To the Editor:-Two independent reports, one to ECRI, the other to the US Food and Drug Administration's Medical Device Reporting System (accession no. MDR-366794), describe almost identical incidents in which an adjustable pressure limiter (APL) valve on a North American Drager (NAD) anesthesia machine blocked the exhaust of gas to the scavenger outlet even though the valve was set appropriately to relieve pressure. Both incidents took place during mask induction. As the patient breathed, the baseline pressure in the patient circuit increased, and the reservoir bag steadily filled. One of the reports states that adjusting the APL valve had no effect, but removing the mask from the patient's face allowed the pressure to drop, avoiding patient injury.

The NAD APL valve has two parts: a needle valve, which is the primary pressure adjustment mechanism, and a disk-in-cage one-way valve (similar to a standard exhalation valve) that is intended to prevent back flow from the scavenger system into the breathing circuit and to provide a slight back pressure to preferentially fill the breathing bag before gas passes through the needle valve into the scavenger port. Sticking of this disk in the closed position blocks entry of gas into the APL valve, so that gas cannot pass to the scavenger system even when the needle valve is adjusted to fully open. Consequently, the breathing bag will gradually expand, and the airway pressure will gradually increase depending on the fresh gas flow.

A blocked NAD APL valve should be evident in the pre-use check of the breathing circuit. One of the incidents was reported to have occurred during the first case of the day, and it is not clear whether a pre-use check had been performed. When discovered, the blockage was eliminated by tapping the APL valve, and the valve remained unblocked afterward. It was hypothesized by the personnel involved that the disk of the one-way valve became stuck as a result of the drying of moisture in the valve from the previous day. A valve that is, or has been, blocked should be tested, repaired, or replaced by an authorized service person.

Most NAD anesthesia machine users are probably not aware that this valve can behave in this way. I am writing this letter to aid clinicians in the differential diagnosis of machine problems and to reinforce the recommendations of ECRI, FDA, and the American Society of Anesthesiologists that pre-use checks be performed before each anesthetic procedure.

Frank P. Primiano, Jr., Ph.D.

Senior Project Engineer; ECRI; Plymouth Meeting, Pennsylvania

(Accepted for publication October 3, 1997.)