Is Hyperaemia Essential for Accurate Functional Assessment of Coronary Stenosis Severity?

Login or register to view PDF.
Abstract

Fractional flow reserve (FFR) requires the use of maximal hyperaemia as described in the original preclinical and clinical validation studies and subsequent practice changing randomized controlled trials. A perception that the need for hyperaemia (usually induced with adenosine) was one of the obstacles to more widespread adoption of FFR has led to interest in the use of resting non-hyperaemic indices to assess the functional significance of coronary stenoses. We examine the current evidence base and conclude that resting indices agree with FFR in only 80 % of lesions when a binary cut-off is employed but closer to 90 % when hybrid strategies utilising both resting indices and FFR are utilised. It seems counter intuitive to sacrifice diagnostic accuracy when in most patients and healthcare systems the induction of hyperaemia with adenosine is safe and emminently affordable.

Disclosure
Dr Hennigan has previously received speaker fees from Volcano Corporation. Dr Robertson has no conflicts of interest to declare. Professor Berry has undertaken consultancy work and given lectures based on institutional contracts between his employer, the University of Glasgow and St Jude Medical. St Jude Medical has also provided support for research with pressure wires. Prof Oldroyd has received speaker fees and institutional grant support from St Jude Medical and speakers fees from Volcano
Correspondence
Dr. Barry Hennigan, Cardiology Department, Golden Jubilee National Hospital, Agamemnon Street, Glasgow, G81 4DY. E: barryhennigan@nhs.net
Received date
05 December 2014
Accepted date
20 April 2015
DOI
https://doi.org/10.15420/icr.2015.10.2.72

The original studies of coronary balloon angioplasty performed by Andreas Gruentzig attempted to employ physiological guidance by measuring trans-stenotic gradients with over-the-wire angioplasty catheters.1 However, these catheters were of such large calibre that they contributed to the stenosis being assessed and in addition, measurements were made at rest as the importance of assessing functional stenosis severity during hyperaemia was not yet fully appreciated. The development of pressure and flow (Doppler) sensing coronary guide wires in the early 1990s led to a re-emergence of physiologically guided percutaneous coronary intervention (PCI). A series of studies using Doppler-derived measurements of coronary flow velocity to calculate coronary flow reserve (CFR) were conducted with variable results, partly related to the technical difficulty of recording stable flow velocity signals but mainly to the fact that CFR cannot separate out the contribution to inadequate myocardial blood flow of resistance in the stenotic large-calibre epicardial conduit vessels (the target for intervention) from that in the microcirculation. The concept of pressure-derived fractional flow reserve (FFR) was first described by Pijls et al. in 19932 and following proof-of-concept studies, several clinical trials demonstrated that the use of FFR to guide PCI was not only associated with improved clinical outcomes but was also highly cost effective.3–6 Despite this, the worldwide use of FFR remains disappointingly low at around 5–10 % of all PCIs.7 This figure has been confirmed in a large recent survey by Toth et al.8

Various reasons have been proposed for the low utilisation of FFR, including the cost of the pressure wires as well as the cost, inconvenience and side effects associated with the induction of hyperaemia. In response to this, investigators have explored the ability of resting pressure-derived indices of stenosis severity to predict FFR and thereby to potentially remove the need for hyperaemia, at least in some patients. In this review we will discuss the role of one of these indices, the instantaneous wave-free ratio (iFR), and examine the strengths and weaknesses of iFR and FFR in detail with emphasis on the available evidence to date.

Physiology

FFR is the ratio of mean distal coronary pressure and mean aortic pressure measured during maximal hyperaemia that is generally achieved through administration of a potent vasodilator such as adenosine, ATP or papaverine either by IV infusion or by intracoronary (IC) bolus injection. By rendering myocardial microvascular resistance constant and minimal, it is possible to separate out the impact of disease in the epicardial conduit artery on myocardial blood flow2.

iFR was first described in 2011 following a proof-of-concept study in 157 coronary stenoses in which FFR was compared with resting (non-hyperaemic) trans-stenotic pressure gradients measured in a time period beginning 25 % into diastole and ending 5 ms before the onset of systole, the ‘wave-free’ period.9 It was proposed that coronary microcirculatory resistance was already constant in this window and as low as whole-cycle resistance during hyperaemia (Figure 1). In theory, this should allow accurate prediction of FFR, and in this initial study iFR was found to have a strong correlation with FFR with a receiver operating

Figure 1: ADVISE Study 2012 Identification of Wave-Free Period in the Cardiac Cycle

Open original
Open in new tab

characteristic area under the curve (ROC AUC) of 93 % for iFR≤0.83 predicting FFR≤0.80.9 For decision making in the cath-lab, the investigators proposed that an iFR value of 0.83 could be equated to an FFR value of 0.80.

