Accurate Calculation of Heart Period and Pulse Wave Transit Time

Abstract

The first step in processing of physiological signals is usually the improvement of the signal-to-noise ratio. However, the applicable filtering methods not only suppress noise but also distort the signal. As a result, parameters derived from filtered physiological signals are also distorted. The paper evaluates filtering methods applied to electrocardiographic (ECG) and photoplethysmographic (PPG) signals in the process of calculating heart period (HP) and pulse wave transit time (PWTT). Band-pass filtering applied in the course of QRS detection causes a shift in R-peak location depending on the QRS waveform. Following QRS detection accurate R-peak localization is suggested using re-filtering of the original signal. The HP was calculated as the time interval between two adjacent R-peaks and also as the time interval between two adjacent local minima in PPG (PPGmin). Both real and simulated data were used for the evaluation. In the latter case the fiducial points of the signals (ECG R-peak and PPGmin locations) were exactly known providing reference positions for the evaluation of filter distortion. Inappropriate filtering shifts the fiducial points. This causes a significant error while calculating parameters by subtraction, like RR interval of heart cycles (t_RR, difference of two consecutive R-peak locations) or PWTT (difference of PPGmin and the associated R-peak location). Even more accurate R-peak and PPGmin localization is needed when variation in t_RR and PWTT is analyzed. These variations characterize the actual stress level of a person before or during blood pressure measurement more accurately than average heart rate displayed by commercially available blood pressure monitors. Based on accurate R-peak and PPGmin localization it has been confirmed that for the same heart cycle the time interval is different between two R-peaks and two PPGmins.