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Passive expiratory flow‐volume curve is not an accurate method to measure the respiratory time constant in rabbits
Author(s) -
Vialet R.,
Monnier A.,
Lagier P.,
Jammes Y.,
Toméi C.,
Martin C.
Publication year - 1999
Publication title -
acta anaesthesiologica scandinavica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.738
H-Index - 107
eISSN - 1399-6576
pISSN - 0001-5172
DOI - 10.1034/j.1399-6576.1999.431008.x
Subject(s) - medicine , bronchoconstriction , respiratory physiology , bronchospasm , airway resistance , lung volumes , anesthesia , respiratory system , functional residual capacity , expiration , occlusion , pulmonary compliance , bronchodilation , tidal volume , airway , lung , cardiology , asthma , bronchodilator
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 (τ), which constitutes an important determinant of mechanical ventilation. The end‐inspiratory occlusion technique is one of the easiest methods to obtain τ 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 τ values given by the end‐inspiratory occlusion technique (τrs) to τ values obtained by references methods for measurements (i.e. the product of static lung compliance by airway resistance: τref) 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 τrs. Then τrs was compared with τref (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 τrs 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.