Hyperinflation and Trapped Gas in Chronic Airflow Limitation

Abstract

Lung volume is determined in part by the elastic forces of the resting thoracic cage. At functional residual capacity (FRC), the elastic recoil of the respiratory system is zero. The lung expands from this point by the application of inspiratory muscle effort along the linear portion of the thoracic compliance curve and in doing so maintains ventilation at the most cost effective point in terms of energy expenditure. In chronic airflow limitation resistance develops in the airways, particularly small airways, and the elastic recoil of the lung is diminished by destruction of elastin in alveolar walls. The resting volume at end expiration is associated with an increased alveolar pressure termed intrinsic PEEP (positive end expiratory pressure). More inspiratory work is required to overcome this pressure and expand the lung which by then may have been pushed into the non-linear part of the compliance curve. As the time constant of alveolar emptying is lengthened increased rate of breathing will further increase lung volume. These mechanical changes are likely to affect ventilation during acute exacerbations and on exercise, but will also be present to some extent in convalescence.