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

J Guttmann, V Kessler, G Mols, R Hentschel, C Haberthür, K Geiger
Critical Care Medicine 2000, 28 (4): 1018-26

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 ETT (Ptrach) 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 ETTs (inner diameter, 2.5-6.5 mm) was determined using a physical model consisting of a tube connector, an anatomically curved ETT, 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 Ptrach was compared with directly measured Ptrach 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 x sec2 (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 Ptrach were within +/- 1 cm H2O. Correspondence between measured and calculated Ptrach is improved even further when the ETT inertance is taken into account.

CONCLUSIONS: Ptrach 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.

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