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Nonstationary analysis of heart rate variability at rest and during exercise

The heart rate variability (HRV) has been used to study the cardiovascular regulation. During incremental exercise, a nonstationary technique is required. We propose the use of an undecimated, 4 voices-wavelet transform. HRV and respiratory amplitude recordings of five healthy young subjects were analyzed at supine rest and during incremental exercise in a cycloergometer. For the supine rest experiments, the scalograms showed high energy levels in the high-frequency band corresponding to the respiratory sinus arrhythmia. Some degree of nonstationarity could be observed. For the incremental exercise, the low-band increased its energy with the load. Only in the recovery the high-frequency components were observable. The wavelet representation allowed the analysis of the variables in a constantly changing environment.