Effects of PEEP on dynamic hyperinflation in patients with airflow limitation.

We have studied the effects of extrinsically applied PEEP (PEEPe) and intrinsic PEEP (PEEPi) on lung volume and peak airway pressure (Paw,peak) in 10 patients with airflow limitation during mechanical ventilation. PEEPe was applied in 2-4 cm H2O increments until values greater than PEEPi were reached. Total lung hyperinflation was quantified by measuring the expired volume resulting from deflation (starting at end inspiration) until cessation of expiratory flow. The previous expired tidal volume was subtracted from this volume to obtain the change in functional residual capacity (delta FRC), the hyperinflation resulting from PEEPi and PEEPe. PEEPi (0.49-1.66 kPa) was demonstrated in all patients before the application of PEEPe and correlated with delta FRC (r = 0.71), with delta FRC increasing by 582 ml/kPa PEEPi (P < 0.05). PEEPe at pressures less than PEEPi increased delta FRC by (mean) 186 (SEM) 34 ml/kPa PEEPe (P < 0.05) and increased Paw,peak by 0.6 (0.12) kPa/kPa PEEPe (P < 0.05). In contrast, PEEPe at pressures greater than PEEPi, increased delta FRC by 695 (128) ml/kPa PEEPe (P < 0.05) and Paw,peak by 1.8 (0.26) kPa/kPa PEEPe. We conclude that PEEPe may be applied cautiously at values less than PEEPi when clinically indicated, but the application of PEEPe at values greater than PEEPi may substantially aggravate lung hyperinflation.