Evidence to Support the Use of FFR

The original validation of FFR demonstrated that below a cut-off value of 0.75 there was 93 % concordance between FFR and a triple gold standard of exercise stress ECG, single-photon emission computed tomography and dobutamine stress echo performed before and after PCI.3 The DEFER study clearly demonstrated that it was safe to postpone intervention on lesions with an FFR≥0.75.5 The FAME study demonstrated that in patients presenting with multi-vessel PCI, FFR guidance was associated with reduced rates of mortality or myocardial infarction (MI) at 2 years: 12.9 % in the angiography-guided group versus 8.4 % in the FFR-guided group
(p=0.02).4 The FAME 2 study demonstrated a markedly reduced rate of death, MI, and urgent revascularisation in patients with stable angina with at least one lesion and an FFR≤0.80 when treated by PCI plus optimal medical therapy compared with optimal medical therapy alone: 4.3 % versus 12.7 % (HR 0.32; 95 % CI 0.19–0.53; p<0.001). In a subsequent landmark analysis of the clinical outcomes from day 7 to 2 years, essentially excluding periprocedural type 4 MI, the combined endpoint of death and non-fatal MI was significantly reduced by PCI (4.6 % versus 8.0 %; p=0.04) despite a 40 % crossover from medical therapy alone to PCI.5

FFR Beyond Physiological Assessment

FFR assessment provides more than just physiological information. The SYNTAX score is a widely used tool that can stratify patients into low, medium and high risk for PCI as opposed to surgical revascularisation.10 The functional SYNTAX score combines FFR data with

Figure 2: Metanalysis Identifying Optimal FFR Cutoffs to Predict MACE at 1 Year in Medically Treated and Revascularised Groups (PCI or CABG)

Open original
Open in new tab

the angiographically derived SYNTAX score and potentially facilitates the selection of patients who will have equivalent outcomes with PCI or surgery.11 In terms of predicting an overall prognostic benefit from PCI, a recent meta-analysis with a combined endpoint of death, MI and revascularisation demonstrated that FFR<0.75–0.76 was associated with a clear beneficial effect of elective revascularisation (Figure 2).12

FFR at the Time of Diagnostic Angiography?

The data from the numerous trials mentioned above have firmly established the safety of FFR guidance in patients already referred for PCI. This has been supplemented by a large French registry in over 1,000 patients who had at least one intermediate lesion of 35 %–65 % diameter stenosis with overall treatment strategy recorded before and after FFR in addition to follow-up for 1-year endpoints.13 Final management changed in 43 % of cases including 33 % of patients initially assigned to receive no revascularisation, 56 % of patients assigned to PCI and in 51 % of patients assigned to coronary artery bypass graft surgery (Figure 3). Major adverse cardiac event rates (death, MI or unplanned PCI) were similar at 1 year for patients whose treatment was reclassified and those

Figure 3: Details of the Final Revascularisation Strategy applied in Each Group of Revascularisation Strategy A Priori

Open original
Open in new tab

Figure 4: iFR Diagnostic Performance in Stable Coronary Disease versus ACS

Open original
Open in new tab

who remained in their original treatment group following FFR (11.2 % versus 11.9 %; p=not significant).13 Similar findings were reported in the RIPCORD study from the UK.14

Problems with FFR

Inducing Hyperaemia

The debate regarding the clinical challenges with adenosine continues and is mainly focused on patient discomfort, cost and additional procedural time. The reality is that most patients, with the exception of those with severe asthma, tolerate adenosine15 and use of intracoronary adenosine in particular is associated with little additional procedural time or cost while preserving diagnostic accuracy. Other agents including regadenoson and nicorandil are also options for inducing hyperaemia and in some situations may be preferable to adenosine.16 There are theoretical concerns that some of the agents used to induce hyperaemia

