Photoplethysmogram's Amplitude is Well Correlated with Beat-By-Beat Changes in Arterial Blood Pressure

Background: To describe the association of the amplitude of the photoplethysmographic waveform (PPG) with arterial pulse pressure (PP) and surrogates of vascular tone. Methods: In 26 anesthetized patients PPG amplitude, invasive arterial blood pressure, estimated vascular compliance (Cvasc) and Doppler resistive index (RI)/mean flow velocity (Vm) were recorded during changes in arterial blood pressure. These variables were analyzed as: 1) Group-averaged analysis: 20 beats were selected at different arterial pressure levels in each patient and then pooled together with data from all patients; 2) Individual analysis: PPG amplitude was correlated with PP and Cvasc beat-by-beat during ~ 5-10 minute recordings. Results: Group-averaged analysis included 111 episodes of arterial pressure variations. PPG amplitude decreased by 38% during hypertension (median 25 and 1st/3rd quartiles [24-41]%, p = 0.0001) and increased by 25% during hypotension (50[40-63]%, p = 0.0205) when compared to normotension (40[30-50]%). PP was higher (65[55-76] mmHg, p < 0.0001) and lower (30[27-39] mmHg, p < 0.0001) during hypertension and hypotension respectively when compared to normotension (44[38-51] mmHg). No statistical differences in RI were observed at different arterial pressure levels. At normotension, Vm was higher (20[14-25] cm/s) than during hypertension (18[10-26] cm/s; p = 0.0238) but lower compared to hypotension (21[15-26] cm/s; p = 0.0043). Beat by beat PPG amplitude was well correlated with PP (median rho = -0.84 [95% CI-0.98 to -0.73] and with CVasc (median rho = 0.96 [95% CI 0.99 to 0.85]; all p < 0.0001). Conclusions: Simple photoplethysmographic waveform analysis can provide reliable, real-time, non-invasive information regarding the status of the vascular tone helping to discriminate the cause of changes in arterial pressure in anesthetized patients.