Impact of stenosis resistance and coronary flow capacity on fractional flow reserve and instantaneous wave-free ratio discordance: a combined analysis of DEFINE-FLOW and IDEAL

BACKGROUND

The pressure-derived parameters fractional flow reserve (FFR) and the emerging instantaneous wave-free ratio (iFR) are the most widely applied invasive coronary physiology indices to guide revascularisation. However, approximately 15-20% of intermediate stenoses show discordant FFR and iFR, and therapeutical consensus is lacking.

AIMS

We sought to associate hyperaemic stenosis resistance index, coronary flow reserve (CFR) and coronary flow capacity (CFC) to FFR/iFR discordance.

METHODS

We assessed pressure and flow measurements of 647 intermediate lesions (593 patients) of two multi-centre international studies.

RESULTS

FFR and iFR were discordant in 15% of all lesions (97 out of 647). FFR+/iFR- lesions had similar hyperaemic average peak velocity (hAPV), CFR and CFC as FFR-/iFR- lesions, whereas FFR-/iFR+ lesions had similar hAPV, CFR and CFC as FFR+/iFR+ lesions (p > 0.05 for all). FFR+/iFR- lesions were associated with lower baseline stenosis resistance, but not hyperaemic stenosis resistance, compared with FFR-/iFR+ lesions (p < 0.001).

CONCLUSIONS

Discordance with FFR+/iFR- is characterised by maximal flow values, CFR, and CFC patterns similar to FFR-/iFR- concordance that justifies conservative therapy. Discordance with FFR-/iFR+ on the other hand, is characterised by low flow values, CFR, and CFC patterns similar to iFR+/FFR+ concordance that may benefit from percutaneous coronary intervention.

Instantaneous wave free ratio value impact on left internal mammary artery graft patency

OBJECTIVES

To assess whether instantaneous wave - free ratio (iFR) value is associated with left internal mammary artery (LIMA) graft failure at 12 months follow-up post coronary artery bypass graft (CABG).

BACKGROUND

Data suggests bypass to a non-significant left anterior descending artery (LAD) lesion due to visual over-estimation may lead to LIMA graft failure. Implementing iFR may result in better arterial graft patency.

METHODS

In iCABG (iFR guided CABG) study patients planned to undergo an isolated CABG procedure was prospectively enrolled and iFR was performed for LAD. Coronary computed tomography angiography was performed at 2 and 12 months follow-up. The primary endpoint of this study was to determine the rate of LIMA graft occlusion or hypoperfusion at 2 and 12-months follow-up. We considered a composite secondary endpoint of Major adverse cardiovascular and cerebrovascular event (MACCE) as a secondary outcome.

RESULTS

In total 69 patients were included with no differences regarding age, sex and risk factors. At 2 months, 50 of LIMAs with pre-CABG iFR median 0.855 (0.785 - 0.892) were patent. Hypoperfusion was found in 8 LIMAs (median iFR 0.88 (0.842 - 0.90)). While, 7 LIMAs (median iFR 0.91 (0.88 - 0.96)) were occluded (p = 0.04). At 12 months, when iFR of LAD was >0.85: just 12 (31.6% out of all patent LIMAS) grafts were patent and 24 (100.0% out of all hypoperfused/occluded) grafts were hypoperfused or occluded (p < 0.001). In terms of MACCE, no difference (p = 1.0) was found between all 3 groups divided according to iFR value.

CONCLUSIONS

Instantaneous wave - free ratio value above 0.85 in LAD is a powerful tool predicting LIMA graft failure at 1-year follow up period.

Coronary Physiology: Modern Concepts for the Guidance of Percutaneous Coronary Interventions and Medical Therapy

Recent evidence on ischemia, rather than coronary artery disease (CAD), representing a major determinant of outcomes, has led to a progressive shift in the management of patients with ischemic heart disease. According to most recent guidelines, myocardial revascularization strategies based on anatomical findings should be progressively abandoned in favor of functional criteria for the guidance of PCI. Thus, emerging importance has been assigned to the assessment of coronary physiology in order to determine the ischemic significance of coronary stenoses. However, despite several indexes and tools that have been developed so far, the existence of technical and clinical conditions potentially biasing the functional evaluation of the coronary tree still cause debates regarding the strategy of choice. The present review provides an overview of the available methods and the most recent acquirements for the invasive assessment of ischemia, focusing on the most widely available indexes, fractional flow reserve (FFR) and instant-wave free ratio (iFR), in addition to emerging examples, as new approaches to coronary flow reserve (CFR) and microvascular resistance, aiming at promoting the knowledge and application of those "full physiology" principles, which are generally advocated to allow a tailored treatment and the achievement of the largest prognostic benefits.

Diagnostic performance of quantitative flow ratio versus fractional flow reserve and resting full-cycle ratio in intermediate coronary lesions

BACKGROUND

Quantitative flow ratio (QFR) is a novel angiography-derived index aimed to assess the functional relevance of coronary stenoses without pressure wires and adenosine. Good diagnostic yield with the hyperemic fractional flow reserve (FFR) have been reported, while data on the comparison of QFR to non-hyperemic pressure ratios (NHPR) are scarce.

METHODS

In this retrospective, observational and single-center study with a large population representative of the real practice, we assessed and compared the diagnostic performance of contrast flow (cQFR) and fixed flow (fQFR) QFR against the NHPR resting full-cyle ratio (RFR) using FFR as reference standard.

RESULTS

A total of 626 lesions from 544 patients were investigated. Mean diameter stenosis, FFR, cQFR, fQFR and RFR were 44.8%, 0.842, 0.847, 0.857 and 0.912, respectively. The correlation between cQFR and FFR was stronger (r = 0.830, P < 0.001) compared to that between FFR and RFR (r = 0.777, P < 0.001) and between cQFR and RFR (r = 0.687, P < 0.001). Using FFR ≤0.80 as reference, the sensitivity, specificity, positive predictive value, negative predictive value, and overall diagnostic accuracy for cQFR were 82%, 95%, 87%, 92%, and 91%, respectively. cQFR displayed a higher area under the curve (AUC) than fQFR and RFR (0.938 vs. 0.891 vs. 0.869, P < 0.01). The good diagnostic yield of cQFR appeared to be maintained in different clinical subsets including female gender, aortic valve stenosis and atrial fibrillation, and in different anatomical subsets including focal and non-focal lesions.

