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Lung emptying in patients with acute respiratory distress syndrome: effects of positive end-expiratory pressure

Dept of Intensive Care Medicine, University Hospital of Heraklion, University of Crete, Heraklion, Crete, Greece

CORRESPONDENCE: D. Georgopoulos, Dept of Intensive Care, University Hospital of Heraklion University of Crete, Heraklion, Crete, Greece. Fax: 30 81392636. E-mail: georgop@med.uoc.gr

Keywords: compliance, flow limitation, resistance, time constant

Received: June 25, 2001
Accepted January 7, 2002

This study was supported by a grand from Cretan Critically Ill Association (CCIA).

The pattern of lung emptying was studied in 10 mechanically-ventilated patients with acute respiratory distress syndrome.

At four levels of positive end-expiratory pressure (PEEP) (0, 5, 10 and 15 cmH₂O) tracheal (P_tr) and airway pressures (P_aw), flow (V') and volume (V) were continuously recorded. Tidal volume was set between 0.5–0.6 L and V'/V curves during passive expiration were obtained. Expired volume was divided into five equal volume slices and the time constant (e) and effective deflation compliance (C_rseff) of each slice was calculated by regression analysis of V/V' and postocclusion V/P_tr relationships, respectively. In each slice, the presence or absence of flow limitation was examined by comparing V'/V curves with and without decreasing P_aw. For a given slice, total expiratory resistance (R_tot) (consisting of the respiratory system (R_rs), endotracheal tube (R_tube) and ventilator circuit (R_vent)) was calculated as the e/C_rseff ratio. In the absence of flow limitation R_rs was obtained by subtracting R_tube and R_vent from R_tot, while in the presence of flow limitation R_rs equaled R_tot. The e of the pure respiratory system (e_rs) was calculated as the product of R_rs and C_rseff.

At zero PEEP, e_rs increased significantly towards the end of expiration (52±31%) due to a significant increase in R_rs (46±36%). Application of PEEP significantly decreased R_rs at the end of expiration and resulted in a faster and relatively constant rate of lung emptying.

In conclusion, without positive end-expiratory pressure the respiratory system in patients with acute respiratory distress syndrome deflates with a rate that progressively decreases, due to a considerable increase in expiratory resistance at low lung volumes. Application of positive end-expiratory pressure decreases the expiratory resistance, probably by preventing airway closure, and as a result modifies the pattern of lung emptying.