Role of variability in microvascular resistance on fractional flow reserve and coronary blood flow velocity reserve in intermediate coronary lesions

Microvascular resistance reserve before and after PCI: A serial FFR and [15O] H2O PET study

BACKGROUND AND AIMS

Microvascular Resistance Reserve (MRR) has recently been introduced as a microvasculature-specific index and hypothesized to be independent of coronary stenosis. The aim of this study was to investigate the change of MRR after percutaneous coronary intervention (PCI).

METHODS

In this post-hoc analysis from the PACIFC trials, symptomatic patients underwent [15O]H2O positron emission tomography (PET) and invasive fractional flow reserve (FFR) before and after revascularization. Coronary flow reserve (CFR) from PET and invasive FFR were used to calculate MRR.

RESULTS

Among 52 patients (87 % male, age 59.4 ± 9.4 years), 61 vessels with a median FFR of 0.71 (95 % confidence interval: 0.55 to 0.74) and a mean MRR of 3.80 ± 1.23 were included. Following PCI, FFR, hyperemic myocardial blood flow (hMBF) and CFR increased significantly (all p-values ≤0.001). MRR remained unchanged after PCI (3.80 ± 1.23 before PCI versus 3.60 ± 0.97 after PCI; p=0.23). In vessels with a pre-PCI, FFR ≤0.70 pre- and post-PCI MRR were 3.90 ± 1.30 and 3.73 ± 1.14 (p=0.56), respectively. Similar findings were observed for vessels with a FFR between 0.71 and 0.80 (pre-PCI MRR 3.70 ± 1.17 vs. post PCI MRR 3.48 ± 0.76, p=0.19).

CONCLUSIONS

Our study indicates that MRR, assessed using a hybrid approach of PET and invasive FFR, is independent of the severity of epicardial stenosis. These findings suggest that MRR is a microvasculature-specific parameter.

Ability of the coronary angiography-derived index of microcirculatory resistance to predict microvascular obstruction in patients with ST-segment elevation

Background

The coronary angiography-derived index of microvascular resistance (caIMR) correlates well with the index of microcirculatory resistance (IMR), which predicts microvascular obstruction (MVO). However, the relationship between caIMR and MVO remains unclear.

Aim

To evaluate the predictive ability of caIMR of MVO after ST-segment elevation myocardial infarction (STEMI).

Methods

CaIMR was calculated using computational flow and pressure simulation in patients with STEMI in whom MVO status had been assessed by cardiac magnetic resonance (CMR) after successful primary percutaneous intervention at Peking University First Hospital between December 2016 and August 2019. The clinical, biochemical, echocardiographic, and CMR characteristics were assessed according to MVO status. The predictive value of the clinical parameters and caIMR was evaluated.

Results

Fifty-three eligible patients were divided into an MVO group (n = 32) and a no-MVO group (n = 21). The caIMR tended to be higher in the MVO group (41.6 U vs. 30.1 U; p = 0.136). CaIMR and peak cardiac troponin-I (cTNI) were independent predictors of MVO (per 1-U increment in caIMR: odds ratio [OR] 1.044, 95% confidence interval [CI] 1.004-1.086, p = 0.030; per 1 ng/L increase in peak cTNI: OR 1.018, 95% CI 1.003-1.033, p = 0.022). In receiver-operating characteristic curve analysis, when a cut-off value of 45.17 U was used, caIMR had some ability to predict MVO (area under the curve 0.622, 95% CI 0.478-0.752, p = 0.127).

Conclusions

CaIMR and peak cTNI were independent predictors of short-term MVO in patients with STEMI who had undergone successful primary percutaneous coronary intervention and may help to identify those at high risk of MVO.

Angiography-based coronary microvascular assessment with and without intracoronary pressure measurements: a systematic review

BACKGROUND

In recent years, several indices have been proposed for quantifying coronary microvascular resistance. We intended to conduct a comprehensive review that systematically evaluates indices of microvascular resistance derived from angiography.

OBJECTIVE

The objective of this study was to identify and analyze angiography-derived indices of microvascular resistance that have been validated against an invasive reference method. We aimed to compare their limits of agreement with their reference methods and explore their advantages and inherent limitations.

