Pulmonary gas exchange after multiple airway occlusion by beads in the dog

Patients with asthma frequently show a bimodal distribution of ventilation-perfusion (VA/Q) ratios. One mode is centered on a VA/Q ratio of near normal value, whereas the other has a very low VA/Q ratio. There is usually almost no perfusion of unventilated units. We have suggested that this bimodal pattern is caused by complete obstruction of some airways with ventilation of their distal alveoli through collateral channels. To test this hypothesis we occluded airways of anesthetized dogs with beads of 1.6, 2.4, and 4.8 mm in diameter, and measured VA/Q distributions using the multiple inert gas elimination technique. Beads were inserted via a bronchoscope until the Pao2 fell by 20 mm Hg, or its value was less than 70 mm Hg. Occlusion of small airways (1.6 mm diameter) resulted in a mild increase in dispersion of VA/Q ratios. With 2.4 mm beads, the dispersion increased further. By contrast, all dogs given 4.8 mm beads showed a bimodal distribution of VA/Q ratios. All animals developed some shunt. We interpret these results as showing that when 1.6 mm airways are occluded, collateral ventilation to distal units is so efficient that ventilation is only mildly impaired; however, as occlusion becomes more proximal, collateral ventilation is less effective until with 4.8 mm beads, the ventilation of distal units is so poor that a bimodal pattern develops. The results suggest that the bimodal distributions seen in asthma are compatible with complete obstruction of some airways, and that the levels of obstruction may affect the pattern of VA/Q distribution.