A novel approach for comparison of heart rate variability derived from synchronously measured electrocardiogram and photoplethysmogram

Currently, heart Rate Variability (HRV) is derived from ECG. Many researchers have explored the usefulness of photoplethysmography (PPG), for the analysis of HRV. However, all these studies are based on statistical approach and used the correlation coefficient for comparing the two data sets obtained using ECG and PPG, which is inappropriate as the causal relationship between the R-peaks in ECG and the peaks in PPG is a physiologically established fact. In this study, the heart beat intervals measured from PPG, are compared, beat by beat, by taking the corresponding beat intervals of same cardiac cycle obtained from ECG and the measurement error is quantized. The error is found to be within 2% across the entire sample of 42 subjects, in spite of diversified constitution of the study group in terms of age, sex and pathology. Errors are also interpreted in terms of variations in pulse transit time. A very good agreement is found between the ECG and PPG derived parameters of HRV. In view of these findings, it may be concluded that PPG provides a safer and low cost option, in place of ECG, as a wearable sensor outside hospital environment, for the analysis of HRV, without compromising on accuracy.