Investigations using logistic regression models on the effect of the LMA on morphine induced vomiting after tonsillectomy

The effect of intraoperative airway management on postoperative vomiting af ter tonsillectomy is unknown. Logistic regression was used in a retrospecti ve study to investigate the effect of the laryngeal mask airway (LMA) on a morphine dose-vomiting response curve. Charts were reviewed in 351 children in whom the airway was managed with either a LMA (n = 177) or a tracheal t ube (n = 174). A mean perioperative morphine dose of 0.10 mg.kg(-1) (SD 0.0 9) was used in 248 children and a further 103 children were given no opioid . One hundred and eighteen of these 248 children vomited (47.6%) compared t o 14 of 103 children given no morphine (13.6%). The probability of vomiting was related to morphine dose using logistic regression with both a linear and an E-max model. Both the calibration (Hosmer-Lemishow goodness of fit c hi-squared test lambda (2), P = 0.81) and discrimination (area under the re ceiver operating characteristic plot, AUC ROC = 0.67) of the E-max model we re better than the linear model (lambda (2), p = 0.49; AUC ROC = 0.64). Pha rmacodynamic parameter estimates for the E-max model were P-o (the baseline probability of vomiting) 0.139, P-max (the maximal probability of vomiting due to morphine) 0.96, ED50 (morphine dose that induces an effect equivale nt to 50% of the logit P-max) 0.09 mg.kg(-1). The probability of vomiting w as 50% after morphine 0.125 mg.kg(-1). The use of the LMA had no effect on this dose-response curve. A covariate analysis investigating propofol for i nduction or isoflurane for the intraoperative maintenance of anaesthesia, h owever, showed that both drugs shifted the curve to the right. The probabil ity of vomiting was 50% after morphine 0.17 mg.kg(-1) and 0.21 mg.kg(-1) fo r the isoflurane and propofol use curves, respectively. The concomitant use of propofol and isoflurane, but not the use of the LMA, decreases the prob ability of vomiting due to morphine.