Assessment of the significance of coronary narrowings

Coronary angiography is often regarded as the gold standard for the diagnosis of
coronary artery narrowing. There is a complex relationship between the degree of narrowing and the effect on blood flow through the coronary artery. At rest there is very little effect on flow until the artery is more than 80% narrowed. This is why chest pain at rest if it is caused by coronary artery disease is associated with critical narrowings in the arteries.
When the narrowing is less severe then it does not affect the blood flow unless the requirment for blood is increased such as during a period of exercise. In this situation coronary narrowings become significant when they occupy more than 60% of the space in the artery.
The difficulty comes because it is difficult to assess the degree of stenosis or narrowing of the artery with a very high degree of accuracy. It is straightfoward to detect a mild narrowing or a critical stenosis but sometimes the degree of narrowing is borderline and in this situation it may be necessary to undertake a functional test with a pressure wire.

How the pressure wire measures the narrowing

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The pressure wire has a pressure sensor near the end. The wire is introduced into the coronary artery and negotiated accross the narrowed artery. Following this the difference between the pressure measured in the artery beyond the narrowing is compared with that in the aorta (main blood vessel). The difference between the two can be expressed as a ratio called the fractional flow reserve (FFR). This is measured at rest and thne after administration of either an infusion or bolus of adenosine. This medicines increases the blood flow through the coronary circulation and simultes the situation of exercise.
Measurement of the FFR can be calculated. if the ratio is >0.8 then the narrowing is not significant and it does not require treatment with an angioplasty. If the ratio is <0.8 then there is evidence that implantation of a coronary stent is beneficial.
Two examples of how FFR can guide the patient management. On the left there is coronary lesion in the LAD artery which may be flow limited based on the angiogram. The FFR was 0.97 which is not significant. On the right the lesion in the LAD does not appear to be flow limiting on the angiogram however the FFR is strongly positive at 0.55 indicating that the narrowing is limiting the flow of blood and that stent implantation is indicated.