Coronary physiology is frequently analysed through the following indexes:
- Fractional flow reserve (FFR)
- Instantaneous wave-free ratio (iFR)
Both are measured using ultra-thin wires with a sensor inside the coronary artery, the pressure before and after the narrowing is compared. If there is a significant drop in pressure indicating poor blood flow, coronary angioplasty (stenting) is recommended. A modest drop in pressure indicates the narrowing is better off treated with medications.
Fractional flow reserve (FFR)
The FFR helps measure how much coronary blood flow is reduced as a result of plaque build-up.
An FFR measurement is defined as the ratio between the maximum actual blood flow in a diseased coronary artery (mean distal pressure) relative to the theoretical maximum flow in a normal coronary artery (mean aortic pressure). Maximum blood flow is induced by vasodilating medications, which causes blood vessels to dilate, very much like simulating an exercise environment, in order to allow maximum blood flow.
For example, an FFR of 0.80 means that blood flow across the coronary artery is reduced by 20% from normal. This result can also be used to assess the progression of coronary artery disease with regards to how it affects blood flow down the vessel.
The value provides a measure of the likelihood that the blocked coronary artery impedes oxygen delivery. There are other factors to consider but in general, an FFR of 0.75 – 0.80 is considered the cut-off value, where anywhere below shows a significant lack of blood flow to the heart muscle and thus would require treatment.
As a guide:
- Patients with an FFR below 0.75 – 0.80 indicate benefit of stenting over medications alone
- Patients with an FFR of 0.80 and above indicate no benefit of stenting over medications alone.
Do note that this is not the only factor to consider. Your cardiologist will use the information of your overall condition together with the results of the coronary physiology study to decide the best treatment option for you.
Instantaneous wave-free ratio (iFR)
The iFR ratio is measured during the wave-free period — a specific period during the heart’s diastole.
The normal value of iFR is 1.0. An iFR score of 0.90 or below, indicates significant blood flow reduction which would call for interventions such as stenting.
It can be calculated using one single heartbeat, but in practice, cardiologists use the averaged value over several heartbeats. As it is measured at rest without vasodilating agents or stressors, the overall procedure is slightly faster compared to the FFR.
Also, it may be necessary to obtain both sets of values if the iFR value is not definitive or falls into the grey area between 0.86 and 0.93. In such situations, your cardiologist may measure the FFR value to gain more insight into the condition.