Figure 5: Clinical Case: iFR versus FFR in Left Main Stem Disease

Open original
Open in new tab

may introduce haemodynamic disturbances including hypotension and bradycardia, which may alter FFR and lesion classification. This was not borne out in a recent study which demonstrated no significant differences in FFR between a variety of hyperaemic agents and routes of administration.16 A strong correlation was found between IV adenosine/ATP and IC nicorandil (r=0.962; p< 0.001 with a classification agreement of 91.2 %). Similarly, IV adenosine and IV regadenoson compared well (r=0.990; p<0.001 with a classification agreeement of 100 %).16

Cost

The technology itself remains reasonably expensive; however, it is likely to become significantly cheaper with increased competition as new wires from alternative manufacturers come to the market. In addition, a pressure-sensing microcatheter is now commercially available. It is important to note that FFR guidance reduces inappropriate PCI and increases quality of life and is therefore cost effective in patients scheduled for multi-vessel PCI despite the associated expense of the pressure wire and hyperaemic agents.17

Grey-Zone FFR Values

The original validation study as well as many subsequent reports confirmed a very high concordance for non-invasive evidence of ischaemia with FFR values ≤0.75.3,5 However, in order to improve sensitivity and minimise the risk of under-treatment an FFR cut-off value of ≤0.80 was adopted in the FAME and FAME 2 trials.3,4 This has resulted in the de facto creation of a so-called FFR grey-zone between 0.75 and 0.80 within which the need to perform revascularisation is unclear and there may in fact be over-treatment. Indeed, preliminary data from our group suggests that only one-third of patients with grey-zone FFRs have perfusion abnormalities in the corresponding artery when assessed using stress MRI.18 All 33 grey-zone stenoses in a recent study were found to be non-ischaemic using a combined reference standard of hyperaemic stenosis resistance index and myocardial perfusion scintigraphy.19 Another recent study assessed 3-year clinical outcomes in 150 patients with intermediate coronary lesions. Of these patients 56 had FFR 0.75–0.80 while the remaining 94 patients had FFR >0.80.20 Although target vessel revascularisation was higher in the patients with FFR 0.75–0.80 (14 % versus 3 %; p=0.020), there was no difference in rates if death or MI. This finding is consistent with the relationship between FFR and prognosis outlined above12 and emphasises that although there is a need for subsequent revascularisation in this group, they require careful consideration of the risk:benefit ratio of intervention.

Studies Comparing iFR with FFR

The VERIFY study demonstrated that, in a consecutive cohort of 206 coronary stenoses of intermediate severity and compared with the FAME trial FFR cut-off value of ≤0.80, the diagnostic accuracy of iFR ≤0.80 was only 60 %.21 The ADVISE clinical registry reported that in 339 stenoses a different iFR cut-off of 0.89 provided a classification agreement between iFR and FFR of 80 % for the same FFR cut-off value of ≤0.80.22 The ADVISE 2 study compared iFR and FFR in 690 coronary stenoses.23 This dataset was used to derive a proposed hybrid strategy, which correctly classified 94 % of lesions by accepting an iFR>0.93 as being negative (non-ischaemic) and an iFR<0.86 as positive (ischaemic) and only requiring FFR to be measured in lesions with iFR values in between these limits. Compared with an FFR for all strategy, this avoided the use of adenosine in 70 % of stenoses (65 % of patients) at the expense of making a different treatment decision in 1 in 16 lesions.23 The RESOLVE study was an independent core laboratory re-analysis of coronary pressure recordings from 1768 patients drawn from several studies including both ADVISE and VERIFY.24 In the RESOLVE study, the diagnostic accuracy of iFR was 80.4 % at an optimal iFR binary cut-off value of ≤0.90. It was also demonstrated that to achieve ≥90 % diagnostic accuracy at each extreme, the adenosine-free iFR range had to be restricted to ≤0.88 and ≥0.93, which was only applicable to 65 % of the study lesions.24 (Figure 5)

In a prospective study of 53 patients with an acute coronary syndrome (ACS) and 29 patients with stable angina, FFR and iFR in the non-culprit lesions of the ACS patients were calculated by two blinded operators.25 Using binary cut-off values, they demonstrated 81.3 % diagnostic concordance with FFR with positive and negative predictive values for iFR of 67 % and 89 %, respectively. Diagnostic accuracy was similar in both stable and unstable patient groups but was greatly improved by adoption of a hybrid approach with adenosine administration when iFR was 0.86–0.94, inclusive (Figure 4).25

