A comparative study of photoplethysmogram and piezoelectric plethysmogram signals

The Photoplethysmogram (PPG) signal is one of the most important vital signals in biomedical applications. The non-invasive property and the convenience in the acquisition of both PPG and Piezoelectric Plethysmogram (PZPG) signals are considered as powerful and accurate tools for biomedical diagnosing applications, such as oxygen saturation in blood, blood flow, and blood pressure measurements. In this paper, a number of features for PPG and PZPG signals (ex. first derivative, second derivative, area under the curve and the ratio of systolic area to the diastolic area) are acquired and compared. The results show that both systems are able to extract the pulse rate (PR) and pulse rate variability (PRV), accurately with an estimation error of less than 10%. The averaged standard deviation of the ratio of the systolic area to the diastolic area for the first derivative of PPG and PZPG signals was small with less than 0.49 and 0.69 for the PPG and PZPG, respectively. Statistical analysis techniques (such as cross-correlation, P-value test, and Bland Altman method) are performed to address the relation between the PPG and PZPG signals. All of these methods showed a strong relationship between the features of the two signals (i.e. PPG and PZPG). The correlation value is found to be 0.954 with a p-value of < 0.05. This opens possibilities for combining both the PPG and PZPG systems to extract more features that can be used in diagnosing cardiovascular diseases. Such a system can provide a possibility to reduce the number of devices connected to patients (especially in emergencies) by means of measuring simultaneously both signals (PZPG and PPG).