Dynamic Hyperinflation: Intrinsic Peep and its Ramifications in Patients with Respiratory Failure

Abstract

Pulmonary hyperinflation, namely a consistent increase in the end-expiratory lung volume above the predicted functional residual capacity (FRC), is a characteristic abnormality in patients with acute or chronic airways obstruction (1). While this may be the result of loss of lung elastic recoil (e.g. pulmonary emphysema), dynamic factors may also be responsible. For example, when there is a significant increase in airway resistance, the rate of lung emptying is unduly slowed and, by necessity, is interrupted by the next inspiratory effort. When the breathing frequency increases, as in exercise or when the ventilatory demands are increased for other reasons, the expiratory time shortens and hence the end-expiratory lung volume may increase above the relaxed FRC position (elastic equilibrium volume). This is referred to as dynamic hyperinflation. Expiratory flow may also be reduced by other mechanisms. These include activity of the inspiratory muscles during expiration (post-inspiration inspiratory activity) as well as activation of laryngeal adductor muscles with expiratory narrowing of the glottic aperture (2–F5).