title: Passive expiratory flow-volume curve is not an accurate method to measure the respiratory time constant in rabbits.
abstract: BACKGROUND
  In mechanically ventilated patients, inspiration is forced by an externally applied positive pressure whereas expiration remains passive and depends on the time constant of the total respiratory system (tau), which constitutes an important determinant of mechanical ventilation. The end-inspiratory occlusion technique is one of the easiest methods to obtain tau values in ventilated patients, especially infants, but its accuracy is not well established. The aim of this study was to compare in anesthetized, paralyzed rabbits tau values given by the end-inspiratory occlusion technique (tau(rs)) to tau values obtained by references methods for measurements (i.e. the product of static lung compliance by airway resistance: tau(ref)) during carbachol-induced bronchospasm eliciting marked modifications of the respiratory mechanics.
  
  
  METHODS
  Comparisons were made in the basal state and after carbachol-induced bronchoconstriction in seven New Zealand adult rabbits. This procedure resulted in a wide range of expiratory time constants. A pneumotachograph was used to measure expiratory flow and volume before and after end-inspiratory occlusion. The slope of the flow volume curve gave tau(rs). Then tau(rs) was compared with tau(ref) (which ranged from 0.30 to 1.96 s).
  
  
  RESULTS
  Statistical analysis revealed a weak correlation between the two methods, and a size-dependent bias of tau(rs) measurements.
  
  
  CONCLUSIONS
  The end-inspiratory occlusion technique leads to a systemic bias in measurements of respiratory time constant, especially when the resistance of the respiratory system is elevated.
authors.affiliation: Department of Anesthesia and Critical Care, Centre Hospitalier Universitaire, Hôpital Nord, Marseille, France.
authors.forename: R
authors.initials: R
authors.lastname: Vialet
authors.forename: A
authors.initials: A
authors.lastname: Monnier
authors.forename: P
authors.initials: P
authors.lastname: Lagier
authors.forename: Y
authors.initials: Y
authors.lastname: Jammes
authors.forename: C
authors.initials: C
authors.lastname: Toméi
authors.forename: C
authors.initials: C
authors.lastname: Martin
mesh_terms: D000403:Airway Resistance; D000818:Animals; D016084:Bronchoconstriction / Q000187:drug effects / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D008170:Lung Compliance; D008449:Maximal Expiratory Flow-Volume Curves; D012123:Pulmonary Ventilation; D011817:Rabbits; D012121:Respiration, Artificial
chemical_list: D002217:Carbachol