Response surface model predictions of wake-up time during scoliosis surgery

With the use of previously published data, new sevoflurane-remifentanil interaction models of various degrees of sedation were created and adapted to desflurane-fentanyl by using minimal alveolar concentration and opioid equivalencies. These models were used to predict return of responsiveness in patients undergoing scoliosis surgery during a wake-up test. Our hypothesis was that one of the interaction models would accurately predict return of responsiveness during a wake-up test.

Three new sevoflurane-remifentanil interaction models were constructed from previous observations in volunteers by using the Observer's Assessment of Alertness/Sedation (OAA/S) scores. These models included predictions of OAA/S<2 (unresponsive), OAA/S< 3, and OAA/S<4 (sedation). Twenty-three patients scheduled for scoliosis surgery received a fentanyl-desflurane anesthetic. With the use of published pharmacokinetic models, predictions of fentanyl and desflurane effect-site concentrations were recorded throughout surgery and converted to equivalent remifentanil and sevoflurane effect-site concentrations. Data were recorded every 10 seconds from the time when desflurane was turned off until 10 minutes after the patients responded by moving their hands and toes. Model predictions were compared with observations with graphical and temporal analyses.

The average difference between the time when a patient first responded and the time when the model predicted that there was a 50% probability that the patient would respond were -2.6 ± 3.6 minutes (mean ± SD) for the OAA/S<2 model, 2.8 ± 5.6 minutes for the OAA/S<3 model and 52.6 ± 32.3 minutes for the OAA/S<4 model.

The results confirmed our study hypothesis; a sevoflurane-remifentanil interaction model built from observations in volunteers and adapted to desflurane and fentanyl accurately predicted patient response during a wake-up test. These results were similar to our previous study comparing model predictions and patient observations after a sevoflurane-remifentanil/fentanyl anesthetic. The OAA/S <2 model most accurately predicted the time patients would respond by moving their fingers and toes. This model may help anesthesiologists better predict return of responsiveness during a wake-up test in patients undergoing spine surgery.