Dynamic hyperinflation and ventilator dependence in chronic obstructive pulmonary disease.

In advanced chronic obstructive pulmonary diseases, functional residual capacity (FRC) can be markedly increased by dynamic mechanisms involving expiratory flow limitation. We studied respiratory mechanics in a seated ventilator-dependent patient with such changes. Relaxed expiration was flow-limited; pressures of 9 to 27 cm H2O (varying with lung volume) could be applied to the airway opening (Pao) without decreasing expiratory flow rate. The FRC was at least 2 L above relaxation volume. Inspiratory total resistance was 16 cm H2O/L/s. Compliance of the lung was 0.16, chest wall was 0.04, and respiratory system was 0.032 L/cm H2O. More importantly, recoil pressures at end inspiration and end expiration, respectively, were 6.5 and 1.5 cm H2O for the lung, 33 and 11 cm H2O for chest wall, and 38 and 12 cm H2O for the respiratory system. Thus the chest wall recoiled inward at all times, pleural pressure was always substantially positive (11 to 33 cm H2O), expiratory flow was maximal, and jugular veins were always full and nonpulsating. Inspiratory work was about 0.27 kg-m per breath (7 times normal), most of it elastic work done on the chest wall.