METHODS AND RESULTS

We searched PubMed from inception until 2022 for studies on different techniques for quantifying microvascular resistance. Seven studies met the inclusion criteria. Five studies included techniques that applied calculations based solely on invasive angiography, and were validated against invasively measured thermodilution-derived index of microvascular resistance. The remaining two studies combined angiography with invasively measured intracoronary pressure data, and were validated against invasive Doppler measurements. We converted the ± 1.96 standard deviation limits of agreement with the reference method from the seven studies into percentages relative to the cut-off value of the reference method. The lower limits of agreement for angiography-based methods ranged from - 122 to - 60%, while the upper limits ranged from 74 to 135%. The range of the limits of agreement was considerably lower for the two combined angiography- and pressure-based methods, standing at - 52 to 60% and - 25 to 27%.

CONCLUSION

Our findings suggest that combined angiography- and pressure-based methods provide a more reliable assessment of microvascular resistance compared to methods relying solely on angiography. Central illustration. Comparative assessment of image-based methods quantifying microvascular resistance with and without intracoronary pressure measurements. Angiography-based methods rely on angiography alone to calculate the microvascular resistance by utilizing angiographic frame counting to extrapolate coronary flow (Q) and subsequently deriving distal coronary pressure using fluid dynamic equations. Combined angiography- and pressure-based methods utilize invasive intracoronary pressure gradients measured during rest and maximal vasodilation to determine coronary flow in their calculation of microvascular resistance. The combined methods showed more acceptable levels of agreement with their reference methods compared to angiography-based methods alone.

Invasive Coronary Assessment in Myocardial Ischemia with No Obstructive Coronary Arteries

PURPOSE OF REVIEW

The purpose of this review is threefold: (i) to give an overview of well-established invasive methods for assessing patients with ischemia with no obstructive coronary arteries (INOCA) in the cardiac catheterization laboratory; (ii) to describe the prognostic and treatment implications based on these findings, and (iii) to discuss current knowledge gaps and future perspectives.

RECENT FINDINGS

Recent studies have demonstrated that invasive coronary function testing not only allows for risk stratification of patients with INOCA but also guides medical therapy with improvement in symptoms and quality of life. Based on these findings, invasive coronary function assessment is now a class 2a recommendation in the 2021 ACC/AHA chest pain guideline to improve the diagnosis of coronary microvascular dysfunction and to enhance risk stratification. Invasive functional testing for patients with INOCA is well established and easily performed in the catheterization laboratory. Comprehensive invasive assessment is a key to differentiating INOCA endotypes and optimizing both medical therapy and preventive strategies including lifestyle modification.

Characterization and implications of intracoronary hemodynamic assessment during coronary spasm provocation testing

BACKGROUND

Current diagnostic criteria for coronary spasm are based on patient's symptoms, ECG shifts and epicardial vasoconstriction during acetylcholine (ACh) spasm testing.

AIMS

To assess the feasibility and diagnostic value of coronary blood flow (CBF) and resistance (CR) assessment as objective parameters during ACh testing.

METHODS

Eighty-nine patients who underwent intracoronary reactivity testing including ACh testing with synchronous Doppler wire-based measurements of CBF and CR were included. Coronary microvascular and epicardial spasm, respectively, were diagnosed based on COVADIS criteria.

RESULTS

Patients were 63 ± 13 years old, predominantly female (69%) and had preserved LV ejection fraction (64 ± 8%). Overall, assessment of CBF and CR during ACh testing revealed a decrease in CBF of 0.62 (0.17-1.53)-fold and an increase of CR of 1.45 [0.67-4.02]-fold in spasm patients compared to 2.08 (1.73-4.76) for CBF and 0.45 (0.44-0.63) for CR in patients without coronary spasm (both p < 0.01). Receiver operating characteristic revealed a high diagnostic ability of CBF and CR (AUC 0.86, p < 0.001, respectively) in identifying patients with coronary spasm. However, in 21% of patients with epicardial spasm and 42% of patients with microvascular spasm a paradoxical response was observed.

