A Comparison of HRV Techniques: The Lomb Periodogram versus The Smoothed Pseudo Wigner-Ville Distribution

The purpose of a tachogram (from the Greek tachos, speed) is to quantify changes in heart rate over short time periods, ordinarily less than one day. Typically, the heart's rate of contraction is sampled once per beat (and hence unevenly in time), being taken as the inverse of the interval which occurs between consecutive QRS complexes (known as an RR interval). Since the Fourier transform algorithm requires even sampling, the tachogram is often re-sampled using interpolation. However, this approach introduces new problems: for example, linear interpolation is a poor approximation, and cubic splines create unacceptable oscillations when one RR interval is unusually longer than its predecessor. Perhaps more importantly for cross-correlation with other signals, such as instantaneous blood pressure, the process of assigning the RR interval to one end of that interval (either at the beginning or end) creates a tachogram out of phase with the true heart rate.