CONCLUSION

cQFR has a high and better diagnostic performance than the NHPR RFR in predicting FFR-based functional significance of coronary stenoses.

5-Year Outcomes of PCI Guided by Measurement of Instantaneous Wave-Free Ratio Versus Fractional Flow Reserve

BACKGROUND

Instantaneous wave-free ratio (iFR) is a coronary physiology index used to assess the severity of coronary artery stenosis to guide revascularization. iFR has previously demonstrated noninferior short-term outcome compared to fractional flow reserve (FFR), but data on longer-term outcome have been lacking.

OBJECTIVES

The purpose of this study was to investigate the prespecified 5-year follow-up of the primary composite outcome of all-cause mortality, myocardial infarction, and unplanned revascularization of the iFR-SWEDEHEART trial comparing iFR vs FFR in patients with chronic and acute coronary syndromes.

METHODS

iFR-SWEDEHEART was a multicenter, controlled, open-label, registry-based randomized clinical trial using the Swedish Coronary Angiography and Angioplasty Registry for enrollment. A total of 2,037 patients were randomized to undergo revascularization guided by iFR or FFR.

RESULTS

No patients were lost to follow-up. At 5 years, the rate of the primary composite endpoint was 21.5% in the iFR group and 19.9% in the FFR group (HR: 1.09; 95% CI: 0.90-1.33). The rates of all-cause death (9.4% vs 7.9%; HR: 1.20; 95% CI: 0.89-1.62), nonfatal myocardial infarction (5.7% vs 5.8%; HR: 1.00; 95% CI: 0.70-1.44), and unplanned revascularization (11.6% vs 11.3%; HR: 1.02; 95% CI: 0.79-1.32) were also not different between the 2 groups. The outcomes were consistent across prespecified subgroups.

CONCLUSIONS

In patients with chronic or acute coronary syndromes, an iFR-guided revascularization strategy was associated with no difference in the 5-year composite outcome of death, myocardial infarction, and unplanned revascularization compared with an FFR-guided revascularization strategy. (Evaluation of iFR vs FFR in Stable Angina or Acute Coronary Syndrome [iFR SWEDEHEART]; NCT02166736).

Instantaneous wave-free ratio compared with fractional flow reserve in PCI: A cost-minimization analysis

BACKGROUND

Coronary physiology is a routine diagnostic tool when assessing whether coronary revascularization is indicated. The iFR-SWEDEHEART trial demonstrated similar clinical outcomes when using instantaneous wave-free ratio (iFR) or fractional flow reserve (FFR) to guide revascularization. The objective of this analysis was to assess a cost-minimization analysis of iFR-guided compared with FFR-guided revascularization.

METHODS

In this cost-minimization analysis we used a decision-tree model from a healthcare perspective with a time-horizon of one year to estimate the cost difference between iFR and FFR in a Nordic setting and a United States (US) setting. Treatment pathways and health care utilizations were constructed from the iFR-SWEDEHEART trial. Unit cost for revascularization and myocardial infarction in the Nordic setting and US setting were derived from the Nordic diagnosis-related group versus Medicare cost data. Unit cost of intravenous adenosine administration and cost per stent placed were based on the average costs from the enrolled centers in the iFR-SWEDEHEART trial. Deterministic and probabilistic sensitivity analyses were carried out to test the robustness of the result.

RESULTS

The cost-minimization analysis demonstrated a cost saving per patient of $681 (95% CI: $641 - $723) in the Nordic setting and $1024 (95% CI: $934 - $1114) in the US setting, when using iFR-guided compared with FFR-guided revascularization. The results were not sensitive to changes in uncertain parameters or assumptions.

CONCLUSIONS

IFR-guided revascularization is associated with significant savings in cost compared with FFR-guided revascularization.

Invasive and non-invasive assessment of ischaemia in chronic coronary syndromes: translating pathophysiology to clinical practice

Intracoronary physiology testing has emerged as a valuable diagnostic approach in the management of patients with chronic coronary syndrome, circumventing limitations like inferring coronary function from anatomical assessment and low spatial resolution associated with angiography or non-invasive tests. The value of hyperaemic translesional pressure ratios to estimate the functional relevance of coronary stenoses is supported by a wealth of prognostic data. The continuing drive to further simplify this approach led to the development of non-hyperaemic pressure-based indices. Recent attention has focussed on estimating physiology without even measuring coronary pressure. However, the reduction in procedural time and ease of accessibility afforded by these simplifications needs to be counterbalanced against the increasing burden of physiological assumptions, which may impact on the ability to reliably identify an ischaemic substrate, the ultimate goal during catheter laboratory assessment. In that regard, measurement of both coronary pressure and flow enables comprehensive physiological evaluation of both epicardial and microcirculatory components of the vasculature, although widespread adoption has been hampered by perceived technical complexity and, in general, an underappreciation of the role of the microvasculature. In parallel, entirely non-invasive tools have matured, with the utilization of various techniques including computational fluid dynamic and quantitative perfusion analysis. This review article appraises the strengths and limitations for each test in investigating myocardial ischaemia and discusses a comprehensive algorithm that could be used to obtain a diagnosis in all patients with angina scheduled for coronary angiography, including those who are not found to have obstructive epicardial coronary disease.

Diagnostic Accuracy of Diastolic Fractional Flow Reserve for Functional Evaluation of Coronary Stenosis

Background

In the resting conditions, narrowing the window of coronary pressure measurements from the whole cardiac cycle to diastole improves diagnostic performance of coronary pressure–derived physiological index. However, whether this also applies to the hyperemic conditions has not yet been thoroughly evaluated.

Objectives

The purpose of this study was to assess whether diastolic fractional flow reserve (diastolic FFR) has better diagnostic performance in identifying ischemia-causing coronary lesions than conventional FFR in a prospective, multicenter, and independent core laboratory–based environment.

