To the Editor:—A 77-yr-old man undergoing insertion of a J-splint for renal obstruction received general anesthesia delivered with an ADU anesthesia machine (Anesthesia Delivery Unit; Datex-Ohmeda, Stockholm, Sweden). A 5% desflurane vaporizer concentration setting with an O2/N2O mixture (2 and 3 l/min, respectively) resulted in stable inspired and expired desflurane concentrations (fig. 1). Immediately after lowering the fresh gas flow (FGF) to 0.35 l/min O2 and 0.35 l/min N2O, and while maintaining the same vaporizer concentration setting, a dramatic increase in inspired and expired desflurane concentrations to about 14% (15:45) was noticed, as shown in figure 1. The duration of this high concentration was short-lived ( 2 min) and did not trigger an alarm; the vaporizer concentration setting was decreased to 4.5% and was left unchanged throughout the remainder of the procedure (until 16:13). After a rapid decrease of the inspiratory and end-expiratory desflurane concentrations to about 7–8.5%, the concentrations started to increase again, leading to a gradual decrease in blood pressure. Because vaporizer malfunction was suspected, the FGF was increased to 5 l/min O2/N2O and was decreased again (to the previous settings) within a period of 1 min (15:56). Inspired and expired concentrations were noticed to decrease and increase again. This maneuver was repeated at 16:03, confirming that indeed something was wrong with the vaporizer output with the use of lower FGF (0.7 l/min). At 16:05, the FGF was therefore increased to 5 l/min. Vaporizer output itself was then checked at low FGF (0.7 l/min) by interrupting ventilation and having the sampling line of the multigas analyzer (Compact Airway Module M-CAiOV, Datex-Ohmeda, Helsinki, Finland) sample gases leaving the common gas outlet (16:10). Desflurane output read 14.5% (at 16:10) during the use of low flows, but matched the dialed 4.5% (16:12) when the FGF was increased again to its previous settings (O2/N2O mixture, 2 and 3 l/min, respectively). An alarm message appeared (“Service fresh gas unit.”). Anesthesia was continued for a few more minutes for the remainder of the surgical procedure with desflurane and high FGF (5 l/min), and the patient was allowed to awaken without further incident. On the same day of our observation, a similar case was reported by the Anesthesiology Discussion Group on GASNet.† With FGF of 0.6 l/min O2 and 0.6 l/min air and a desflurane dial setting at 8%, the desflurane concentration on the agent analyzer display slowly approached 4.5–5.5%. Then, without warning, the desflurane concentration suddenly increased to 15%. It is unclear whether the events were the same as in our case. The ADU vaporizing unit is an electronically controlled, flow-over, variable bypass, and measured flow vaporizer, and its mechanism of action and performance have been described recently. Vaporizer output increased with lower FGF, with the largest error with FGF of 0.2 l/min (4.3 and 7.3% absolute output measured with 3% and 6% dialed, respectively, in a single instance). In the current case, however, substantially higher total FGFs (0.7 l/min) were used. Very preliminary testing by Datex-Ohmeda indicates that the one-way valve that prevents backflow of saturated vapor from the cassette via inspiratory channel toward the bypass channel may have failed to close after lowering the FGF (fig. 2). This problem may be more significant when desflurane is used because the pressure in the desflurane Aladin cassette (Datex-Ohmeda, Stockholm, Sweden) may exceed 1 atm because of its high vapor pressure when the temperature is greater than 22.8°C (boiling point of desflurane at 1 atm pressure). A similar problem in 1999 prompted a redesign of this one-way valve and an upgrading of all ADU anesthesia machines in service worldwide (Mr. Ola Lassborn, Quality Manager, Datex-Ohmeda, Stockholm, Sweden, verbal personal communication, March 2003). Despite the new design, this report suggests a continued problem with this valve with the possible delivery of unintended high concentrations of inhaled anesthetics. It is unclear whether the valve still has a design problem or whether only a few defective valves exist (a manufacturing issue). This safety issue is being addressed by Datex-Ohmeda. For now, it is advisable to monitor carefully for excessive agent concentrations when using the ADU Datex-Ohmeda anesthesia machines, especially if desflurane is administered.