TIME-COURSE AND PATTERN OF PULMONARY FLOW DISTRIBUTION FOLLOWING UNILATERAL AIRWAY OCCLUSION IN SHEEP

1. Unilateral bronchial occlusion causes ipsilateral hypoxic pulmonary vasoconstriction, which shifts blood flow to towards the other lung. We studied the time course of how diversion following acute bronchial occlusion, and the temporal effect of the latter on blood gases and ve rtical distribution of blood flow within the two lungs. 2. Serial infu sions of radioactive or fluorescent microspheres were given to each of seven adult standing sheep before, during occlusion of the left mains tem bronchus for up to 6 min, and after release of occlusion, Pulmonar y and systemic arterial pressures were recorded continuously and arter ial and mixed venous blood gases were determined intermittently, Post- mortem, the lungs were inflated, dried and cut into slices. Relative b lood how at the time of infusion was expressed as the weight-normalize d intensity of each tracer in each slice or lung divided by the weight -normalized intensity in the two lungs. 3. Within 30 s, 1 min and 2 mi n after onset of occlusion, how in the occluded lung had decreased to 68-84% (range), 51-78% and 43-79% respectively, of the initial value. In the contralateral lung, how increased by 10-24%, 14-37% and 23-39% respectively. the distribution of flow along the gravitational axis wi thin each lung varied widely between animals, both before and during o cclusion, The during-occlusion profiles in the occluded lung differed from those in the non-occluded lung. In either lung, during-occlusion profiles could not be predicted with certainty from the pre-occlusion profiles. Two minutes post-occlusion, inter-and intra-lung flow distri bution were nearly the same as before occlusion, Arterial oxygen tensi on fell in the first minute of occlusion, but never below 7.5 kPa, and increased slowly thereafter. Arterial carbon dioxide tension increase d slightly throughout the occlusion period. No appreciable changes in systemic or pulmonary artery pressure were observed. Post-occlusion, a rterial oxygen tension was still sub-normal, while carbon dioxide tens ion continued to increase. 4. We conclude that acute unilateral bronch ial occlusion diverts blood dow within 30 s towards the contralateral lung. This rapidly occurring flow diversion prevents the development o f severe arterial hypoxaemia. The variable and largely unpredictable d istribution of blood flow in the hyperfused non-occluded lung might ex plain some of the gas-exchange abnormalities observed in physiological ly hyperfused lungs and in patients with one hyperfused lung.