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(®), 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 μg/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.