971-56 Myocardial Contrast Echocardiography can be Used to Assess Dynamic Changes in Microvascular Function In-Vivo

The transit rate of sonicated albumin microbubbles (Albunex®, mean size=4.3 μ) has been shown to correlate with that of radiolabelled red blood cells in the blood perfused beating heart. We have previously demonstrated that the transit rate of these microbubbles is decreased during hyperthermia-induced microvascular injury In this study, we hypothesized that microbubble transit rate could be used as a marker of reversible endothelial injury during myocardial contrast echocardiography (MCE).We produced endothelial injury by inducing different degrees of myocardial hypoxia. This was accomplished by perfusing an arrested heart with either arterial blood, venous blood, or blood diluted to different degrees with a crystalloid cardioplegia solution. The flow rate into the cross-clamped aorta was held constant in each dog (mean=170mi), as was the perfusate temperature (mean = 27°C). Perfusate hematocrit varied from 0–27%, while perfusate pO2 ranged from 15–600mmHg. MCE was performed by injecting 2ml of a 1:1 dilution of Albunex® into the perfusate line and images were acquired at a sampling rate of 30 Hz. The mean transit rate of Albunex® through the myocardium was derived by fitting a λ-variate function to the background-subtracted time-intensity plots. The mean transit rate of Albunex® decreased as the hematocrit decreased. On multivariate analysis, the hematocrit and the pO2 were the best correlates of mean microbubble transit rate (y=0.06 hematocrit-0.002 pO2+0.72, r=0.72, p<0.001).We conclude that MCE, can be used to assess reversible microvascular injury. This technique, therefore, has great potential for understanding dynamic changes in microvascular function in-vivo, particularly those modulated by the endothelium