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The velocity of flow and pressure in the venae cavae of four normal conscious subjects was studied. Velocity was measured with a catheter-tip electromagnetic transducer. The effects of respiration, Valsalva and Müller maneuvers, coughing, and exercise were studied. Caval blood velocities during breath holding showed marked cardiac pulsations, being maximal at the time of ventricular systole and minimal or reversed at atrial systole. Peak velocities during ventricular systole ranged from 30 to 45 cm/sec in the inferior, and from 10 to 35 cm/sec in the superior, vena cava. A second diastolic forward flow velocity ranged from 36 to 76% of the systolic peak. During inspiration, velocity transiently increased. Reduction of flow velocity in abdominal breathing and the Müller maneuver is consistent with the formation of a local area of inferior vena caval collapse at the diaphragm. During the Valsalva maneuver, abrupt reduction in caval flow was seen that persisted throughout the strain. There was immediate overshoot when the strain was released. Coughing produced a reduction of flow velocity with backflow in the superior vena cava. In leg exercise, inferior caval flow velocity rose immediately, and it remained high during recovery. Marked respiratory velocity variations with inspiratory increases occurred during and after exercise.

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.

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