Estimation of inspiratory pressure drop in neonatal and pediatric endotracheal tubes.

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Abstract
Endotracheal tubes (ETTs) constitute a resistive extra load for intubated patients. The ETT pressure drop (ΔPETT) is usually described by empirical equations that are specific to one ETT only. Our laboratory previously showed that, in adult ETTs, ΔPETT is given by the Blasius formula (F. Lofaso, B. Louis, L. Brochard, A. Harf, and D. Isabey. Am. Rev. Respir. Dis. 146: 974–979, 1992). Here, we also propose a general formulation for neonatal and pediatric ETTs on the basis of adimensional analysis of the pressure-flow relationship. Pressure and flow were directly measured in seven ETTs (internal diameter: 2.5–7.0 mm). The measured pressure drop was compared with the predicted drop given by general laws for a curved tube. In neonatal ETTs (2.5–3.5 mm) the flow regime is laminar. The ΔPETT can be estimated by the Ito formula, which replaces Poiseuille’s law for curved tubes. For pediatric ETTs (4.0–7.0 mm), ΔPETT depends on the following flow regime: for laminar flow, it must be calculated by the Ito formula, and for turbulent flow, by the Blasius formula. Both formulas allow for ETT geometry and gas properties.