Cardiac rhythm problems during spaceflight
At present, there is little evidence suggesting that cardiovascular adaptation to microgravity or space flight increases susceptibility to life-threatening arrhythmias in astronauts.
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.
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.
Virtually no changes in arrhythmias were documented in flights of 4 to 16 days during either intravehicular or extravehicular operations compared to preflight measurements.
[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.
Twenty-one months later the crew member suffered from coronary artery disease and a cardiac infarction without suggestive ECG changes.
The arrhythmias occurred during effort tests, extravehicular activities (EVAs), lower body negative pressure sessions, and throughout the entire mission.
[10] In addition, an isolated incident of a non-sustained 14-beat ventricular tachycardia (Figure 1), with a maximum heart rate of 215 beats per minute, was recorded using in-flight Holter monitoring aboard the Mir.
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.
In one of these publications,[9] cardiac MRI was used and showed a reduction in left ventricular mass on landing day; however, extended recovery data were not obtained (Figure 3).
[8] Unpublished data (also measured with ultrasound) show decreases in left ventricular mass after 6-month missions aboard the ISS.
Not all cells within the heart share identical action potentials; therefore, a certain degree of variability, or inhomogeneity, in their repolarization time exists.
[32] A systems analysis using the computer model of human physiology developed at the University of Mississippi Medical Center also predicts a loss in left ventricular mass following short-duration space flight.
According to the model predictions, the reductions in left ventricular mass observed after short-duration exposure to microgravity may be the result of a contraction of the myocardial interstitial fluid space secondary to a loss in the plasma volume (see Figure 6).
At least two potential risk factors for arrhythmias have been reported either during or immediately after space flight: cardiac atrophy and a prolonged QTc interval.
This article incorporates public domain material from Human Health and Performance Risks of Space Exploration Missions (PDF).
