Heart rate variability during exercise can quantify cardiorespiratory deconditioning in 55- to 65-year-old men and women exposed to head-down bed rest.

Abstract

The heart rate variability (HRV) features HRV_0.75 and HRV_0.5, obtained by detrended fluctuation analysis during incremental cycle ergometer tests, have been shown to correlate with indices of cardiorespiratory fitness. Here, we evaluate the ability of these features to track individual changes in cardiorespiratory fitness induced by head-down bed rest (HDBR) among late middle-aged adults. Twenty-two healthy participants (55-65 years old, 11 women) were exposed to 14-day HDBR. Eleven participants were assigned to a group performing daily aerobic and resistive exercises during HDBR, while the others were assigned to a control group with no countermeasure intervention. We measured gas exchange and electrocardiography during incremental cycle ergometer tests pre- and post-HDBR. The countermeasure preserved oxygen uptake (V̇O₂) at the two ventilatory thresholds (VT1 and VT2), HRV_0.75 and HRV_0.5. However, these four features decreased in the control group after HDBR. A large and proportional bias was evidenced between V̇O₂ at VT1 and HRV_0.75, which was less the case for V̇O₂ at VT2 versus HRV_0.5. However, the intra-individual changes in the control group were correlated (r_rm = 0.66 for VT1 vs. HRV_0.75 and r_rm = 0.82 for V̇O₂ at VT2 versus HRV_0.5, both p < 0.001). Overall, this study indicated that HRV_0.75 and HRV_0.5 can be used to longitudinally monitor changes in cardiorespiratory fitness in late middle-aged adults with physical deconditioning. Future research can apply these methods to other study populations.