The effects of exercise on heart rate variability in healthy subjects and clinical populations
Heart rate variability (HRV) is a sensitive, non-invasive measure of cardiac autonomic modulation and a known risk factor for cardiac event and death in certain populations. The acute effects of exercise are to drastically alter HRV. To a degree, the changes tend to reflect the known rearrangement of autonomic control which occurs during exercise. Data concerning chronic effects of exercise training on HRV are mixed. However, some indication of increases in vagally modulated measures exists. The aim of this thesis was to examine the acute and chronic effects of exercise on short-term measures of HRV in healthy subjects and clinical populations. Initial investigations assessed the agreement and reliability of different, commercially available measurement systems for short-term HRV. The initial literature review concluded that data were mixed concerning the reliability of short-term measurement. The consequent chapters and papers found that agreement was dependent on the systems used and that reliability was fair under only certain physiological conditions. Reviewing the literature concerning the chronic effects of exercise on HRV also provided mixed results. However, a meta-analysis of these data confirmed a strong, highly significant main effect for increased global (SDNN) vagal (HF) measures of HRV. This effect was modulated by: subject age, sex and the length of the exercise intervention. Empirical work on healthy adults showed that the application of recommended HRV measures to exercise ECG data was limited mainly to lower intensities. Alternative measurement techniques were also employed with only moderate success. Prior to this thesis, little was known about the autonomic function of patients with peripheral artery disease (PAD). A randomised controlled trial of exercise training in 52 PAD patients was successfully increasedmaximal walking time in patients on a supervised walking program but did not alter any resting HRV measures. However, HRV measured during exercise sessions, demonstrated significant increases in global and vagally mediated frequency domain measures. Changes in HRV in cardiac rehabilitation patients have been examined fairly thoroughly in the past but baseline measures are usually made very soon after MI or surgery. This study into the effects of eight weeks of cardiac rehabilitation on HRV measures showed that measurescould be positively affected in patients with a delayed entry to cardiac rehabilitation. Indeed, all expected resting HRV measures increased significantly demonstrating increased vagal modulation of heart rate. In both PAD and post-MI patients, HF power was found to be a significant predictor of improvement in exercise performance. Previously this relationship has only been shown in healthy subjects. In conclusion, HRV is a significant risk factor in a number of populations. Exercise is capable or modifying HRV in certain cases. HRV should, therefore, be viewed as a therapeutic target.