RESPIRATORY TRANSFER IMPEDANCE BETWEEN 8 AND 384 HZ IN GUINEA-PIGS BEFORE AND AFTER BRONCHIAL CHALLENGE

We report a forced oscillatory technique for noninvasively measuring r espiratory transfer impedance (Ztr) between 8 and 384 Hz in guinea pig s. This technique uses a device consisting of two chambers: one surrou nding the animal's head that is used as a plethysmograph to measured f low through the airway opening and the other that surrounds the animal 's body and is used to apply pressure oscillations to the body surface . Ztr was measured in spontaneously breathing awake guinea pigs and wh ile the animals were anesthetized in normal and methacholine-challenge d conditions. An eight-element model consisting of an airway compartme nt separated from a tissue compartment by a shunt gas compression comp artment was fit to the data. Anesthesia increased central and peripher al airway resistance and bronchial airway wall compliance by 13, 31, a nd 44%, respectively, whereas it decreased tissue compliance by 37%. C ompared with the unanesthetized condition, the methacholine challenge (20 mu g/mu g) resulted in an increase in central and peripheral airwa y resistance (69 and 319%, respectively) and a decrease in bronchial a irway wall and tissue compliance (37 and 79%, respectively). This tech nique is capable of measuring Ztr in anesthetized and awake guinea pig s. Analysis of these data with this eight-element model provides reaso nable estimates of airway and tissue parameters.