Indirect measurement of absolute cardiac output during exercise in simulated altered gravity is highly dependent on the method

Purpose Altered gravity environments introduce cardiovascular changes that may require continuous hemodynamic monitoring in both spaceflight and terrestrial analogs. Conditions in such environments are often prohibitive to direct/invasive methods and therefore, indirect measurement techniques must be used. This study compares two common cardiac measurement techniques used in the human spaceflight domain, pulse contour analysis (PCA—Nexfin) and inert gas rebreathing (IGR—Innocor), in subjects completing ergometer exercise under altered gravity conditions simulated using a tilt paradigm. Methods Seven subjects were tilted to three different angles representing Martian, Lunar, and microgravity conditions in the rostrocaudal direction. They completed a 36-min submaximal cardiovascular exercise protocol in each condition. Hemodynamics were continuously monitored using Nexfin and Innocor. Results Linear mixed-effects models revealed a significant bias of 25.7 ± 1.7 ml ( p < 0.0001 ) in stroke volume and 3.19 ± 0.18 l/min ( p < 0.0001 ) in cardiac output, with Nexfin measuring greater than Innocor in both variables. These values are in agreement with a Bland-Altman analysis. The correlation of stroke volume and cardiac output measurements between Nexfin and Innocor were r = 0.54 ( p < 0.0001 ) and r = 0.76 ( p < 0.0001 ) respectively. Conclusion There is a poor agreement in absolute stroke volume and cardiac output values between measurement via PCA (Nexfin) and IGR (Innocor) in subjects who are exercising in simulated altered gravity environments. These results suggest that the chosen measurement method and device greatly impacts absolute measurements of cardiac output. However, there is a good level of agreement between the two devices when measuring relative changes. Either of these devices seem adequate to capture cardiac changes, but should not be solely relied upon for accurate measurement of absolute cardiac output.