Influence of endotracheal tube diameter on forced deflation flow-volume curves in rhesus monkeys.

The forced deflation (FD) technique is the recommended gold standard to generate forced expiratory vital capacity (FVC) curves and to measure maximum expiratory flow-volume (MEFV) relationships in intubated infants and children. However, the influence of the endotracheal tube (ETT) on the site of flow limitation, the shape and the analysis of the resultant MEFV curves have not been defined. Nine anaesthetized (thiopentone, 8 mg x kg(-1) x h(-1)) rhesus monkeys (mean weight (+/-SEM) 10+/-1 kg) were intubated consecutively with ETTs of different internal diameters (ID 3.0-5.5 mm, at intervals of 0.5 mm); the largest representing the appropriate ETT size for the animal. Quadruplicate MEFV curves were generated by FD using the standard +40 cmH2Oinsp/-40 cmH2Oexp pressures and recorded at each ETT diameter. The effect of the different ETT diameters on the FD flow-volume curve was analysed by comparing maximum expiratory flows at isovolume points at 50, 25 and 10% FVC (MEF50, MEF25 and MEF10, respectively). The shape of the resulting MEFV curves could be divided into an initial horizontal part (tube-dependent), and a final descending slope (airway-dependent). No difference in FVC occurred irrespective of the ETT size (p>0.05 by analysis of variance (ANOVA)). MEF50 increased with increasing tube diameter (p<0.05). MEF25 remained unchanged using ETTs which were less than 1.5 mm smaller than the largest ETT. Smaller ETTs resulted in significantly reduced MEF25 (p<0.05). MEF10 was not influenced by the ETT size. We conclude that maximum expiratory flows measured by the forced deflation technique are not influenced by an appropriately sized endotracheal tube at lung volumes below 25% forced vital capacity in our monkey model with normal lungs. We postulate that the effect of endotracheal tubes on maximum expiratory flow volume curves in intubated infants might be of similar or even smaller magnitude, which remains to be established.