Hypoxic mixtures of oxygen and nitrous oxide were demonstrated with three anaesthesia machines which had cyclopropane flowmeters. In one machine the bobbin did not prevent back flow and the hypoxic mixture occurred when the cyclopropane flow control was left open. Two other machines only delivered hypoxic mixtures if the cyclopropane bobbins were removed from their seats and the flow controls opened. The vaporizer on one machine increased the hypoxia when turned on.
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