Probability to tolerate laryngoscopy and noxious stimulation response index as general indicators of the anaesthetic potency of sevoflurane, propofol, and remifentanil

Background: The probability to tolerate laryngoscopy (P-TOL) and its derivative, the noxious stimulation response index (NSRI), have been proposed as measures of potency of a propofol-remifentanil drug combination. This study aims at developing a triple drug interaction model to estimate the combined potency of sevoflurane, propofol, and remifentanil in terms of P-TOL. We compare the predictive performance of P-TOL and the NSRI with various anaesthetic depth monitors.

Methods: Data from three previous studies (n=120) were pooled and reanalysed. Movement response after laryngoscopy was observed with different combinations of propofol-remifentanil, sevoflurane-propofol, and sevoflurane-remifentanil. A triple interaction model to estimate P-TOL was developed. The NSRI was derived from P-TOL. The ability of P-TOL and the NSRI to predict observed tolerance of laryngoscopy (TOL) was compared with the following other measures: (i) effect-site concentrations of sevoflurane, propofol, and remifentanil (Ce-SEVO, Ce-PROP, and Ce-REMI); (ii) bispectral index; (iii) two measures of spectral entropy; (iv) composite variability index; and (v) surgical pleth index.

Results: Sevoflurane and propofol interact additively, whereas remifentanil interacts in a strongly synergistic manner. The effect-site concentrations of sevoflurane and propofol at a P-TOL of 50% (Ce50;SE) were 2.59 (0.13) vol % and 7.58 (0.49) mu g ml(-1). ACe(REMI) of 1.36 (0.15) ng ml-1 reduced the Ce50 of sevoflurane and propofol by 50%. The common slope factor was 5.22 (0.52). The P-TOL and NSRI predict the movement response to laryngoscopy best.

Conclusions: The triple interaction model estimates the potency of any combination of sevoflurane, propofol, and remifentanil expressed as either P-TOL or NSRI.