Continuous calculation of intratracheal pressure in the presence of pediatric endotracheal tubes

Objective: To measure the pressure-flow relationship of pediatric endotracheal tubes (ETTs) in trachea models, to mathematically describe this relationship, and to evaluate in trachea/lung models a method for calculation of pressure at the distal end of the err (P-trach) by subtracting the flow-dependent pressure drop across the ETT from the airway pressure measured at the proximal end of the ETT. Design:Trachea models and trachea/lung models. Setting: Research laboratory in a university medical center. Interventions: The pressure-flow relationship of pediatric errs (inner diameter, 2.5-6.5 mm) was determined using a physical model consisting of a tube connector, an anatomically curved err, and an artificial trachea. The model was ventilated with sinusoidal gas flow (12-60 cycles/min), The coefficients of an approximation equation considering ETT resistance and inertance were fitted separately to the measured pressure-flow curves for inspiration and expiration, Calculated P-trach was compared with directly measured P-trach in mechanically ventilated physical trachea/lung models. Measurements and Main Results: The pressure-flow relationship was considerably nonlinear and showed hysteresis around the origin caused by the inertia of accelerated gas, ETT inertance ranged from 0.1 to 0.4 cm H2O/L.sec(2) (inner diameter, 6-2.5 mm), The abrupt change in cross-sectional area at the tube connector caused an inspiration-to-expiration asymmetry. Calculated and measured P-trach were within +/- 1 cm H2O. Correspondence between measured and calculated P-trach is improved even further when the ETT inertance is taken into account. Conclusions: P-trach can continuously be monitored in the presence of pediatric ETT by combining ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.