Methods

In this prospective multicenter registry at 29 Japanese centers, we compared the diagnostic performance of FFR, diastolic FFR, resting distal to aortic coronary pressure (Pd/Pa), and diastolic pressure ratio (dPR) using myocardial perfusion scintigraphy (MPS) as the reference standard in 378 patients with single-vessel coronary disease.

Results

Inducible myocardial ischemia was found on MPS in the relevant myocardial territory of the target vessel in 85 patients (22%). In the receiver-operating curve analyses, diastolic FFR had comparable area under the curve (AUC) compared with FFR (AUC_{diastolic FFR}: 0.66; 95% confidence interval [CI]: 0.58-0.73, vs AUC_FFR: 0.66; 95% CI: 0.58-0.74, P = 0.624). FFR and diastolic FFR showed significantly larger AUCs than resting Pd/Pa (0.62; 95% CI: 0.54-0.70; P = 0.033 and P = 0.046) but did not show significantly larger AUCs than dPR (0.62; 95% CI: 0.55-0.70; P = 0.102 and P = 0.113).

Conclusions

Diastolic FFR showed a similar diagnostic performance to FFR as compared with MPS. This result reaffirms the use of FFR as the most accurate invasive physiological lesion assessment. (Diagnostic accuracy of diastolic fractional flow reserve (d-FFR) for functional evaluation of coronary stenosis; UMIN000015906)

Contribution of Age-Related Microvascular Dysfunction to Abnormal Coronary: Hemodynamics in Patients With Ischemic Heart Disease

OBJECTIVES

This study sought to investigate the contribution of age-related microcirculatory dysfunction to abnormal coronary hemodynamics in patients with coronary atherosclerosis.

BACKGROUND

Impairment in myocardial blood supply in patients with coronary atherosclerosis can be accentuated due to age-related changes in microcirculatory function.

METHODS

Intracoronary pressure and flow were measured with the Doppler technique in 299 vessels (228 patients), and the thermodilution technique in 120 vessels (99 patients). In 172 patients, Doppler measurements were also performed in unobstructed vessels. Associations of coronary hemodynamics with aging were studied in both the stenosed and unobstructed arteries.

RESULTS

Aging was associated with a progressive increase in minimal microvascular resistance and a progressive decrease in hyperemic flow in both obstructed and nonobstructed coronary arteries. As such, coronary flow reserve decreased with advancing age. Epicardial stenosis severity assessed by resting Pd/Pa, basal stenosis resistance index, and hyperemic stenosis resistance index was equivalent across age groups. By contrast, fractional flow reserve increased with advancing age. Consequently, the adjusted risk of a fractional flow reserve/coronary flow reserve pattern reflective of concomitant focal epicardial and diffuse or microvascular disease (relative risk: 1.6; 95% confidence interval: 1.1 to 2.3; p = 0.017) increased with advancing age, whilst the adjusted risk of a fractional flow reserve/coronary flow reserve pattern reflective of non-flow-limiting stenosis with a healthy microcirculation decreased (relative risk: 0.7; 95% CI: 0.5 to 1.0; p = 0.022).

CONCLUSIONS

Aging is associated with progressive pan-myocardial impairment of coronary vasodilatory capacity due to an increase in minimal microvascular resistance. Concomitant aging-related impairment in microvascular function impacts the pathophysiology of ischemic heart disease in the individual patient and is not adequately identified by hyperemic coronary pressure measurements alone.

Comparison of fractional flow reserve, instantaneous wave-free ratio and a novel technique for assessing coronary arteries with serial lesions

AIMS

Physiological indices such as fractional flow reserve (FFR), instantaneous wave-free ratio (iFR) and resting distal coronary to aortic pressure (Pd/Pa) are increasingly used to guide revascularisation. However, reliable assessment of individual stenoses in serial coronary disease remains an unmet need. This study aimed to compare conventional pressure-based indices, a reference Doppler-based resistance index (hyperaemic stenosis resistance [hSR]) and a recently described mathematical correction model to predict the contribution of individual stenoses in serial disease.

METHODS AND RESULTS

Resting and hyperaemic pressure wire pullbacks were performed in 54 patients with serial disease. For each stenosis, FFR, iFR, and Pd/Pa were measured by the translesional gradient in each index and the predicted FFR (FFRpred) derived mathematically from hyperaemic pullback data. "True" stenosis significance by each index was assessed following PCI of the accompanying stenosis or measurements made in a large disease-free branch. In 27 patients, Doppler average peak flow velocity (APV) was also measured to calculate hSR (hSR=∆P/APV, where ∆P=translesional pressure gradient). FFR underestimated individual stenosis severity, inversely proportional to cumulative FFR (r=0.5, p<0.001). Mean errors for FFR, iFR and Pd/Pa were 33%, 20% and 24%, respectively, and 14% for FFRpred (p<0.001). Stenosis misclassification rates based on FFR 0.80, iFR 0.89 and Pd/Pa 0.91 thresholds were not significantly different (17%, 24% and 20%, respectively) but were higher than FFRpred (11%, p<0.001). Apparent and true hSR correlated strongly (r=0.87, p<0.001, mean error 0.19±0.3), with only 7% of stenoses misclassified.

CONCLUSIONS

Individual stenosis severity is significantly underestimated in the presence of serial disease, using both hyperaemic and resting pressure-based indices. hSR is less prone to error but challenges in optimising Doppler signals limit clinical utility. A mathematical correction model, using data from hyperaemic pressure wire pullback, produces similar accuracy to hSR and is superior to conventional pressure-based indices.

Invasive coronary physiology: a Dutch tradition

Invasive coronary physiology has been applied since the early days of percutaneous transluminal coronary angioplasty, and has become a rapidly emerging field of research. Many physiology indices have been developed, tested in clinical studies, and are now applied in daily clinical practice. Recent clinical practice guidelines further support the use of advanced invasive physiology methods to optimise the diagnosis and treatment of patients with acute and chronic coronary syndromes. This article provides a succinct review of the history of invasive coronary physiology, the basic concepts of currently available physiological parameters, and will particularly highlight the Dutch contribution to this field of invasive coronary physiology.

