Fractional flow reserve (FFR) is the fraction of the maximal coronary flow that remains in the presence of an epicardial coronary lesion. Hence it is useful in assessing the severity of lesions, especially in case of lesions thought to be borderline on angiography. The flow has contributions from the factors due to the epicardial coronary artery, myocardium and the collateral flow. Hence a good collateral flow can lead to underestimation of the severity of stenosis estimated by FFR. Advantage of FFR over coronary flow reserve is that as it is calculated only at peak hyperemia, it is mostly independent of basal flow, aortic pressure, heart rate and the status of the microcirculation. There is a strong correlation between FFR and inducible myocardial ischemia. An FFR below 0.75 identified physiologically significant stenosis. In patients with coronary lesions, FFR can be used to identify patients who will benefit from from percutaneous coronary interventions. An FFR of less than 0.90 after coronary stenting would predict a higher event rate of 20% and an FFR below 0.80 an event rate of 30% compared with 5-6% event rates in those with FFR above 0.90.
Guide catheters without side holes are preferred for FFR estimation. Contrast should be flushed out and the guide catheter filled with saline for good pressure measurements. Excess adenosine will have to be given to get adequate hyperemia if there are side holes for the catheter, due to extra spillage of drug into the aorta. Matching of the wire tip pressure and the aortic pressure should be done before crossing the lesion. The level of the external pressure transducer should be adjusted to get a match of the tracings if necessary. Conventionally the transducer position corresponds to a level 5 cm below the sternal level indicating the level of the right atrium. A loose Y connector and a retained guide wire introducer can cause pressure leakage. Drift in signals can be identified by repeat matching of the pressures after the wire pull back. There is a limitation while assessing serial lesions as the true FFR across a lesion can be known only after complete relief of the other lesion. There are formulae available for calculating the significance of individual lesions. But this will require measurement of wedge pressure which can be done only during interventions and not during diagnostic catheterizations.