Changes in regional lung impedance after intravenous histamine bolus in dogs: effects of lung volume.

We measured the effect of lung volume on the time course of regional lung input impedance (ZA) after bolus intravenous administration of 2 mg of histamine in seven open-chest dogs using alveolar capsule oscillators. ZA (24-200 Hz) was obtained during apnea at constant lung volume every 2 s for 80 s at lung inflation pressures of 0.1, 0.2, 0.3, 0.5, 0.7, and 1.0 kPa. Local airway resistance (RA) and elastance of the local lung region were calculated by fitting a four-parameter model to the measured ZA. Total lung resistance and lung elastance were also calculated from tracheal pressure and flow measured during mechanical ventilation (0.3 Hz) just before and after each set of ZA measurements. We found the histamine-induced changes in both lung resistance and lung elastance to decrease with increasing lung volume. RA also showed a large negative dependency on lung volume, and the variation between different RA measurements became markedly increased as lung volume decreased. In contrast, local airway elastance was essentially unaffected by lung volume. These results support the idea that parenchymal tethering of the very distal airways impedes their narrowing during bronchoconstriction. They also indicate that reduced parenchymal tethering causes airway narrowing to become markedly more inhomogeneous.