QUANTITATIVE ASPECTS IN MYOCARDIAL CONTRAST ECHOCARDIOGRAPHY

Myocardial tissue perfusion is not currently quantified in the clinica l setting. Thus the aim of this paper is to review the quantitative in formation on myocardial perfusion provided by contrast echocardiograph y. In a circulatory model-without capillary network interposed between injection and sampling point of contrast-the transit time of microbub bles (source of the echo contrast effect) is inversely related to abso lute flow, thus providing accurate quantitation. A similar situation i s represented by blood flow inside a vessel or a cardial cavity, where , it the prerequisites for quantitation are respected, it is possible to measure blood flow by contrast echocardiography. In the coronary ci rculation, the transit time of contrast microbubbles varies according to their interaction with coronary microcirculation, and to the charac teristics of contrast agents as flow tracer. Echo contrast agents with small microbubbles have been injected into the coronary branches of e xperimental animals, under both coronary autoregulation and maximal co ronary dilation, providing good estimated of coronary blood flow. The accuracy of these measurement might improve when new contrast agents, with characteristics closer to those of a flow tracer, are available. If a tracer in injected before a bifurcation, and provided it mixes ad equately, the amount of tracer distributed to each branch is proportio nal to the corresponding blood flow. A similar situation is encountere d when an echo contrast agent is injected into the aortic root or into the left main coronary artery. Here, the ration between myocardial si gnal intensity in the different perfusion territories reflects the cor responding ratio of blood flows. The validity of this approach has bee n previously demonstrated in experimental animals and validated in pat ients with coronary stenoses. The injection of contrast agents into th e coronary circulation at baseline and under coronary hyperaemia has t he potential for measuring coronary bloodflow reserve. However, what i s still unclear is whether contract echo changes reflect changes in co ronary blood flow (i.e. flow reserve), coronary blood volume (ie. coro nary recruitment) or both, and also whether they influence the differe nt types of contrast agent. Finally, myocardial contrast echocardiogra phy 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 ab sent, contrast echo can provide an accurate estimate of perfusion abno rmalities.