Effects of upper airway obstruction on respiratory mechanics in a variable compliance model

ABSTRACT

BACKGROUND: Upper airway obstruction is caused by an intrinsic or extrinsic neck mass and vocal cord paralysis. A recognized hazard of prolonged endotracheal intubation is progressive airway occlusion resulting from deposition of secretions. If the obstruction persists, it may be life threatening condition. However, early diagnosis of partial airway obstruction is very difficult because patients are asymptomatic and do not have lesions with abnormal radiological characteristics. METHODS: In the test lung model, lung compliances were set to normal (25 ml/cmH2O; [control, C25 group]) and to levels seen in chronic obstructive pulmonary disease (40 ml/cmH2O; [C40 group]), and acute respiratory distress syndrome (20 ml/cmH2O; [C20 group] and 15 ml/cmH2O; [C15 group]). A ventilator (Drager Fabius GS, Germany) was attached to a test lung, and a series of endotracheal tubes (ETTs) ranging in size from 7.5 to 2.5 mm in inner diameter (ID) of the connector were used to simulate progressive occlusion. During the lung compliance and the connector size were changed, we measured some respiratory mechanics. RESULTS: Progressive ETT occlusion induced an increase in the peak inspiratory pressure. In the C40 group, the inspiratory pause pressure spontaneously increased on repeated ventilation. Auto- positive end-expiratory pressure (Auto-PEEP) was observed under the condition of high compliance and occlusion. Dynamic compliance decreased at an ID of 5.5 mm in all groups. Respiratory resistance was inversely proportional to the ID of the connector. CONCLUSIONS: The dynamic compliance and resistance were significantly changed. However the change of static compliance had little effect on respiratory mechanics.