Consecutive ultra-short-term heart rate variability to track dynamic changes in autonomic nervous system during and after exercise

Objective: Quantitative measurement of the dynamic changes in autonomic nervous system (ANS) during and after exercise has great significance in clinical, sports training and other fields. A consecutive ultra-short-term (30 s, UST) heart rate variability (HRV) method was proposed to track the exercise-induced autonomic control of heart rate (HR). Approach: Twenty-three healthy young men participated in the study. The first four stages of the Modified Bruce Protocol (S0-S3) were performed. Six HRV indices, i.e. HF (power of high frequency ranged from 0.15 to 0.4 Hz), LF (power of low frequency ranged from 0.04 to 0.15 Hz), LF/HF, SD1 and SD2 of Poincare plot, and SD2/SD1, over 30 s were calculated every 5 s over 3 min RR time series during, as well as after, exercise. Main results: The results showed that during exercise, SD1, SD2, HF and LF dropped down quickly and tended to stabilize. Particularly, SD1 and HF showed a slight upward trend in the lower three stages while the declining time of SD2 in S3 lasted longer than the other stages. SD2/SD1 increased rapidly first and then decreased slowly. The values of SD2/SD1 in S3 remained higher than those in the other stages. After exercise, SD1, SD2, HF and LF kept increasing first and then declined slowly or fluctuated with decaying amplitudes. SD2/SD1 increased initially, then decreased and fluctuated slightly. Significance: Compared with the indices in frequency domain, the Poincare indices were more sensitive and accurate in UST measurement of ANS during exercise. The results demonstrated that the UST method could characterize the dynamic changing tendency of ANS during and after exercise and quantify the differences of changes in ANS induced by exercise with different intensities. In particular, the vagal branch functioned dominantly in controlling HR in S0 but the effect of the sympathetic branch on HR enhanced with the increase of exercise intensity. In addition, the transient changes of ANS related with the sudden onset of exercise could also be reflected, despite perhaps being limited by the computation window width to some extent. Thus, the consecutive UST Poincare indices could provide a feasible and simple method to measure quantitatively the exercise-induced dynamic changes in ANS.