How to guide PCI?: A network meta-analysis

BACKGROUND

Traditional coronary angiography (CA) as a main technique has been used to determine the coronary artery anatomy and guide percutaneous coronary intervention (PCI). We mainly focused on whether the new techniques could improve the patients' mortality, major adverse cardiovascular events (MACEs), and myocardial infarction.

METHODS

For the network meta-analysis, we searched the trials of different PCI guidances from MEDLINE, Current Contents Connect, Google Scholar, EMBASE, Cochrane Library, PubMed, Science Direct, and Web of Science. The last search date was December 10, 2018.

RESULTS

The analyses of all results found that there was no significant difference in mortality among the groups. Randomized clinical trials (RCT) analysis showed that intravascular ultrasound (IVUS)-guided PCI was significantly superior to CA, fractional flow reserve, instantaneous wave-free ratio, optical coherence tomography. However, CA, fractional flow reserve, instantaneous wave-free ratio, and optical coherence tomography showed no difference in reducing mortality. The analyses of all results found that there was no significant difference in the incidence of MACEs among the groups. RCTs analysis showed that IVUS-guided PCI was significantly superior to CA, but there was no significant difference among the other groups. The analyses of all results or RCTs showed that there was no significant difference in myocardial infarction incidence among the groups.

CONCLUSION

IVUS-guided PCI is an effective method to decrease all-cause death MACEs.

Reclassification of Treatment Strategy With Instantaneous Wave-Free Ratio and Fractional Flow Reserve: A Substudy From the iFR-SWEDEHEART Trial

OBJECTIVES

The authors sought to compare reclassification of treatment strategy following instantaneous wave-free ratio (iFR) and fractional flow reserve (FFR).

BACKGROUND

iFR was noninferior to FFR in 2 large randomized controlled trials in guiding coronary revascularization. Reclassification of treatment strategy by FFR is well-studied, but similar reports on iFR are lacking.

METHODS

The iFR-SWEDEHEART (Instantaneous Wave-Free Ratio Versus Fractional Flow Reserve in Patients With Stable Angina Pectoris or Acute Coronary Syndrome Trial) study randomized 2,037 participants with stable angina or acute coronary syndrome to treatment guided by iFR or FFR. Interventionalists entered the preferred treatment (optimal medical therapy [OMT], percutaneous coronary intervention [PCI], or coronary artery bypass grafting [CABG]) on the basis of coronary angiograms, and the final treatment decision was mandated by the iFR/FFR measurements.

RESULTS

In the iFR/FFR (n = 1,009/n = 1,004) populations, angiogram-based treatment approaches were similar (p = 0.50) with respect to OMT (38%/35%), PCI of 1 (37%/39%), 2 (15%/16%), and 3 vessels (2%/2%) and CABG (8%/8%). iFR and FFR reclassified 40% and 41% of patients, respectively (p = 0.78). The majority of reclassifications were conversion of PCI to OMT in both the iFR/FFR groups (31.4%/29.0%). Reclassification increased with increasing number of lesions evaluated (odds ratio per evaluated lesion for FFR: 1.46 [95% confidence interval: 1.22 to 1.76] vs. iFR 1.37 [95% confidence interval: 1.18 to 1.59]). Reclassification rates for patients with 1, 2, and 3 assessed vessels were 36%, 52%, and 53% (p < 0.01).

CONCLUSIONS

Reclassification of treatment strategy of intermediate lesions was common and occurred in 40% of patients with iFR or FFR. The most frequent reclassification was conversion from PCI to OMT regardless of physiology modality. Irrespective of the physiological index reclassification of angiogram-based treatment strategy increased with the number of lesions evaluated.

Diagnostic Performance of the Instantaneous Wave-Free Ratio: Comparison With Fractional Flow Reserve

BACKGROUND

Aim of the present study was to perform a meta-analysis of all available studies comparing the instantaneous wave-free ratio (iFR) with fractional flow reserve (FFR).

METHODS AND RESULTS

Published trials comparing the iFR with FFR were searched for in PubMed, Google Scholar, and Scopus electronic databases. A total of 23 studies were available for the analysis, including 6381 stenoses. First, a meta-analysis of all studies was performed exploring the correlation between FFR and iFR. Interestingly, we found good correlation (0.798 [0.78-0.82]) between the 2 indices (P<0.001). In addition, to evaluate the diagnostic performance of iFR to identify FFR-positive coronary stenoses, we performed an additional meta-analysis, summarizing the results of receiver operating characteristics analyses from individual studies reporting the area under the curve. Summing the results of these studies, we found that iFR has a good diagnostic performance for the identification of FFR-positive stenoses (area under the curve=0.88 [0.86-0.90]; P<0.001). Furthermore, our search results included 5 studies that compared iFR and FFR to a third independent reference standard. Interestingly, no significant differences between iFR and FFR were reported in those studies.

CONCLUSIONS

The present meta-analysis shows that iFR significantly correlates with standard FFR and shows a good diagnostic performance in identifying FFR-positive coronary stenoses. Finally, iFR and FFR have similar diagnostic efficiency for detection of ischemia-inducing stenoses when tested against a third comparator.

Association between fractional flow reserve, instantaneous wave-free ratio and dobutamine stress echocardiography in patients with stable coronary artery disease

AIMS

The association between fractional flow reserve (FFR) and dobutamine stress echocardiography (DSE) in real-world stable angina patients is scant and controversial whereas no such comparison exists with instantaneous wave-free ratio (iFR). The current retrospective study aimed to investigate the associations among these modalities in patients with stable coronary artery disease (CAD) and intermediate coronary lesions.

METHODS AND RESULTS

We studied 62 consecutive stable angina patients who underwent DSE and subsequently coronary angiography with FFR (in all 62) and iFR (in 46/62 patients) assessment of intermediate single-vessel lesions between 2014 and 2015. Using receiver operating characteristic (ROC) curves we sought to identify the optimal FFR and iFR cut-off points with the highest discriminative power to predict the DSE result. The kappa coefficient was used to assess the agreement between FFR, iFR and DSE. The mean age of the study cohort was 63.5±12 years and 35 (56.5%) were males. Thirteen (21%) lesions were adjudicated as causing reversible ischaemia on DSE. Using ROC (FFR predicting DSE result), the area under the curve was 0.952 (95% CI: 0.902 to 1), whereas for iFR it was 0.743 (95% CI: 0.560 to 0.927), pAUC comparison=0.03. The optimal FFR cut-off point predicting positive DSE was 0.80. There was strong agreement between DSE and FFR (kappa 0.682, p<0.001). There was only modest agreement between iFR and DSE (kappa 0.258, p=0.068) using a cut-off value of 0.9.

