Myocardial contrast two-dimensional echocardiography can be an index of in vivo red blood cell transit rate

We have previously shown the rheology of sonicated albumin microbubbles (size = 4.5 μ) to be similar to that of red blood cells (RBCs) in the microcirculation. Our hypotheses was that the myocardial transit of these microbubbles as measured by 2D-echo would be an index of myocardial RBC transit rate in vivo. We cannulated the left anterior descending (LAD) artery in 8 dogs and perfused it with blood from the right carotid artery. LAD blood flow was varied from 13 to 110 ml/min using a roller pump. At each stage, 100 μCi of ^99mTechnetium labeled RBCs was injected into the LAD bed and gamma emissions were sampled over the bed every 0.5 sec to generate time-activity curves. Sonicated albumin (0.5 ml) was also injected at each stage and 2D echo was performed to obtain time-intensity curves. There was an excellent correlation between actual blood flow (ml/min) and the rate of transit of both RBC's and microbubbles (R² = 0.98 and r² = 0.90, respectively). There was also a close correlation between microbubble and labeled RBC transit rate during the 39 stages analyzed (y = 1.04x, SEE = 0.05, R² = 0.92). We conclude that myocardial contrast echocardiography can be used to assess RBC transit rate in vivo. This offers a powerful tool for measuring regional myocardial blood flow.