It has previously been stated that the reason the VERIFY study reported an overall diagnostic accuracy for iFR versus FFR of only 60 % compared with 88 % in the ADVISE study is because in VERIFY, iFR was calculated using an in-house algorithm written in MatLab rather than the commercially available proprietary software.21 In fact, all raw data from VERIFY were provided to the independent core laboratory in the University of Columbia, New York for the RESOLVE study analysis. The correlation between iFR and FFR was stronger in the VERIFY study (r2=0.70 using the MatLab programme) than in the RESOLVE study (r2=0.66 using the proprietary software).24 Furthermore, the RESOLVE core lab analysis of the VERIFY dataset showed a mean difference in iFR values between the two different methodologies of -0.007, which is less than the mean difference between repeated measures of either parameter and therefore represents no difference at all.

Table 1: Comparison of the Diagnostic Accuracy of Resting and Hyperemic Indices in Reference to Combined Reference Standard Comprising MPS and HSR

Open original
Open in new tab

iFR versus Perfusion Scintigraphy and/or Hyperaemic Stenosis Resistance as the Reference Standard

A retrospective analysis of 85 stenoses with adequate resting measurements from which to derive iFR out of a dataset of 299 combined pressure and doppler flow coronary physiology studies was performed and compared with a combined reference standard comprising myocardial perfusion scintigraphy (MPS) and hyperaemic stenosis resistance index (HSR).19 Both of HSR and MPS needed to be positive for the lesion to be considered ischaemic. The optimal ischaemic cut-off values were 0.66 mmHg/cm/sec for basal stenosis resistance (BSR) (sensitivity 84.6 %, specificity 79.7 %), 0.82 for iFR (sensitivity 69.2 %, specificity 88.1%), and 0.75 for FFR (sensitivity 88.5 %, specificity 76.3 %). There was no significant difference in AUC for either resting parameter (BSR or iFR) and FFR with respect to the combined reference standard (Table 1).19

Although HSR has previously identified as being highly sensitive and specific for ischemia,26 this particular study has a number of significant limitations including a small sample size and the fact that the original validation of HSR was itself based on the MPS. This imaging modality has a high false-positive rate and importantly it was not repeated post revascularisation, which is possiby the most favourable means of assessing the diagnostic accuracy of any test for ischaemia. In addition, intracoronary adenosine was used to measure FFR in the low dose range of 20–40 μg. Previous studies have demonstrated that intracoronary doses of at least 100–200 μg are required to approximate the FFR achieved by IV adenosine infusion (Figure 6).27 This may lead to an underestimation of FFR, which in turn will erroneously increase the levels of agreement between the hyperaemic and non-hyperaemic indices. Sub-maximal hyperamiea would also be consistent with the absence of any difference between iFR and iFR during hyperaemia, a result entirely inconsistent with previously published data using IV adenosine. This study, however, showed no difference between Pd/Pa and iFR, consistent with the results of the RESOLVE meta-analysis.24

The CLARIFY registry reported data from 51 stenoses that had also been interrogated using combined pressure and flow velocity recordings.28 The authors defined HSR as the reference standard for ischaemia and determined that iFR at a cut-off value of <0.86 had an ROC AUC of 0.93 (95 % CI 0.85–1.00), while FFR<0.75 had an ROC AUC of 0.96 (95 % CI 0.89–1.00).29

Figure 6: Cumulative Percentage of Patients with FFR≤0.75 (n=46)

Open original
Open in new tab

Figure 7. Correlation of iFR versus Pd/Pa

Open original
Open in new tab

iFR Versus Coronary Flow Velocity Reserve as the Reference Standard

In the JUSTIFY study, a retrospective dataset compared iFR with FFR in terms of their ability to predict coronary flow velocity reserve (CFVR).30 While CFVR is a useful parameter, it has generally been superseded by FFR due to marked dependence on systemic haemodynamics (heart rate, blood pressure, contractility) and difficulties in obtaining reproducible flow measurements. It is known that at least 30 % of lesions have discordant CFVR and FFR results and therefore it was unsurprising that FFR ROC AUC was 0.72 (CI 0.65–0.79). In this population iFR appeared to have a stronger correlation with CFVR with an ROC AUC of 0.82 (CI 0.76–0.88; p<0.001).30 The scenario of a lesion with a conventionally ischaemic FFR value but ‘preserved’ CFVR will be discussed later in this review.