CONCLUSIONS

This study demonstrates feasibility and potential diagnostic value of intracoronary physiology assessments during ACh testing. We observed opposite responses of CBF and CR to ACh in patients with positive vs. negative spasm test. While a decrease in CBF and an increase in CR during ACh seem pathognomonic for spasm, some patients with coronary spasm demonstrate paradoxical ACh response demanding further scientific investigations.

Particularities of coronary physiology in patients with atrial fibrillation: insights from combined pressure and flow indices measurements

Background

Symptoms suggestive of myocardial ischemia are frequently encountered in patients with atrial fibrillation (AF) even in the absence of obstructive coronary artery disease. Nevertheless, an in-depth characterisation of coronary physiology in patients with AF is currently lacking.

Objectives

We aim to provide an insight into the characteristics of coronary physiology in AF, by performing simultaneous invasive measurements of coronary flow- and pressure- indices in a real-life population of patients with AF and indication of coronary angiography.

Methods

This is a prospective open label study including patients with permanent or persistent AF and indication of coronary angiography showing intermediate coronary stenosis requiring routine physiological assessment (n = 18 vessels from 14 patients). We measured FFR (fractional flow reserve), and Doppler-derived coronary flow indices, including CFR (coronary flow reserve) and HMR (hyperaemic microvascular resistance).

Results

From the analysed vessels, 18/18 vessels (100%) presented a pathological CFR (<2.5), indicative of coronary microvascular dysfunction (CMD), and 3/18 (17%) demonstrated obstructive epicardial coronary disease (FFR ≤ 0.8). A large proportion of vessels (15/18; 83%) showed discordant FFR/CFR with preserved FFR and low CFR. 47% of the coronary arteries in patients with AF and non-obstructive epicardial coronary disease presented structural CMD (HMR ≥ 2.5 mmHg/cm/s), and were associated with high BMR and an impaired response to adenosine. Conversely, vessels from patients with AF and non-obstructive epicardial coronary disease with functional CMD (HMR < 2.5 mmHg/cm/s) showed higher bAPV. The permanent AF subpopulation presented increased values of HMR and BMR compared to persistent AF, while structural CMD was more often associated with persistent symptoms at 3 months, taking into account the limited sample size of our study.

Conclusion

Our findings highlight a systematically impaired CFR in patients with AF even in the absence of obstructive epicardial coronary disease, indicative of CMD. In addition, patients with AF presented more prevalent structural CMD (HMR ≥ 2.5 mmHg/cm/s), characterized by reduced hyperaemic responses to adenosine, possibly interfering with the FFR assessment.

Prospects for Precision Medicine in Acute Myocardial Infarction: Patient-Level Insights into Myocardial Injury and Repair

The past decade has seen a marked expansion in the understanding of the pathobiology of acute myocardial infarction and the systemic inflammatory response that it elicits. At the same time, a portfolio of tools has emerged to characterise some of these processes in vivo. However, in clinical practice, key decision making still largely relies on assessment built around the timing of the onset of chest pain, features on electrocardiograms and measurements of plasma troponin. Better understanding the heterogeneity of myocardial injury and patient-level responses should provide new opportunities for diagnostic stratification to enable the delivery of more rational therapies. Characterisation of the myocardium using emerging imaging techniques such as the T1, T2 and T2* mapping techniques can provide enhanced assessments of myocardial statuses. Physiological measures, which include microcirculatory resistance and coronary flow reserve, have been shown to predict outcomes in AMI and can be used to inform treatment selection. Functionally informative blood biomarkers, including cellular transcriptomics; microRNAs; extracellular vesicle analyses and soluble markers, all give insights into the nature and timing of the innate immune response and its regulation in acute MI. The integration of these and other emerging tools will be key to developing a fuller understanding of the patient-level processes of myocardial injury and repair and should fuel new possibilities for rational therapeutic intervention.

Invasive Detection of Coronary Microvascular Dysfunction: How It Began, and Where We Are Now

The landscape of interventional cardiology is ever evolving. Contemporary practice has shifted from a stenosis-centred approach to the total characterisation of both the epicardial and microcirculatory vessels. Microcirculatory dysfunction plays an important role in the pathophysiology of acute and chronic coronary syndromes, and characterisation of the microcirculation has important clinical consequences. Accordingly, the invasive diagnosis of microcirculatory dysfunction is becoming a key feature of the interventional cardiologist's toolkit. This review focuses on the methodology underpinning the invasive diagnosis of microvascular dysfunction and highlights the indices that have arisen from these methodologies.

