Low measured auto-positive end-expiratory pressure during mechanical ventilation of patients with severe asthma: Hidden auto-positive end-expiratory pressure

Objective: To describe the occurrence of low measured auto-end expiratory pressure (auto PEEP) during mechanical ventilation of patients with severe asthma. Design: Observational clinical study. Setting: Medical intensive care unit of a university affiliated county hospital. Patients: Four mechanically ventilated patients with severe asthma who had low measured auto-PEEP despite marked increase in both peak and plateau airway pressures. Interventions: None. Measurements and Main Results: Peak 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 +/- 7 to 53 +/- 6 cm H2O; p < .001) and in plateau pressure (28 +/- 2 to 18 +/- 3 cm H2O; p < .001), but only minimal change in auto-PEEP (5 +/- 3 to 4 +/- 3 cm H2O). The difference between plateau pressure and auto-PEEP decreased between the early and late time points (23 +/- 1 to 14 +/- 1 cm H2O; p < .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 +/- 4 to 5 +/- 4 cm H2O), but a much smaller change in plateau pressure (31 +/- 3 to 29 +/- 3 cm H2O). Conclusions: We 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.