FFR to Predict CABG Result: All Benefits in a Population Much Too Pure?

Coronary artery bypass graft (CABG) anastomosed to one vessel with normal or nearly normal fractional flow reserve (FFR) have poorer graft patency at one year compared against anastomosed grafts to vessels with functionally significant lesions shown by FFR.

However, the recent study FARGO (Fractional Flow Reserve Versus Angiography Randomization for Graft Optimization) did not show differences in patency in the 100 patients it included (14.5% vs 14.3%, p=0.97). This study, and the scarce evidence available, show FFR might not have a significant role in patients undergoing myocardial revascularization surgery, regardless our physiopathological intuition.

The online European Heart Journal has recently released the IMPAG study ahead of print (Impact of Preoperative FFR on Arterial Bypass Graft Anastomotic Function) which suggests a clear association between preoperative FFR values and graft patency 6 months after CABG with all arterial grafts. The authors examined 199 lesions in 67 patients with target vessel mean preoperative FFR of 0.73.

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The protocol included a blind angiogram at 6 months where function was scored as 0 for an occluded graft, 1 when flow was predominantly native, 2 when flow supplies from the native coronary and from the graft where balanced, and 3 when the native coronary was fully opacified by the graft.

Scores 0 to 2 were considered non-functional and 3 was regarded afunctional.

At 6 months, 49 of the original 199 grafts were considered non-functional (24%), and half of these were straight occluded. Preoperative FFR was significantly associated to graft function at 6 months for all conduits and all target vessels (p<0.001).

The best FFR cutoff value to predict this was ≤0.78 vs. the classic 0.8.

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No one frowned upon the fact that FFR values and stenosis percentage in the angiogram showed poor correlation. This is why the study concluded that FFR (not stenosis diameter by angiogram) is what predicts graft patency.

This shows the already familiar limitation of angiograms, though in the daily clinical practice grafts are done to all lesions with >50% occlusion by angiogram.

The study has a few limitations, such as the small number of patients, the short follow-up period (6 months) and the fact that all grafts were arterial (the latter is somewhat far from the daily practice).

In prior studies, the same author had shown distal pressure in a vein graft is nearly the same as aortic pressure (0.4 mmHg difference), which minimizes the risk of competitive flow. However, vein grafts are more likely to occlude than arterial grafts.

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Arterial grafts, on the contrary, show greater pressure drop compared to the aorta (resting 1-3 mmHg and hyperemic up to 9 mmHg), which is what make them more susceptible to chronic flow competition. The latter could explain why arterial grafts benefit more from preoperative FFR.

Original Title: Impact of preoperative fractional flow reserve on arterial bypass graft anastomotic function: the IMPAG trial.

Reference: David Glineur et al. European Heart Journal (2019) 0, 1–9.

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