Fractional flow reserve
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Priyantha Ranaweera, M.D.
Synonyms and Keywords: FFR, coronary fractional flow reserve
Overview
While coronary angiography assess the extent and the structural severity of fixed coronary stenoses, angiography does not provide functional information regarding flow limitations. While exercise tolerance testing may provide this functional information, only a quarter to a third of patients undergoing cardiac catheterization may have undergone stress test before cardiac catheterization. Thus there is the need for diagnostic studies to assess the functional significance of stenoses in the cardiac catheterization laboratory. While intervention in a critically narrowed lesion is appropriate, a considerable number of lesions are of “intermediate” severity on coronary angiography. Assessment of fractional flow reserve (FFR) may improve the ability to assess the functinal consequences of such lesions in the cardiac catheterization laboratory [1].
Definition
Fractional flow reserve is defined as the fraction of maximal achievable blood flow that can still be maintained to the myocardium despite the presence of a stenosis.
FFR = Pd/Pa ( Pd = pressure distal to the lesion measured by the pressure wire, Pa = pressure at the tip of the guide/catheter)
This measure is a surrogate marker of relative ischemia during exercise.
Physiologic Basis for FFR Measurements
- During maximal hyperemia the FFR can be calculated by dividing the distal pressure by proximal pressure.
- Pressure drop across a lesion is proportional to:
- length of the lesion
- flow across the lesion.
- It is inversely related to:
- square root of the area of the stenosis.
FFR Vs Coronary flow reserve CFR
The FFR assesse the significance of epicardial stenosis independent of the distal vascular bed, where as the CFR is a measure of the distal vascular bed which may be affected by conditions such as hypertension and diabetes.
FFR - 0.75 threshold/cut off
Pijls, et al. in a landmark study validated the clinical use of FFR against ischemai on nuclear imaging studies. A threshold of 0.75 is associated with a sensitivity of 90% and a specificity of 100% [2].
Setting up the equipment
- Open the Radi wire from the packet and flush it with saline. Do not pull it out of the protective tubing.
- Zero the arterial and distal pressure ports. ( Pa and Pd). (If the pressure tracings need to be displayed in the cardiac cath lab display rather than on the Radi console display, then connect the two connector cables from the Radi console to the cath lab system.)
- Do ACT ( Do not introduce the guide wire in to the coronary artery until the ACT > 200)
- After confirming an ACT > 200 secs, equlize the pressure with the Radi wire at the tip of the guide.
- Cross the lesion with the pre-planned wire. (The Radi wire can be used for lesions that are easy to cross)
- Obsreve the FFR at rest.
- Induced coronary hyperemia.
- Measure FFR
- Could also perform pull back FFR if use the iv route for drugs.
Inducing maximum coronary hyperemia
This can be done either by delivering one or more of intracoronary boluses of adenosine (20 mcg dose, peaks in 5 seconds), or using 140 mcg/kg/min over two minutes intravenously from the antecubital fossa or up to 180 mcg/kg/min intravenously from the femoral vein.
Examples of FFR Measurements
Different types of tracings
As the lesion progresses in severity, the pressure drops first in diastole, and then both in systole and diastole.
FFR correlation with IVUS
Using FFR as the gold standard for lesion severity, Brigouri et al demonstrated
- Optimal sensitivity (sens) and specificity (spec) of IVUS to discriminate significant from non-significant stenoses when area stenosis was >70% (sen 100%, spec 68%)
- Cut-off values fitted to a FFR of <0.75
- MLD <1.8 mm (sen 100%, spec 66%),
- MLA <4.0 mm2 (sens 92%, spec 56%)
- (All lesions with area stenosis <70%had FFR >0.75 but 50% of lesions with area stenosis >70% had FFR <0.75) [3].
FFR in clinical decision making
The Outcomes of Lesions that are not Hemodynamically Significant by FFR: The DEFER Trial
The DEFER trial demonstrated that among intermediate lesions that were not hemodynamically significant (i.e. the FFR was greater than 0.75), there was no impact on clinical outcomes (Death or death / MI). Furthermore the risk of adverse events was quite low and mortality attributable to these lesions was under 1% per year[4].
In the DEFER trial, a total of 325 patients undergoing PCI of an intermediate lesion were randomized to deferral of the PCI if the FFR was > 0.75 (n=91) or performance of a PCI (n=90) even though the lesion was not functionally significant. In other words this trial test what would happen if lesions that are not functionally significant either are or are not dilated. If the FFR was < 0.75, the lesion was dilated.
