"Intrinsic" PEEP (PEEPi): role of expiratory muscles

Abstract

In normal subjects breathing at rest, lung volume at end-expiration corresponds with the elastic equilibrium volume, or relaxation volume (Vr), of the respiratory system. If expiratory duration is not long enough, then inspiration may begin before the system has returned to Vr, such that end-expiratory lung volume is higher than Vr. This condition is named dynamic pulmonary hyperinflation and intrinsic positive end-expiratory pressure (PEEPi), also called auto PEEP, corresponds to the elastic recoil of the lung at end-expiration [1, 2]. In the spontaneously breathing subject, this dynamic hyperinflation occurs mainly when the rate of lung emptying is decreased owing to high respiratory resistance or flow limitation. The highest degrees of dynamic hyperinflation are, thus, encountered in patients with airway diseases, particularly chronic obstructive pulmonary disease (COPD) patients. In the ventilated patient, external determinants may also induce dynamic pulmonary hyperinflation, or increase the magnitude of dynamic hyperinflation that was already present before the onset of mechanical ventilation. These additional external determinants include a preceding inflation volume that is too large, a time available for expiration that is reduced or a decreased rate of lung emptying owing to high resistance associated with the tube, the ventilator tubing or devices [3]. Dynamic pulmonary hyperinflation has important pathophysiological and therapeutic implications mainly in critical care medicine [3]. In particular, it implies that as the inspiratory muscles start to contract they must first offset the elastic recoil pressure of the system before inspiratory flow can begin and lung volume can increase. PEEPi associated with dynamic pulmonary hyperinflation thus corresponds to an inspiratory threshold load and is associated with an increased inspiratory work of breathing. This has led several investigators to give external PEEP or continuous positive airway pressure (CPAP) in the belief that this would unload the inspiratory muscles and, therefore, reduce the inspiratory work of breathing [4–6]. It must be stressed, however, that PEEPi and dynamic pulmonary hyperinflation are not always linked. In fact, alveolar pressure can also remain positive throughout expiration when the expiratory muscles contract [1, 5]. With respect to this, recent studies have shown that in many patients with stable but severe COPD [7] as well as in many mechanically ventilated patients [8, 9], expiration is a mechanically active process. These observations, thus, provide strong evidence that the active nature of expiration is an important determinant of PEEPi in such patients and, therefore, that the importance of dynamic pulmonary hyperinflation has been overestimated. Since a positive end-expiratory alveolar pressure resulting from an expiratory muscle contraction does not represent an inspiratory threshold load, the level of external PEEP that is needed in such patients must be significantly less than conventionally thought. The contribution of expiratory muscle contraction to the increase in end-expiratory alveolar pressure has to be taken into account when applying external PEEP or CPAP to offset PEEPi or these procedures might, in fact, result in enhanced pulmonary dynamic hyperinflation with further deleterious consequences [4, 10]. In this issue of the European Respiratory Journal, ZAKYNTHINOS and co-workers [11] report on a method for detecting expiratory muscle use and quantifying its contribution to PEEPi. The two points at issue in this and other studies [7–9] are: