Transient changes in heart rate variability in response to orthostatic task, endurance exersice and training : with special reference to autonomic blockades and time-frequency analysis

Heart rate variability (HRV) is generally accepted as an estimate of the autonomic, particularly vagal, control of the heart. The quantitative relationship of HRV to vagal heart rate (HR) control is, however, unclear. In addition, the conventional analysis methods of HRV are not suitable for analyzing rapid adjustments of autonomic control. Time-frequency analysis methods, including short-time Fourier transform (STFT), have been developed for analyzing non-stationary signals.The present study was designed to evaluate the ability of HRV to quantify within-subject changes in autonomic HR control by using autonomic blockades and an active orthostatic task, and to study transient changes in HRV in response to a bout of endurance exercise and to endurance training. The present blockade experiments indicated that the spectral power of HRV at all frequencies, particularly high frequency power [HFP, in ln(ms2)], was predominantly mediated by the vagal system and that the within-subject relationship between HFP and vagal HR control was essentially linear. HRV provided no specific index of sympathetic HR control.During the active orthostatic task, the STFT method was able to detect and quantify transient changes in vagal HR control. Application of the STFT method to the exercise data showed that HFP decreased during the incremental maximal exercise test up to an intensity of 60-70% of maximal power, reflecting vagal withdrawal. Further, the increase in absolute HFP immediately after exercise was faster after the low than high intensity exercise, suggesting faster recovery of vagal HR control after the low intensity exercise. The effects of aerobic fitness on HRV response to exercise were evaluated by using cross-sectional and longitudinal designs.Two groups differing in age-related aerobic fitness were formed by using age-related aerobic fitness norms as classification criteria. Comparisons between the high and moderate age-related fitness groups showed that high age-related aerobic fitness was related to improved vagal HR control at rest and during recovery from submaximal endurance exercise. In the longitudinal design, previously untrained individuals exercised twice a week for 14 weeks. The results indicated that the low-dose endurance training programme improved vagal HR control at the same absolute submaximal exercise intensities but did not alter vagal HR control at rest in previously untrained individuals.It also increased maximal oxygen uptake. To summarize, the STFT method provided a non-invasive tool for assessing transient changes in vagal HR control without the need to interfere with normal control, e.g. by using blocking drugs. The changes in HRV in response to endurance exercise yielded information on the function of the autonomic nervous system in addition to that obtained by resting measurements of HRV.