Photoplethysmography and nociception

Photoplethysmography (PPG), i.e. pulse oximetric wave, is a non-invasive technique that is used in anaesthesia monitoring primarily to monitor blood oxygenation. The PPG waveform resembles that of the arterial blood pressure but instead of pressure it is related to the volume changes in the measurement site and hence contains information related to the peripheral blood circulation, including skin vasomotion, which is controlled by the sympathetic nervous system. Because of this link, skin vasomotor response and PPG amplitude response have been associated with nociception under general anaesthesia. Recently, there has been interest in monitoring nociception during general anaesthesia. In many of the published studies, PPG waveform information has been included. The focus of this topical review is to provide an overview on the information embedded in the PPG waveform especially in the context of the autonomic nervous system and analgesia monitoring.