CONCLUSIONS

In patients referred for evaluation of stable CAD, there was good agreement between DSE and FFR (87%) but less so with iFR (71.7%).

Coronary circulation: Pressure/flow parameters for assessment of ischemic heart disease

Both invasive and non-invasive parameters have been reported for assessment of the physiological status of the coronary circulation. Fractional flow reserve and coronary (or myocardial) flow reserve may be obtained by invasive or non-invasive means. These metrics of coronary stenosis severity have achieved wide clinical acceptance for guiding revascularization decisions and risk stratification. Other indices are obtained invasively (e.g., instantaneous wave-free ratio, iFR; hyperemic stenosis resistance) or non-invasively (e.g., PET absolute myocardial blood flow (mL/min/g)) and have been used for the same purposes. Both iFR, and whole-cycle distal coronary to aortic mean pressure (Pd/Pa) are measured under basal condition and used for assessment of hemodynamic stenosis severity as is index of basal stenosis resistance (BSR). These metrics typically are dichotomized at an empirically derived cut point into "normal" and "abnormal" categories for purposes of clinical decision making and data analysis. Once dichotomized the indices do not always point in the same direction and so confusion may arise. This review, therefore, will present basic principles relevant to understanding commonly employed metrics of the physiological status of the coronary circulation, potential strengths and weaknesses, and hopefully an improved appreciation of the clinical information provided by each.

Coronary Angiography With Pressure Wire and Fractional Flow Reserve

BACKGROUND

Approximately 800 000 coronary angiography procedures are per- formed in Germany each year, mainly in order to identify coronary artery stenoses. As a rule, revascularization is indicated only when coronary artery stenoses cause relevant ischemia, but this cannot be determined unequivocally by angiography alone. Pressure wire measurement and the measurement of fractional flow reserve (FFR) enable direct evaluation of the hemodynamic relevance of coronary artery stenoses during diagnostic coronary angiography.

METHODS

This review is based on publications retrieved by a selective search in PubMed focusing especially on large randomized trials, registry studies, and meta- analyses on either pressure wire measurement or FFR.

RESULTS

According to a registry study from France, the hemodynamic evaluation of a stenosis during coronary angiography affected decisions about revascularization in 43% of cases. Randomized multicenter trials have shown that a combined end- point consisting of death, myocardial infarction, or revascularization arose signifi- cantly less commonly in the FFR group than in the group receiving angiography without FFR (13.2% versus 18.3%; p = 0.02), and that patients with one or more hemodynamically significant coronary artery stenoses (FFR ≤ 0.80) benefited more from revascularization than from medical management alone (event rate, 8.1% versus 19.5%; p <0.001). It was also shown that revascularization yields no benefit if relevant ischemia has been ruled out, even if the angiogram shows high-grade coronary artery stenoses.

CONCLUSION

All cardiac catheter laboratories should be capable of performing pres- sure wire measurements and measurements of FFR and should do so regularly if ischemia due to coronary artery stenosis cannot be demonstrated non-invasively. A pathological FFR measurement is an indication for revascularization.

Qualitative resting coronary pressure wave form analysis to predict fractional flow reserve

AIMS

The aim of this study was to evaluate the predictability of resting distal coronary pressure wave forms for fractional flow reserve (FFR).

METHODS AND RESULTS

Resting coronary wave forms were qualitatively evaluated for the presence of (i) dicrotic notch, (ii) diastolic dipping, and (iii) ventricularisation. In a development cohort (n=88), a scoring system was developed that was then applied to a validation cohort (n=428) using a multivariable linear regression model to predict FFR and receiver operating characteristics (ROC) to predict FFR ≤0.8. In the development cohort, all three qualitative parameters were independent predictors of FFR. However, in a multivariable linear regression model in the validation cohort, qualitative wave form analysis did not further improve the ability of resting distal coronary to aortic pressure ratio (Pd/Pa) (p=0.80) or instantaneous wave-free ratio (iFR) (p=0.26) to predict FFR. Using ROC, the area under the curve of resting Pd/Pa (0.86 versus 0.86, p=0.08) and iFR (0.86 versus 0.86, p=0.26) did not improve by adding qualitative analysis.

CONCLUSIONS

Qualitative coronary wave form analysis showed moderate classification agreement in predicting FFR but did not add substantially to the resting pressure gradients Pd/Pa and iFR; however, when discrepancies between quantitative and qualitative analyses are observed, artefact or pressure drift should be considered.

Comparison between instantaneous wave-free ratio versus morphometric assessments by intracoronary imaging

Anatomical measurements obtained by intracoronary imaging devices are reported to correlate significantly with fractional flow reserve (FFR). Instantaneous wave-free ratio (iFR) is a nonhyperemic index of stenosis severity with discordant reports regarding its accuracy in relation to FFR. There is no information on the correlation of iFR with measurements derived from intracoronary imaging devices. The purpose of this study was to assess the relationship among iFR, intravascular ultrasound (IVUS), and optical frequency domain imaging (OFDI) parameters. Eighty lesions in 72 patients who underwent elective angiography and had intermediate lesions were enrolled. All lesions were assessed by iFR, FFR, IVUS, and OFDI. iFR was ≤ 0.89 in 21 (26%) lesions and FFR was ≤ 0.80 in 41 (51%) lesions. iFR correlated significantly with both IVUS-derived minimum lumen area (MLA) (r = 0.375, p = 0.003) and OFDI-derived MLA (r = 0.357, p = 0.005). FFR also correlated significantly with both IVUS-derived MLA (r = 0.472, p < 0.001) and OFDI-derived MLA (r = 0.445, p < 0.001). Among the lesions with FFR ≤ 0.80, iFR > 0.89 (mismatch) was observed in 20 lesions. There was no lesion with iFR ≤ 0.89 (reverse mismatch) among the lesions with FFR > 0.80. The lesion location among three major coronary vessels was related with the discrepancy between iFR and FFR (p = 0.007). In conclusion, iFR and FFR showed a significant correlation with IVUS and OFDI measurements. The discrepancy of iFR and FFR was associated with the lesion locations.

