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The purpose of this clinical trial was to investigate the safety and efficacy of semi-closed circle ventilation of the Dräger anaesthesia ventilators (Cicero, Cato), using a fresh gas flow (FGF) of 600 ml.min-1. Twenty infants, weighing less than 6000 g, without cardiorespiratory abnormalities who required general anaesthesia of at least 30 min were included. The FGF was reduced to 600 ml.min-1 after 10 min of denitrogenation with a FGF of 4 to 61.min-1. The composition of the FGF (600 ml.min-1) was calculated as follows: oxygen necessary for consumption (60 ml.min-1) plus the remaining FGF in a 1:2 relationship for oxygen. The inspiratory nitrogen fraction was calculated to exclude accumulation. Inspiratory fractions of O2 and N2O plus inspiratory and endtidal CO2 partial pressures and noninvasive oxygen saturation were the control parameters. The gas concentrations (O2 and N2O) remained within safe limits. Hypoxic gas concentrations were not observed. Neither nitrogen nor CO2 accumulated in the circle system. In conclusion, low flow anaesthesia can be performed safely in infants under 6000 grams with the Dräger Cicero and Cato anaesthesia ventilators.