A New Concept in Controlled Ventilation of Children with the Bain Anesthetic Circuit

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Abstract
The Bain anesthesia circuit was studied as a semi-open or partial rebreathing system during controlled ventilation in 16 children weighing 7.5-48 kg. During anesthesia the lungs were ventilated with a volume ventilator set at 3 times the calculated alveolar ventilation to provide optimum mixing in the exhalation tube of the Bain circuit. Fresh gas inflow rates initially were set equal to the calculated alveolar ventilation, and after 30 to 45 min, PCO [partial pressure of CO2], PO2, and pH values were measured. At the same time, the fractional concentration of mixed expired CO2 [F.hivin.ECO2 was recorded from a capnograph inserted between the ventilator and the Bain circuit. After initial readings, the fresh gas inflow was varied over a range of 1400-3000 ml/m2 per min at 20 min intervals, with the arterial blood-gas values and F.hivin.ECO2 recorded at each setting. A lower fresh gas inflow than previously recommended can be used safely in children. When the minute ventilation is 3 times the fresh gas inflow, values for F.hivin.ECO2 correlate closely with PaCO2 [arterial CO2 tension] values; with a fresh gas inflow of 2500 ml/m2 per min, PaCO2 values can be maintained near 40 torr.