Factitious reading by gas monitor.

Sir,

In the era of low flow anesthesia, gas monitors have become an integral part of anesthesia work station. They provide critical information about inspired and expired concentrations of O2, N2O, and CO2 and also provide a guard against the possibility of intraoperative awareness by giving accurate minimal alveolar concentration (MAC) value of various volatile anesthetic agents. However, as with any technology gas monitoring is also associated with inherent errors and fallacies, which if overlooked can potentially lead to patient morbidity. In a recent critical incidents analysis involving anesthesia equipment, 13.4% incidents were related to problems with gas monitoring.[1] Here, we report a case of erroneous detection of enflurane despite of the agent not being used and explore a possible mechanism behind the error.

To anesthetize a 1-year-old child, we used Drager Primus ASAE-0129 workstation with a side stream gas analyzer. Anesthesia was induced using sevoflurane in O2 and N2O mixture of 50:50 ratios at 5 L/min of fresh gas flow (FGF). When a MAC value of 3.0 was reached, intravenous cannula was secured, and sevoflurane vaporizer was turned off and airway was secured using laryngeal mask airway device. The isoflurane vaporizer was turned on and anesthesia was maintained using isoflurane in O2 and N2O mixture of 50:50, at 2 L/min of FGF. The gas analyzer detected the volatile anesthetic agents as sevoflurane and enflurane along with their inspired and expired concentrations and MAC value [Figure 1], although enflurane had never been used with this anesthesia machine. MAC value was not relied thereafter and FGF was not decreased below 2 L. The monitor showed MAC values for both iso- and en-flurane throughout the procedure.

Figure 1

Gas monitor displaying sevoflurane, enflurane and minimal alveolar concentration value

Anesthetic gases are most commonly analyzed by infrared method (dispersive or nondispersive) to isolate the absorbance characteristics of the gas sample. Many gases absorb infrared energy at a wavelength specific to the gas being analyzed[2]. Infrared absorbance spectra for CO2 and N2O are located at the 4–5 μm range and for volatile anesthetic agents; it ranges between 8 and 13 μm. The refractive index of en-, iso- and sevo-flurane is overlapping (1,540.4, 1,563.3 and 1,538.3, respectively).[3] Thus, they require complex methods of discrimination along with advanced mathematical techniques to identify and measure them.

Wrong detection as halothane for patient's expired breath methane[4] or isopropyl alcohol has been reported at lower wavelengths (3.0–3.3 μm). Misidentification can also occur if a wrong agent has been filled in the vaporizer. However, since the Drager Primus uses higher wavelengths for its volatile anesthetic agents analysis (8-13 μm) and has a fool proof filling method, both the above events can not be true in our case. We postulate that the use of both agents that is, sevoflurane during induction and isoflurane during maintenance of anesthesia may have led to the wrong detection of enflurane whose peak wavelength lies around the wavelengths of both these agents. Fault in expired gas sensor membrane may have contributed to this factitious enflurane identification.

The technical support team from Drager medicals were contacted for the cause and the fault was found to be in the sensor of gas monitor (ILCA2 infrared gas analyzer Drager Medical, Lubeck, Germany). After replacing the part with the new one, the sensor started to identify the agents correctly. This gas analyzer works by pulsed nondispersive infrared method and a multispectral detector. The measurement method is not susceptible to cross-sensitivities from water vapor, ethanol, and acetone.

These sensors need to be checked periodically as per the manufacture's recommendations and moisture should be prevented from entering into the sampling line. This particular sensor needed to be replaced monthly according to the manufacturers recommendation, but the step had been overlooked which probably lead to the misidentification. Agents like enflurane, which are no longer in use, should not be encoded into the sensors. When such a problem is encountered, the FGF should not be decreased to <1 L, as the disparity between delivered and inspired concentration of anesthetic agents becomes more marked at low flows. Sensors which do not cross-react with the gases, which are exhaled by the patient such as alcohol, acetone, carbon monoxide, and methane, should be used.

Standards for basic intraoperative monitoring presently do not require volatile anesthetic agents measurement. However, erroneous readings may lead to over or under dosing of these agents leading to patient morbidity including delayed awakening, hemodynamic instability and intraoperative awareness. Wrong identification of volatile anesthetic agents by the gas monitor may lead to unnecessary hassle at the same time giving a hint that the ongoing gas monitoring may not be reliable to ensure adequate depth of an ongoing general anesthetic.