Volume dependence of respiratory system resistance during artificial ventilation in rabbits

The volume dependence of respiratory resistance (Rrs), usually observed dur ing normal breathing, is expected to be accentuated during expiratory flow limitation (EFL). In order to quantify this dependence we studied the press ure, flow and volume data obtained from eight New Zealand rabbits, artifici ally ventilated at different levels of applied expiratory pressure (0-10 hP a), before and during histamine i.v. infusion. EFL was provoked by lowering the expiratory pressure and was detected by the application of an addition al negative expiratory pressure and by forced oscillations. The analysis of respiratory system mechanics was performed by multiple regression, using t he classical linear first-order model and also a nonlinear model, accountin g for volume dependence of Rrs. Both models satisfactorily fitted the data in the absence of EFL. The nonlinear model proved to be more appropriate in the presence of EFL. The coefficient expressing the volume dependence of R rs (Rvd) was significantly more negative during EFL. Rvd values were highly correlated with the fraction of the tidal volume left to be expired at the onset of EFL. A threshold Rvd value of -1,000 (hPa(.)s(.)l(-2)) detected E FL with high sensitivity and specificity. We conclude that a strongly negat ive volume dependence of Rrs is a reliable and noninvasive index of EFL dur ing artificial ventilation.