Infrared analysis of volatile anesthetics: impact of monitor agent setting, volatile mixtures, and alcohol.

Infrared analysis can determine exhaled concentrations of the three volatile anesthetics in common use because each absorbs infrared light. Many infrared analyzers use a single source of infrared light at a wavelength of 3.3 microns for measurements of all three agents but cannot identify which agent is in use. Organic gases such as ethanol also absorb infrared light. This study determined the effects on the accuracy of a single-wavelength infrared anesthetic monitor (Capnomac, Puritan-Bennett PB254) of (1) incorrect anesthetic agent setting, (2) mixtures of volatile anesthetics, and (3) ethanol vapor in the analyzed gas. Changing the agent setting on the monitor during steady-state delivery of an agent resulted in readings for the halothane setting five times higher than those for the enflurane setting, and six times higher than those for the isoflurane setting. These ratios reflect the monitor's fixed internal gain for each agent setting. Mixtures of anesthetics had a simple additive effect on the monitor's accuracy. With the monitor set for halothane, 0.2 vol% isoflurane mixed with halothane caused readings 1.2 vol% higher than the true halothane concentration. Conversely, with the monitor set for isoflurane, 1 vol% halothane mixed with isoflurane resulted in readings 0.2 vol% too high. In a model simulating alveolar gas, ethanol vapor corresponding to blood alcohol levels of 0.10, 0.30, and 0.50% had a slight but not clinically significant effect on readings for enflurane and isoflurane but increased readings with the halothane setting 3.5 times the corresponding level of blood alcohol. Clinicians can test for an interfering gas such as ethanol before induction by checking the reading in the halothane setting during preoxygenation.