Influence of bronchomotor tone on airway dimensions and resistance in excised dog lungs

In 10 excised dog lobes we measured total lobar resistance (Rlo) by forced oscillation at various transpulmonary pressures (Ptp) before and after bronchodilators and obtained simultaneous tantalum bronchograms in 5. Mean airway diameter increased up to a Ptp of 30 cmH2O, although the increase was greater between 0 and 10 cmH2O. In contrast, Rlo decreased between 0 and 10 cmH2O but increased again between 10 and 30 cmH2O. At a Ptp of 30 cmH2O, the mean diameter of small airways (< 2 mm) increased 31%, that of large airways (> 7 mm) 2% after aerosolized bronchodilators. At a Ptp of 1 cmH2O, increases were 44 and 15%, respectively. Values of Rlo were reduced at all Ptp after bronchodilator, but minimum Rlo occurred at lower Ptp than before, and the absolute increase in Rlo at greater Ptp was preserved. In four additional lobes, we partitioned Rlo using a retrograde catheter. All of the increase in Rlo at large Ptp was peripheral to the retrograde catheter. We conclude that excised lungs have bronchomotor tone that narrows airways even at high Ptp and that loss of tone magnifies the relative increase in Rlo at high Ptp, supporting suggestions that the increase is due to tissue resistance and hysteresis.