Effect of a fluid challenge on the Surgical Pleth Index during stable propofol-remifentanil anaesthesia

Background The Surgical Pleth Index (SPI), derived from pulse amplitude and heartbeat interval, is proposed to monitor anti-nociception during anaesthesia. Its response to noxious stimulation can be affected by the intravascular volume status. This study investigated the effect of a fluid challenge (FC) on SPI during steady-state conditions. Methods After Institutional Review Board approval, 33 consenting patients undergoing neurosurgery received a 4?ml/kg starch FC over less than 5?min under stable surgical stimulation conditions and stable propofol (CePPF) and remifentanil (CeREMI) effect-site concentrations as estimated by target-controlled infusion systems. Intravascular volume status was assessed using the Delta Down (DD). We looked at the SPI response to FC according to DD, CePPF, and CeREMI. Results Following FC, SPI did not change in 16, increased in 12, and decreased in 3 patients. CeREMI poorly affected the SPI response to FC. In normovolaemic patients, the probability of an SPI change after FC was low under common CePPF (0.9 to 3.9 mu g/ml). A decrease in SPI was more probable with worsening hypovolaemia and lowering CePPF, while an increase in SPI was more probable with increasing CePPF. SPI changes were only attributable to modifications in pulse wave amplitude and not in heart rate. Conclusions During stable anaesthesia and surgery, SPI may change in response to FC. The effect of FC on SPI is influenced by volaemia and CePPF through pulse wave amplitude modifications. These situations may confound the interpretation of SPI as a surrogate measure of the nociceptionanti-nociception balance.