Innovations with iFR

A particularly challenging group to assess physiologically are patients with tandem lesions or diffuse disease. In a proof-of-concept study coronary arteries were assessed using a motorised pullback system with calculation of iFR values per millimeter length of vessel.31 The investigators suggested that iFR may have the potential to be used in PCI planning for the determination of physiological lesion length as well as predicting the physiological response to PCI of single lesions when a number of tandem lesions are assessed.31 Given that iFR can be performed rapidly in multiple vessels, there are also potential practical and theoretical advantages in bifurcation disease.

Resting Pd/Pa

The relationship between resting gradient (Pd/Pa) and FFR was described in advance of the development of iFR in a retrospective analysis of 528 pressure-wire studies in 483 patients demonstrating a strong correlation between resting Pd/Pa and FFR (Spearman’s rank correlation coefficient 0.74).32 In fact, it was shown that a resting Pd/Pa of ≤0.87 had a positive predictive value (PPV) of 100 %, while a resting Pd/Pa of ≥0.96 had a negative predictive value (NPV) of 93.9 % in reference to FFR≤0.80. Similarly, when FFR ≤0.75 was used to define an ischaemic stenosis, Pd/Pa≤0.85 had a PPV of 95 %, while a resting Pd/Pa of ≥0.93 had an NPV of 95.7 %. The authors calculated that using these cut-off values, adenosine interrogation would have been avoided in 53 % of the FAME study population (FFR≤0.80) and 34 % of the DEFER study population (FFR<0.75), effectively coining the hybrid method for pressure wire assessment.32 Another, more recent, study retrospectively analysed 570 lesions in 527 patients applying a Pd/Pa adenosine zone range of 0.87–0.99 to achieve 100 % diagnostic accuracy with reference to FFR≤0.80, which would have avoided the use of adenosine in only 14.6 % of lesions.33 The authors reported that with a Pd/Pa adenosine zone of 0.88–0.96, 46.8 % of lesions would have been assessed without adenosine while maintaining 96 % agreement with an FFR for all strategy.

Resting Pd/Pa Versus iFR

In the large RESOLVE meta-analysis, which used an independent core-lab, no significant difference was found between iFR and Pd/Pa in reference to FFR≤0.80 with an overall diagnostic accuracy of 80 % for both indices (using the optimal ROC determined cut-off points of 0.90 for iFR and 0.92 for Pd/Pa.24 The relationship between iFR and Pd/Pa was explored again in a separate study of 1,000 patients with no observable diagnostic advantage of using iFR over Pd/Pa (Figure 7).34 These are important observations for operators who elect to trade off concordance with FFR against the need for hyperaemia, as although iFR analysis is currently only possible using proprietary software, all pressure wire systems are capable of providing resting Pd/Pa.

Problems with iFR

iFR is not Independent of Hyperaemia

iFR assumes that microvascular resistance is already constant and minimal in the wave-free period. Both the VERIFY study and the analysis from Johnson et al. clearly demonstrate that this is not the case.22,34 In the VERIFY study, average iFR values fell significantly following the induction of hyperemia from 0.82 ± 0.16 versus 0.64 ± 0.18 with hyperaemia (95 % CI 0.17–0.20; p=0.0001). At an individual level, iFR decreased with hyperaemia in 97 % of the lesions studied (Figure 8). In the study by Johnson et al, myocardial resistance during the diastolic wave-free period, calculated from simultaneous pressure and flow data, was 250 % higher at rest than during hyperaemia.34 The authors recommend that if clinicians have access to iFR in their cath-labs they should measure at rest and again during hyperaemia; this will make it abundantly clear that it is not, as advertised, an ‘adenosine-independent index of stenosis severity’.

Loss of Diagnostic Accuracy

When binary cut-off values are used for decision making, as in the ongoing randomised trials of iFR guidance versus FFR guidance, DEFINE FLAIR (NCT02053038) and iFR Swedeheart (NCT02166736), the currently available data clearly confirm that avoiding the use of adenosine altogether translates into a different treatment decision in 20 % of lesions compared with an FFR for all strategy. Given wide enough non-inferiority margins and the knowledge that 80 %
of the population recruited will have convergent iFR and FFR results, it is likely that these studies will show non-inferiority. However,
such a result would conceal a mix of both inappropriate PCI and incomplete revascularisation in the iFR-guided group compared with an FFR-guided strategy.

