Monitoring: Equipment and Computers
Background and Goal of Study: There have been several studies published about the presence of propofol particles in exhaled air. However, it is not clear whether this technique can be reliable and reproducible as to have a clear impact on research or clinical practice. In the past years we have been working on improving the methodology and optimizing the results, improving sampling and data collection to increase the sensitivity and accuracy. A LabView (National Instruments) application developed allows the connection of the infusion pumps, vital signs monitor, BIS and PTR-MS (QMS Ionicon High Sensitivity Proton Transfer Reaction Mass Spectrometer), which allows automatic real-time data collection. We have now developed a new sampling cannula of low absorbent material (PEEK) which introduced into the oro-tracheal tube allows taking the sample. Simultaneously, the sampling system has been improved by heating it and including a micro valve that allows air sampling, exclusively on the expiratory phase.
Materials and Methods: 300 patients, 18-60 years old both sexes ASA I II, scheduled for surgery under general anesthesia were involved. Vital signs, TCI parameters and the propofol concentration (178+1 amu), acetone (58+1 amu) and isoprene (68+1 amu) in expired air are recorded. Propofol concentrations in expired air are being compared with the plasmatic concentration and effect offered by the TCI, as well as its correlation with BIS.
Results and Discussion: With the improvements introduced, the exhaled propofol can now be monitored with a reproducible method, in which variations in the propofol infusion generate changes in exhaled propofol concentration. In the preliminary results, these changes correlate with all plasma concentration, effect concentration and BIS. Preliminary results reveal that the average concentrations of propofol in air are of 48ppb for Plasmatic TCI concentrations of 2.5 mcg/ml, 55ppb for 3mcg/ml and 68ppb for 4mcg/ml we will have to wait for the completion of the study to offer more consistent and definitive results.
Conclusion(s): Improvements introduced in the sample system together with the automation of data collection, allow us to perform studies in large series of patients with reproducibility and accuracy. If the results are confirmed, it could be possible to use this technique as a non invasive propofol monitoring. It would also lead to think that, in the future, a propofol pharmacokinetic model of the lung could be defined.