**3.7 Postflight orthostatic intolerance**

Orthostatic intolerance is usually observed after returning to 1 G on Earth. The definition of orthostatic intolerance usually includes simple syncope, lightheadedness, or >20-mmHg reduction in systolic blood pressure.

Astronauts usually notice orthostatic intolerance during prolonged upright standing rather than while standing up. Just before fainting, they sometimes have tachycardia, suggesting that they have postural orthostatic tachycardia syndrome (POTS). This phenomenon is due to a state in which fluid shift easily triggers tachycardia, which also easily triggers Bezold-Jarisch reflex, and the vagal response suppresses the systolic blood pressure. Although all astronauts stood upright for 10 min, 63% were unable to finish the stand test in 10 min.

Less important factors for postflight orthostatic intolerance are reduced compliance of the lower legs, reduced baroreflex sensitivity, and increased basal sympathetic tone.

Reduction of the circulatory blood volume is the most important factor for postflight orthostatic intolerance. The decrease in stroke volume after spaceflight reflects this circulatory blood volume loss. Although this is the main cause, the recovery of circulatory blood volume to the normal state is not complete. The crew members are recommended to take 8 g of salt and 1 L of water, which ameliorates the orthostatic tolerance, albeit not completely.

Another factor is the limitation by vasoconstriction. The postflight blood pressure of non-finishers cannot be increased by the total peripheral resistance compared with the preflight state. During the postflight upright standing 70° tilt test, the total peripheral resistance cannot increase despite activation of muscle sympathetic nerve activity, probably due to the alterations in venoarterial reflex and smooth muscle atrophy of the resistant vessels. Overall, circulatory blood volume reduction and attenuated vasoconstriction are the main factors for orthostatic intolerance.
