Positive End-Expiratory Pressure (PEEP); Indications and Physiologic Considerations

This report describes a patient in the recovery phase of the adult respiratory distress syndrome in whom the persistence of severe hypoxemia was not corrected by a high fractional concentration of oxygen in the inspired gas and positive end-expiratory pressure. A right-to-left interatrial shunt was diagnosed by M-mode and cross-sectional echocardiography with saline injection, and the presence of a patent foramen ovale was confirmed at the time of cardiac surgery.

A bronchopleural fistula (BPF) may complicate the management of patients with major pulmonary disease or thoracic surgery. Neonates with idiopathic respiratory distress syndrome and requiring ventilation are especially susceptible to pulmonary barotrauma, which may results in a BPF. Morbidity and mortality are consistently high. In ventilating patients with BPF, the effects of peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP) on air leak have not been documented. These relationships were studied in rabbits prepared by thoracotomy and creation of a standardized BPF. Randomized trials of various levels of PIP and PEEP were applied, and the percent of inspired tidal volume lost through the BPF calculated. The percent of inspired volume lost does not increase significantly from 10 to 30 cm H₂O PIP (p>0.05). Percent leak does increase significantly when increasing PEEP from 0 to 16 cm H₂O (p<0.001). Any PEEP greater than 6 cm H₂O results in more air loss through the BPF than any level of PIP (p<0.01). Linear regressions through a common origin were calculated to illustrate the relationship of PIP versus leak and PEEP versus leak. The slopes of these lines (0.572 and 3.97, respectively) are significantly different (p<0.001). When using equal increments of PIP or PEEP PEEP will have over a sixfold greater effect on air leak than does PIP. These data suggests that PIP should be increased preferentially when ventilating patients with BPF in order to minimize air leak. PEEP less than 6 cm H₂O can be used without any significant increase in the volume of air lost.

Structural and mechanical changes were correlated in 29 dogs with acute alveolar injury induced by the subcutaneous administration of N-nitroso-N-methylurethane (NNNMU). The injury was characterized by necrosis and repair of alveolar epithelium while the vascular endothelium remained essentially intact Compliance of the lung (Cl) decreased and elastic recoil increased as epithelial necrosis occurred. During recovery, improved elastic recoil coincided with epithelial regeneration, although Cl remained abnormal. The late phase was characterized histologically by widespread closure of clusters of alveoli alternating with dilated small air spaces.

The process resulted in distortion of lung architecture and resembled interstitial fibrosis. Reduced lung volume and decreased distensibility of dilated small air spaces may have accounted for the persistently abnormal Cl. Because of the specific site of anatomic involvement, the predictable evolution of deranged lung mechanics, and the similarity to human lung injury (adult respiratory distress syndrome), the lung injured by administration of N-nitroso-N-methylurethane is a suitable model to study pathophysiology and types of therapy in a controlled setting.

The cardiopulmonary effects of CPAP under conditions of increased intra-abdominal pressure were studied using the rabbit as an experimental model. Central venous pressure and cardiac output were reduced when the intra-abdominal pressure was increased. Addition of CPAP caused a further reduction in C.V.P. and C.O. Despite these reductions CPAP significantly improved arterial P_aO₂ without significant effects on pH and P_aCO₂. The use of CPAP in conditions associated with pulmonary dysfunction secondary to increased intra-abdominal pressure may be a definite therapeutic modality.