Online exhaled propofol monitoring in normal-weight and obese surgical patients

Background Ion mobility spectrometry (IMS) allows for online quantification of exhaled propofol concentrations. We aimed to validate a bedside online IMS device, the Edmon(R), for predicting plasma concentrations of propofol in normal-weight and obese patients. Methods Patients with body mass index (BMI) >20 kg/m(2) scheduled for laparoscopic cholecystectomy or bariatric surgery were recruited. Exhaled propofol concentrations (C-A), arterial plasma propofol concentrations (C-P) and bispectral index (BIS) values were collected during target-controlled infusion (TCI) anaesthesia. Generalised estimation equation (GEE) was applied to all samples and stable-phase samples at different delays for best fit between C-P and C-A. BMI was evaluated as covariate. BIS and exhaled propofol correlations were also assessed with GEE. Results A total of 29 patients (BMI 20.3-53.7) were included. A maximal R-2 of 0.58 was found during stable concentrations with 5 min delay of C-A to C-P; the intercept a = -0.69 (95% CI -1.7, 0.3) and slope b = 0.87 (95% CI 0.7, 1.1). BMI was found to be a non-significant covariate. The median absolute performance error predicting plasma propofol concentrations was 13.4%. At a C-A of 5 ppb, the model predicts a C-P of 3.6 mu g/ml (95% CI +/- 1.4). There was a maximal negative correlation of R-2 = 0.44 at 2-min delay from C-A to BIS. Conclusions Online monitoring of exhaled propofol concentrations is clinically feasible in normal-weight and obese patients. With a 5-min delay, our model outperforms the Marsh plasma TCI model in a post hoc analysis. Modest correlation with plasma concentrations makes the clinical usefulness questionable.