Upright Cardiac Output Measurements in the Transition to Weightlessness during Parabolic Flights

OBJECTIVE: Aims of this study were 1) to determine cardiac output by inert gas rebreathing (COreb) during transition into 0 Gz in the standing position 2) to compare impedance cardiography (ICG) and pulse contour method (PCM) with COreb as a reference method. METHODS: We measured baseline COreb and heart rate (HR) on ground, and COreb, COpcm, COicg and HR in standing and supine positions in the transition to weightlessness in 6 subjects. We conducted repeated measures ANOVA, Bland and Altman analysis and analysis of percentage error of each data set. RESULTS: COreb rose from 5.03 ±0.7 upright ground control to 11.45 ±3.6 L.min-1 in 0 Gz. HR and stroke volume (SV) rose from 83 ±14 to 113 ±19 beats per min and from 61 ±6 to 99 ±18 mL, respectively. Mean COreb, COpcm and COicg across all conditions were 10.45 ±3.04, 7.42 ±1.71, and 6.57 ±2.46 L/min, respectively. Overall Bland and Altman analysis showed poor agreement for COpcm and COicg compared to COreb. DISCUSSION: Large bias for both comparisons indicated that both PCM and ICG underestimate the true CO value. Paired CO values of individual subjects showed a better correlation between methods and a broad bias range, indicating a preponderant role for large between-subjects variability. Repeated COreb determinations in 1 Gz (i.e., when the cardiovascular system is in a steady state) should be used for calibration of the PCM and of ICG data. PCM and ICG can then be used to track CO dynamics during rapid changes of acceleration profiles