The reduction in heart rate (HR) during the first minute of recovery immediately after a graded maximal exercise stress test (GXT) has recently been found to be a powerful and independent predictor of cardiovascular and all-cause mortality. Reduced vagal activity has been postulated as the cause, but this has not been proven in a population with slow HR recovery (HRR). Purpose: To investigate autonomic contributions to HRR using time-frequency analysis in a group of individuals demonstrating slow HRR. Methods: HRR was defined as the difference in HR between peak exercise and 1 min later; a value <= 18 bpm was set as threshold and considered abnormal. A modified continuous wavelet transform (CWT) was used to perform time-dependent spectral analysis during the baseline steady state and the following non-steady-state conditions created by GXT. This method provides dynamic measures of low-frequency (LF) and high-frequency (HF) peaks associated with autonomic activity. Individuals (N = 20) with a previous slow HRR underwent a second GXT within 3 months after their initial test. An additional eight subjects whose first GXT disclosed normal HRR were taken as a control group. Results: Seven of 20 subjects demonstrated slow HRR (14 +/- 5 bpm) on the repeat test, and 13 subjects displayed normal HRR (29 +/- 5 bpm). Subjects with slow HRR in both GXT displayed significantly (P < 0.05) lower HF and LF fluctuations during recovery than those with normal HRR. Conclusions: Attenuated HRR after GXT, assessed by CWT, is indeed associated with abnormal vagal reactivation and prolonged sympathetic stimulation after termination of maximal exercise.
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