The current status of CZT SPECT myocardial blood flow and reserve assessment: Tips and tricks

Cardiac PET-derived measurements of myocardial blood flow (MBF) and myocardial flow reserve (MFR) are proven robust indexes of the severity of coronary artery disease (CAD). They facilitate the diagnosis of diffuse epicardial and microvascular disease and are also of prognostic significance. However, low availability and high cost have limited their wide clinical implementation. Over the last 15 years, cadmium zinc telluride (CZT)-based detectors have been implemented into SPECT imaging devices. Myocardial perfusion scintigraphy can be performed faster and with less radiation exposure as compared with standard gamma cameras. Rapid dynamic SPECT studies with higher count rates can be performed. This technological breakthrough has renewed the interest in SPECT MBF assessment in patients with CAD. Currently, two cardiac-centered CZT gamma cameras are available commercially-Discovery NM530c and D-SPECT. They differ in parameters such as collimator design, number of detectors, sensitivity, spatial resolution and image reconstruction. A number of publications have focused on the feasibility of dynamic CZT SPECT and on the correlation with cardiac PET and invasive coronary angiography measurements of fractional flow reserve. Current study reviews the present status of MBF and MFR assessment with CZT SPECT. It also aims to provide an overview of specific issues related to acquisition, processing and interpretation of quantitative studies in patients with CAD.

Angiography derived assessment of the coronary microcirculation: is it ready for prime time?

INTRODUCTION

Non-obstructive coronary arteries (NOCA) are present in 39.7% to 62.4% of patients who undergo elective angiography. Coronary microcirculation (<400 µm) is not visible on angiography therefore functional assessment, invasive or non-invasive plays a prior role to help provide a more personalized diagnosis of angina.

AREA COVERED

In this review, we revise the pathophysiology, clinical importance and invasive assessment of the coronary microcirculation, and discuss angiography-derived indices of microvascular resistance. A comprehensive literature review over four decades is also undertaken.

EXPERT OPINION

The coronary microvasculature plays an important role in flow autoregulation and metabolic regulation. Invasive assessment of microvascular resistance is a validated modality with independent prognostic value, nevertheless, its routine application is hampered by the requirement of intravascular instrumentation and hyperaemic agents. The angiography-derived index of microvascular resistance has emerged as a promising surrogate in pilot studies, however, more data are needed to validate and compare the diagnostic and prognostic accuracy of different equations as well as to illustrate the relationship between angiography-derived parameters for epicardial coronary arteries and those for the microvasculature.

Three-Dimensional Angiographic Characteristics versus Functional Stenosis Severity in Fractional and Coronary Flow Reserve Discordance: A DEFINE FLOW Sub Study

Background: Coronary angiography alone is insufficient to identify lesions associated with myocardial ischemia that may benefit from revascularization. Coronary physiology parameters may improve clinical decision making in addition to coronary angiography, but the association between 2D and 3D qualitative coronary angiography (QCA) and invasive pressure and flow measurements is yet to be elucidated. Methods: We associated invasive fractional flow reserve (FFR), coronary flow reserve (CFR) and coronary flow capacity (CFC) with 2D- and 3D-QCA in 430 intermediate lesions of 366 patients. Results: Overall, 2D-QCA analysis resulted in less severe stenosis severity compared with 3D-QCA analysis. FFR+/CFR− lesions had similar 3D-QCA characteristics as FFR+/CFR+ lesions. In contrast, vessels with FFR−/CFR+ discordance had 3D-QCA characteristics similar to those of vessels with concordant FFR−/CFR−. Contrarily, FFR+/CFR− lesions had CFC similar to that of as FFR-/CFR- lesions. Conclusions: Non-flow-limiting lesions (FFR+/CFR−) have 3D-QCA characteristics similar to those of FFR+/CFR+, but the majority are not associated with inducible myocardial ischemia as determined by invasive CFC. FFR−/CFR+ lesions have 3D-QCA characteristics similar to those of FFR−/CFR− lesions but are more frequently associated with a moderately to severely reduced CFC, illustrating the angiographic–functional mismatch in discordant lesions.

