Passive expiratory flow-volume curve is not an accurate method to measure the respiratory time constant in rabbits

Background: Ln mechanically ventilated patients, inspiration is forced by a n externally applied positive pressure whereas expiration remains passive a nd depends on the time constant of the total respiratory system (tau), whic h constitutes an important determinant of mechanical ventilation. The end-i nspiratory 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, par alyzed 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) duri ng carbachol-induced bronchospasm eliciting marked modifications of the res piratory mechanics. Methods: Comparisons were made in the basal state and after carbachol-induc ed bronchoconstriction in seven New Zealand adult rabbits. This procedure r esulted in a wide range of expiratory time constants. A pneumotachograph wa s used to measure expiratory flow and volume before and after end-inspirato ry occlusion. The slope of the flow volume curve gave tau rs. Then tau rs w as compared with tau ref (which ranged from 0.30 to 1.96 s). Results: Statistical analysis revealed a weak correlation between the two m ethods, and a size-dependent bias of tau rs measurements. Conclusions: The end-inspiratory occlusion technique leads to a systemic bi as in measurements of respiratory time constant, especially when the resist ance of the respiratory system is elevated.