Controlled ventilation with the Bain co-axial system. A rationalisation of gender-related minute volume requirements and carbon dioxide replacement therapy.

The minute ventilation required to maintain either hypocapnia (FE'CO2 = 4.0 kPa, group A, n = 36) or normocapnia (FE'CO2 = 5.0 kPa, group B, n = 38) in 74 adult patients was determined for the Bain system with controlled ventilation at 10 breath.min-1 and fresh gas flow equal to minute ventilation. Males required a greater minute ventilation and fresh gas flow than females (p < 0.01) in each group (group A, males approximately 90 ml.min-1.kg-1, females approximately 80 ml.min-1.kg-1; group B, males approximately 75 ml.min-1.kg-1; females approximately 65 ml.min-1.kg-1). Where capnography is not available, it may be safer to ventilate patients' lungs using the minute volume and fresh gas flow requirements established in group A patients. The administration of 5% carbon dioxide in oxygen for 2 min safely re-established normocapnia and spontaneous respiration in hypocapnic patients when ventilated with the same minute volume, fresh gas flow and respiratory rate required during surgery. Only one patient required greater than 400 ml.min-1 of carbon dioxide to produce a 5% concentration. Maximum flow from the carbon dioxide flowmeter may be restricted to 400 ml.min-1 rather than the 500 ml.min-1 currently recommended.