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.