Monitoring of respiratory rate in postoperative care using a new photoplethysmographic technique

Photoplethysmography (PPG) is a non-invasive optical technique that measures variations in skin blood volume and perfusion. The PPG signal contains components that are synchronous with respiratory and cardiac rhythms. We undertook this study to evaluate PPG for monitoring patients' respiratory rate in the postoperative care unit, using a new prototype device. We compared it with the established technique, transthoracic impedance (TTI).

PPG signals from 16 patients (ASA classes 1-2, mean age 43 years) who were recovering from general anaesthesia after routine operations were recorded continuously for 60 minutes/patient. The respiratory synchronous part of the PPG signal was extracted by using a bandpass filter. Detection of breaths in the filtered PPG signals was done both visually and by using an automated algorithm. In both procedures, the detected breaths were compared with the breaths detected in the TTI reference.

A total of 10,661 breaths were recorded, and the mean +/- SD respiratory rate was 12.3 +/- 3.5 breaths/minute. When compared with TTI, the rates of false positive and false negative breaths detected by PPG (visual procedure) were 4.6 +/- 4.5% and 5.8 +/- 6.5%, respectively. When using the algorithm for breath detection from PPG, the rates of false positive and false negative breaths were 11.1 +/- 9.7% and 3.7 +/- 3.8%, respectively, when compared to TTI. Lower respiratory rates increased the occurrence of false-positive breaths that were detected by the PPG using visual identification (p < 0.05). The same tendency was seen with the automated PPG procedure (p < 0.10).

Our results indicate that PPG has the potential to be useful for monitoring respiratory rate in the postoperative period.