To the Editor:—We wish to report a serious machine fault that developed with a Modulus II Plus anesthesia machine (Ohmeda, Madison, WI). The hypoxic guard linking the oxygen and nitrous oxide flow control values failed, causing the oxygen knob to turn on the nitrous oxide, such that a hypoxic mixture was created.
The machine had been used without problems by the same anesthesiologist during the previous case and had been serviced by the Ohmeda field service representative 3 weeks before the incident. When the anesthesiologist attempted to administer 100% Oxygen before induction of anesthesia, the flow of oxygen did not increase above 1 l/min. Further, counterclockwise rotation of the oxygen flow control knob in an attempt to increase the oxygen flow led to the nitrous oxide control knob being engaged and nitrous oxide flow increasing until a hypoxic gas mixture resulted. The machine was exchanged immediately for another machine, and the anesthetic continued uneventfully.
Examination of the machine revealed that the flow control knob for oxygen was “loose” and protruded further than usual. However, the knob could not be pulled off the spindle as it was restrained both by the cover plate and by the proportioning chain.
Part of the Link 25 Proportion-Limiting System used on the Ohmeda Modulus machine consists of a 14-tooth sprocket fixed to the nitrous oxide control knob, with the sprocket linked by a chain to a 29-tooth sprocket, which is mounted on a threaded sleeve attached to the oxygen control knob. The oxygen flow control knob is secured to its spindle by two recessed set screws (Figure 1(A)). Normally, turning the oxygen flow control knob counterclockwise turns the spindle, which in turn opens the flow control valve.
Figure 1. (A) Twenty-nine-tooth sprocket fixed to oxygen flow control knob via a threaded sleeve on the Modulus II Plus anesthetic machine. There is a gap between the collar (marked A) and the end of the threaded sleeve. One of the two set screws holding the oxygen finger control knob onto the spindle is marked B. (B) Twenty-nine-teeth sprocket fixed to oxygen flow control knob via a threaded sleeve on Modulus II Plus anesthesia machine. The collar at the end of the threaded sleeve has impinged tightly against the sprocket causing the proportioner chain to increase the flow of nitrous oxide when the oxygen flow control knob was turned counterclockwise to increase oxygen flow.
Figure 1. (A) Twenty-nine-tooth sprocket fixed to oxygen flow control knob via a threaded sleeve on the Modulus II Plus anesthetic machine. There is a gap between the collar (marked A) and the end of the threaded sleeve. One of the two set screws holding the oxygen finger control knob onto the spindle is marked B. (B) Twenty-nine-teeth sprocket fixed to oxygen flow control knob via a threaded sleeve on Modulus II Plus anesthesia machine. The collar at the end of the threaded sleeve has impinged tightly against the sprocket causing the proportioner chain to increase the flow of nitrous oxide when the oxygen flow control knob was turned counterclockwise to increase oxygen flow.
In this machine, on removing the cover to examine the flow control valves, we discovered that the set screws fixing the oxygen flow control knob and threaded sleeve assembly to the spindle of the flow control valve were loose. When the oxygen knob was turned counterclockwise to increase the flow of oxygen, the oxygen knob and sleeve assembly “slipped” on the spindle, and the oxygen flow only increased to a maximum of 1 l/min. However, the threaded sleeve on the oxygen control knob, which is fixed to the knob, not to the spindle, continued to turn within the sprocket, screwing the sprocket further away from the oxygen control knob. As the oxygen knob was further turned counterclockwise, it eventually caused the collar (marked A in Figure 1) to impinge against the oxygen sprocket of the proportioner system. Further attempts to increase oxygen flow caused the proportioning chain to turn the nitrous oxide knob until a maximum of 12 l nitrous oxide to 1 l oxygen flowed from the common gas outlet.
Other causes of failures of the link 25 Proportion Limiting Systems have been described. [1,2] As far as we are aware, this is the first time that such a grossly hypoxic mixture could be delivered from turning on the oxygen knob. The failure of the chain-linked hypoxic guard to prevent the administration of a hypoxic mixture is of great concern to us and could have serious consequences for a patient. It is of particular concern that the loosening of the screws occurred during use, as this resulted in the fault not being detectable during routine preoperative checking. Although the machine had been fully checked before the first case, the proportioning system had not been rechecked between patients on the schedule.
This case report emphasizes the need to check the anesthesia machine between cases, in addition to conducting a full check at the beginning of the list. It also illustrates the necessity of using an oxygen analyzer that will warn of hypoxic mixtures that can occur despite the presence of a proportioning system, if the proportioning system should fail or if a third gas is administered.
Peter C. Gordon, M.B.B.Ch., F.F.A.(S.A.); Michael F. M. James, M.B.Ch.B., F.R.C.A., Ph.D.; Hilary Lapham, M.B.Ch.B.; Michael Carboni, Technologist; Department of Anesthesia; Groote Schuur Hospital and University of Cape Town; Observatory, Cape Town; South Africa 7925.
(Accepted for publication October 26, 1994.)
