Phase locked loop algorithm of longterm ECG induces low frequency oscillations in heart rate variability spectra

Spectral analysis of heart rare variability (HRV) becomes a promising tool in detecting patients with high risk for sudden death. The calculations are based on the exact evaluation of the QRS distances from beat to beat. Phase locked loop technique should avoid errors from speed variations of the tape recorder. In the authors' investigation they compared spectra of QRS interval tachograms (RR-tachograms) from Holter recordings (ELATEC Holter System) with spectra calculated from directly stored ECG signals. The authors calculated the HRV spectra for healthy volunteers, for patients with proven coronary artery disease and for patients after heart transplantation with very low HRV. The spectral densities from Holter compared with direct recordings showed no differences for healthy volunteers, but for patients with low HRV who showed significant differences in the LF region (0.05-0.15 Hz). Only in the spectra of the Holter ECG system using a phase locked loop technique systemic oscillations around 0.125 Hz were present.<>