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The aim of this study was to determine the resistances of endotracheal tubes (ETTs) commonly used in neonatal and pediatric intensive care units and the relationship of resistance to flow rate, size, and shape of ETT.Laboratory-based measurements.We examined straight tubes with inner diameters between 2.5 and 6 mm and shouldered (Cole) tubes with inner diameter/outer diameter between 2.5/4 and 3.5/5 mm. We assessed ETT resistance at standard and "appropriate for patient use" lengths at flow rates from 0 L/min to 30 L/min. We used calibrated rotameters to control the flow of gas and proximal static pressure measured by using either an industrial draft gauge or a differential pressure transducer. The ETT resistance was calculated by dividing the proximal ETT pressure by the measured flow and expressed as the mean of three measurements at each flow rate.Resistance increased as ETT diameter decreased; at flows of 5 L/min and 10 L/min, the resistances of the 6 mm inner diameter ETT were 3.1 H2O/L/sec and 4.6 cm H2O/L/sec, respectively, and the resistances of the 2.5 mm inner diameter ETT were 81.2 H2O/L/sec and 139.4 cm H2O/L/sec, respectively. Shortening an ETT to a length appropriate for patient use (e.g., a 4.0 mm inner diameter, from 20.7 to 11.3 cm) reduced its resistance on average by 22%. The resistance of a Cole tube was ∼50% lower than that of a straight tube with an inner diameter corresponding to the narrow part of the shouldered tube.Our results suggest that the use of a small-diameter, straight ETT will significantly increase the work of breathing.