**3.8 Cardiovascular deconditioning**

What is the cause of this cardiovascular deconditioning? NASA's criteria of orthostatic intolerance are (1) presyncopal symptoms (pallor, cold sweat, nausea, blackout, and fainting), (2) gradual systolic blood pressure decrease <80 mmHg, (3) sudden systolic blood pressure decrease >15 mmHg, or (4) sudden heart rate decrease >15 bpm while on the 70° tilt bed for 15 min. A recent report stated that 65% of astronauts satisfied these criteria. Previously, this cardiovascular deconditioning was considered to be solely due to circulatory fluid loss, but other causes have also been explored.

In addition to the decrease in circulatory blood volume, other causes, i.e., altered arterial baroreflex gain, altered leg venous volume, easy fluid pooling in the space of atrophied skeletal muscles, attenuated muscle pump effects due to skeletal muscle atrophy, hypersensitivity of β-adrenergic receptors, and altered influence of vestibular (especially otolith) input, have been considered. Moreover, increased venous permeability of lower leg vessels and attenuated cardiopulmonary volume receptor reflex after −6° head-down tilt for 14 days were observed in our bedrest experiment. These changes are not the only cause of cardiovascular deconditioning, and multiple factors act in concert.
