Second derivative photoplethysmographic signal analysis of differences between fingers in healthy subjects

To prevent and diagnose cardiovascular diseases, it is important to detect the increase in arterial stiffness. According to previous studies, alterations in the amplitudes of the second derivative photoplethysmographic (SDPPG) signal distinctive wave peaks 'a', 'b', 'c', 'd', and 'e' can be related through the regression models to the cardiovascular age of the subject. However, it is not known, if these models are applicable bilaterally and how the SDPPG signal amplitudes are influenced by the measurement location. This study focuses on amplitude differences in the SDPPG signal distinctive wave peaks between the left and the right hand fingers. The study was conducted on 30 subjects who had no disease diagnosed. The photoplethysmographic (PPG) signal was registered from the left and right hand fingers using 8 channel analog front-end and identical finger-clip PPG sensors for the synchronous registration of the signals. The amplitudes of the SDPPG signal distinctive wave peaks were detected from each period of the recorded PPG signals. It was found that the normalized amplitudes b/a from the left hand fingers were significantly higher $(P \lt 0.03)$ and c/a were significantly lower than those of the right hand fingers. In conclusion, the regression models for cardiovascular age estimation are bilaterally inapplicable at normalized amplitudes b/a and c/a. Further studies have to address the causes of the bilateral differences.