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 t he measurement of pulmonary mechanics during mechanical ventilation. W e also evaluated the influence of different ventilator settings on the magnitude of leak. An adjustable leak was created at the end of the E TT in a lung model with constant compliance. Flow, tidal volume, and p ressure changes were measured above and below the leak. Compliance (C- i) and resistance (R(i)) during inspiration were determined by linear regression analysis (LRA) using the equation of motion and the Mead an d Wittenberger method (MWM). The ventilatory change that influenced th e degree of leak most was prolongation of inspiratory time (T-i). The presence of a leak around the ETT resulted in an overestimation of the C-i and R(i) values, which was proportional to the size of the leak. This overestimation was also influenced by the method used to determin e C-i and R(i). Because the contribution of the leak to the tidal volu me progressively increased as inspiration continued, methods of analys is that depended mainly on measurement points at the end of inspiratio n showed a larger deviation from the true C-i and R(i) values than met hods mainly influenced by measurement points at the beginning of inspi ration. Because of this, shortening of inspiration, or analysis of poi nts at the beginning of inspiration reduces the error in the measureme nts of C-i and R(i) when a leak is present. Breaths with a large leak should be excluded from any analysis of pulmonary mechanics. (C) 1996 Wiley-Liss, Inc.