Relationship of central and peripheral airway resistance to lung volume in dogs

We could not reconcile reported relationships between lung resistance measurements and lung volume with bronchographic and anatomic studies showing that airway diameters change monotonically with lung volume and that small airways change diameter proportionately at least as much as large ones. Accordingly we measured central and peripheral airways resistances with a new technique. The relevant pressures were measured with a tracheal cannula, a wedged retrograde catheter, and two parenchymal needles in seven open-chested dogs while pleural pressure was oscillated at 1 Hz. In contrast to previous studies, the volume dependency of peripheral resistance was at least as great as that of central resistance with vagi intact, the volume dependencies of central and peripheral resistances were not abolished by vagotomy, and neither resistance increased systematically at high volumes. Volume dependency of central resistance resembled predictions for isotropic expansion of airways with vagi cut but increased with bronchomotor tone. These results fit generally with bronchographic data. Previous studies may have been affected by volume dependency due to "tissue resistance" and catheter phase lags.