Estimation of inspiratory pressure drop in neonatal and pediatric endotracheal tubes

Endotracheal tubes (ETTs) constitute a resistive extra load for intubated patients. The ETT pressure drop (Delta P-ETT) is usually described by empirical equations that are specific to one ETT only. Our laboratory previously showed that;, in adult ETTs, Delta P-ETT 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 Delta P-ETT can be estimated by the Ito formula, which replaces Poiseuille's law for curved tubes. For pediatric ETTs (4.0-7.0 mm), Delta P-ETT 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.