ObjectiveTo describe the occurrence of low measured auto-end-expiratory pressure (auto-PEEP) during mechanical ventilation of patients with severe asthma.DesignObservational clinical study.SettingMedical intensive care unit of a university-affiliated county hospital.PatientsFour mechanically ventilated patients with severe asthma who had low measured auto-PEEP despite marked increase in both peak and plateau airway pressures.InterventionsNone.Measurements and Main ResultsPeak pressure, plateau pressure, and auto-PEEP were measured at an early time point, when airflow obstruction was most severe, and again at a later time after clinical improvement. Auto-PEEP was measured by the method of end-expiratory airway occlusion. From the early to the late time point, there was a marked decrease in peak pressure (76 plus minus 7 to 53 plus minus 6 cm H2O; p less than .001) and in plateau pressure (28 plus minus 2 to 18 plus minus 3 cm H2O; p less than .001), but only minimal change in auto-PEEP (5 plus minus 3 to 4 plus minus 3 cm H2O). The difference between plateau pressure and auto-PEEP decreased between the early and late time points (23 plus minus 1 to 14 plus minus 1 cm H2O; p less than .01), even though tidal volume was larger at the late time point. In three patients, low auto-PEEP and a large difference between plateau pressure and auto-PEEP was only seen after expiratory time was prolonged. In these three patients, prolongation of expiratory time resulted in a large decrease in measured auto-PEEP (14 plus minus 4 to 5 plus minus 4 cm H2O), but a much smaller change in plateau pressure (31 plus minus 3 to 29 plus minus 3 cm H2O).ConclusionsWe conclude that measured auto-PEEP may underestimate end-expiratory alveolar pressure in severe asthma, and that marked pulmonary hyperinflation may be present despite low measured auto-PEEP, especially at low respiratory rates. This phenomenon may be due to widespread airway closure that prevents accurate assessment of alveolar pressure at end-expiration.(Crit Care Med 1996; 24:541-546)