To the authors’ knowledge, there are no randomised controlled trials comparing the hybrid iFR/FFR decision tree with an FFR for all strategy. SYNTAX II (NCT02015832) does employ this option but is an observational study. The currently available data are less comprehensive than for binary cut-off decision-making but avoiding the use of adenosine in between 50–60% of patients may translate into a different treatment decision in around 10 % of patients. Again, this will conceal a mixture of inappropriate PCI and incomplete revascularisation.

Beyond these concerns, if iFR was employed at the time of initial diagnostic angiography, as in the RIPCORD study, then patients will likely have divergent management decisions compared with an FFR for all strategy.14 This will translate not just into different patterns of stent implantation but different overall management in terms of which patients undergo revascularisation and by which technique.

Whether employed in patients already scheduled for PCI or within a binary cut-off or hybrid strategy there is another major concern about using iFR or indeed any resting index of stenosis severity to determine the need for revascularisation. The flow increase associated with hyperaemia when interrogating a large-calibre vessel with a proximal stenosis is much greater than that of a small diameter vessel supplying a small myocardial territory.35 This means that some prognostically important lesions may exhibit small resting pressure gradients and therefore non-ischaemic iFR results which become manifestly ischaemic FFR results during hyperaemia. On the basis that some of these lesions have ‘non-ischaemic’ CFR values >2.0, it has been argued that the FFR result is false and the iFR result is correct. Thus, in the study by van de Hoef et al, half of the patients with discordant iFR and FFR values were ‘ischaemic’ according to the combined reference standard of MPS/HSR, and the other half were ‘non-ischaemic’.20 Notwithstanding the limitations of this study as described above, this indicates that approximately 50 % of treatment decisions will likely be incorrect. Furthermore, a CFR >2.0 does not in any way exclude myocardial ischaemia and may still be too low for a given coronary vessel and myocardial territory at high myocardial oxygen demand.

Figure 8: iFR Readings Before (REST) and After Induction of Hyperemia (HYP) in the VERIFY Study

Open original
Open in new tab

Recommendations

iFR was originally described as an adenosine-independent index of stenosis severity, but it has since been unequivocally demonstrated otherwise. The use of iFR is promoted by some groups on the grounds that the loss of diagnostic accuracy is an acceptable price to pay in return for reducing (hybrid iFR/FFR) or avoiding altogether (binary iFR) the need to use adenosine. Given the large evidence base for FFR that establishes clinical efficacy, cost-effectiveness and prognostic benefit, we do not consider this to be an acceptable trade-off. Beyond this, there are currently no clinical outcome studies with iFR. As such, it is counterintuitive and contrary to current clinical guidelines to recommend iFR over FFR. IV adenosine is the gold standard technique for inducing steady state maximal hyperaemia, but if operators do not wish to use it due to concerns over cost, time or side-effects, we suggest a policy of high-dose IC adenosine, which should minimise procedural delays as well as costs while providing high-quality data with minimal adverse effects. Healthcare providers need to examine their reimbursement policies to enable cardiologists to utilise this proven adjunct to PCI to the benefit of patients without financially stressing their institutions. n

