Comparison of the Magill and Lack anaesthetic breathing systems in anaesthetized patients.

The Magill and Lack anaesthetic breathing systems were compared by measuring inspired and expired carbon dioxide concentrations and expired minute volumes in lightly anaesthetized, unstimulated subjects. There were no significant differences between the two breathing systems at fresh gas flow rates of approximately 50 and 70 ml kg-1 min-1. Inspired carbon dioxide concentrations increased in one of six subjects at the higher fresh gas flow rate using the Magill system and in two using the Lack system. Inspired carbon dioxide concentration did not increase in only one of six subjects at the lower fresh gas flow rate with both systems. Expired carbon dioxide concentrations and expired minute volume increased in the majority of subjects at both fresh gas flow rates using each system. We conclude that a fresh gas flow rate greater than 70 ml kg-1 min-1 (which approximated to alveolar minute volume in our subjects) should be supplied to the Magill and Lack breathing systems.