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

Ion mobility spectrometry (IMS) allows for online quantification of exhaled propofol concentrations. We aimed to validate a bedside online IMS device, the Edmon^® , for predicting plasma concentrations of propofol in normal-weight and obese patients.

Patients with body mass index (BMI) >20 kg/m² 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.

A total of 29 patients (BMI 20.3-53.7) were included. A maximal R² 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 μg/ml (95% CI ±1.4). There was a maximal negative correlation of R²  = 0.44 at 2-min delay from C_A to BIS.

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.