FRACTAL DIMENSION AND APPROXIMATE ENTROPY OF HEART PERIOD AND HEART-RATE - AWAKE VERSUS SLEEP DIFFERENCES AND METHODOLOGICAL ISSUES

1. Investigations that assess cardiac autonomic function include non-l inear techniques such as fractal dimension and approximate entropy in addition to the common time and frequency domain measures of both hear t period and heart rate. This article evaluates the differences in usi ng heart rate versus heart period to estimate fractal dimensions and a pproximate entropies of these time series. 2. Twenty-four-hour ECG was recorded in 23 normal subjects using Holter records. Time series of h eart rate and heart period were analysed using fractal dimensions, app roximate entropies acid spectral analysis for the quantification of ab solute and relative heart period variability in bands of ultra low (< 0.0033 Hz), very low (0.0033-0.04 Hz), low (0.04-0. 15 Hz) and high (0 .15-0.5 Hz) frequency. 3. Linear detrending of the time series did not significantly change the fractal dimension or approximate entropy val ues. We found significant differences in the analyses using heart rate versus heart period between waking up and sleep conditions for fracta l dimensions, approximate entropies and absolute spectral powers, espe cially for the power in the band of 0.0033-0.5 Hz. Log transformation of the data revealed identical fractal dimension values for both heart rate and heart period. Mean heart period correlated significantly bet ter with fractal dimensions and approximate entropies of heart period than did corresponding heart rate measures. 4. Studies using heart per iod measures should take the effect of mean heart period into account even for the analyses of fractal dimension and approximate entropy. As the sleep-awake differences in fractal dimensions and approximate ent ropies are different between heart rate and heart period, the results should be interpreted accordingly.