Carbon monoxide within circle systems.

After reports of carbon monoxide toxicity from the United States, the Medicines Control Agency issued a warning which recommended that soda lime used in circle breathing systems should not be allowed to dry out. We have measured carbon monoxide levels within a circle system in vitro (under a variety of conditions) and in vivo. Carbon monoxide was detected only when a patient was connected to the circle system. As levels in the expiratory limb of the circle system were significantly higher than those in the inspiratory limb, we conclude that the patient acts as the source of carbon monoxide.