The Complementary Value of Absolute Coronary Flow in the Assessment of Patients with Ischaemic Heart Disease

Fractional flow reserve (FFR) is the current gold-standard invasive assessment of coronary artery disease (CAD). FFR reports coronary blood flow (CBF) as a fraction of a hypothetical and unknown normal value. Although used routinely to diagnose CAD and guide treatment, how accurately FFR predicts actual CBF changes remains unknown. Here we compared fractional CBF with the absolute CBF (aCBF in mL/min), measured with a computational method during standard angiography and pressure-wire assessment, on 203 diseased arteries (143 patients). We found a substantial correlation between the two measurements (r 0.89, Cohen’s Kappa 0.71). Concordance between fractional and absolute CBF reduction was high when FFR was >0.80 (91%), but reduced when FFR was ≤0.80 (81%), 0.70-0.80 (68%) and, particularly 0.75-0.80 (62%). Discordance was associated with coronary microvascular resistance, vessel diameter and mass of myocardium subtended, all factors to which FFR is agnostic. Assessment of aCBF complements FFR, and may be valuable to assess CBF, particularly in cases within the FFR ‘grey-zone’.

Comparison of Doppler Flow Velocity and Thermodilution Derived Indexes of Coronary Physiology

OBJECTIVES

The aim of this study was to compare Doppler flow velocity and thermodilution-derived indexes and to determine the optimal thermodilution-based diagnostic thresholds for coronary flow reserve (CFR).

BACKGROUND

The majority of clinical data and diagnostic thresholds for flow-based indexes are derived from Doppler measurements, and correspondence with thermodilution-derived indices remain unclear.

METHODS

An international multicenter registry was conducted among patients who had coronary flow measurements using both Doppler and thermodilution techniques in the same vessel and during the same procedure.

RESULTS

Physiological data from 250 vessels (in 149 patients) were included in the study. A modest correlation was found between thermodilution-derived CFR (CFRthermo) and Doppler-derived CFR (CFRDoppler) (r2 = 0.36; P < 0.0001). CFRthermo overestimated CFRDoppler (mean 2.59 ± 1.46 vs 2.05 ± 0.89; P < 0.0001; mean bias 0.59 ± 1.24 by Bland-Altman analysis), the relationship being described by the equation CFRthermo = 1.04 × CFRDoppler + 0.50. The commonly used dichotomous CFRthermo threshold of 2.0 had poor sensitivity at predicting a CFRDoppler value <2.5. The optimal CFRthermo threshold was 2.5 (sensitivity 75.54%, specificity 81.25%). There was only a weak correlation between hyperemic microvascular resistance and index of microvascular resistance (r2 = 0.19; P < 0.0001), due largely to variation in the measurement of flow by each modality. Forty-four percent of patients were discordantly classified as having abnormal microvascular resistance by hyperemic microvascular resistance (≥2.5 mm Hg · cm-1 · s) and index of microvascular resistance (≥25).

CONCLUSIONS

CFR calculated by thermodilution overestimates Doppler-derived CFR, while both parameters show modest correlation. The commonly used CFRthermo threshold of 2.0 has poor sensitivity for identifying vessels with diminished CFR, but using the same binary diagnostic threshold as for Doppler (<2.5) yields reasonable diagnostic accuracy. There was only a weak correlation between microvascular resistance indexes assessed by the 2 modalities.

Functional significance of intermediate coronary stenosis in patients with single-vessel coronary artery disease: A comparison of dynamic SPECT coronary flow reserve with intracoronary pressure-derived fractional flow reserve (FFR)

BACKGROUND

The aim of this study was to investigate the correlation of coronary flow reserve (CFR) assessed by rest/stress myocardial perfusion imaging with dynamic single-photon emission computed tomography (SPECT) with intracoronary pressure-derived fractional flow reserve (FFR) in patients with single-vessel coronary artery disease (CAD).

