AIRWAY AND TISSUE MECHANICS DURING PHYSIOLOGICAL BREATHING AND BRONCHOCONSTRICTION IN DOGS

In five open-chest dogs and with four to five alveolar capsules we use d an optimal ventilator waveform (OVW) to follow frequency and tidal v olume (VT) dependence of lung, airway, and tissue resistance (R) and e lastance (E) before and during constant infusion of histamine (16 mu g .kg(-1).min(-1)). OVW contains sufficient flow energy between 0.234 an d 4.7 Hz, avoids nonlinear harmonic interactions, and simultaneously v entilates with physiological VT, Each OVW breath permits a smooth esti mate of frequency dependence of R and E for the whole lung. A constant -phase model analysis provided estimates of purely viscous resistance (Rvis), which represents the sum of airway resistance (Raw) and any pu rely newtonian component of tissue resistance (Rti), and parameters G and H, which govern frequency dependence of Rti and tissue elastance ( Eti), respectively. Tissue structural damping (eta) is calculated as G /H. This model was applied to the whole lung and tissue impedance as e stimated from each capsule. We found a small but inconsequential purel y newtonian component of Rti, even during constriction. Four dogs show ed a peak response at similar to 4 min in lung Rvis coupled (in time) to initial increases in G, H, eta, and airway inhomogeneities. In two of these dogs the response was severe. Tissue properties estimated fro m whole lung impedance (G, H, and eta) were nearly identical to values estimated from unobstructed capsules throughout infusion. By using a technique independent of alveolar capsules, our results indicate that a major if not dominant response to a constrictive agonist occurs in l ung tissues, resulting in a large increase in Rti and Eti. With severe constriction, significant increases occur in Raw and airway inhomogen eities as well. Finally, separation of airway and tissue properties us ing input impedance estimated from the frequency-rich OVW avoids use o f alveolar capsules and may prove an effective tool for partitioning a irway and tissue properties in humans.