Quantitative aspects in myocardial contrast echocardiography

Myocardial tissue perfusion is not currently quantified in the clinical setting. Thus the aim of this paper is to review the quantitative information on myocardial perfusion provided by contrast echocardiography. In a circulatory model-without the capillary network interposed between injection and sampling point of contrast-the transit time of microbubbles (source of the echo contrast effect) is inversely related to absolute flow, thus providing accurate quantitation. A similar situation is represented by blood flow inside a vessel or a cardiac cavity, where, if the prerequisites for quantitation are respected, it is possible to measure blood flow by contrast echocardiography. In the coronary circulation, the transit time of contrast microbubbles varies according to their interaction with coronary microcirculation, and to the characteristics of contrast agents as flow tracers. Echo contrast agents with small microbubbles have been injected into the coronary branches of experimental animals, under both coronary autoregulation and maximal coronary dilation, providing good estimates of coronary blood flow. The accuracy of these measurements might improve when new contrast agents, with characteristics closer to those of a flow tracer, are available. If a tracer is injected before a bifurcation, and provided it mixes adequately, the amount of tracer distributed to each branch is proportional to the corresponding blood flow. A similar situation is encountered when an echo contrast agent is injected into the aortic root or into the left main coronary artery. Here, the ratio between myocardial signal intensity in the different perfusion territories reflects the corresponding ratio of blood flows. The validity of this approach has been previously demonstrated in experimental animals and validated in patients with coronary stenoses. The injection of contrast agents into the coronary circulation at baseline and under coronary hyperaemia has the potential for measuring coronary blood flow reserve. However, what is still unclear is whether contrast echo changes reflect changes in coronary blood flow (i.e. flow reserve), coronary blood volume (i.e. coronary recruitment) or both, and also whether they influence the different types of contrast agent. Finally, myocardial contrast echocardiography can provide information on the spatial distribution of myocardial perfusion, i.e. the presence, site and extent of perfused myocardium. Thus, in models where myocardial perfusion may be either present or absent, contrast echo can provide an accurate estimate of perfusion abnormalities.