Network meta-analysis comparing iFR versus FFR versus coronary angiography to drive coronary revascularization

AIMS

Instantaneous free-wave ratio (iFR) has been recently demonstrated non-inferior to fractional flow reserve (FFR) to drive coronary revascularization; however, no study has compared iFR versus coronary angiography (CA). We performed a network meta-analysis to evaluate efficacy and safety of iFR- versus CA-guided strategy.

METHODS AND RESULTS

We searched for randomized trials and studies with propensity score matching in The Cochrane Collaboration Central Register of Controlled Trials, EMBASE, and MEDLINE/Pubmed. CA, FFR, and iFR were the three competitive arms, MACE (a composite endpoint of death, myocardial infarction [MI], and target vessel revascularization [TVR]) was the primary endpoint, while its single components the secondary ones. Subgroup analysis was performed for patients presenting with stable coronary artery disease. Eight studies were selected: 4126 patients were evaluated with FFR, 2160 with iFR, and 2214 with CA, acute coronary syndrome (ACS) was the most frequent admission diagnosis. After 12 months, rates of MACE and all-cause death did not differ between groups (respectively OR 1.04 and OR 0.86 for iFR vs FFR). Both FFR and iFR reduced TVR compared to CA (respectively OR 0.68 and OR 0.70). In patients with stable CAD both FFR and iFR reduced risk of subsequent MI compared to CA (respectively OR 0.66 and OR 0.79).

CONCLUSION

Compared to CA alone, both FFR and iFR are safe and effective in guiding coronary revascularization at 12 months. In patients with stable CAD, both FFR and iFR-guided revascularization reduce the risk of subsequent MI at 12 months.

Fractional Flow Reserve or Coronary Flow Reserve for the Assessment of Myocardial Perfusion : Implications of FFR as an Imperfect Reference Standard for Myocardial Ischemia

Purpose of Review

Accumulating evidence exists for the value of coronary physiology for clinical decision-making in ischemic heart disease (IHD). The most frequently used pressure-derived index to assess stenosis severity, the fractional flow reserve (FFR), has long been considered the gold standard for this purpose, despite the fact that the FFR assesses solely epicardial stenosis severity and aims to estimate coronary flow impairment in the coronary circulation. The coronary flow reserve (CFR) directly assesses coronary blood flow in the coronary circulation, including both the epicardial coronary artery and the coronary microvasculature, but is nowadays less established than FFR. It is now recognized that both tools may provide insight into the pathophysiological substrate of ischemic heart disease, and that particularly combined FFR and CFR measurements provide a comprehensive insight into the multilevel involvement of IHD. This review discusses the diagnostic and prognostic characteristics, as well as future implications of combined assessment of FFR and CFR pressure and flow measurements as parameters for inducible ischemia.

Recent Findings

FFR and CFR disagree in up to 40% of all cases, giving rise to fundamental questions regarding the role of FFR in contemporary ischemic heart disease management, and implying a renewed approach in clinical management of these patients using combined coronary pressure and flow measurement to allow appropriate identification of patients at risk for cardiovascular events.

Summary

This review emphasizes the value of comprehensive coronary physiology measurements in assessing the pathophysiological substrate of IHD, and the importance of acknowledging the broad spectrum of epicardial and microcirculatory involvement in IHD. Increasing interest and large clinical trials are expected to further strengthen the potential of advanced coronary physiology in interventional cardiology, consequently inducing reconsideration of current clinical guidelines.

Past, Present and Future of Coronary Physiology

It is well known that the apparent significant coronary stenosis on angiography sometimes does not cause significant ischemia, and vice versa. For this reason, decision-making based on coronary physiology is becoming more and more important. Fractional flow reserve (FFR), which has emerged as a useful tool to determine which lesions need revascularization in the catheterization laboratory, now has a class IA indication in the European Society of Cardiology guidelines. More recently, the instantaneous wave-free ratio, which is considered easier to use than FFR, has been graded as equivalent to FFR. This review discusses the concepts of FFR and instantaneous wave-free ratio, current evidence supporting their use, and future directions in coronary physiology.

The Evolving Future of Instantaneous Wave-Free Ratio and Fractional Flow Reserve

In this review, the authors reflect upon the role of coronary physiology in the modern management of coronary artery disease. They critically appraise the scientific background of the instantaneous wave-free ratio (iFR) and fractional flow reserve (FFR), from early experimental studies to validation studies against indexes of ischemia, to clinical trials assessing outcome. At this important juncture for the field, the authors make predictions for the future of physiological stenosis assessment, outlining developments for both iFR and FFR in new clinical domains beyond the confines of stable angina. With a focus on the evolving future of iFR and FFR, the authors describe how physiological assessment with iFR may advance its application from simply justifying to guiding revascularization.

Diagnostic accuracy of instantaneous wave free-ratio in clinical practice

AIMS

To evaluate the correlation between iFR and FFR in real-world clinical practice.

METHODS AND RESULTS

Retrospective, single-centre study of 229 consecutive pressure-wire studies (n_p  = 158). Real-time iFR and FFR measurements were performed for angiographically borderline stenoses. Functionally significant stenoses were defined as iFR <0.86 or FFR ≤0.80. An iFR between 0.86 and 0.93 was considered within the grey zone (Hybrid approach). Median iFR and FFR (IQR) were 0.92 (0.87-0.95) and 0.83 (0.76-0.89), respectively. Pearson's correlation coefficient was 0.75 (P < 0.001). Bland-Altman plot showed a mean difference between iFR and FFR that remained consistent throughout the range of values. The optimal iFR cutoff was 0.91-sensitivity 80%, specificity 82% with ROC area under curve of 89%. Using the Hybrid iFR-FFR strategy, we demonstrated high accuracy of iFR results-sensitivity 95%, specificity 96%, PPV 95%, and NPV 96%. In addition, this method would have avoided adenosine in 56% of patients. Mean follow-up period was 17.2 (±3.4) months. All-cause mortality was 3.2% (n_p  = 5) and repeat intervention was required in six lesions (2.6%).

