Time and frequency domain analysis

The HRV time domain measures included the mean RR intervals (Mean RR), standard deviation of the RR intervals (SDNN), square root of the mean of sum of squares of the differences between adjacent RR intervals (RMSSD), and pNN50. The latter the proportion of the NN50 (successive RR intervals differing by more than 50 ms) divided by the total number of RR intervals in the ECG recordings. The SDNN provides information about all the components contributing to the HRV during the recording period and is, therefore, a fairly global measure of HRV. The RMSSD and pNN50 reflects the cardiac parasympathetic control of the heart rate^22–24. Fast Fourier transform was used to calculate the four main spectral components for the total power (TP) for the frequency range 0.0033–0.40 Hz; the very low frequency power (VLF) for the frequency range 0.0033–0.04 Hz; the low frequency power (LF) for the frequency range 0.04–0.15 Hz and the high frequency power (HF) for the frequency range 0.15–0.40 Hz. The VLF component is a major determinant of the physical activity and possibly reflects sympathetic activity, though its origin remains controversial²². The LF component reflects both sympathetic and parasympathetic control of the heart rate, while the HF component is generally interpreted as a marker of vagal activity and is respiration mediated²². The ratio of LF to HF (LF/HF) reflects the global sympatho-vagal balance or reflects the sympathetic activity²².