Similarity in bilateral photoplethysmographic peripheral pulse wave characteristics at the ears, thumbs and toes

The characteristics of the photoplethysmography (PPG) pulse signal are body site specific, with pulses from the various peripheral sites showing differences in pulse transit time, strength and shape, and variation of each over time. The aim of this study was to determine the similarity in the right-left pulse characteristics for a group of normal subjects with pulses obtained simultaneously From six peripheral sites (ears, thumbs and big toes). A multi-site photoplethysmography pulse measurement and analysis system is described and comprised six sets of pulse probes and amplifiers, arranged and electronically matched in pairs for the right and left side comparisons. Two sets of data were collected: firstly a set of validation data to test the electronic matching of right-left channels, and secondly a set of physiological data for pulses from 40 normal subjects. The right-to-left side similarity in pulse waveform shape at the three segmental levels (ears, thumbs and toes) was assessed using two types of analysis: root mean square error (RMSE) providing a measure of differences, and cross correlation analysis providing a measure of the degree of similarity. Very low levels of RMSE and correlation coefficients close to unity were obtained for the system validation data, demonstrating good right-left channel matching. The RMSE was an order of magnitude lower than that calculated for the normative physiological data, where median RMSE levels were between 0.053 and 0.060 relative to a peak-to-peak pulse amplitude of unity. The median correlation coefficients were greater than 0.98 for all three segmental levels studied, with the maximum values approaching those of the validation data. We have shown that pulses from the right and left sides of normal subjects are highly correlated at each segmental level. We have obtained a normative range of pulse data, with which specific (vascular) patient groups can be compared.