title: Low measured auto-positive end-expiratory pressure during mechanical ventilation of patients with severe asthma: hidden auto-positive end-expiratory pressure.
abstract: OBJECTIVE
  To describe the occurrence of low measured auto-end-expiratory pressure (auto-PEEP) during mechanical ventilation of patients severe asthma.
  
  
  METHODS
  Observational clinical study.
  
  
  METHODS
  Medical intensive care unit of a university-affiliated county hospital.
  
  
  METHODS
  Four mechanically ventilated patients with severe asthma who had low measured auto-PEEP despite marked increase in both peak and plateau airway pressures.
  
  
  METHODS
  None.
  
  
  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 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.
authors.affiliation: Department of Medicine, Hennepin County Medical Center, Minneapolis, MN 55415, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Leatherman
authors.forename: S A
authors.initials: SA
authors.lastname: Ravenscraft
mesh_terms: D000208:Acute Disease; D000328:Adult; D001249:Asthma / Q000150:complications* / Q000503:physiopathology / Q000628:therapy; D006801:Humans; D018467:Positive-Pressure Respiration, Intrinsic / Q000175:diagnosis / Q000209:etiology* / Q000503:physiopathology; D011446:Prospective Studies; D012121:Respiration, Artificial; D015656:Respiratory Mechanics; D013997:Time Factors