Resistance and inertia of endotracheal tubes used in infants during periodic flow.

To study the effects of a periodic pattern of gas flow on the dynamic behavior of infant endotracheal tubes, we measured the resistance (expressed as a function of gas flow) and inertia of endotracheal tubes of 2.5, 3.0, and 3.5-mm internal diameter under conditions of both periodic and quasisteady gas flow. We examined how resistance and inertia are affected by ventilatory rate, the direction of gas flow through the tube, and the expansion of the airway caliber at the junction of the tube and the trachea. The resistance of the endotracheal tubes was proportional to gas flow for ventilatory rates of 60, 90, and 120 breath/min and for quasisteady flow, indicating turbulence even at low flow rates. The resistance of the tubes was less sensitive to ventilatory rate, direction of flow, and expansion of the airway caliber than that of larger endotracheal tubes, a finding explained by the flow conditions defined by the Womersley and Reynolds numbers in these tubes. All tubes exhibited measureable inertia, as predicted from their small diameter. The resistive and inertial properties of the endotracheal tubes described in this report can be used to evaluate the contribution of the endotracheal tube to the dynamics of breathing in intubated infants.