Measurement of bronchial blood flow with radioactive microspheres in awake sheep

Distribution of bronchial blood flow was measured in unanesthetized sheep by the use of two modifications of the microsphere reference sample technique that correct for peripheral shunting of microspheres: 1) A double microsphere method in which simultaneous left and right atrial injections of 15μm microspheres tagged with different isotopes allowed measurement of both pulmonary blood flow and shunt-corrected bronchial blood flow, and 2) a pulmonary arterial occlusion method in which left atrial injection and transient occlusion of the left pulmonary artery prevented delivery to the lung of microspheres shunted through the peripheral circulation and allowed systemic blood flow to the left lung to be measured. Both methods can be performed in unanesthetized sheep. The pulmonary arterial occlusion method is less costly and requires fewer calculations. The double microsphere method requires less surgical preparation and allows measurement without perturbation of pulmonary hemodynamics. There was no statistically significant difference between bronchial blood flow measured with the two methods. However, total bronchial blood flow measured during pulmonary arterial occlusion (1.52 ± 0.98% of cardiac output, n = 9) was slightly higher than that measured with the double microsphere method (1.39 ± 0.88% of cardiac output, n = 9). In another series of experiments in which sequential measurements of bronchial blood flow were made, there was a significant increase of 15% in left lung bronchial blood flow during the first minute of occlusion of the left pulmonary artery. Thus pulmonary arterial occlusion should be performed 5 s after microsphere injection as originally described by Baile et al. (1). Blood flow (ml·min^-1·100 g^-1) to airways of 11 unanesthetized sheep measured with both methods was 15.5 ± 7.1 to the trachea, 16.6 ± 6.5 to main stem bronchi, 17.8 ± 7.8 to lobar bronchi, and 21.6 ± 11.4 to whole lung including intraparenchymal airways. Mean blood flow per weight of tracheal mucosa/submucosa was 10-fold larger than perfusion of whole tracheal wall.