Vaporizer interlock malfunctioning.

Sir,

A 50 years old female was posted for modified radical mastectomy under general anesthesia. The anesthesia workstation in use was Drager Fabius with Drager 2000 sevoflurane and isoflurane vaporizers attached to the Selectatec back bar mounting of the machine. During the course of the anesthesia, we noticed that the vaporizer interlock, which allows switching on of one vaporizer at a time was malfunctioning and allowed the switching on of second vaporizers simultaneously [Figure 1]. Normally, the interlock mechanism on top of the vaporizer has a pin in the slot that fits the interlock bar in a closed position only [Figure 2a]. The interlock closes only with the vaporizer in the off position and not when it is in use with a maximum dial concentration.

Figure 1

Two vaporizers in use simultaneously

Figure 2

(a) Intact interlock pin and (b) A broken interlock pin

In our case, the vaporizer interlock malfunctioned allowing the use of the second vaporizer while one vaporizer was already in use. Buettner[1] reported as case of vaporizer malfunction and subsequent switching on the second vaporizer. In that case, error was in the distance between the center line and the locking pin, which was 1.63 mm shorter than the manufacturer specifications. Kim and Kim[2] described a case in which there was interruption in the supply of breathing gas during anesthesia caused by malposition of the Drager Vapor 2000 vaporizer, which was accidentally tilted and lifted off the Selectatec manifold of the anesthesia machine. Cooper et al.[3] mentioned that human error was involved in 82% while, equipment failure in 14% of preventable incidents. In our case, we found that the pin inside this slot was broken [Figure 2b] and the interlock could be shut with the dial concentration in the maximum position, thereby allowing the second vaporizer to be switched on simultaneously. This would be significant if by an error or oversight two vaporizers would be on causing harm to the anaesthetized patient. In this case it is purely a human error of not finding the broken pin of vaporizer while doing preuse check, but it may go unnoticed because the pin is slightly deep inside the slot for the interlocking bar. Purpose of reporting this case is to highlight the preuse checking of vaporizer for any broken pin inside the slot for interlocking bar besides checking for another safety test. This vaporizer interlock safety mechanism is not a part of routine safety check but needs incorporation in the revised safety checklist with the advent of newer technology.