References
  1. Gruntzig A. Transluminal dilatation of coronary-artery stenosis. Lancet 1978;1(8058):263.
    Crossref | PubMed
  2. Pijls NH, van Son JA, Kirkeeide RL, et al. Experimental basis of determining maximum coronary, myocardial, and collateral blood flow by pressure measurements for assessing functional stenosis severity before and after percutaneous transluminal coronary angioplasty. Circulation 1993;87(4):1354–67.
    Crossref | PubMed
  3. Pijls NHJ, Fearon WF, Tonino PAL, et al. Fractional flow reserve versus angiography for guiding percutaneous coronary intervention in patients with multivessel coronary artery disease: 2-year follow-up of the FAME (Fractional Flow Reserve Versus Angiography for Multivessel Evaluation) study. J Am Coll Cardiol 2010;56(3):177–84.
    Crossref | PubMed
  4. De Bruyne B, Pijls NH, Kalesan B, et al; FAME 2 Trial Investigators. Fractional flow reserve-guided PCI versus medical therapy in stable coronary disease. N Engl J Med 2012;367(11):991–1001.
    Crossref | PubMed
  5. De Bruyne B, Fearon WF, Pijls NH, et al; FAME 2 Trial Investigators. Fractional flow reserve-guided PCI or stable coronary artery disease. N Engl J Med 2014;371(13):1208–17.
    Crossref | PubMed
  6. Pijls NH, van Schaardenburgh P, Manoharan G, et al. Percutaneous coronary intervention of functionally nonsignificant stenosis: 5-year follow-up of the DEFER Study. J Am Coll Cardiol 2007;49(21):2105–11.
    Crossref | PubMed
  7. Dattilo PB, Prasad A, Honeycutt E, et al. Contemporary patterns of fractional flow reserve and intravascular ultrasound use among patients undergoing percutaneous coronary intervention in the United States: insights from the National Cardiovascular Data Registry. J Am Coll Cardiol 2012;60(22):2337–9.
    Crossref | PubMed
  8. Toth GG, Toth B, Johnson NP, et al. Revascularization decisions in patients with stable angina and intermediate lesions: results of the international survey on interventional strategy. Circ Cardiovasc Interv 2014;7(6):751–9.
    Crossref | PubMed
  9. Sen S, Escaned J, Malik IS, et al. Development and validation of a new adenosine-independent index of stenosis severity from coronary wave-intensity analysis: results of the ADVISE (ADenosine Vasodilator Independent Stenosis Evaluation) study. J Am Coll Cardiol 2012;59(15):1392–402.
    Crossref | PubMed
  10. Mohr FW, Morice MC, Kappetein AP, et al. Coronary artery bypass graft surgery versus percutaneous coronary intervention in patients with three-vessel disease and left main coronary disease: 5-year follow-up of the randomised, clinical SYNTAX trial. Lancet 2013;381(9867):629–38.
    Crossref | PubMed
  11. Nam CW, Mangiacapra F, Entjes R, et al; FAME Study Investigators. Functional SYNTAX score for risk assessment in multivessel coronary artery disease. J Am Coll Cardiol 2011;58(12):1211–8.
    Crossref | PubMed
  12. Johnson NP, Tóth GG, Lai D, et al. Prognostic value of fractional flow reserve: linking physiologic severity to clinical outcomes. J Am Coll Cardiol 2014;64(16):1641–54.
    Crossref | PubMed
  13. Van Belle E, Rioufol G, Pouillot C, et al; Investigators of the Registre Français de la FFR–R3F. Outcome impact of coronary revascularization strategy reclassification with fractional flow reserve at time of diagnostic angiography: insights from a large French multicenter fractional flow reserve registry. Circulation 2014;129(2):173–85.
    Crossref | PubMed
  14. Curzen N, Rana O, Nicholas Z, et al. Does routine pressure wire assessment influence management strategy at coronary angiography for diagnosis of chest pain?: the RIPCORD study. Circ Cardiovasc Interv 2014;7(2):248–55.
    Crossref | PubMed
  15. Layland J, Carrick D, Lee M, et al. Adenosine: physiology, pharmacology, and clinical applications. JACC Cardiovasc Interv 2014;7(6):581–91.
    Crossref | PubMed
  16. Lim WH, Koo BK, Nam CW, et al. Variability of fractional flow reserve according to the methods of hyperemia induction. Catheter Cardiovasc Interv. 2015;85(6):970–6.
    Crossref | PubMed
  17. Siebert U, Arvandi M, Gothe RM, et al. Improving the quality of percutaneous revascularisation in patients with multivessel disease in Australia: cost-effectiveness, public health implications, and budget impact of FFR-guided PCI. Heart Lung Circ 2014;23(6):527–33.
    Crossref | PubMed
  18. Watkins S. Validation of First Pass Magnetic Resonance Myocardial Perfusion Imaging using Fractional Flow Reserve. MD thesis. University of Glasgow, Glasgow, 2009.