CONCLUSIONS

This study demonstrates that iFR is a valuable adjunct to FFR using the Hybrid iFR-FFR strategy in a real-world population. The use of adenosine may be avoided in about half the cases.

Discrepancy between fractional flow reserve and instantaneous wave-free ratio: Clinical and angiographic characteristics

BACKGROUND

The invasive physiologic index such as fractional flow reserve (FFR) or instantaneous wave-free ratio (iFR) is used in clinical practice to identify ischemia-causing stenosis and to guide treatment strategy. We investigated clinical and angiographic characteristics of lesions with discrepancy between FFR and iFR.

METHODS

From the 3V FFR-FRIENDS study, 975 vessels (393 patients) with available pre-intervention FFR and iFR were included in this study. The vessels were classified according to FFR and iFR into: concordant normal (Group 1 [n=724]: FFR>0.80 and iFR≥0.90); high FFR and low iFR (Group 2 [n=33]: FFR>0.80 and iFR<0.90); low FFR and high iFR (Group 3 [n=82]: FFR≤0.80 and iFR≥0.90); and concordant abnormal (Group 4 [n=136]: FFR≤0.80 and iFR<0.90).

RESULTS

Angiographic stenosis severity assessed by percent diameter stenosis, minimum lumen diameter and lesion length was increased from Group 1 to Group 4 (all p<0.001). SYNTAX score increased and FFR decreased proportionally from Group 1 to Group 4 (all p<0.001). In multivariable GEE model, female, diabetes mellitus, smaller reference vessel diameter, and higher %DS were significantly associated with low iFR among high FFR groups (Group 2 discordance). Conversely, males, absence of diabetes mellitus and lower %DS were significantly associated with high iFR among low FFR groups (Group 3 discordance).

CONCLUSIONS

Four groups classified according to FFR and iFR were different in clinical and angiographic characteristics, SYNTAX score, and predictors of discordance. The lesions with discordant FFR and iFR may need to be interpreted as a different clinical entity.

Basal stenosis resistance index derived from simultaneous pressure and flow velocity measurements

AIMS

Vasodilator-free basal stenosis resistance (BSR) equals fractional flow reserve (FFR) accuracy for ischaemia-inducing stenoses. Nonetheless, basal haemodynamic variability may impair BSR accuracy compared with hyperaemic stenosis resistance (HSR). We evaluated the influence of basal haemodynamic variability, as encountered in practice, on BSR accuracy versus HSR when derived from simultaneous pressure and flow velocity measurements, and determined its diagnostic performance for HSR-defined significant stenoses.

METHODS AND RESULTS

Simultaneous coronary pressure and flow velocity were obtained in 131 stenoses. The impact of basal haemodynamic conditions on BSR was evaluated by means of their relationship with the relative difference between BSR and HSR. Diagnostic performance of BSR, FFR, iFR, and resting Pd/Pa was assessed by comparing the area under the curve (AUC), using HSR as reference standard. The relative difference between BSR and HSR was not associated with basal heart rate, aortic pressure or rate pressure product. Among all stenoses, as well as within the 0.6-0.9 FFR range, BSR AUC was significantly greater than resting Pd/Pa and iFR AUC; all other AUCs were equivalent.

CONCLUSIONS

With simultaneous pressure and flow velocity measurements, basal conditions do not systematically limit BSR accuracy compared with HSR. Consequently, diagnostic performance of BSR is equivalent to FFR, and closely approximates HSR.

Use of the Instantaneous Wave-free Ratio or Fractional Flow Reserve in PCI

BACKGROUND

Coronary revascularization guided by fractional flow reserve (FFR) is associated with better patient outcomes after the procedure than revascularization guided by angiography alone. It is unknown whether the instantaneous wave-free ratio (iFR), an alternative measure that does not require the administration of adenosine, will offer benefits similar to those of FFR.

METHODS

We randomly assigned 2492 patients with coronary artery disease, in a 1:1 ratio, to undergo either iFR-guided or FFR-guided coronary revascularization. The primary end point was the 1-year risk of major adverse cardiac events, which were a composite of death from any cause, nonfatal myocardial infarction, or unplanned revascularization. The trial was designed to show the noninferiority of iFR to FFR, with a margin of 3.4 percentage points for the difference in risk.

RESULTS

At 1 year, the primary end point had occurred in 78 of 1148 patients (6.8%) in the iFR group and in 83 of 1182 patients (7.0%) in the FFR group (difference in risk, -0.2 percentage points; 95% confidence interval [CI], -2.3 to 1.8; P<0.001 for noninferiority; hazard ratio, 0.95; 95% CI, 0.68 to 1.33; P=0.78). The risk of each component of the primary end point and of death from cardiovascular or noncardiovascular causes did not differ significantly between the groups. The number of patients who had adverse procedural symptoms and clinical signs was significantly lower in the iFR group than in the FFR group (39 patients [3.1%] vs. 385 patients [30.8%], P<0.001), and the median procedural time was significantly shorter (40.5 minutes vs. 45.0 minutes, P=0.001).

CONCLUSIONS

Coronary revascularization guided by iFR was noninferior to revascularization guided by FFR with respect to the risk of major adverse cardiac events at 1 year. The rate of adverse procedural signs and symptoms was lower and the procedural time was shorter with iFR than with FFR. (Funded by Philips Volcano; DEFINE-FLAIR ClinicalTrials.gov number, NCT02053038 .).

Instantaneous Wave-free Ratio versus Fractional Flow Reserve to Guide PCI

BACKGROUND

The instantaneous wave-free ratio (iFR) is an index used to assess the severity of coronary-artery stenosis. The index has been tested against fractional flow reserve (FFR) in small trials, and the two measures have been found to have similar diagnostic accuracy. However, studies of clinical outcomes associated with the use of iFR are lacking. We aimed to evaluate whether iFR is noninferior to FFR with respect to the rate of subsequent major adverse cardiac events.

