Changes in cardiac output and orthostatic tolerance of astronauts

Examinations of 14 cosmonauts who performed orbital flights of 14 to 175 days were used to correlate cardiac output (CO) inflight with orthostatic tolerance and LBNP reactions postflight. In 3 crewmembers CO was lower than or close to the preflight level. In 4 cosmonauts CO was higher thant preflight. The remaining 7 crewmembers showed lower orthostatic tolerance and stronger LBNP reactions. The difference between mean CO values before and during flight was in negative correlation with orthostatic tolerance (r = -0.6) and in positive correlation with LBNP reactions (r = 0.7). The correlation coefficients were derived from small samples but an identical relationship between the two different tests with inflight CO variations gives evidence that such a relationship actually exists.