title: Non-pulmonary factors strongly influence the stress index.
abstract: OBJECTIVE
  A quantitative measure of the airway pressure-time tracing during passive inflation [stress index (SI)] has been suggested as an indicator of tidal lung recruitment and/or overinflation. If reliable, this simple index could help guide positive end-expiratory pressure (PEEP) and tidal volume selection. The compartment surrounding the lungs should impact airway pressure and could, therefore, affect SI validity. To explore the possibility, we determined SI in a swine model of pleural effusion (PLEF).
  
  
  METHODS
  Unilateral PLEF was simulated by instilling fluid (13 ml/kg-moderate, 26 ml/kg-large) into the right pleural space of five anesthetized, paralyzed, mechanically ventilated pigs. Animals were ventilated with constant flow ventilation: tidal volume (V (T)) 9 ml/kg, f set to end-tidal CO₂ (ETCO₂) of 30-40 mmHg, inspiratory to expiratory ratio (I/E) 1:2, PEEP 1 or 10 cmH₂O. Respiratory system mechanics and computed tomography (CT) were acquired at end-inspiration and end-expiration to determine % tidal recruitment and overinflation.
  
  
  RESULTS
  Prior to PLEF instillation, SI values derived at PEEP = 1 and 10 cmH₂O were 0.90 and 1.22, respectively. Moderate PLEF increased these SI values to 1.06 and 1.24 and large PLEF further increased SI to 1.23 and 1.27 despite extensive tidal recruitment and negligible overdistention by CT. The initial half of the tidal pressure curve produced SI values (range 0.82-1.17) that were significantly lower than those of the second half (0.98-1.37).
  
  
  CONCLUSIONS
  In the presence of pleural fluid, SI indicated overinflation when virtually none was present and tidal lung recruitment predominated. When the extrapulmonary environment is abnormal, caregivers are advised to interpret the SI with caution.
authors.affiliation: Dipartimento di Anestesiologia, Terapia Intensiva e Scienze Dermatologiche, Università delgi Studi, Milan, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Formenti
authors.forename: Jeronimo
authors.initials: J
authors.lastname: Graf
authors.forename: Arnoldo
authors.initials: A
authors.lastname: Santos
authors.forename: Arnoldo Santos
authors.initials: AS
authors.lastname: Olveido
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Gard
authors.forename: Kate
authors.initials: K
authors.lastname: Faltesek
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Adams
authors.forename: David J
authors.initials: DJ
authors.lastname: Dries
authors.forename: John J
authors.initials: JJ
authors.lastname: Marini
mesh_terms: D000818:Animals; D005260:Female; D008170:Lung Compliance / Q000502:physiology*; D008176:Lung Volume Measurements / Q000379:methods; D008991:Monitoring, Physiologic / Q000379:methods; D010996:Pleural Effusion; D011175:Positive-Pressure Respiration; D012121:Respiration, Artificial; D015656:Respiratory Mechanics / Q000502:physiology*; D013552:Swine