Effect of airway diameter, suction catheters, and the bronchofiberscope on airflow in endotracheal and tracheostomy tubes.

Pressure-flow relationships of an artificial airway-mechanical ventilator system were investigated. Endotracheal tubes varying from 7 to 9.5 mm. internal diameter and tracheostomy tubes from 7 to 10 mm. internal diameter were tested over flow-rates from 0.3 to 1.4 L. per second using a tidal volume of 500 ml. The effect of partial obstruction within these airways by suction catheters and the bronchofiberscope was examined. Depending on the internal diameter and the flow-rate, the driving pressure necessary to overcome the resistance of the tubes ranged from 5 to 20 cm. H2O. Partial obstruction of the artificial airways by suction catheters further increased this pressure. The introduction of the bronchofiberscope through an endotracheal tube of 7.5 mm. internal diameter, leaving an effective diameter of 4.9 mm., produced a driving pressure as high as 70 cm. H2O and reduced the flow rate to 50 per cent and the tidal volume to 80 per cent of its set value. Attention should be paid to duration of tracheobronchial suction while the patient is removed from the respirator since a high-grade resistance is created by passage of a suction catheter through an endotracheal tube. Bronchofiberscopy should be cautiously performed through cuffed artificial airways with at least 8 mm. internal diameter because the airway may be too comprised even if mechanical ventilatory assistance is employed.