Airway and tissue impedances of canine lungs after step volume changes

We investigated the changes in pulmonary mechanics in five anesthetized paralyzed tracheostomized open-chest dogs after step changes in lung volume. We applied small-amplitude (10-ml) volume oscillations at 6 Hz at the tracheal opening for 50-s periods, during which we applied a step volume change of 250, 500, or 750 ml to the lungs. Alveolar capsule measurements of alveolar pressure allowed us to calculate cycle-by-cycle values for airway resistance (Raw) and reactance (Xaw) and lung tissue resistance (Rti) and reactance (Xti). Before the step changes in lung volume, when transpulmonary pressure (Ptp) had a mean value of 0.65 kPa, Raw was markedly greater than Rti. The situation was reversed after the step changes, however, when Raw decreased and Rti increased. Both Raw and Xaw showed negative dependences on Ptp and hence on airway caliber, as expected, and also decreased transiently after the step volume changes, almost certainly due to a vagally mediated bronchodilation reflex. Both Rti and Xti showed clear linear dependences on Ptp and were themselves tightly coupled. Furthermore, our estimate of bulk modulus for lung tissue at 6 Hz is comparable to its previously reported values at much lower oscillation frequencies.