How do cardiologists determine what percentage of the coronary artery is blocked during an angiogram, and what is the significance of this?

Doctor's Answers (1)

The coronary angiogram is the current gold standard to establish the degree of coronary artery disease and with experience one can usually determine the stenosis with just “eyeballing”.

However, I do use adjunct imaging inside the blood vessel itself, and this helps to clearly establish the area and character of the narrowing or stenosis in the coronary vessel. 

Evidence from trials clearly show that a narrowing is critical when it approaches 70% as it affects blood flow to the heart muscle. However, we also know that there are areas of the blood vessel filled with fat with a very thin covering layer, liable to rupture, called the “vulnerable” plaque, which is not visible on the angiogram.

This thin layer can burst and cause blood to clot, stopping the flow of blood and ultimately a heart attack. These vulnerable plaques can be seen with OCT or Optical Coherence Tomography which can visualize the inside of the arteries with a resolution of upto 20microns. There are other newer techniques on the horizon to identify such “vulnerable” plaques in a routine manner but this is still some way off being applied on a routine basis.

However even the “70%” rule does not always hold and sometimes even 40-50% narrowing can be significant in certain critical coronary vessels such as the left main stem or proximal left anterior descending artery and affect survival.  In borderline narrowing the operator’s experience comes into play and this maybe when other techniques such as the above techniques such as intravascular imaging (IVUS or OCT) or pressure gradient measurement (FFR or Fractional Flow reserve) maybe necessary to determine the significance of the narrowing.

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