This article confirms a recently developed distal jet centerline technique for noninvasively quantifying regurgitant cardiac valve flows. The basic principle that allows flow rate to be calculated is conservation of momentum. As the jet entrains more mass, its centerline velocity decays inversely with distance from the orifice. Under pulsatile flow conditions, it has been shown that this technique remains applicable at peak flow for a normal resting adult heart rate of 60 to 70 beats/min. It is, however, conceivable that at higher heart rates, the same inverse relation between centerline velocity and distance is never fully established, because there is a finite time interval required for the jet to penetrate the receiving chamber. Therefore, the purpose of this study was to determine whether the inverse relationship of centerline velocity to distance develops sufficiently rapidly so that quantitative techniques based on that decay would be applicable over a wide range of heart rates. Two different techniques, an engineering tool, laser Doppler anemometry, and a clinical tool, Doppler ultrasound, were used for measuring jet centerline velocities (averaged over multiple beats). Physiologic pulsatile flows were pumped through two circular orifices, 4 and 6 mm in diameter, at 60 to 150 beats/min; peak orifice velocities ranged from 2 to 5 m/sec. Steady flow experiments were also performed with the same orifice diameters and over the same velocity range. Peak centerline velocities in the fully developed turbulent jet region decayed inversely with distance at all heart rates studied. With laser Doppler anemometry, the proportionality constant of the decay curve was found to be in the range 6.4 +/- 0.5. The pulsed Doppler results provided a jet constant in the range 6.7 +/- 0.3 with the 4-mm orifice diameter, whereas the constant was 6.5 +/- 0.3 with the 6-mm orifice diameter. In steady flow, the proportionality constant was found to be 6.1 +/- 0.2. Therefore, within a wide range of physiologic heart rates, full jet development occurs with sufficient speed so that the expected centerline velocity decay is established (jet empirical constant of 6.3). The conservation of momentum technique for calculating orifice flow rate on the basis of these centerline velocities is thus applicable under physiologic conditions.