Can tracking of contrast echocardiographic targets be used to measure intracardiac flow velocities?

Recent studies suggest that the slopes of linearly moving echocardiographic contrast targets (microbubbles) can be used to predict flow velocities in the right heart. The purpose of this study was to assess the accuracy of velocities measured with contrast echocardiography by comparing them with those recorded with a previously calibrated quantitative range-gated 2-dimensional echocardiographic Doppler flow meter. Venous saline echocardiographic contrast injections and Doppler studies were performed in 10 patients, aged 6 months to 16 years, who had been operated on for lesions in the left side of the heart. Blood velocities were measured on a Doppler flow meter along the direction of flow in the main pulmonary artery just distal to the pulmonary valve in a short-axis plane. Immediately after the Doppler study, an M-mode echocardiogram of the pulmonary valve was derived from the same plane as that used for the preceding Doppler sample volume, passing through the pulmonary valve; 2 to 10 ml of normal saline solution was forcefully hand-injected through a previously positioned peripheral venous line and recorded on strip chart and videotape at a paper speed of 100 mm/s. Pulmonary flow velocities by contrast echocardiography were measured as the slopes of the moving contrast echocardiographic targets seen just beyond the pulmonary valve leaflets. Measurable microbubbles had to follow the constraints described in a previous study by Shiina et al.³ Doppler velocities were read directly from the machine calibration marks (centimeters per second) recorded with the fast Fourier output. Instantaneous and peak velocities were determined at time-matched systolic points in cardiac cycles of identical R-R intervals. Significant but low level correlations were found between contrast echocardiography and Doppler instantaneous (r = +0.45) or peak (r = +0.11) systolic velocities. Our results suggest that while contrast echocardiographic microbubbles can be measured to yield a velocity, they do not behave like moving red blood cells that reflect ultrasound to produce a Doppler shift. Therefore, contrast echocardiography cannot be used reliably to measure flow velocities in the right heart.