Recent work in patients with acute respiratory failure (ARF) due to exacerbation of chronic airflow obstruction (CAO) suggests that application of low degrees of positive end-expiratory pressure (PEEP) can improve rather than impair respiratory mechanics, because PEEP replaces intrinsic PEEP (PEEPi). However, the impact of PEEP on pulmonary gas exchange has not been fully investigated. We designed this study to examine the effects of PEEP and those of PEEPi on ventilation/perfusion (VA/Q) mismatching in mechanically ventilated patients with CAO. Eight patients were studied under four conditions: (1) during controlled mechanical ventilation with the ventilatory setting established by the attending physicians (PEEPi-100%), according to standard criteria; (2) after application of PEEP amounting to 50% (PEEP-50%), and then (3) to 100% (PEEP-100%) of the original PEEPi; and finally, (4) after reduction of PEEPi to 50% of the initial value (PEEPi-50%), obtained by increasing expiratory time and decreasing respiratory rate and tidal volume. Respiratory mechanics, hemodynamics, respiratory blood gases, and VA/Q distributions were measured during each ventilatory mode. At low values of PEEP (PEEP-50%) no changes in respiratory mechanics nor in hemodynamics were observed, but PaO2 moderately increased (from 103 +/- 25.2 to 112 +/- 29.6 mm Hg) and PaCO2 slightly decreased (from 42 +/- 3.7 to 40 +/- 3.3 mm Hg) essentially because of an increase in the mean VA/Q ratio (first moment) of both flood flow (Q, from 0.65 +/- 0.28 to 0.78 +/- 0.29) and ventilation (V, from 4.02 +/- 1.55 to 4.93 +/- 2.00) distributions (p < 0.05, each).(ABSTRACT TRUNCATED AT 250 WORDS)