METHODS

Patients with suspected or known stable CAD who were referred for invasive coronary angiography were prospectively enrolled. Both invasive FFR and SPECT were performed in subjects with single-vessel intermediate coronary stenosis. A cutoff value of < 0.8 was used to define abnormal FFR.

RESULTS

A total of 34 patients were enrolled. The mean age of the subjects was 62.1 ± 6.7 years, and 79.4% were male. SPECT-derived CFR showed a significantly moderate correlation with FFR (r = 0.505, P = .003). The diagnostic performance for the identification of abnormal FFR in terms of sensitivity, specificity, and accuracy was 88.9%, 83.3%, and 87.9%, respectively, for CFR, with an optimized cutoff value of 1.73.

CONCLUSION

In patients with single-vessel CAD, SPECT CFR was useful for the detection of functionally significant stenosis. Our data support the use of this technique as an optional method for hemodynamic assessment, especially when FFR results are in normal range.

Heart Team risk assessment with angiography-derived fractional flow reserve determining the optimal revascularization strategy in patients with multivessel disease: Trial design and rationale for the DECISION QFR randomized trial

In patients with multivessel disease (MVD), functional information on lesions improves the prognostic capability of the SYNTAX score. Quantitative flow ratio (QFR®) is an angiography‐derived fractional flow reserve (FFR) that does not require a pressure wire or pharmacological hyperemia. We aimed to investigate the feasibility of QFR‐based patient information in Heart Teams' discussions to determine the optimal revascularization strategy for patients with MVD. We hypothesized that there is an acceptable agreement between treatment recommendations based on the QFR approach and recommendation based on the FFR approach. The DECISION QFR study is a prospective, multicenter, randomized controlled trial that will include patients with MVD who require revascularization. Two Heart Teams comprising cardiologists and cardiac surgeons will be randomized to select a revascularization strategy (percutaneous coronary intervention or coronary artery bypass graft) according to patient information either based on QFR or on FFR. All 260 patients will be assessed by both teams with reference to the anatomical and functional SYNTAX score/SYNTAX score II 2020 derived from the allocated physiological index (QFR or FFR). The primary endpoint of the trial is the level of agreement between the treatment recommendations of both teams, assessed using Cohen's κ. As of March 2022, the patient enrollment has been completed and 230 patients have been discussed in both Heart Teams. The current trial will indicate the usefulness of QFR, which enables a wireless multivessel physiological interrogation, in the discussions of Heart Teams to determine the optimal revascularization strategy for MVD.

Model-based analysis of the sensitivities and diagnostic implications of FFR and CFR under various pathological conditions

Although fractional flow reserve (FFR) and coronary flow reserve (CFR) are both frequently used to assess the functional severity of coronary artery stenosis, discordant results of diagnosis between FFR and CFR in some patient cohorts have been reported. In the present study, a computational model was employed to quantify the impacts of various pathophysiological factors on FFR and CFR. In addition, a hyperemic myocardial ischemic index (HMIx) was proposed as a reference for comparing the diagnostic performances of FFR and CFR. Obtained results showed that CFR was more susceptible than FFR to the influence of many pathophysiological factors unrelated to coronary artery stenosis. In particular, the numerical study proved that increasing hyperemic coronary microvascular resistance significantly elevated FFR while reducing CFR despite fixed severity of coronary artery stenosis, whereas introducing aortic valve disease only caused a significant decrease in CFR with little influence on FFR. These results provided theoretical evidence for explaining some clinical observations, such as the increased risk of discordant diagnostic results between FFR and CFR in patients with increased hyperemic microvascular resistance, and significant increase in CFR after surgical relief of severe aortic valve disease. When evaluated with respect to the predictive value for hyperemic myocardial ischemia, the performance of FFR was found to be considerably compromised in the presence of severe coronary vasodilation dysfunction or aortic valve disease, whereas the relationship between CFR and HMIx remained relatively stable, suggesting that CFR may be a more reliable indicator of myocardial ischemia under complex pathophysiological conditions.