METHODS

We conducted a multicenter, randomized, controlled, open-label clinical trial using the Swedish Coronary Angiography and Angioplasty Registry for enrollment. A total of 2037 participants with stable angina or an acute coronary syndrome who had an indication for physiologically guided assessment of coronary-artery stenosis were randomly assigned to undergo revascularization guided by either iFR or FFR. The primary end point was the rate of a composite of death from any cause, nonfatal myocardial infarction, or unplanned revascularization within 12 months after the procedure.

RESULTS

A primary end-point event occurred in 68 of 1012 patients (6.7%) in the iFR group and in 61 of 1007 (6.1%) in the FFR group (difference in event rates, 0.7 percentage points; 95% confidence interval [CI], -1.5 to 2.8; P=0.007 for noninferiority; hazard ratio, 1.12; 95% CI, 0.79 to 1.58; P=0.53); the upper limit of the 95% confidence interval for the difference in event rates fell within the prespecified noninferiority margin of 3.2 percentage points. The results were similar among major subgroups. The rates of myocardial infarction, target-lesion revascularization, restenosis, and stent thrombosis did not differ significantly between the two groups. A significantly higher proportion of patients in the FFR group than in the iFR group reported chest discomfort during the procedure.

CONCLUSIONS

Among patients with stable angina or an acute coronary syndrome, an iFR-guided revascularization strategy was noninferior to an FFR-guided revascularization strategy with respect to the rate of major adverse cardiac events at 12 months. (Funded by Philips Volcano; iFR SWEDEHEART ClinicalTrials.gov number, NCT02166736 .).

Instantaneous wave-free ratio derived from coronary computed tomography angiography in evaluation of ischemia-causing coronary stenosis: Feasibility and initial clinical research

The instantaneous wave-free ratio (iFR) closely related to fractional flow reserve (FFR) is a adenosine-independent physiologic index of coronary stenosis severity. We sought to evaluate whether iFR derived from coronary computed tomographic angiography (iFRCT) can be used as a novel noninvasive method for diagnosis of ischemia-causing coronary stenosis.We retrospectively enrolled 33 patients (47 lesions) with coronary artery disease (CAD) and examined with coronary computed tomographic angiography (CTA), invasive coronary angiography (ICA), and FFR. Patient-specific anatomical model of the coronary artery was built by original resting end-diastolic CTA images. Based on the model and computational fluid dynamics, individual boundary conditions were set to calculate iFRCT as the mean pressure distal to the stenosis divided by the mean aortic pressure during the diastolic wave-free period of rest state. Ischemia was assessed by an FFR of up to 0.8, while anatomically obstructive CAD was defined by a stenosis of at least 50% by ICA. The correlation between iFRCT and FFR was evaluated. The receiver operating characteristic (ROC) curve was used to select the cut-off value of iFRCT for diagnosis of ischemia-causing stenosis. The diagnostic performances of iFRCT, coronary CTA, and iFRCT plus CTA for ischemia-causing stenosis were compared with ROC curve and Delong method.On a per-vessel basis, iFRCT and FFR had linear correlation (r = 0.75, p < 0.01). ROC analysis identified an optimal iFRCT cut-off value of 0.82 for categorization based on an FFR cut-off value 0.8, and the diagnostic accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of iFRCT were 78.72%,70.59%, 83.33%,70.59%, and 83.33%, respectively. Compared with obstructive CAD diagnosed by coronary CTA (AUC = 0.60), iFRCT yielded diagnostic improvement over stenosis assessment with AUC increasing from 0.6 by CTA to 0.87 (P < 0.01) and 0.90 (P < 0.01) when iFRCT plus CTA.In conclusion, iFRCT is a promising index improving diagnostic performance over coronary CTA for detection of ischemia-causing coronary stenosis.

Diagnostic performance of instantaneous wave-free ratio for the evaluation of coronary stenosis severity confirmed by fractional flow reserve: A PRISMA-compliant meta-analysis of randomized studies

BACKGROUND

The instantaneous wave-free ratio (iFR) is a new vasodilator-free index of coronary stenosis severity. The aim of this meta-analysis is to assess the diagnostic performance of iFR for the evaluation of coronary stenosis severity with fractional flow reserve as standard reference.

METHODS

We searched PubMed, EMBASE, CENTRAL, ProQuest, Web of Science, and International Clinical Trials Registry Platform (ICTRP) for publications concerning the diagnostic value of iFR. We used a random-effects covariate to synthesize the available data of sensitivity, specificity, positive likelihood ratio (LR+), negative likelihood ratio (LR-), and diagnostic odds ratio (DOR). Overall test performance was summarized by the summary receiver operating characteristic curve (sROC) and the area under the curve (AUC).

RESULTS

Eight studies with 1611 subjects were included in the meta-analysis. The pooled sensitivity, specificity, LR+, LR-, and DOR for iFR were respectively 73.3% (70.1-76.2%), 86.4% (84.3-88.3%), 5.71 (4.43-7.37), 0.29 (0.22-0.38), and 20.54 (16.11-26.20). The area under the summary receiver operating characteristic curves for iFR was 0.8786. No publication bias was identified.

CONCLUSION

The available evidence suggests that iFR may be a new, simple, and promising technology for coronary stenosis physiological assessment.

Current developments and future applications of intracoronary hemodynamics

Intracoronary hemodynamic assessment of the physiologic significance of coronary lesions improves clinical outcomes in patients with coronary artery disease. Coronary flow velocity reserve, fractional flow reserve, instantaneous wave-free ratio, and index of microcirculatory resistance utilize sensor-mounted guidewires to approximate coronary flow. Coronary flow velocity reserve and fractional flow reserve rely on pharmacologic administration of adenosine to achieve hyperemia and diagnose epicardial lesion severity. As an adenosine-free index, the instantaneous wave-free ratio utilizes a wave-free period in the mid-late diastole during which resistance is constant and low to assess lesion significance. The index of microcirculatory resistance combines hyperemic pressure measurements with thermodilution to quantify microvascular resistance. We review the physiology, clinical trials, and clinical applications of these invasive hemodynamic assessments.