cardiac arrhythmias, there are observational data that have been documented over many years that are suggestive of cardiac electrical changes during long flights. For example, during Skylab, all 9 American crewmembers exhibited some form of rhythm disturbance. Most of these rhythm disturbances consisted of single PVCs and were clinically insignificant. However, one crewmember experienced a 5-beat run of ventricular tachycardia during a lower-body negative pressure protocol, and another had periods of “wandering supraventricular pacemaker” during rest and following exercise. More recently, it has been shown that the corrected QT interval (QTc), a marker of ventricular repolarization, was prolonged slightly in a small number of astronauts after long-duration space flight. In-flight Holter monitoring was not performed during these space flights. Thus, it is not known whether this prolongation was associated with any known arrhythmias. In-flight Holter monitoring was undertaken in the early Space Shuttle era.
Virtually no changes in arrhythmias were documented in flights of 4 to 16 days during either intravehicular or extravehicular operations compared to preflight measurements.4,5 Indeed, in these studies, the frequency of arrhythmias may actually have been reduced in flight, though the day-to-day variability of these arrhythmias, which is known to be quite wide, was not quantified. However, aboard the Mir space station, PVCs were detected that were not present before flight6 and a 14-beat run of ventricular tachycardia was documented.3
More recently, several conditions that may predispose crewmembers to arrhythmias have been identified. D’Aunno et al.7 found that after long-duration missions QTc intervals are slightly prolonged in crewmembers who did not have prolonged QTc intervals after their short-duration Space Shuttle flights, and several investigators have found decreases in left ventricular mass following space flight.8,9
All of these findings raise the concern that cardiac rhythm disturbances may become an issue during the long in-flight tours of duty planned for ISS and interplanetary missions. The degree to which space flight and its many variables can be considered arrhythmogenic is not clear, but the possibility that serious cardiac rhythm disturbances might occur during space flight is a concern to NASA.
Spaceflight evidence
There have been no systematic studies of the arrhythmogenic potential of long-duration space flight, and only two studies of short-duration space flight. There have been, however, a number of published reports detailing in-flight arrhythmias. Table 1 includes a summary of some of these reports.
| Program | Launch | Flight | EVA | Re-Entry or Landing | Post-flight |
|---|---|---|---|---|---|
| Mercury | PVCs, PACs | Sinus Dysrhythmia, 1 PVC, 1 PAC, One fusion beat | |||
| Gemini | Rare PACs | ||||
| Apollo | Lunar surface: atrial bigeminal rhythm (extreme fatigue), PVCs, PACs | ||||
| Skylab | PVCs, AV block, ectopic beats, AV junctional rhythm, ST segment and Twave alterations during max stress, ventricular couplet, 3-beat V-tach | Ventricular Tachycardia | |||
| Space Shuttle | PVCs, PACs | PVCs, PACs | |||
| Table adapted from Charles, JB, Frey, MA, Fritsch-Yelle JM, Fortner GW. Chapter 3: Cardiovascular and Cardiorespiratory Function in Space Biology and Medicine. Nicogossian AE, Mohler SR, Gazenko OG, Grigoriev AI, eds. AIIA, Reston VA. 1996. p73. | |||||
Leguay and Seigneuric also compiled some of the reports from the pre-Shuttle era of manned space flight.10 Several of these reports are briefly described below.
One crewmember during Apollo 15 experienced a 22-beat nodal bigeminal rhythm, which was followed by premature atrial beats.5 This crewmember reported extreme fatigue during the incident, but only when questioned about it by crew surgeons; thus, it was not severe enough to impact the mission. Twenty-one months later the crew member suffered from coronary artery disease and a cardiac infarction without suggestive ECG changes.10