Assessment of local lung impedance by the alveolar capsule oscillator in dogs: a model analysis

Abstract

The alveolar capsule oscillator technique has shown that the response of the lung periphery to intravenous histamine injection in dogs is extremely inhomogeneous both in terms of local peripheral airway resistance (RA) and local peripheral elastance (EA) (M. Mishima, Z Balassy, and J. H. T. Bates. J. Appl. Physiol. 77: 2140-2148, 1994). To assess the physical extent of the local lung region identified by this technique, we performed computer simulations using an asymmetrical branching model of the canine lung proposed by K. Horsfield, W. Kemp, and S. Philips (J. Appl. Physiol. 52: 21-26, 1982). The acoustic impedance of the model from 26 to 200 Hz as seen from the alveolar capsule oscillator was calculated. RA and EA were estimated from the simulated acoustic impedance between 26 and 200 Hz and were found to be 492 hPa.s.l-1 and 156,300 hPa/l, respectively. These values are similar to those found experimentally in previous studies. By simulating data using the model in various stages of completeness, we determined that approximately 50% of RA is determined by the acinus to which the alveolar capsule is attached, whereas the remainder is determined by airways < or = 1 mm diameter that converge on this acinus. By contrast, EA was determined almost entirely (95%) by the acinus directly under the capsule. Inhomogeneous peripheral airway constriction altered RA severalfold but did not affect EA by >5%. This suggests that the previously observed changes induced in EA by bronchial challenge reflect real changes in intrinsic tissue elastance rather than merely regional mechanical inhomogeneities.