At 5 years, the survival did not differ between those patients who had a a PCI versus those who did not have a PCI (the deferred group) of a stenosis that was not associated with ischemia. Likewise there was no difference in the risk of death or MI or the number of patients who were free of chest pain on follow-up between the two groups. The risk of cardiovascular death was less than 1% per year in vessels that were either treated or untreated and was not affected by stenting.
The Role of FFR in Selecting Which Lesions to Dilate in the Patient with Multivessel Disease: Results of the FAME Trial
The FAME trial randomized 1005 patients with multivessel disease (excluding those with left main disease or STEMI) who were undergoing intracoronary stent implantation to one of the two following strategies:
- Drug-eluting stent implantation guided by angiography alone in which case all significant lesions were stented versus
- Drug-eluting stent implantation guided by FFR measurements in addition to angiography. patients who were randomly assigned to the FFR strategy had stents implanted in those lesions with an FFR of 0.80 or less.
Patients in the FFR guided strategy had fewer stents placed than those in the strategy relying upon angiography alone (2.7±1.2 versus 1.9±1.3 stents, p<0.001). At one year, the primary end point of death, nonfatal myocardial infarction, and repeat revascularization was observed in 18.3% of patients in the angiography group versus 13.2% patients) in the angiography plus FFR group (P=0.02). there was no difference in the incidence of survival free from angina at one year: 78% of patients in the angiography group versus 81% of patients in the FFR group (P=0.20). The risk of death was 3.0% vs 1.8% (p=0.19). The risk of death or MI was 11.1% in the angiography group versus 7.3% in the FFR group (p=0.04). The number of patients who were event free and free of angina were 73% in the FFR group and 67.6% in the angiography group (p=0.07). Procedure costs and the use of contrast was lower in the FFR group as well. It should be noted that only 63% of lesions that were measured had an FFR less than 0.80.
The authors speculate that the results of COURAGE and SYNTAX might have been different had stenting been performed with FFR guidance in which case only those lesions that were ischemic would have been treated. THE FFR cutpointis 075 to 0.80 and FAME used the higher end of this range to that fewer lesions were left unrevascularized.
By not dilating and stenting lesions that appear to be severe angiographically but are not associated with ischemia, you avoid the placement of a stent which could result in a risk of thrombosis or restenosis in a lesion that was not associated with ischemia. The FAME authors concluded that "Routine measurement of FFR in patients with multivessel coronary artery disease who are undergoing PCI with drug-eluting stents significantly reduces the rate of the composite end point of death, nonfatal myocardial infarction, and repeat revascularization at 1 year."
The Role of FFR in Left Main Coronary Artery Disease (LMCA)
In a small non-randomized study, 30 patients who had a hemodynamically significant FFR of <0.75 underwent coronary artery bypass grafting, and an accompanying 24 patients who had a non-hemodynamically significant FFR > 0.75 were managed medically. There was no difference in clinical outcomes between these two groups[5]. In this very small study no patients in the medically managed group died or had AMI.
Likewise, in a small study of 51 patients, FFR measurenet was helpful to identify patients with intermediate left main disease in whom deferral of surgical revascularization was associated with excellent survival and low event rates[6].
FFR in Acute Coronary Syndromes
The use of FFR has been validated in patients with previous myocardial infarction. The same study also demonstrated the relation between stenosis severity, coronary blood flow, myocardial ischemia, and extent of perfusion territory[7].
- Recommendations from Eindhoven and Aalst
- Acute chest pain without ECG changes or enzyme elevation: FFR applicable as usual.
- In patients with acute chest pain with ECG changes or elevated cardiac enzymes / non-STEMI
- Single vessel disease or culprit lesion clear from ECG - FFR indicated, treat culprit. Use FFR, if indicated, in case of other lesions.
- If culprit lesion is not clear - use FFR as usual[8].
- (Note: if time interval >48 h use FFR as usual.)*
- STEMI/transmural myocardial infarction: FFR – Do not apply in < 5 days.
FFR in lesions involving bifurcations and sidebranches.
In this study, measurement of FFR in jailed side branch lesions was shown to be both safe and feasible. It also showed that most of these lesions did not have functional significance, despite morphologic appearance[9].
FFR Measurement In Renal Artery Stenosis
Using PressureWire and vasodilatory stimulus, and during diagnositc catheterization, pressure measurement and calculation of Pd/Pa ratio can be used to guide treatment of renal artery stenosis. The cutoff value is found to be a Pd/Pa ratio of 0.90[10].
FFR and Procedural Time Saved
Using FFR for decision making was shown to cut down the overall hospital stay with better outcomes when compared to using angiography alone to guide angiographic findings [11].
