title: Assessment of airway closure from deflation lung volume-pressure curve: sigmoidal equation revisited.
abstract: OBJECTIVE
  To assess a sigmoidal equation for describing airway closure.
  
  
  METHODS
  Experimental study.
  
  
  METHODS
  University laboratory.
  
  
  METHODS
  Eight piglets mechanically ventilated on zero end-expiratory pressure (ZEEP).
  
  
  METHODS
  Control and lung saline lavage.
  
  
  RESULTS
  Lungs were inflated up to transpulmonary pressure of 30 cmH(2)O at constant flow (0.12l s(-1)) then deflated at the same flow rate up to the point at which oesophageal pressure was constant, which was assumed to represent complete airway closure. The deflation volume-transpulmonary pressure curve was fitted to: (1) a sigmoidal equation focusing on inflexion point and pressure at maximal compliance increase and (2) an exponential equation above an inflexion point determined by eyeballing. Data deviate from the exponential equation at the point of airway closure onset. The zero-volume intercept was determined. Complete airway closure was reached at -8.3+/-3.5cmH(2)O in control conditions and at -1.3+/-3.7 cmH(2)O after lavage (p < 0.05). Between control and lavage, onset of airway closure was 3.0+/-1.9 vs. 6.0+/-2.8 cmH(2)O (p <0.05), inflexion point 3.2+/-1.8 vs. 7.7+/-2.6 cmH(2)O (p <0.001), pressure at maximal compliance increase -1.9+/-0.7 vs. -0.03+/-2.1cmH(2)O (p <0.05) and zero-volume intercept -1.5+/-1.4 vs. 0.3+/-2.3cmH(2)O (p <0.05).
  
  
  CONCLUSIONS
  During mechanical ventilation airways stay open and close around ZEEP in control but are closed above ZEEP after lavage. Inflexion point might reflect onset of airways closure in control. Pressure at maximal compliance increase was not a marker of complete airways closure. In control and lavage, pressure at maximal compliance increase and zero-volume intercept were reasonably equivalent.
authors.affiliation: Hôpital de la Croix-Rousse, Service de Réanimation Médicale et d'Assistance Respiratoire, 103 Grande Rue de la Croix-Rousse, 69004 Lyon, France.
authors.forename: Frédérique
authors.initials: F
authors.lastname: Bayle
authors.forename: Claude
authors.initials: C
authors.lastname: Guérin
authors.forename: Sophie
authors.initials: S
authors.lastname: Debord
authors.forename: Michel
authors.initials: M
authors.lastname: Badet
authors.forename: Stéphane
authors.initials: S
authors.lastname: Lemasson
authors.forename: Jean-Charles
authors.initials: JC
authors.lastname: Poupelin
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Richard
mesh_terms: D000402:Airway Obstruction / Q000503:physiopathology*; D000818:Animals; D005260:Female; D005602:France; D007442:Intubation, Intratracheal; D008176:Lung Volume Measurements; D015233:Models, Statistical; D012128:Respiratory Distress Syndrome / Q000503:physiopathology; D013552:Swine