Pharmacodynamic modelling of the effect of remifentanil using the Pupillary Pain Index

Using a targeted controlled infusion of remifentanil during total intravenous anesthesia, we investigated the effect-site concentrations of remifentanil that correlate with different values of the Pupillary Pain Index and which concentrations were necessary for achieving a Pupillary Pain Index ≤ 4 and its usefulness in titrating opioids. The Pupillary Pain Index was measured in 54 patients prior to surgery under different remifentanil effect-site concentrations and subsequently modeled. One hundred and twenty-eight measurements were taken at different remifentanil concentrations while titrating propofol for a similar depth of hypnosis using a BIS monitor. Our modeled Hill equation revealed a remifentanil of 2.96 ng/mL for a PPI of 4, and the probability model a Ce of 3.22 ng/mL for the probability of 50% of patients achieving a PPI score ≤ 4. For the probability of 80% of patients achieving a PPI score ≤ 4 the Ce of remifentanil was 4.39 ng/mL. We conclude that concentrations of remifentanil that have been shown to suppress movement in response to noxious stimulation correspond to a Pupillary Pain Index ≤ 4.