Propofol concentration in exhaled air and arterial plasma in mechanically ventilated patients undergoing cardiac surgery

Measuring propofol concentration in plasma (cPPL) and in exhaled alveolar gas (cPG) during constant infusion provides information about their respective time courses. In the present study, we compared these time courses in patients undergoing cardiac surgery from the beginning of propofol anaesthesia until eye opening upon awakening. The cPG was measured before, during, and after continuous infusion of propofol for general anaesthesia in 12 patients at two randomly allocated doses (3 or 6 mg kg−1 h−1). Gas samples were collected on Tenax tubes. After thermodesorption, cPG was measured by gas chromatography mass spectrometry. Simultaneously with exhaled gas, arterial blood was sampled for measuring cPPL by reversed-phase high-performance liquid chromatography with fluorescence detection. In order to compare the time courses of cPPL and cPG as dimensionless values directly, each gas and plasma value was normalized by relating it to the corresponding value at the end of the initial infusion after 40 min. The cPG ranged between 2.8 and 22.5 ppb, whereas the corresponding cPPL varied between 0.3 and 3.3 μg ml−1. Normalized concentration values showed a delayed increase in cPG compared with cPPL under constant propofol infusion before the onset of cardiopulmonary bypass, and a delayed decrease after stopping the propofol at the end of anaesthesia. Propofol can be measured in exhaled gas from the beginning until the end of propofol anaesthesia. The different time courses of cPPL and cPG have to be considered when interpreting cPG.