The oxygen concentration of fresh gas mixtures delivered to a Bain circuit by a Boyle's anaesthetic machine equipped with leaking oxygen flowrneters was measured at the common gas outlet during free flow and during controlled ventilation of a model lung. The results demonstrate that, despite satisfactory oxygen and nitrous oxide rotameter settings, such a machine can deliver hypoxic fresh gas mixtures irrespective of whether the leaking flowmeter is mounted downstream or upstream. This selective loss of oxygen increases with the size of the leak and increases with the back pressure on the flowmeter assembly during controlled ventilation. A downstream position of the oxygen flowmetre should not be regarded as foolproof. Continuous monitoring of the oxygen concentration of fresh gas mixtures with an in-line oxygen analyzer is recommended.EGER, et al. t WERE THE FIRST to point out that anaesthetic machines equipped with an upstream oxygen flowmeter could deliver a hypoxic mixture, despite deceptively satisfactory flowmeter readings, should there be a leak in the oxygen or other flowmeters. They suggested that this danger could be reduced if the oxygen flowmeter was mounted in a downstream position immediately proximal to the outlet of the common collecting chamber. Since then, fatal and nearfatal accidents due to hypoxic fresh gas mixtures delivered by anaesthetic machines equipped with an upstream oxygen flowmeter have been reported. 2-s In 1978 the Canadian Standards Association adopted Eger's recommendation 6 which was endorsed by the Canadian Anaesthetists' Society 7 and later implemented by anaesthetic departments across the country.Many newer anaesthetic machines are equipped with only nitrous oxide and oxygen flowmeters. There is an equal chance for a leak to occur in either flowmeter in such a machine. When the leak is in the upstream nitrous oxide flowmeter, selective loss of oxygen will not occur.t Should the leak occur in the downstream
Four commonly used vaporizers were studied for the effect of carrier gas density on vaporizer output. Vapour concentrations from a halothane Cyprane (Fluotec) Mark 2 increased in relation to the density of carrier gas, whereas the concentrations delivered by an enflurane Ohio vaporizer decreased. The halothane Cyprane (Fluotec) Mark 3 and enflurane Cyprane vaporizers were largely independent of density. Of clinical importance, nitrous oxide/oxygen (75/25), compared with oxygen alone, increased the vapour concentration outputs of the halothane Mark 2 up to 30 per cent and decreased the outputs of the enflurane Ohio unit up to 20 per cent.PREVIOUS OBSERVATIONS suggest that the anaesthetic outputs of commonly used vaporizers vary with the composition of carrier gas L2. Nitrous oxide, 60 to 70 per cent with oxygen, was reported to increase the vapour concentration outputs of a halothane vaporizer (Cyprane MK.2) t and two enflurane vaporizers (Ohio and Cyprane) z. These effects were attributed to redistribution of flow resistances within these vaporizers by increased carrier gas density.To explore further the hypothesis that vaporizer function depends upon the density of carrier gas, we measured the output responses of four commonly used vaporizers to carrier gases of widely varying density and viscosity. METHODSThe four vaporizers studied were two models of the Cyprane (Fluotec) halothane vaporizer, the Mark 2 and Mark 3, and the enflurane vaporizers manufactured by Ohio and Cyprane. All four units had been calibrated recently by the manufacturer. Each vaporizer was placed on a flat surface and fitted with a rigid metal extension incorporating a sampling poll 35 cm distal to the vaporizer outlet. For this study, we selected carrier gases and carrier gas mixtures which have a wide range of gas density; these were helium with oxygen (50150), nitrogen, oxygen, carbon dioxide with oxygen (10/90), nitrous oxide with oxygen (80/20), nitrous oxide and SF6 with nitrous oxide (9.5/90.5), Each gas or gas mixture was delivered from calibrated rotameters set to give a total flow rate of 6 l/min, this rate having been verified by a timed collection of gas with a Collins watersealed spirometer.Carrier gases were introduced suddenly into the inlet of each vaporizer and kept constant while outlet vapour concentrations were measured at dial settings of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 per cent. Vapour concentrations were recorded two to three minutes after establishing each condition of carrier gas and dial setting; in preliminary studies we found that changes in vaporizer output were always complete within 120 seconds and remained stable for at least 30 minutes.The concentrations of both vapours (halothane and enflurane) and all carrier gases except helium were measured by a Perkin-Elmer #1100 mass spectrometer. Helium cannot be analyzed in our laboratory and its concentration was only inferred from rotameter settings. Vapour concentrations in the helium/oxygen mixture were determined by a Beckman infrared analyzer with a h...
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