Objective: To assess the effects of different positive end-expiratory pressure (PEEP) levels (0, 5, 10, and 15 cm H2O) on tidal expiratory flow limitation (FL), regional intrinsic positive end-expiratory pressure (PEEPi) inhomogeneity, alveolar recruited volume (Vrec), respiratory mechanics, and arterial blood gases in mechanically ventilated patients with acute respiratory distress syndrome (ARDS).
Design: Prospective clinical study.
Setting: Multidisciplinary intensive care unit of a university hospital.
Patients: Thirteen sedated, mechanically ventilated patients during the first 2 days of ARDS.
Interventions: Detection of tidal FL and evaluation of total dynamic PEEP (PEEPt,dyn), total static PEEP (PEEPt,st), respiratory mechanics, and Vrec from pressure, flow, and volume traces provided by the ventilator. The average (+/-sd) tidal volume was 7.1 +/- 1.5 mL/kg, the total cycle duration was 2.9 +/- 0.45 secs, and the duty cycle was 0.35 +/- 0.05.
Measurements: Tidal FL was assessed using the negative expiratory pressure technique. Regional PEEPi inhomogeneity was assessed as the ratio of PEEPt,dyn to PEEPt,st (PEEPi inequality index), and Vrec was quantified as the difference in lung volume at the same airway pressure between quasi-static inflation volume-pressure curves on zero end-expiratory pressure (ZEEP) and PEEP.
Results: On ZEEP, seven patients exhibited FL amounting to 31 +/- 8% of tidal volume. They had higher PEEPt,st and PEEPi,st ( p<.001) and lower PEEPi inequality index ( p<.001) than the six nonflow-limited (NFL) patients. Two FL patients became NFL with PEEP of 5 cm H2O and five with PEEP of 10 cm H2O. In both groups, PaO2 increased progressively with PEEP. In the FL group, there was a significant correlation of PaO2 to PEEPi inequality index ( p=.002). For a given PEEP, Vrec was greater in NFL than FL patients, and a significant correlation of Pao to Vrec ( p<.001) was found only in the NFL group.
Conclusions: We conclude that on ZEEP, tidal FL is common in ARDS patients and is associated with greater regional PEEPi inhomogeneity than in NFL patients. With PEEP of 10 cm H2O, flow limitation with concurrent cyclic dynamic airway compression and re-expansion and the risk of "low lung volume injury" were absent in all patients. In FL patients, PEEP induced a significant increase in PaO2, mainly because of the reduction of regional PEEPi inequality, whereas in the NFL group, arterial oxygenation was improved satisfactorily because of alveolar recruitment.