To the Editor:—
An oxygen flush valve is a useful feature of most anesthesia machines. Its malfunction is rare but potentially harmful.1–3In one instance, a fatigued metal spring left a valve wide open during a preoperative machine check.1Another valve became slow to release because of inadequate lubrication.2We report a relatively insidious valve problem.
A patient was to undergo percutaneous closure of an atrial septal defect. Because transesophageal sonographic guidance was to be employed, we opted for general anesthesia with the aid of tracheal intubation. After an uneventful check of the machine, anesthesia was induced intravenously. For maintenance, our machine was set to deliver 2% isoflurane in air. However, our gas analyzers indicated delivery of 1% isoflurane and 60% oxygen. A sevoflurane vaporizer also failed to deliver expected concentrations of vapor. Repetition of the machine check revealed a leak of oxygen, approximately 4 l/min, into the breathing circuit. The oxygen flush button superficially appeared to be undamaged, and its spring exhibited the usual force. After the button was pressed a few times, the leak disappeared. The machine was taken to our service area, where routine external checks revealed no problems. However, we found an unstable “orthopedic” condition inside of our 7-yr-old Excel 210 SE machine (Datex-Ohmeda, Madison, WI). The push button for its flush valve is attached to a shaft that transmits the finger pressure to open the valve. Our machine had suffered a spiral fracture of that plastic shaft (fig. 1). As long as the fracture was “reduced,” the assembly functioned normally. However, the two pieces could “displace” upon rotation with respect to each other, and then the shaft was too long and so held the flush valve partially open. With this diagnostic information available, we could deliberately make the leak come and go, and we observed that different operators inadvertently applied various degrees of torque as they pressed the button. The broken part was easily replaced.
Fig. 1. Exaggerated diagrams of the fractured plunger. At top, the fracture is shown reduced, and the shaft is of normal length. Below, the two pieces are rotated into displacement with respect to each other, and the shaft is effectively too long. In our case, the excessive length held the oxygen flush valve partially open at rest.
Fig. 1. Exaggerated diagrams of the fractured plunger. At top, the fracture is shown reduced, and the shaft is of normal length. Below, the two pieces are rotated into displacement with respect to each other, and the shaft is effectively too long. In our case, the excessive length held the oxygen flush valve partially open at rest.
Gross trauma to the assembly has been reported to lock open an oxygen flush valve.3Because of that and other possibilities, federal regulations require the oxygen flush valve to be protectively housed.3Because of the intermittent character of our valve malfunction, it is not clear if and when our machine suffered a strong impact. The machine does have a sturdy protective rim around the push button, and the fracture may have occurred through routine stress on the plastic shaft.
Transient malfunction of a valve should not be dismissed. Our colleagues in anesthesia, engineering, and manufacturing do not recall an event similar to the one we describe. This problem, although rare, is illustrative of the need for constant assessment of the proper function of anesthesia equipment. The aircraft industry speaks of gremlins. Our intermittent gremlin was treacherous indeed.
Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts. talston@partners.org
