Hypoxic gas mixtures delivered by anaesthetic machines equipped with a downstream oxygen flowmeter

The oxygen concentration of fresh gas mixtures delivered to a Bain circuit by a Boyle's anaesthetic machine equipped with leaking oxygen flowmeters 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 flowmeter should not be regarded as foolproof. Continuous monitoring of the oxygen concentration of fresh gas mixtures with an in-line oxygen analyzer is recommended.