Lung expansion, airway pressure transmission, and positive end-expiratory pressure.

Transmission of airway pressure to the intrapleural space and change in functional residual capacity by positive end-expiratory pressure (PEEP) were measured in ten anesthetized swine. Measurements and calculations were performed with varying lung and chest wall compliances. When both compliances were normal, approximately half of the applied airway pressure was transmitted. Aspiration of hydrochloric acid reduced lung compliance approximately fourfold and decreased airway pressure transmission. Increased thoracic compliance also reduced airway pressure transmission. When acid aspiration reduced lung compliance and sternotomy simultaneously increased thoracic compliance, pressure transmission was maximally reduced. Decreases in either thoracic or lung compliance reduced the volume-expanding effects of PEEP. Positive end-expiratory pressure was least effective when thoracic and lung compliances were reduced simultaneously. Careful assessment of both lung and thoracic compliances may be helpful in treating patients requiring elevated airway pressure.