Effects of endotracheal tube size and ventilator settings on the mechanics of a test system during intermittent flow ventilation.

The effect of varying the size of standard neonatal endotracheal tubes on delivered tidal volumes (VT), resistance (R), dynamic compliance (Cdyn), and resistive work of breathing (WOB) was measured in a test system during intermittent flow ventilation at different ventilator settings. The experiments were performed with a Sechrist infant ventilator connected to a Dräger Test Lung via standard neonatal endotracheal tubes. R, inspiratory (Ri), and expiratory resistance (Re) as well as WOB were significantly affected by endotracheal tube size. The calculated difference in Re between endotracheal tubes of 2.5 and 3.5 mm I.D. was 93 cm H2O/L/S (mean value for all studies). Cdyn and VT were also affected by endotracheal tube size. However, although statistically significant differences were found in Cdyn (mean, 0.584 mL/cm H2O and 0.567 mL/cm H2O) and VT (mean, 13.0 mL and 12.7 mL) for the tube sizes 3.5 mm and 2.5 mm I.D., respectively, the absolute numerical differences were small. Also, ventilator settings with respect to the peak inspiratory pressure (PIP) - positive end-expiratory pressure (PEEP) difference had a significant influence on Cdyn for both tube sizes. On the other hand, flow and inspiratory time adjustments had no significant effect on ventilatory parameters. Endotracheal tube size and ventilator settings should be considered when comparing the pulmonary function tests in intubated and non-intubated newborn infants.