  19. van de Hoef TP, Meuwissen M, Escaned J, et al. Head-to-head comparison of basal stenosis resistance index, instantaneous wave-free ratio, and fractional flow reserve: diagnostic accuracy for stenosis-specific myocardial ischaemia. EuroIntervention 2014 [Epub ahead of print].
    PubMed
  20. Shiono Y, Kubo T, Tanaka A, et al. Long-Term Outcome After Deferral of Revascularization in Patients With Intermediate Coronary Stenosis and Gray-Zone Fractional Flow Reserve. Circ J 2015;79(1):91–5.
    Crossref | PubMed
  21. Berry C, van ‘t Veer M, Witt N, et al. VERIFY (VERification of Instantaneous Wave-Free Ratio and Fractional Flow Reserve for the Assessment of Coronary Artery Stenosis Severity in EverydaY Practice): a multicenter study in consecutive patients. J Am Coll Cardiol 2013;61(13):1421–7.
    Crossref | PubMed
  22. Petraco R, Escaned J, Sen S, et al. Classification performance of instantaneous wave-free ratio (iFR) and fractional flow reserve in a clinical population of intermediate coronary stenoses: results of the ADVISE registry. EuroIntervention 2013 May 20;9(1):91-101.
    Crossref | PubMed
  23. Escaned J. Oral Presentation of ADVISE 2 Results. Presented at: TCT Conference, San Francisco, CA; 30 October 2013.
  24. Jeremias A, Maehara A, Généreux P, et al. Multicenter core laboratory comparison of the instantaneous wave-free ratio and resting Pd/Pa with fractional flow reserve: the RESOLVE study. J Am Coll Cardiol 2014;63(13):1253–61.
    Crossref | PubMed
  25. Indolfi C, Mongiardo A, Spaccarotella C, et al. The instantaneous wave-free ratio (iFR) for evaluation of non-culprit lesions in patients with acute coronary syndrome and multivessel disease. Int J Cardiol 2014;178C:46–54.
    Crossref | PubMed
  26. Meuwissen M, Siebes M, Chamuleau SA, et al. Hyperemic stenosis resistance index for evaluation of functional coronary lesion severity. Circulation 2002;106(4):441–6.
    Crossref | PubMed
  27. Leone AM, Porto I, De Caterina AR, et al. Maximal hyperemia in the assessment of fractional flow reserve: intracoronary adenosine versus intracoronary sodium nitroprusside versus intravenous adenosine: the NASCI (Nitroprussiato versus Adenosina nelle Stenosi Coronariche Intermedie) study. JACC Cardiovasc Interv 2012;5(4):402–8.
    Crossref | PubMed
  28. De Luca G, Venegoni L, Iorio S, et al. Effects of increasing doses of intracoronary adenosine on the assessment of fractional flow reserve. JACC Cardiovasc Interv 2011;4(10):1079–84.
    Crossref | PubMed
  29. Sen S, Asrress KN, Nijjer S, et al. Diagnostic classification of the instantaneous wave-free ratio is equivalent to fractional flow reserve and is not improved with adenosine administration. Results of CLARIFY (Classification Accuracy of Pressure-Only Ratios Against Indices Using Flow Study).
    J Am Coll Cardiol 2013;61(13):1409–20.
    Crossref | PubMed
  30. Petraco R, van de Hoef TP, Nijjer S, et al. Baseline Instantaneous Wave-Free Ratio as a Pressure-Only Estimation of Underlying Coronary Flow Reserve: Results of the JUSTIFY-CFR Study (Joined Coronary Pressure and Flow Analysis to Determine Diagnostic Characteristics of Basal and Hyperemic Indices of Functional Lesion Severity-Coronary Flow Reserve). Circ Cardiovasc Interv 2014;7(4):492–502
    Crossref | PubMed
  31. Nijjer SS, Sen S, Petraco R, et al. Pre-angioplasty instantaneous wave-free ratio pullback provides virtual intervention and predicts hemodynamic outcome for serial lesions and diffuse coronary artery disease. JACC Cardiovasc Interv 2014;7(12):1386–96.
    Crossref | PubMed
  32. Mamas MA, Horner S, Welch E, et al. Resting Pd/Pa measured with intracoronary pressure wire strongly
    predicts fractional flow reserve. J Invasive Cardiol 2010;22(6):260–5.
    PubMed
  33. Kwon TG, Matsuzawa Y, Li J, et al. Clinical usefulness of nonhyperemic baseline Pd/Pa as a hybrid baseline Pd/Pa-fractional flow reserve strategy. Coron Artery Dis 2015;26(1):49–55.
    Crossref | PubMed
  34. Johnson NP, Kirkeeide RL, Asrress KN, et al. Does the instantaneous wave-free ratio approximate the fractional flow reserve? J Am Coll Cardiol 2013;61(13):1428–35.
    Crossref | PubMed
  35. De Bruyne B, Pijls NH, Bartunek J, et al. Fractional flow reserve in patients with prior myocardial infarction. Circulation 2001;104(2):157–62.
    Crossref | PubMed