Comparison of haemodynamic- and electroencephalographic-monitored effects evoked by four combinations of effect-site concentrations of propofol and remifentanil, yielding a predicted tolerance to laryngoscopy of 90%

This prospective study evaluates haemodynamic and electroencephalographic effects observed when administering four combinations of effect-site concentrations of propofol (Ce_PROP) and remifentanil (Ce_REMI), all yielding a single predicted probability of tolerance of laryngoscopy of 90% (P_TOL = 90%) according to the Bouillon interaction model. We aimed to identify combinations of Ce_PROP and Ce_REMI along a single isobole of P_TOL that result in favourable hypnotic and haemodynamic conditions. This knowledge could be of advantage in the development of drug advisory monitoring technology. 80 patients (18–90 years of age, ASA I–III) were randomized into four groups and titrated towards Ce_PROP (Schnider model, ug⋅ml⁻¹) and Ce_REMI (Minto model, ng⋅ml⁻¹) of respectively 8.6 and 1, 5.9 and 2, 3.6 and 4 and 2.0 and 8. After eleven minutes of equilibration, baseline measurements of haemodynamic endpoints and bispectral index were compared with three minutes of responsiveness measurements after laryngoscopy. Before laryngoscopy, bispectral index differed significantly (p < 0.0001) between groups in concordance with Ce_PROP. Heart rate decreased with increasing Ce_REMI (p = 0.001). The haemodynamic and arousal responses evoked by laryngoscopy were not significantly different between groups, but Ce_PROP = 3.6 μg⋅ml⁻¹ and Ce_REMI = 4 ng⋅ml⁻¹ evoked the lowest median value for ∆HR and ∆SAP after laryngoscopy. This study provides clinical insight on the haemodynamic and hypnotic consequences, when a model based predicted P_TOL is used as a target for combined effect-site controlled target- controlled infusion of propofol and remifentanil. Heart rate and bispectral index were significantly different between groups despite a theoretical equipotency for P_TOL, suggesting that each component of the anaesthetic state (immobility, analgesia, and hypnotic drug effect) should be considered as independent neurophysiological and pharmacological phenomena. However, claims of (in)accuracy of the predicted P_TOL must be considered preliminary because larger numbers of observations are required for that goal.