title: Topographic distribution of tidal ventilation in acute respiratory distress syndrome: effects of positive end-expiratory pressure and pressure support.
abstract: OBJECTIVE
  Acute respiratory distress syndrome is characterized by collapse of gravitationally dependent lung regions that usually diverts tidal ventilation toward nondependent regions. We hypothesized that higher positive end-expiratory pressure and enhanced spontaneous breathing may increase the proportion of tidal ventilation reaching dependent lung regions in patients with acute respiratory distress syndrome undergoing pressure support ventilation.
  
  
  METHODS
  Prospective, randomized, cross-over study.
  
  
  METHODS
  General and neurosurgical ICUs of a single university-affiliated hospital.
  
  
  METHODS
  We enrolled ten intubated patients recovering from acute respiratory distress syndrome, after clinical switch from controlled ventilation to pressure support ventilation.
  
  
  METHODS
  We compared, at the same pressure support ventilation level, a lower positive end-expiratory pressure (i.e., clinical positive end-expiratory pressure = 7 ± 2 cm H2O) with a higher one, obtained by adding 5 cm H2O (12 ± 2 cm H2O). Furthermore, a pressure support ventilation level associated with increased respiratory drive (3 ± 2 cm H2O) was tested against resting pressure support ventilation (12 ± 3 cm H2O), at clinical positive end-expiratory pressure.
  
  
  RESULTS
  During all study phases, we measured, by electrical impedance tomography, the proportion of tidal ventilation reaching dependent and nondependent lung regions (Vt%dep and Vt%(nondep)), regional tidal volumes (Vt(dep) and Vt(nondep)), and antero-posterior ventilation homogeneity (Vt%nondep/Vt%dep). We also collected ventilation variables and arterial blood gases. Application of higher positive end-expiratory pressure levels increased Vt%dep and Vtdep values and decreased Vt%nondep/Vt%dep ratio, as compared with lower positive end-expiratory pressure (p < 0.01). Similarly, during lower pressure support ventilation, Vt%dep increased, Vtnondep decreased, and Vtdep did not change, likely indicating a higher efficiency of posterior diaphragm that led to decreased Vt%nondep/Vt%dep (p < 0.01). Finally, PaO2/FIO2 ratios correlated with Vt%dep during all study phases (p < 0.05).
  
  
  CONCLUSIONS
  In patients with acute respiratory distress syndrome undergoing pressure support ventilation, higher positive end-expiratory pressure and lower support levels increase the fraction of tidal ventilation reaching dependent lung regions, yielding more homogeneous ventilation and, possibly, better ventilation/perfusion coupling.
authors.affiliation: Department of Health Sciences, University of Milan-Bicocca, Monza, Italy.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Mauri
authors.forename: Giacomo
authors.initials: G
authors.lastname: Bellani
authors.forename: Andrea
authors.initials: A
authors.lastname: Confalonieri
authors.forename: Paola
authors.initials: P
authors.lastname: Tagliabue
authors.forename: Marta
authors.initials: M
authors.lastname: Turella
authors.forename: Andrea
authors.initials: A
authors.lastname: Coppadoro
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Citerio
authors.forename: Nicolo'
authors.initials: N
authors.lastname: Patroniti
authors.forename: Antonio
authors.initials: A
authors.lastname: Pesenti
mesh_terms: D018806:APACHE; D000328:Adult; D000368:Aged; D015992:Body Mass Index; D018592:Cross-Over Studies; D017097:Electric Impedance; D005260:Female; D006785:Hospitals, University; D006801:Humans; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D011175:Positive-Pressure Respiration / Q000379:methods*; D011446:Prospective Studies; D012128:Respiratory Distress Syndrome / Q000628:therapy*; D013990:Tidal Volume; D014054:Tomography