Wavelet transform: A better approach for the evaluation of instantaneous changes in heart rate variability

The aim of our study was to validate the Vaidyanathan wavelet tool for HRV analysis during orthostatic testing. Two groups of normotensive male subjects were studied: 13 adolescents and 27 young adults. Both groups consisted of subjects with negative, (N-), and with positive family history for hypertension, (N+). These subjects underwent 5-minute active standing upright, preceded and followed by 5-minute periods in supine position. Continuous electrocardiogram (ECG) was recorded and HRV indices were calculated using wavelet (WT) and fast Fourier transform (FFT) simultaneously. WT and FFT data showed high level of correlation (>0.9). Due to its inherent properties, WT proved to be more informative than FFT in the analysis of the non-stationary ECG signal during orthostatic testing. WT revealed HRV dynamics more accurately since it allowed HRV evaluation for shorter intervals (60 s) than FFT (256 s). During the initial and recovery period lower parasympathetic activity (P < 0.0001; P < 0.02) and higher ratio of autonomic balance (sympathetic vs. parasympathetic) (P < 0.0001; P < 0.02) were evidenced in (N+) as compared to (N-). The upright posture was accompanied by a prompt decrease in HRV and by an elevation of the index of autonomic balance. These alterations were more pronounced in N(+). In conclusion, we believe that wavelet analysis is an appropriate approach for the estimation of HRV dynamics in non-stationary conditions. Furthermore, we demonstrate certain essential alterations in the autonomic modulation of the cardiovascular system in young normotensives with positive family history for essential hypertension.