Microvascular Resistance Reserve for Assessment of Coronary Microvascular Function: JACC Technology Corner

The need for a quantitative and operator-independent assessment of coronary microvascular function is increasingly recognized. We propose the theoretical framework of microvascular resistance reserve (MRR) as an index specific for the microvasculature, independent of autoregulation and myocardial mass, and based on operator-independent measurements of absolute values of coronary flow and pressure. In its general form, MRR equals coronary flow reserve (CFR) divided by fractional flow reserve (FFR) corrected for driving pressures. In 30 arteries, pressure, temperature, and flow velocity measurements were obtained simultaneously at baseline (BL), during infusion of saline at 10 mL/min (rest) and 20 mL/min (hyperemia). A strong correlation was found between continuous thermodilution-derived MRR and Doppler MRR (r = 0.88; 95% confidence interval: 0.72-0.93; P < 0.001). MRR was independent from the epicardial resistance, the lower the FFR value, the greater the difference between MRR and CFR. Therefore, MRR is proposed as a specific, quantitative, and operator-independent metric to quantify coronary microvascular dysfunction.

Physiology-Based Revascularization of Left Main Coronary Artery Disease

It is of critical importance to correctly assess the significance of a left main lesion. Underestimation of significance beholds the risk of inappropriate deferral of revascularization, whereas overestimation may trigger major but unnecessary interventions. This article addresses the invasive physiological assessment of left main disease and its role in deciding upon revascularization. It mainly focuses on the available evidence for fractional flow reserve and instantaneous wave-free ratio, their interpretation, and limitations. We also discuss alternative invasive physiological indices and imaging, as well as the link between physiology, ischemia, and prognosis.

Invasive Diagnosis of Coronary Functional Disorders Causing Angina Pectoris

Coronary vasomotion disorders represent a frequent cause of angina and/or dyspnoea in patients with non-obstructed coronary arteries. The highly sophisticated interplay of vasodilatation and vasoconstriction can be assessed in an interventional diagnostic procedure. Established parameters characterising adequate vasodilatation are coronary blood flow at rest, and, after drug-induced vasodilation, coronary flow reserve, and microvascular resistance (hyperaemic microvascular resistance, index of microcirculatory resistance). An increased vasoconstrictive potential is diagnosed by provocation testing with acetylcholine or ergonovine. This enables a diagnosis of coronary epicardial and/or microvascular spasm. Ischaemia associated with microvascular spasm can be confirmed by ischaemic ECG changes and the measurement of lactate concentrations in the coronary sinus. Although interventional diagnostic procedures are helpful for determining the mechanism of the angina, which may be the key to successful medical treatment, they are still neither widely accepted nor applied in many medical centres. This article summarises currently well-established invasive methods for the diagnosis of coronary functional disorders causing angina pectoris.

Coronary Angiography-Derived Index of Microvascular Resistance

A coronary angiography-derived index of microvascular resistance (caIMR) is proposed for physiological assessment of microvasular diseases in coronary circulation. The aim of the study is to assess diagnostic performance of caIMR, using wire-derived index of microvascular resistance (IMR) as the reference standard. IMR was demonstrated in 56 patients (57 vessels) with stable/unstable angina pectoris and no obstructive coronary arteries in three centers using the Certus pressure wire. Based on the aortic pressure wave and coronary angiograms from two projections, the caIMR was computed and assessed in blinded fashion against the IMR at an independent core laboratory. Diagnostic accuracy, sensitivity, specificity, positive predictive value and negative predictive value of the caIMR with a cutoff value of 25 were 84.2% (95% CI: 72.1% to 92.5%), 86.1% (95% CI: 70.5% to 95.3%), 81.0% (95% CI: 58.1% to 94.6%), 88.6% (95% CI: 76.1% to 95.0%), and 77.3% (95% CI: 59.5% to 88.7%) against the IMR with a cutoff value of 25. The receiver-operating curve had area under the curve of 0.919 and the correlation coefficient equaled to 0.746 between caIMR and wire-derived IMR. Hence, caIMR could eliminate the need of a pressure wire, reduce technical error, and potentially increase adoption of physiological assessment of microvascular diseases in patients with ischemic heart disease.

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.

