Continuous Heart Rate Variability Monitoring Through Complex Demodulation Implemented with the Fast Fourier Transform and Its Inverse

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Abstract
A new method for the analysis of 24-hour heart rate variability (HRV) using complex demodulation (CDM) implemented with the fast Fourier transform (FFT) and its inverse is described. In a control group with palpitations and dizzy spells (n = 30, 47.2 +/- 16.7 years) the relationship between HRV parameters and subject age was investigated. CDM was used to obtain the amplitude and frequency of the low frequency (LF) and high frequency (HF) oscillations for 8 diurnal hours and 4 nocturnal hours. Differences between the two periods were seen in the LF/HF ratio (2.2 +/- 0.6 vs 1.5 +/- 0.6; P < 0.0001), HF amplitudes (12 +/- 6 vs 17 +/- 7 normalized units, P < 0.05), and in the mean frequency of the LF oscillations (0.078 +/- 0.008 vs 0.073 +/- 0.007 Hz, P < 0.01). During the daytime, age was inversely correlated to HF amplitude (r = -0.60), directly correlated to HF mean central frequency (r = 0.40), inversely correlated to LF amplitude (r = -0.55), and likewise inversely correlated to LF mean central frequency (r = -0.74, P < 0.001). At night, age was only inversely correlated to HF amplitude and to LF mean central frequency. Continuous HRV monitoring through CDM implemented with the FFT and its inverse differentiates the periods of diurnal activity and nocturnal rest as an expression of two different activity states of the autonomic nervous system. It allows nonstationary analysis, and separately provides mean and instantaneous oscillation amplitude and frequency. Subject age is not equally related to mean amplitude and frequency of a given oscillation.