Single-breath estimates of cardiac output prior to and in microgravity

Cardiac output (CO) measured at rest and during exercise using a single-breath (SB) method has been shown to be reliable and valid, however under estimates CO. This may be due to the effect of gravity on blood or gas distribution affecting the R-PCO2 relationship. Six astronauts were studied erect and supine at rest and during rest and exercise prior to and during space flights (9-14 days). SB and re-breathing (RB) CO's were determined alternately in each condition. Rest CO increased from erect to supine (17%) and from pre-flight to flight (36%) with both techniques. CO increased linearly with VO2 for RB and SB respectively (4.86 and 4.72, r= 0.92), however the intercept for the SB was lower (3.68 vs. 5.29 1/min). The slopes of the regression of CO measured by SB and RB for ground and flight ranged from 0.74 to 0.84 with r's of .82 to .95. These data agree with previous comparisons of CO measured by SB and RB. Therfore: 1) gravity does not explain the differences and 2) SB techniques yield valuable information as to changes in CO.