Nonhomogeneous alveolar pressure swings in the presence of airway closure

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Abstract
When thoracic gas volume (TGV) is determined plethysmographically, it is assumed that the alveolar pressure swings are homogeneous and are appropriately represented by pressure swings at the mouth. There are differences in total lung capacities derived from TGV measurements made at different levels in the vital capacity. In the presence of airway closure alveolar pressure swings may be nonhomogeneous during a TGV determination. This possibility was tested in 6 dogs. Pressure [P] at the airway opening (ao) was measured from an endotracheal catheter. A balloon-tipped catheter was passed into the right lower lobe (RLL) bronchus for measurement of RLL pressure. .DELTA.PRLL-.DELTA.Pao was monitored during inspiratory efforts with the airway opening occluded. With the RLL balloon inflated, .DELTA.PRLL always exceeded .DELTA.Pao by an amount averaging 8.2%. Induction of a pneumothorax eliminated all differences between .DELTA.PRLL and .DELTA.Pao. During a TGV measurement the chest wall may apply to the lungs nonhomogeneous forces that, in the presence of airway closure (e.g., chronic obstructive pulmonary disease and asthma), would result in nonhomogeneous alveolar pressure swings and potentially significant errors in the plethysmographic determination of TGV.