Real-time monitoring of propofol in expired air in humans undergoing total intravenous anesthesia

Background: The physicochemical properties of propofol could allow diffusion across the alveolocapillary membrane and a measurable degree of pulmonary propofol elimination. The authors tested this hypothesis and showed that propofol can be quantified in expiratory air and that propofol breath concentrations reflect blood concentrations. This could allow real-time monitoring of relative changes in the propofol concentration in arterial blood during total intravenous anesthesia. Methods: The authors measured gas-phase propofol using a mass spectrometry system based on ion-molecule reactions coupled with quadrupole mass spectrometry which provides a highly sensitive method for on-fine and off-fine measurements of organic and inorganic compounds in gases. In a first sequence of experiments, the authors sampled blood from neurosurgery patients undergoing total intravenous anesthesia and performed propofol head-space determination above the blood sample using an auto-sampler connected to the mass spectrometry system. In a second set of experiments, the mass spectrometry system was connected directly to neurosurgery patients undergoing target-controlled infusion via a T piece inserted between the endotracheal tube and the Y connector of the anesthesia machine, and end-expiratory propofol concentrations were measured on-line. Results: A close correlation between propofol whole blood concentration and propofol headspace was found (range of Pearson r, 0.846-0.957; P < 0.01; n = 6). End-expiratory propofol signals mirrored whole blood values with close intraindividual correlations between both parameters (range of Pearson 0.784-0-985; n = 11). Conclusion: Ion-molecule reaction mass spectrometry may allow the continuous and noninvasive monitoring of expiratory propofol levels in patients undergoing general anesthesia.