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

Objective: To measure the pressure-flow relationship of pediatric endotrach eal tubes (ETTs) in trachea models, to mathematically describe this relatio nship, and to evaluate in trachea/lung models a method for calculation of p ressure at the distal end of the err (P-trach) by subtracting the flow-depe ndent 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 diam eter, 2.5-6.5 mm) was determined using a physical model consisting of a tub e connector, an anatomically curved err, and an artificial trachea. The mod el was ventilated with sinusoidal gas flow (12-60 cycles/min), The coeffici ents 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-t rach in mechanically ventilated physical trachea/lung models. Measurements and Main Results: The pressure-flow relationship was considera bly 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 a nd measured P-trach were within +/- 1 cm H2O. Correspondence between measur ed and calculated P-trach is improved even further when the ETT inertance i s taken into account. Conclusions: P-trach can continuously be monitored in the presence of pedia tric ETT by combining ETT coefficients and the flow and airway pressure con tinuously measured at the proximal end of the ETT.