The amplitude (AMP) of Photoplethysmogram (PPG) is used as a marker of vasodilatation. The pulse transit time (PTT), which shows a good correlation with blood pressure (BP), is not strong enough to detect the changes in BP. This study examined the sensitivity of the combined effect of the finger and toe AMP, and the PTT of PPG as a marker of the changes in BP during general anesthesia.

Forty patients receiving maxillofacial surgery under general anesthesia were enrolled in this study. During surgery, the intra-arterial BP, ECG, finger and toe PPG signals were measured. Using the R-wave from the ECG, the AMP and PTT was derived from PPG data. The correlation between BP and PPG parameters (AMP and PTT) were compared. New parameters that show high correlation with the BP were found. Regression equations for calculating the BP using the PPG parameters were formulated.

The new parameter, log (fingerAMP/toeAMP), showed the highest correlation in each patient (mean correlation coefficient in the systolic BP: -0.846, diastolic BP: -0.858). However, when the data from all 40 patients were combined, the correlation coefficient of the toe PTT was highest (systolic BP: -0.726, diastolic BP: -0.646). The regression equation showed the highest correlation between the actual BP and calculated BP when the toe PTT and log (fingerAMP/toeAMP) were included.

The AMP of the toe and finger PPG can be used to estimate the invasive continuous blood pressure.