To the Editor:--We would like to report a case in which we encountered two problems with the Ohmeda Tec-6 vaporizer: (1) a significant fresh gas leak developed after Tec-6 was turned off, and (2) neither the Tec-6 nor other vaporizers (isoflurane and enflurane), which were mounted on the same anesthesia machine, could be turned on after the Tec-6 was turned off.
The patient was a 44-yr-old man who had undergone septorhinoplasty under general anesthesia with nitrous oxide and desflurane in oxygen using a Tec-6 vaporizer mounted on a North American Drager anesthesia machine (Narkomed 2B). The induction and intraoperative course were uneventful. At the end of the procedure, desflurane was turned off, and the patient's trachea was extubated. On attempting to assist his ventilation via mask and despite a tight seal of the mask and a high flow of oxygen, it was noted that the reservoir bag could not be filled with oxygen. The fresh gas flush button was pushed several times while the adjustable pressure limiting valve was closed to allow filling of the reservoir bag. However, the reservoir bag only minimally filled while the oxygen flush valve was continuously activated. During these maneuvers, the selector switch was turned to the "bag" mode. While flushing, we could hear a leak around the area of the Tec-6 desflurane vaporizer. Because the patient was still unconscious and required a more secure airway, his trachea was electively reintubated and the lungs were ventilated using a manual self-inflation resuscitation (Ambu) bag. We tried to manipulate the dial of the desflurane vaporizer as well as the concentration dials of the other vaporizers that were mounted on the anesthesia machine to determine whether the "leak" situation could be corrected; we found that all of the vaporizers were in a locked "off" position and could not be turned on. Another anesthesia machine was brought into the operating room, and we subsequently ventilated the patient's lungs in a normal and uneventful manner.
The Tec 6 vaporizer used to deliver desflurane differs in design from the variable bypass concentration-calibrated vaporizers (such as the Ohmeda Tec 4 and Tec 5 and the Dragerwerk Vapor 19.1), used to deliver halothane, enflurane, and isoflurane, because of the physical properties of desflurane. The principles of operation of the Tec 6 are described elsewhere. [1,2,3].
The investigation conducted by Ohmeda identified a possible situation in which the valve piston could stay in the depressed position when the vaporizer control dial was returned to the stand-by position (Figure 1). This situation would be a result of the piston becoming temporarily lodged into a cross-hole feature of the valve body. In this condition, a gas path would be created that could vent fresh gas flow to the atmosphere.
Figure 1. An engineering illustration of section of Tec-6 vaporizer head and interface manifold. This adapter manifold required for use of the Tec 6 NAD Variant Vaporizer developed a significant fresh gas leak when the vaporizer was turned off after use, such as at the end of our case. This condition is caused by a manifold valve being held in an open position. The internal plunger of this valve can become lodged against the flow control holes of the valve body, creating a passageway for a gas leak. A leaking sound, such as a hissing noise, was present. (Courtesy of Ohmeda, Inc.)
Figure 1. An engineering illustration of section of Tec-6 vaporizer head and interface manifold. This adapter manifold required for use of the Tec 6 NAD Variant Vaporizer developed a significant fresh gas leak when the vaporizer was turned off after use, such as at the end of our case. This condition is caused by a manifold valve being held in an open position. The internal plunger of this valve can become lodged against the flow control holes of the valve body, creating a passageway for a gas leak. A leaking sound, such as a hissing noise, was present. (Courtesy of Ohmeda, Inc.)
To address this possibility, Ohmeda revised the valve body design to remove the cross-hole feature. Further, Ohmeda has conducted a field action to notify customers with affected units (Ohmeda Tec 6, desflurane vaporizer for use with North American Drager Anesthesia Systems), provided appropriate additional instructions for the user in the event of such a problem, and replaced the affected units with vaporizers containing the revised valve body component. The second issue involved the requirement to check and adjust the North American Drager anesthesia machine vaporizer interlock mechanism. This should be conducted on replacement of any vaporizer by authorized service personnel. After adjustment on the referenced machine, the interlock system operated appropriately, allowing the selection of any of the mounted vaporizers.
Salahadin Abdi, M.D., Ph.D., Department of Anesthesiology, The Massachusetts General Hospital, Harvard Medical School.
Martin A. Acquadro, M.D., D.M.D., F.A.C.P.M., Department of Anesthesiology, The Massachusetts Eye and Ear Infirmary, Harvard Medical School, 32 Fruit Street, Boston, Massachusetts 02114.
(Accepted for publication April 10, 1995.)
