Microbubbles have intracardiac velocities similar to those of red blood cells

The slope of an individual contrast trajectory on M-mode echocardiography represents the projection of the intracardiac velocity vector of a microbubble in the direction of the sound beam. Doppler echocardiography measures this projection of red blood cell velocity. To ascertain whether microbubbles have similar intracardiac velocities to those of red blood cells, 11 subjects were studied during intravenous injections of 5% dextrose solution. The flow across the tricuspid and pulmonary valves was examined. Microbubble velocity was measured by M-mode contrast slope analysis and simultaneously by Doppler technique. Results from both methods were correlated with red blood cell velocity measured by Doppler recording at the same time in the cardiac cycle, shortly before appearance of contrast medium (3 to 8 beats before the corresponding contrast velocity measurements). In all subjects, 10 sets of three velocities each (M-mode slope and Doppler data before and during contrast injection) were obtained for each valve. Visual inspection of the Doppler tracings showed similar velocity profiles before and during contrast appearance; the signal intensity was greater with contrast. Quantitatively, microbubble velocity assessed by M-mode trajectory slopes correlated well with the Doppler-de-rived velocity of red blood cells (r = 0.98, p < 0.001, slope of the regression line = 0.99, standard error of the estimate = 7 cm/s). Doppler velocities measured with and without contrast medium showed a similar correlation (r = 0.99, p < 0.001, slope of regression = 1.01, standard error of the estimate = 6 cm/s). In individual subjects, the correlation coefficient between microbubble and red blood cell velocities ranged from 0.978 to 0.998. It is concluded that microbubbles derived from intravenous injections travel with velocities similar to those of red blood cells across the tricuspid and pulmonary valves. Thus, contrast echocardiography can be used to estimate intracardiac blood velocity.