In vivo myocardial kinetics of air-filled albumin microbubbles during myocardial contrast echocardiography: Comparison with radiolabeled red blood cells

Myocardial contrast echocardiography (MCE) is a new technique for assessing myocardial perfusion that uses intracoronary injections of microbubbles of air. Because these microbubbles have a mean diameter of 4.3±0.3 μm and an intravascular rheology similar to that of red blood cells (RBCs), we hypothesized that their mean myocardial transit rates recorded on echocardiography would provide an estimation of regional myocardial blood flow in the in vivo beating heart. Accordingly, blood flow to the left anterior descending coronary artery (LAD) of 12 open-chest anesthetized dogs (group I) was adjusted to 4 to 6 flows (total of 60 flows), and microbubbles and radiolabeled RBCs were injected into the LAD in a random order at each stage. The mean myocardial RBC transit rates were measured by fitting a gamma-variate function to time-activity plots generated by placing a miniature CsI₂ probe over the anterior surface of the heart, and the mean myocardial microbubble transit rates were measured from time-intensity plots derived from off-line analysis of MCE images obtained during the injection of microbubbles. An excellent correlation was noted between flow (measured with an extracorporeal electromagnetic flow probe) and mean myocardial RBC transit rate (y = 2.83 x 10^-3x + 0.01, r = .96, SEE = 0.02, P<.001). A close correlation was also noted between mean RBC and microbubble myocardial transit rates (y = 1.01x + 0.01, r = .89, SEE = 0.02, P<.001). Despite its theoretical advantages, a lagged normal density function did not provide a better fit to the MCE data than the gamma-variate function. Experiments performed in six additional dogs (group II, 33 stages) demonstrated that removing the effect by deconvolution of the bolus spread before reaching the myocardium is not necessary to maintain the relation between mean myocardial transit rate and LAD blood flow if the duration and site of the tracer injection are constant. Additionally, the coefficient of variation for determining the mean microbubble transit rate during MCE was 21% in these dogs (five injections at a single flow in each dog). We conclude that the transit rates of microbubbles during MCE can be used to assess regional myocardial blood flow in the in vivo beating heart as long as the blood volume remains constant.