Vaporizers

Definition

Assuming that the agent analyzer reading is correct, the patient is receiving isoflurane instead of the intended sevoflurane, and at a concentration much greater than that set on the vaporizer concentration dial. The difference in anesthetic may be due to the unintentional or deliberate filling of the agent-specific sevoflurane vaporizer with a different anesthetic (in this case, isoflurane). Situations that result in delivery of increased anesthetic vapor concentration include vaporizer misfilling, malfunction, tipping, overfilling, and gas flow reversal through the vaporizer.

Recognition

The end result of these situations is an anesthetic agent and/or concentration that differs from that intended. This manifests as clinical and hemodynamic signs of anesthetic overdose, including profound hypotension, as occurred in this case. Identification of the agent and measurement of its concentration in the breathing circuit are critical to making a correct diagnosis. Before attempting to troubleshoot the cause of the excess and incorrect agent, the clinician should immediately discontinue use of the vaporizer in question and purge the breathing system with oxygen to rapidly decrease the anesthetic concentration. The cardiovascular depression must be corrected. The anesthesiologist might also consider disconnecting the patient from the anesthesia machine’s breathing circuit and ventilating the patient’s lungs using an alternative system, such as a self-inflating manual ventilation device (e.g., Ambu bag) or a different anesthesia circuit (e.g., Bain) connected to an O ₂ source.

Some investigative work is required to determine whether the anesthetic overdose is due to a vaporizer or a breathing circuit problem. Because modern potent inhaled volatile anesthetics are delivered using machine-mounted, calibrated, agent-specific vaporizers, the vaporizer concentration dial is set to a certain value (e.g., 1%), and the concentration of agent in the gas flowing from the common gas outlet (CGO) of the anesthesia machine is sampled and analyzed. The CGO is where the gas mixture created by the anesthesia machine flowmeters and vaporizer leaves the machine to be conducted to the fresh gas inlet of the breathing system. The CGO is therefore the sampling location in the delivery system that is closest to the vaporizer outlet. With the fresh gas flow set to 5 L/min of the carrier gas with which the vaporizer is calibrated by the manufacturer (air for Dräger vaporizers; O ₂ for GE, Datex-Ohmeda, and Penlon vaporizers), the concentration dial is set to 1%, and the concentration of agent in the gas flowing from the CGO is measured using a calibrated anesthetic agent analyzer. The measured concentration should be within 10% to 15% of the vapor concentration dial setting (e.g., if the dial is set to 1%, the concentration measured should be between 0.85% and 1.15%) if the vaporizer is in calibration (according to the manufacturers’ specifications).

If a different anesthetic is detected, the agent-specific vaporizer does not contain the agent for which it is labeled. The vaporizer (and if necessary the anesthesia machine) should be removed from service until it has been inspected and tested in a nonclinical environment. If a higher-than-expected concentration is detected, it is likely that the problem is with the vaporizer and not the breathing circuit. If the measured agent concentration agrees with the dialed-in concentration, a possible problem is liquid agent in the anesthesia circuit. In the event that a freestanding vaporizer is being used (i.e., one placed in series between the fresh gas source and the breathing circuit [and one used by perfusionists during cardiopulmonary bypass]), it should be inspected to ensure that the direction of fresh gas flow through the vaporizer is correct (i.e., the fresh gas enters via the vaporizer inflow, not the outflow, connection).