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Twenty-three patients were investigated during diagnostic right and left cardiac catheterization with an electromagnetic catheter-tip velocity probe. The catheter contained a pressure lumen for simultaneous measurements of intravascular pressure. Average peak and mean blood velocities were 66 and 11 cm/sec in the ascending aorta, 57 and 10 cm/sec in the pulmonary artery, 28 and 12 cm/sec in the superior vena cava, and 26 and 13 cm/sec in the inferior vena cava. The velocity pattern in the ascending aorta was similar to that obtained by other methods. Positioning of the catheter in the ascending aorta required care; in one patient with aortic stenosis the recorded blood velocity pattern was unsatisfactory. In the pulmonary artery flicking of the catheter often produced artifacts in the records. The effect of deep respiration on blood velocity in the ascending aorta and pulmonary artery was studied. In the ascending aorta the highest velocities and stroke volumes were achieved during late expiration while in the pulmonary artery blood velocity and stroke volume were greatest in inspiration. In nine patients the cardiac outputs calculated from the product of mean velocity and radiologically measured cross-sectional area of the ascending aorta or pulmonary artery were compared with cardiac outputs determined by the indicator-dilution method; the correlation coefficient was 0.73. There were no complications, and the probe proved reliable.

Measurement of Instantaneous Blood Flow Velocity and Pressure in Conscious Man with a Catheter-Tip Velocity Probe