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Cardiac rhythm variability (CRV) studies are wide ly used in medicine. Spectral CRV composition is important for medical practice. CRV signals are usually used to measure CRV spectrum. In practice CRV signals are:-rhythmocardiograms (RCG) [1 3] (graphical presentation of RR interval duration of ECG signal; abscissa is time; ordinate is RR interval duration); -cardiointervalograms (CIG) [4] (abscissa is RR interval number; ordinate is RR interval duration); CIG signal is rarely used because its spectrum is an inverse function of readings rather than frequency;-tachograms of heart rate (THR) [5] (abscissa is time; ordinate is momentary heart rate (HR) (inverse RR interval time value)).CRV realizations using other signals are also described in the literature. Most CRV realizations using other signals were introduced to maximize the model of cardiac rhythm modulation by integral pulse frequency modulation (IPFM) [6 8].CRV spectra are now usually estimated using RCG and THR signals. The literature gives virtually no theo retical-experimental analysis of convergence of heart rate variability spectral measures estimated via heart rate and heart period signals (spectral analysis, window type, etc.). Correlation between Tachograms of Heart Rate and RhythmocardiogramsCorrelation between RCG and THR is nonlinear:where HP (t) is RCG value; k is coefficient of signal dimension (60,000 factor between msec and beat/min), HR(t) is THR value. The simplest RCG model is described in [13]:where HP(t) is continuous model RCG signal, t is record ing time moment, f 1 is cardiac rhythm harmonic frequen cy, A is oscillation amplitude, and HP 0 is mean period of cardiac contraction. THR signal corresponding to RCG signal is: Comparative Analysis of Tachogram of HR and RhythmocardiogramComplete analytical analysis of amplitude spectrum of periodic signal described by Eq. (3) (Fourier series coef ficients) requires integration of the following equation:where A n is amplitude of harmonic n of RCG signal (fre quency nf 1 ), Т is signal period (f 1 = 1/T), and s(t) is trans formed signal. Numerical simulation of signal spectrum is rather simple. However, simulation results should meet strict requirements.

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