The Cardiovascular Alterations Team has conducted studies to determine what alterations in hemodynamic regulation result from sixteen days of simulated microgravity exposure in normal human subjects. In this project we made additional measurements on these same study subjects in order to determine whether there is an increase in susceptibility to ventricular arrhythmias resulting from simulated microgravity exposure.
Numerous anecdotal and documented reports from the past 30 years suggest that the incidence of ventricular arrhythmias among astronauts is increased during space flight. For example, documented runs of ventricular tachycardia have been recorded from crew members of Skylab and Mir, there was much attention given by the lay press to Mir Commander Vasily Tsibliyev's complaints of heart rhythm irregularities in July of 1997, and cardiovascular mechanisms may have been causal in the recent death of an experimental primate shortly after return from space. In 1986, a Mir cosmonaut, Alexander Laveikin, was brought home and replaced with an alternate cosmonaut as a result of cardiac dysrhythmias that began during extravehicular activity. Furthermore, at a joint NASA/NSBRI workshop held in January 1998, cardiac arrhythmias were identified as the highest priority cardiovascular risk to a human Mars mission. Despite the evidence for the risk of a potentially lethal arrhythmia resulting from microgravity exposure, the effects of space flight and the associated physiologic stresses on cardiac conduction processes are not known, and an increase in cardiac susceptibility to arrhythmias has never been quantified.
In this study we found that 16 days of head down bed rest appears to increase the incidence of microvolt level T-wave alternans, which reverts to baseline levels two-three days after the bed rest period. This is the first data obtained under control conditions which indicates that simulated microgravity alters cardiac electrical processes. The presence of T-wave alternans (although with a lower onset heart rate than observed) in clinical patient populations has been found to indicate an increased risk of ventricular arrhythmias.
The data presented here indicate the need to further investigate the effect of space flight on the heart's susceptiblity to ventricular arrhythmias, and if necessary develop appropriate countermeasures.
Microvolt level T-wave alternans testing developed under NASA and NSBRI support has now been successfully commercialized and was FDA cleared in April 1999 as a non-invasive means of identifying patients at increased risk of ventricular arrhythmias and sudden cardiac death. Three hundred thousand Americans die each year of sudden cardiac death. Effective treatment is available in the form of the implantable cardioverter/defibrillator. The problem has been that until now there has not been an effective means of identifying who is at risk. T-wave alternans testing is now in clinical use and promises to have a major role in reducing sudden cardiac death.