Drawover anaesthetic systems. Factors determining the inspired oxygen concentration

The factors that determine the inspired oxygen concentration delivered by drawover anaesthetic systems were studied in the laboratory and in anaesthetised patients. The inspired oxygen concentration is dependent upon oxygen flow, reservoir volume and total ventilation; respiratory rate and inspiration:expiration ratio have a small influence. A reservoir of at least of 1 m corrugated tube (internal volume 415 ml) makes optimal use of a 4 litres/minute oxygen flow. Delivery of a safe inspired oxygen concentration can be ensured with economical oxygen flows.