title: Determination of 'recruited volume' following a PEEP step is not a measure of lung recruitability.
abstract: BACKGROUND
  It has been proposed that the analysis of positive end-expiratory pressure (PEEP)-induced volume changes can quantify alveolar recruitment. The potential of a lung to be recruited is expected to be high in acute respiratory distress syndrome (ARDS), where collapsed lung tissue is very common. The volume change that is beyond the delta volume because of the patient's compliance has been termed 'recruited volume' (RecV). However, data of patients with low and high RecV showed less severe lung disease in high 'recruiters', indicating that RecV may not equal the 'potentially recruitable lung tissue' seen in computed tomography scans. We hypothesized that RecV is higher in lung-healthy (LH) patients with little collapsed lung compared with ARDS patients.
  
  
  METHODS
  RecV and inspiratory capacity (IC) were determined in 12 LH and in 25 ARDS patients during incremental PEEP (steps of 2 cmH2 O). RecV was determined as the time-dependent increase in end-expiratory volume following the first expiration to the new PEEP level (ΔTDV). Gas distribution in LH patients was analyzed by electric impedance tomography.
  
  
  RESULTS
  Cumulative RecV(ΔTDV) and IC were higher (P < 0.01) in LH compared with ARDS patients, 1739 ml vs. 832 ml and 4432 ml vs. 2020 ml, respectively. In both groups, RecV correlated excellently with IC (R(2) = 0.86). In LH, RecV emanated mainly from nondependent lung regions at PEEP below 15 cmH2O. Maximum plateau pressure was reached with fewer PEEP steps in ARDS compared with LH patients (11 vs. 14, P < 0.01).
  
  
  CONCLUSIONS
  Our findings suggest that RecV predominately measures a slow fraction of inflation of already aerated lung tissue and not recruitment of collapsed alveoli.
authors.affiliation: Department of Anesthesiology and Intensive Care Medicine, University Medical Center Freiburg, Freiburg, Germany.
authors.forename: C A
authors.initials: CA
authors.lastname: Stahl
authors.forename: K
authors.initials: K
authors.lastname: Möller
authors.forename: D
authors.initials: D
authors.lastname: Steinmann
authors.forename: D
authors.initials: D
authors.lastname: Henzler
authors.forename: S
authors.initials: S
authors.lastname: Lundin
authors.forename: O
authors.initials: O
authors.lastname: Stenqvist
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D008170:Lung Compliance / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D011175:Positive-Pressure Respiration; D012128:Respiratory Distress Syndrome / Q000503:physiopathology*