To the Editor: In a 73-yr-old woman undergoing mitral valve repair, mechanical ventilation using a previously checked Draeger Fabius GS anesthesia machine (Draeger Medical, Telford, PA) was instituted and was uneventful, before initiating extracorporeal circulation. Before separation from cardiopulmonary bypass, the lungs were re-expanded using the anesthesia machine's reservoir bag. The mechanical ventilator was then activated using the volume control mode. The electrically powered piston ascended and descended, and the reservoir bag inflated and deflated in the characteristic manner of fresh gas-decoupled systems during mechanical ventilation. The capnograph revealed a minimum carbon dioxide concentration as expected during extracorporeal circulation. However, her lungs were not inflating and the breathing circuit pressure gauge did not register any significant change. The anesthesiologist immediately noticed the absence of tidal volumes. Manual ventilation was instituted without difficulty; multiple attempts at volume control and pressure control mechanical ventilation failed. The breathing circuit was disconnected from the endotracheal tube and pressurized in an attempt to identify a leak. No leak was found. Manual ventilation continued until an operational anesthesia machine was brought into the room and used for mechanical ventilation. The patient suffered no adverse consequences from this incident. We examined the malfunctioning machine with a biomedical engineer and identified a source of mechanical ventilator failure in the Draeger Fabius GS which, to the best of our knowledge, has not been reported. Upon opening the fresh gas decoupling valve compartment, we observed a warped decoupling valve (Fig. 1). When this valve is not seated properly during mechanical ventilator inspiration, gas may flow away from the lungs, following the path of least resistance. This results in ineffective tidal volumes.Figure 1.: Open decoupling valve housing showing a warped valve.How might a previously working valve suddenly malfunction? Draeger Fabius GS machines may be equipped with an electrically heated breathing system. The manufacturer recommends turning the heater off when the machine is not in use to minimize carbon dioxide absorber desiccation. Perhaps a sustained increase in temperature may affect the shape. It is also possible for the valve to have been structurally defective. Contrary to the unidirectional valves in the Draeger Fabius GS machines which are made of inflexible ceramic, the decoupling valves are made of a bendable black rubber. As a result, the decoupling valves may warp without cracking. Cracked unidirectional valves may be readily observed through the transparent valve housing. A warped decoupling valve, located under a metallic cap, is not visible unless the compartment is open, as exemplified in this case. Regardless of the reason for failure, a defective decoupling valve should be suspected if the machine does not pass its automated leak test or if mechanical ventilation fails during an anesthetic. It must be understood that in the Draeger Fabius GS, piston and bag movement are not indicative of effective positive pressure mechanical ventilation. Rafael A. Ortega, MD Emily R. Zambricki, BA Department of Anesthesiology Boston University School of Medicine Boston, MA [email protected]