The Constant Search for Indices of Coronary Flow and Perfusion

Rafael Beyar, MD, DSc
Rafael Beyar, MD, DSc
Assessment of coronary flow is an important task for diagnosis and assessment of interventions in ischemic heart disease. Since the early days of coronary catheterizations, a reliable index of epicardial coronary flow and myocardial perfusion has been sought by the cardiologists and concepts such as “slow flow” or no flow has been primarily based on the angiographic appearance of the injected dye in its passage through the epicardial arteries and the myocardial perfusion bed. In a search for better tools for assessment of coronary flow, perfusion and the effect of coronary stenosis on coronary hemodynamics, several devices that provide us with accurate measurements of pressures and flow in the coronary system have been developed and are available today in the catheterization laboratory.1,2 The flow wire based on Doppler velocity measurements1 has been mostly used for the assessment of coronary flow reserve (CFR) as a physiologic index reflecting the significance of coronary stenosis. While the Doppler-based CFR has been clearly proven as a valuable index in clinical studies, it is limited by its sensitivity to position and motion of the wire on the one hand, and by its cost on the other hand. Pressure-based indices such as the fractional flow reserve2 has been also developed for assessments of coronary hemodynamics, and they may also provide the operator with information of pressure and flow through a stenosis segment. Yet, pressure-based indices do not provide information on flow in the absence of coronary stenosis. With the race for a realistic indices reflecting coronary flow and perfusion in the catheterization laboratory, the simplicity of visual assessment of the coronary flow, whether qualitative or quantitative, and the availability of this indices in all the catheterization laboratories is the major advantage over the more accurate, but more expensive and more time-consuming wire based methods that are available today in the catheterization laboratory. The TIMI flow grading proposed almost two decade ago3 reflects the speed and completeness of passage of the injected contrast agent through the relevant coronary artery has an established prognostic value and can be easily assessed by the operator. The definitions of the TIMI grading system are easily conceived, independent of the quality or the projection of the image and can be widely applied to angiographic-based studies. It is typically assessed immediately during the operation, requiring no time for analysis and provides a tool that is used continuously through the procedure. The same principle of the speed and completeness of passage of the contrast agent through the coronary arteries is also used in the TIMI frame count method suggested by Gibson et al.4,5 While the TIMI frame count with its corrected versions accounting for the different lengths of the coronary arteries provides a quantitatively, continuous variable reflecting coronary flow, its assessment is more laborious than the qualitative TIMI grade. See Bickel et al. on pages 590–596 The paper by Bickel et al in this issue of the Journal of Invasive Cardiology6 have shown that there is a considerable overlap between TIMI II and TIMI III flow when as assessed quantitatively using the TIMI frame count. The TIMI flow grading system is not able to separate completely perused and partially perused coronary arteries. It has been shown that the use of a frame-count based continuous variable, rather than the crude TIMI classification, may provide more accurate data regarding the success of revascularization and also more accurate prognostic information. While its assessment is not instantaneous, the TIMI frame count is a relatively simple index that can be easily measured by the operator at the end of the procedure, does not require special equipment and requires only a short time. While the TIMI indices do provide us with important and reliable information regarding epicardial coronary flow, these indices do not necessarily reflect myocardial perfusion. The blush score,7 which is the amount of staining of the myocardium by the injected contrast agent, has been suggested as an important index reflecting myocardial perfusion following successful revascularization in acute myocardial infarction. This index has primarily been used in a qualitative way, and methods to measure it qualitatively have not been developed. While all the angiographic visualization methods are important, it should be recognized that they all dependent on multiple factors that are not always accounted for. These factors are the amount and rate of the contrast agent injection, the image projection, the length of the specific coronary artery, the size of its perfusion zone and the diameter of the artery. Each of these factors may have an independent effect on these indices and may introduce variability into this index. In order to explore these indices widely, qualitative image-based methods that will provide additional information to crude visualization should be developed. Effort should be directed into integration of these indices into the angiographic unit, in a way similar to online quantitative coronary angiography packages. In the meantime, we continuously rely on visual assessment of coronary flow using TIMI grading in the catheterization laboratory, and may use the corrected TIMI count suggested by Gibson or the Blush score if a more accurate and continuous number is sought to reflect coronary flow and perfusion.
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