Editor:

We report a problem hidden behind the vaporizers on a Datex-Ohmeda Excel 210 SE (Datex-Ohmeda Limited, Madison, WI, USA) anaesthetic machine that resulted in a vaporizer leak. A 70-yr-old ASA I male required a general anaesthetic for a Thompson’s hemiarthroplasty. Before starting the case, the anaesthetic machines were checked and no faults were identified. There were two vaporizers on the Excel 210 SE anaesthetic machine in the operating theatre, an Isotec 4 isoflurane vaporizer (Datex-Ohmeda Limited) and a Datum (Blease Medical Equipment Limited, Buckinghamshire, UK) sevoflurane vaporizer. These were properly seated, locked into position and contained an anaesthetic agent. There was no evidence of a gas leak.

Following intravenous induction and insertion of a laryngeal mask in the anaesthetic room, the patient was transferred to the operating theatre. The Isotec 4 vaporizer on the Excel 210 SE anaesthetic machine was set to deliver 3% isoflurane with a fresh gas flow of 1.5 L min⁻¹ (oxygen 0.5 L min⁻¹; air 1 L min⁻¹). Spontaneous ventilation was allowed on a circle system with a soda lime carbon dioxide absorber and confirmed to be effective (saturation 99%; F_iO₂ 0.35; end-tidal CO₂(etCO₂) 5.5 kPa; end-tidal isoflurane 1.2%).

During the case it was noted that the isoflurane vaporizer was almost empty. This was removed and replaced with a full Isotec 4 vaporizer. This was seated properly, locked into position on the back bar and set to deliver 3% isoflurane. However, it was quickly noted that a hypoxic gas mixture (F_iO₂ 0.15) was being delivered and the patient’s oxygen saturation had fallen (92%) but his etCO₂ remained stable (5.5 kPa). The supply of oxygen was increased to 10 L min⁻¹, the delivery of air was stopped and all connections were checked. However, the level of F_iO₂ only rose to 0.33 and the oxygen saturations continued to fall. Replacement of the anaesthetic machine immediately corrected the problem with delivery of fresh gas and subsequently improved oxygenation.

Removal of the sevoflurane vaporizer revealed that one of the captive fasteners (1001-4086-000; Datex-Ohmeda Limited) which held the two-stage cover manifold onto the back bar of the Excel 210 SE had been lost. Removal of the isoflurane vaporizer revealed that the other captive fastener was partially displaced. The plate had pivoted on the displaced captive fastener and slipped up approximately 1 mm above the back bar behind the isoflurane vaporizer. Although the vaporizer was correctly seated and locked into position on the back bar, the plate prevented a tight seal on the o-ring and allowed a significant gas leak. On realignment of the plate, re-insertion of the only captive fastener and replacement of the vaporizer, the gas leak was abolished. However, the captive fastener was damaged and could easily be displaced by slight movement of the two-stage cover manifold. We hypothesized that this had occurred when the vaporizer was changed.

This cause of a leak of fresh gas flow has not been previously reported. The captive fasteners which hold the two-stage cover manifold in position should be checked during routine machine maintenance. However, this is particularly important for the Datex-Ohmeda Excel 210 SE as the captive fasteners are hidden behind the vaporizers and can be dislodged easily. In our department, the presence of the captive fasteners and the stability of the two-stage cover manifold are now confirmed at each vaporizer change when the Datex-Ohmeda Excel 210 SE anaesthetic machine is in use.