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1. A patient complaining of chest pain with exercise is evaluated by cardiac catheterization. The left anterior descending (LAD) branch of the coronary artery is visualized but the contrast angiography is poor. A Doppler-tipped catheter is inserted and the blood velocity is observed to increase transiently from 10 cm/sec to 70 cm/sec and then decrease back to 10 cm/sec as the probe passes a particular location in the artery. What was the cause of these changes in velocity measurements?