Dr P Norsk

On the ground, blood and fluid are drawn towards the legs in upright humans. To maintain an adequate perfusion of the brain, reflexes are initiated by pressure receptors to stimulate the heart and contract the vessels. During acute weightlessness, the opposite occurs: Blood and fluid are no longer drawn towards the legs leading to expansion of central blood volume and the heart. In fact, results from parabolic flights have shown that the cardiac atriae are distended even more than during supine conditions on the ground (Videbæk and Norsk, J. Appl. Physiol., 83: 1862-66, 1997). Thus, gravity constantly compresses the heart in supine individuals, which may have a deleterious impact on cardiac function in heart failure patients.

Another question is how prolonged weightlessess affects cardiac function in humans. Already after 4-5 days of flight, cardiac output and stroke volume are decreased to a level close to those of the upright position on the ground. Furthermore, Perhonen et al. (J. Appl. Physiol. 91: 645-53, 2001) have shown that left ventricular mass decreases by some 12±7% in 4 astronauts after 10 days of spaceflight. Thus, atrophy of the cardiac muscle might be one of the mechanisms of the lower cardiac output in space. Another reason could be that intravascular volume is decreased by spaceflight (Alfrey et al. J. Appl. Physiol. 81: 98-104, 1996) so that the venous return to the heart is decreased. The quantitative contribution of each of these effects, cardiac atrophy and blood volume depletion, should be the focus of investigative efforts in the future.

We have observed an augmented sympathetic nervous activity and attenuated renal responses to saline and water stimuli in astronauts during spaceflight (Norsk et al., J. Appl. Physiol. 78: 2253-59, 1995; Norsk et al., The Lancet, 356: 1577-78, 2000). These observations might be consequences of the lower cardiac output in space. Whether this is the case will be investigated in the future on the International Space Station. In addition, we plan to investigate, whether the decrease in cardiac function is to some degree similar to that of heart failure. In this way, we hope to define the effects of gravity on heart failure patients.

To measure cardiac output in space, a rebreathing technique has been developed. This technique is now being used for treatment of patients and represents a spin-off of space technology to medical treatment.

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