Spaceflight and the telltale heart.

Cardiac arrhythmias have not been a common occurrence during manned spaceflights. Cardiovascular deconditioning, on the other hand, is a universal finding both during and after exposure to microgravity. Exercise capability is reduced only in the immediate postflight period as is cardiac output. These adaptive changes do not impair crew health or their ability to perform effectively in weightless flight. We believe that these effects are mediated by a decreased effective circulating blood volume and that they are completely reversible. In any case, an effective countermeasure is available to obviate or mitigate any untoward deconditioning effects on deorbit and return to Earth's gravity.