Abstract 18500: Determinants of Cardiac Loading During Long Duration Spaceflight

Abstract

INTRODUCTION: Prolonged exposure to microgravity (bedrest) leads to a decrease in cardiac mass by ~1%/week due to reduced cardiac work. However, the magnitude of reduction with long duration spaceflight with modern countermeasures is unknown. One purpose of this study (Integrated CardioVascular) is to quantify cardiac work and mass in astronauts during 6 months on the International Space Station (ISS). METHODS: Data from 5 astronauts who completed a long duration stay on the ISS (~6 months) were analyzed. Cardiac mass (MRI) was measured pre- and post-flight. Continuous finger blood pressure waveforms (Volume-Clamp) for 24 hours were measured before and the end of flight (15 days before they returned to the earth). Cardiac mass was estimated by (epicardial volume - endocardial volume)*1.05 g/mL. Daily stroke work and cardiac work were calculated by the sum of each stroke volume (ΣSV, SV; stroke voluem) and stroke work (ΣSV*cmBPs, cmBPs; mean central blood pressure during systole) per day, respectively. The central blood pressure waveform was mathematically reconstructed from the finger blood pressure waveform. Beat-to-beat SV was estimated with BP waveform analysis (Modelflow). RESULTS: The %change in cardiac mass was variable (+1% on average) with divergent responses among astronauts (-4%~ +14%). All astronauts showed the reduction of cardiac work (-15±9%), while 4 of 5 showed the reduction of stroke work (-8±12%). A strong positive linear relationship was shown between % changes in cardiac work vs. mass (R 2 =0.471) as well as %changes in stroke work vs. cardiac mass (R 2 =0.905). CONCLUSIONS: Cardiac atrophy during long duration spaceflight is highly dependent on the relative changes in cardiac work and stroke work from preflight to the end of the mission. Current countermeasures may be protective in some, but not all astronauts.