Cardiac output by pulse contour analysis does not match the increase measured by rebreathing during human spaceflight

Pulse contour analysis of the non-invasive finger arterial pressure waveform provides a convenient means to estimate cardiac output (Q ). The method has been compared to standard methods under a range of conditions but never before during spaceflight. We compared pulse contour analysis with the Modelflow algorithm to estimates of Q obtained by rebreathing during pre-flight baseline testing and during the final month of long-duration spaceflight in nine healthy male astronauts. By Modelflow analysis, stroke volume was greater in supine baseline than seated baseline or inflight. Heart rate was reduced in supine baseline so that there were no differences in Q by Modelflow estimate between the supine (7.02±1.31 L/min, mean±SD), seated (6.60±1.95 L/min) or inflight (5.91±1.15 L/min) conditions. In contrast, rebreathing estimates of Q increased from seated baseline (4.76±0.67 L/min) to inflight (7.00±1.39 L/min, significant interaction effect of Method and Spaceflight, P<0.001). Pulse contour analysis utilizes a three element Windkessel model that incorporates parameters dependent on aortic pressure - area relationships that are assumed to represent the entire circulation. We propose that a large increase in vascular compliance in the splanchnic circulation invalidates the model under conditions of spaceflight. Future spaceflight research measuring cardiac function needs to consider this important limitation for assessing absolute values of Q and stroke volume.

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