Intrinsic positive end-expiratory pressure in ambulatory patients with airways obstruction

In patients with severe expiratory airflow limitation, dynamic hyperinflation often occurs when inspiratory efforts are initiated at a thoracic volume above the relaxation point of the respiratory system. The result is intrinsic positive end-expiratory alveolar pressure (PEEPi). To determine whether PEEPi occurs in ambulatory patients, we measured alveolar pressure (Palv) noninvasively during tidal breathing in 8 normal subjects, 15 asthmatic subjects, and 19 patients with COPD, using a body plethysmographic technique that includes computerized corrections for nonlinear pneumotachometer output and for plethysmograph leakage. In all 8 normal subjects, 9 asthmatic subjects, and 3 COPD patients, Palv descended smoothly to zero at end expiration. In contrast, among each of the remaining 22 patients, there was an abrupt change in slope of the Palv tracing near end expiration, identifying the onset of the next inspiratory effort and indicating the presence of PEEPi, ranging from 0.2 to 9.5 cm H₂O. PEEPi was significantly correlated with FRC (% of predicted); PEEPi = (0.040 x %FRC) - 3.65, r = 0.73, p < 0.001, and with the reciprocal of FEV₁ (% of predicted), PEEPi = (138/%FEV₁) - 1.34, r = 0.69, p < 0.001. PEEPi could be elicited in normal subjects by severe expiratory resistive loading but not by the increased expiratory muscle activity occurring during an MVV maneuver. We conclude that PEEPi is common in patients with airways obstruction, even without overt ventilatory failure, and that its severity is generally in proportion to the severity of the hyperinflation and the airways obstruction.