Changes in a surgical stress index in response to standardized pain stimuli during propofol-remifentanil infusion

Background. The surgical stress index (SSI) is based on a sum of the normalized pulse beat interval (PBI) and the pulse wave amplitude (PPGA) time series of the photoplethysmography. As a measure of the nociception-anti-nociception balance in response to a standardized pain stimulus, SSI was compared with EEG changes in state and response entropy (SE and RE), PPGA, and heart rate (HR) during various targeted pseudo-steady-state concentrations of propofol and remifentanil. Methods. Forty ASA I patients were allocated to one of the four groups to receive a remifentanil step-up/-down effect-compartment target-controlled infusion (Ce-remi) of 0, 2, 6, 2, 0 ng ml(-1), or 6, 2, 0, 2, 6 ng ml(-1), and an effect-compartment target-controlled propofol infusion (Ce-prop) to keep the SE between 30 and 50 or 15 and 30, respectively. At each steady-state Ce-remi, maximum change in SSI, SE, RE, PPGA, and HR after a noxious stimulus was compared with the baseline value. A correlation and prediction probability (PK) with Ce-prop and Ce-remi, was measured. Results. Static and dynamic values of SSI correlated to Ce-remi better than SE, RE, HR, and PPGA. SSI was independent of Ce-prop, in contrast to SE and RE. The PK for Ce-remi both before and during a noxious stimulus was better with SSI. Conclusions. SSI appeared to be a better measure of nociception-anti-nociception balance than SE, RE, HR, or PPGA