EFFECT OF LOCAL BRONCHIAL OBSTRUCTION ON GAS-EXCHANGE IN DOG

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Abstract
To study the effect of local airway obstruction on gas exchange, balloon catheters were inflated in segmental, lobar or left main bronchi. Three techniques were used: an inert gas method to determine the distribution of ventilation-perfusion ratios, radioactive microspheres to measure regional blood flow and a mass spectrometer to measure alveolar PO2 and alveolar PCO2 in obstructed segments. Obstruction of segmental bronchi produced no shunt or measurable change in the distribution of ventilation-perfusion ratios. Obstruction of a lobar bronchus produced a shunt, but this was considerably less than the blood flow in the lobe before branchial obstruction, as measured by injected radioactive microspheres. The degree of decrease in blood flow with bronchial obstruction diminished as the volume of obstructed lung increased. Alveolar gas analysis in obstructed segments showed that the inspired PO2 and PCO2 of the obstructed region were very close to the end-tidal PO2 and PCO2 values of the unobstructed lung. This reflects the effectiveness of collateral ventilation. A surprising finding was that the PCO2 in the obstructed segment was substantially higher than that in mixed venous blood and was an average of 3.9 mm Hg higher than that predicted from the Haldane effect. The high PCO2 may be an example of the negative arterial-alveolar difference for this gas, and it may play a role in promoting collateral ventilation.