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

Abstract

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 cmH 2 O) tracheal (Ptr) and airway pressures (Paw), flow (V9) and volume (V) were continuously recorded. Tidal volume was set between 0.5-0.6 L and V9/V curves during passive expiration were obtained. Expired volume was divided into five equal volume slices and the time constant (te) and effective deflation compliance (Crs eff ) of each slice was calculated by regression analysis of V/V9 and postocclusion V/Ptr relationships, respectively. In each slice, the presence or absence of flow limitation was examined by comparing V9/V curves with and without decreasing Paw. For a given slice, total expiratory resistance (Rtot) (consisting of the respiratory system (Rrs), endotracheal tube (Rtube) and ventilator circuit (Rvent)) was calculated as the te/Crs eff ratio. In the absence of flow limitation Rrs was obtained by subtracting Rtube and Rvent from Rtot, while in the presence of flow limitation Rrs equaled Rtot. The te of the pure respiratory system (ters) was calculated as the product of Rrs and Crs eff .At zero PEEP, ters increased significantly towards the end of expiration (52¡31%) due to a significant increase in Rrs (46¡36%). Application of PEEP significantly decreased Rrs 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.

Effect of Albuterol on Expiratory Resistance in Mechanically Ventilated Patients

“…The protocol for the evaluation of expiratory resistance and flow limitation includes 3 steps, previously described. 6,7 The airway is occluded at end-inspiration for 3 s (with the end-inspiratory hold button on the ventilator), then:…”

“…We measured flow, volume, airway pressure, and tracheal pressure at the airway opening, as previously described. 6,7 All inhaled bronchodilators were withheld for 6 hours before the study period. PEEP was zero during the study period.…”

“…We assessed flow limitation by comparing the expiratory flows at a given portion of lung volume, as previously described. 6,7 …”

“…6,7 At each experimental condition we plotted volume against flow and identified the inflection point of the curve, defined as the point of maximum slope following the expiratory peak flow. We then subdivided the expired volume from the inflection point to the end of expiration into 5 equal slices (Fig.…”

“…6,7 The present physiology study evaluates the effect of inhaled albuterol on expiratory re-sistance in patients with COPD exacerbation, and examines the correlation of albuterol-induced changes in expiratory and inspiratory resistance.…”