PEEP-induced changes in lung volume in acute respiratory distress syndrome. Two methods to estimate alveolar recruitment
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Lung volumes, especially functional residual capacity (FRC), are decreased in acute respiratory distress syndrome (ARDS). Positive end-expiratory pressure (PEEP) contributes to increased end-expiratory lung volume (EELV) and to improved oxygenation, but differentiating recruitment of previously nonaerated lung units from distension of previously open lung units remains difficult. This study evaluated simple methods derived from bedside EELV measurements to assess PEEP-induced lung recruitment while monitoring strain.
Prospective multicenter study in 30 mechanically ventilated patients with ARDS in five university hospital ICUs. Two PEEP levels were studied, each for 45 min, and EELV (nitrogen washout/washin technique) was measured at both levels, with the difference (Δ) reflecting PEEP-induced lung volume changes. Alveolar recruitment was measured using pressure-volume (PV) curves. High and low recruiters were separated based on median recruitment at high PEEP. Minimum predicted increase in lung volume computed as the product of ΔPEEP by static compliance was subtracted from ΔEELV as an independent estimate of recruitment. Estimated and measured recruitments were compared. Strain induced by PEEP was also calculated from the same measurements.
FRC was 31 ± 11% of predicted. Median [25th–75th percentiles] PEEP-induced recruitment was 272 [187–355] mL. Estimated recruitment correlated with recruited volume measured on PV curves (ρ = 0.68), with a slope close to identity. The ΔEELV/FRC ratio differentiated high from low recruiters (110 [76–135] vs. 55 [23–70]%, p = 0.001). Strain increase due to PEEP was larger in high recruiters (p = 0.002).
PEEP-induced recruitment and strain can be assessed at the bedside using EELV measurement. We describe two bedside methods for predicting low or high alveolar recruitment during ARDS.
KeywordsNitrogen washout/washin End-expiratory lung volume Functional residual capacity Acute respiratory distress syndrome Mechanical ventilation Positive end-expiratory pressure Lung recruitment
General Electric provided the “Engström” ventilators for the study and a research grant, but had no access to the data, analysis, interpretation or writing of the manuscript.
Conflict of interest
Four authors and their institution are involved in a patent with General Electric describing the method used to estimate alveolar recruitment described in the manuscript. The following persons and their institutions are involved: Jean Dellamonica for CHU de Nice, Hôpital L’Archet, Université de Nice Sophia Antipolis, France; Alain Mercat for CHU Angers, Angers, France; Jean-Christophe M. Richard for CHU Charles Nicolle, Rouen, France; Laurent Brochard for Assistance Publique-Hopitaux de Paris, groupe hospitalier Henri Mondor, Créteil, France and Université Paris EST, Créteil, France. A grant was also received from General Electric for the conduct of the study. General Electric had no access to the data nor to the content of the manuscript. All authors kept full control of the analysis of the data and the writing of the manuscript.
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