Cost-Effectiveness of FFR
Using FFR for decision making was shown to cut down the overall cost with better outcomes when compared to using angiography alone to guide PCI [11].
Precautions
- Ensure that there is no pressure gradient between the pressure wire (placed at the tip of the guide) and the guiding catheter. The pressures should be equalized at the onset.
- The two pressures should be compared and noted before inducing coronary hyperemia.
- Use a standard wire first to cross difficult lesions. The pressure wire may not be the most suitable in complex lesions.
- Pressure wire may track under a plaque and cause a dissection.
- Repeated attempts in trying to negotiate tortuous anatomy may cause trauma to intact endothelium and promote thrombosis and dissection.
References
- ↑ Pijls NH (2004). "Optimum guidance of complex PCI by coronary pressure measurement". Heart. 90 (9): 1085–93. doi:10.1136/hrt.2003.032151. PMC 1768417. PMID 15310716. Unknown parameter
|month=ignored (help) - ↑ Pijls NH, De Bruyne B, Peels K; et al. (1996). "Measurement of fractional flow reserve to assess the functional severity of coronary-artery stenoses". N. Engl. J. Med. 334 (26): 1703–8. PMID 8637515. Unknown parameter
|month=ignored (help) - ↑ Briguori C, Anzuini A, Airoldi F; et al. (2001). "Intravascular ultrasound criteria for the assessment of the functional significance of intermediate coronary artery stenoses and comparison with fractional flow reserve". Am. J. Cardiol. 87 (2): 136–41. PMID 11152827. Unknown parameter
|month=ignored (help) - ↑ Pijls NH, van Schaardenburgh P, Manoharan G; et al. (2007). "Percutaneous coronary intervention of functionally nonsignificant stenosis: 5-year follow-up of the DEFER Study". J. Am. Coll. Cardiol. 49 (21): 2105–11. doi:10.1016/j.jacc.2007.01.087. PMID 17531660. Unknown parameter
|month=ignored (help) - ↑ Bech GJ, Droste H, Pijls NH; et al. (2001). "Value of fractional flow reserve in making decisions about bypass surgery for equivocal left main coronary artery disease". Heart. 86 (5): 547–52. PMID 11602550. Unknown parameter
|month=ignored (help) - ↑ Lindstaedt M, Yazar A, Germing A; et al. (2006). "Clinical outcome in patients with intermediate or equivocal left main coronary artery disease after deferral of surgical revascularization on the basis of fractional flow reserve measurements". Am. Heart J. 152 (1): 156.e1–9. doi:10.1016/j.ahj.2006.03.026. PMID 16824848. Unknown parameter
|month=ignored (help) - ↑ De Bruyne B, Pijls NH, Bartunek J; et al. (2001). "Fractional flow reserve in patients with prior myocardial infarction". Circulation. 104 (2): 157–62. PMID 11447079. Unknown parameter
|month=ignored (help) - ↑ Potvin JM, Rodés-Cabau J, Bertrand OF; et al. (2006). "Usefulness of fractional flow reserve measurements to defer revascularization in patients with stable or unstable angina pectoris, non-ST-elevation and ST-elevation acute myocardial infarction, or atypical chest pain". Am. J. Cardiol. 98 (3): 289–97. doi:10.1016/j.amjcard.2006.02.032. PMID 16860011. Unknown parameter
|month=ignored (help) - ↑ Koo BK, Kang HJ, Youn TJ; et al. (2005). "Physiologic assessment of jailed side branch lesions using fractional flow reserve". J. Am. Coll. Cardiol. 46 (4): 633–7. doi:10.1016/j.jacc.2005.04.054. PMID 16098427. Unknown parameter
|month=ignored (help) - ↑ De Bruyne B, Manoharan G, Pijls NH; et al. (2006). "Assessment of renal artery stenosis severity by pressure gradient measurements". J. Am. Coll. Cardiol. 48 (9): 1851–5. doi:10.1016/j.jacc.2006.05.074. PMID 17084261. Unknown parameter
|month=ignored (help) - ↑ 11.0 11.1 Leesar MA, Abdul-Baki T, Akkus NI, Sharma A, Kannan T, Bolli R (2003). "Use of fractional flow reserve versus stress perfusion scintigraphy after unstable angina. Effect on duration of hospitalization, cost, procedural characteristics, and clinical outcome". J. Am. Coll. Cardiol. 41 (7): 1115–21. PMID 12679210. Unknown parameter
|month=ignored (help)