Effect of leak around the endotracheal tube on measurements of pulmonary compliance and resistance during mechanical ventilation: a lung model study.

We studied the effect of leaks around the endotracheal tube (ETT) on the measurement of pulmonary mechanics during mechanical ventilation. We also evaluated the influence of different ventilator settings on the magnitude of leak. An adjustable leak was created at the end of the ETT in a lung model with constant compliance. Flow, tidal volume, and pressure changes were measured above and below the leak. Compliance (Ci) and resistance (Ri) during inspiration were determined by linear regression analysis (LRA) using the equation of motion and the Mead and Wittenberger method (MWM). The ventilatory change that influenced the degree of leak most was prolongation of inspiratory time (Ti). The presence of a leak around the ETT resulted in an overestimation of the Ci and Ri values, which was proportional to the size of the leak. This overestimation was also influenced by the method used to determine Ci and Ri. Because the contribution of the leak to the fidal volume progressively increases as inspiration continued, methods of analysis that depended mainly on measurement points at the end of inspiration showed a larger deviation from the true Ci and Ri values than methods mainly influenced by measurement points at the beginning of inspiration. Because of this, shortening of inspiration, or analysis of points at the beginning of inspiration reduces the error in the measurements of Ci and Ri when a leak is present. Breaths with a large leak should be excluded from any analysis of pulmonary mechanics.