The effect of hyperinflation on lung elasticity in healthy subjects.

In acute severe asthma, lung distensibility may increase. To determine whether hyperinflation alone can increase lung distensibility acutely total lung capacity (TLC) and static volume-pressure (VP) properties were measured in six healthy subjects after quiet breathing, and two periods of hyperinflation of 20-60 min, with negative pressure assistance at the chest wall (NPA) in one period and positive airways pressure assistance (PPA) in the other. In five subjects there was no change with NPA; with PPA lung volume at a static transpulmonary pressure of 10 cm H2O (VL 10) increased by 0.3 L (P less than 0.01) and K, the shape constant of an exponential function fitted to the deflation VP curve, increased (P less than 0.001) without a change in TLC. In Subject 6, with both NPA and PPA, VL 10 increased by 1.3 L (P less than 0.001). K increased (P less than 0.001 with NPA; P less than 0.05 with PPA) and TLC increased (P less than 0.001 with NPA; P less than 0.01 with PPA); volume hysteresis was unchanged. The small increase of lung distensibility in five subjects after PPA we attribute to decreased pulmonary blood volume; in Subject 6 the changes with hyperinflation suggest decreased tissue forces. We conclude that hyperinflation can cause an acute increase in lung distensibility in some individuals. The variable response between subjects parallels that seen in asthma.