Different effects of inspiratory duration and expiratory duration on heart rate deceleration capacity and heart rate asymmetry

Low values of heart rate deceleration capacity (DC) and heart rate asymmetry (HRA) are associated with cardiovascular risks. Slow respiration has been proven to enhance the magnitudes of these indexes, but individual inspiratory (TI) and expiratory (TE) durations were not controlled in most studies. This study aims to examine whether the effects of TI and TE on these indexes would be the same and, if not, how to adjust TI and TE to maximize the effect of slow respiration. We evaluated 14 seated healthy young adults who randomly controlled their breathing to nine combinations of TI and TE, each chosen respectively from 2, 4, and 6 s. A 5-min R-R interval time series was obtained from each study period for further analysis. The magnitude of DC increased when TI or TE increased, while that of acceleration capacity (AC) remained almost unchanged by TI. We further defined a new index as 100 × DC2/(DC2 + AC2) and found it to be correlated with conventional Guzik’s (r = 0.94) and Porta's (r = 0.99) indexes of HRA during different combinations of TI and TE. Increasing TI and increasing TE both enhanced the magnitudes of HRA indexes, with TI taking effect when ≤ 4 s, and TE taking effect when > 4 s. DC and HRA indexes were maximized with a TI of 4 s and a TE of 6 s. We suggest that a TI of 3–4 s with a TE of 7–6 s is an appropriate standard for slow respiration.