Potential for Lung Recruitment Estimated by the Recruitment-to-Inflation Ratio in Acute Respiratory Distress Syndrome.

Abstract

RATIONALE\nResponse to PEEP in acute respiratory distress syndrome (ARDS) depends on recruitability. We propose a bedside approach to estimate recruitability accounting for the presence of complete airway closure.\n\n\nOBJECTIVES\nTo validate a single-breath method for measuring recruited volume, and test whether it differentiates patients with different responses to PEEP.\n\n\nMETHODS\nPatients with ARDS were ventilated at 15 and 5 cmH2O of PEEP. Multiple pressure-volume curves were compared to a single-breath technique. Abruptly releasing PEEP (15 to 5 cmH2O) increases expired volume: the difference between this volume and the volume predicted by compliance at low PEEP (or above airway opening pressure) estimates the recruited volume by PEEP. This recruited volume divided by the effective pressure change gave the compliance of the recruited lung; the ratio of this compliance to the compliance at low PEEP gave the Recruitment-to-Inflation ratio. Response to PEEP was compared between high and low recruiters based on this ratio.\n\n\nMEASUREMENTS AND MAIN RESULTS\nForty-five patients were enrolled. Four patients had airway closure higher than high PEEP and recruitment could not be assessed. In others, recruited volume measured by the experimental and the reference methods were strongly correlated (P< 0.0001, R2= 0.798) with small bias (-21 mL). The Recruitment-to-Inflation ratio (median 0.5, range 0 to 2.0) correlated with both oxygenation at low PEEP and the oxygenation response; at PEEP 15, high recruiters had better oxygenation (P = 0.004), whereas low recruiters experienced lower systolic arterial pressure (P = 0.008).\n\n\nCONCLUSIONS\nA single-breath method quantifies recruited volume. The Recruitment-to-Inflation ratio might help to characterize lung recruitability at the bedside.