Objective Identification of Intermediate Lesions Inducing Myocardial Ischemia Using Sequential Intracoronary Pressure and Flow Measurements

Background

Although ischemic heart disease has a complex and multilevel origin, the diagnostic approach is mainly focused on focal obstructive disease as assessed by pressure‐derived indexes. The prognostic relevance of coronary flow over coronary pressure has been suggested and implies that identification of relevant perfusion abnormalities by invasive physiology techniques is critical for the correct identification of patients who benefit from coronary revascularization. The purpose of this study was to evaluate the diagnostic potential of a sequential approach using pressure‐derived indexes instantaneous wave‐free ratio (iFR), fractional flow reserve (FFR), and coronary flow reserve (CFR) measurements to determine the number of intermediate lesions associated with flow abnormalities after initial pressure measurements.

Methods and Results

A total of 366 intermediate lesions were assessed with simultaneous intracoronary pressure and flow velocity measurements. Contemporary clinical iFR, FFR, and CFR cut points for myocardial ischemia were applied. A total of 118 (32%) lesions were FFR+ and 136 (37%) lesions were iFR+. Subsequent CFR assessment resulted for FFR in a total of 91 (25%) FFR+/CFR+ and for iFR a total of 111 (30%) iFR+/CFR+ lesions. An iFR, FFR, and invasive flow velocity assessment approach would have yielded 20% of lesions (74 of 366) as ischemic.

Conclusions

Ultimately, 20% of intermediate lesions are associated with flow abnormalities after applying a pressure and flow velocity sequential approach. If iFR is borderline, FFR has limited additional value, in contrast with CFR. These results emphasize the use of coronary physiology in assessing stenosis severity but may also further question the contemporary reputation of a pressure‐based approach as a gold standard for the detection of myocardial ischemia in ischemic heart disease.

An intact animal model for the assessment of coronary blood flow regulation "Coronary blood flow regulation"

Coronary blood flow adapts to metabolic demand ("metabolic regulation") and remains relatively constant over a range of pressure changes ("autoregulation"). Coronary metabolic regulation and autoregulation are usually studied separately. We developed an intact animal experimental model to explore both regulatory mechanisms of coronary blood flow. Coronary pressure and flow-velocities were measured in four anesthetized and closed-chest pigs using an intracoronary Doppler wire. Metabolic regulation was assessed by coronary flow reserve defined as the ratio between the maximally vasodilated and the basal flow, with hyperemia achieved using intracoronary administration of adenosine (90 µg) or bradykinin (10-6  M) as endothelium-independent and -dependent vasodilators respectively. For both vasodilators, we found a healthy coronary flow reserve ≥ 3.0 at baseline, which was maintained at 2.9 ± 0.2 after a 6-hr period. Autoregulation was assessed by the lower breakpoint of coronary pressure-flow relationships, with gradual decrease in coronary pressure through the inflation of an intracoronary balloon. We found a lower limit of autoregulation between 42 and 55 mmHg, which was stable during a 6-hr period. We conclude that this intact animal model is adequate for the study of pharmacological interventions on the coronary circulation in health and disease, and as such suitable for preclinical drug studies.

Influence of microcirculation load on FFR in coronary artery stenosis model

BACKGROUND

The coronary artery hemodynamics are impacted by both the macrocirculation and microcirculation. Whether microcirculation load impact the functional assessment of a coronary artery stenosis is unknown. The purpose of this study is to investigate the effect of porous media of the microcirculation on fractional flow reserve (FFR) in stenotic coronary artery model.

METHODS

A three dimensional computational simulation of blood flow in coronary artery symmetric stenotic model was constructed. The computational fluid dynamics (CFD) model was developed with Fluent 16.0. Blood was modeled as a shear thinning, non-Newtonian fluid with the Carreau model. A seepage outlet boundary condition and transient inlet conditions were imposed on the model. Coronary physiologica diagnostic parameter such as pressure, velocity and fractional flow reserve (FFR) were investigated in the model and compared with the microcirculation load (ML) and constant pressure load (PL) condition.

RESULTS

The present study showed the different hemodynamics in the ML and PL condition. The pre-stenotic pressure is almost the same in the two model. However the pressure in the post-stenotic artery domain is much lower in the PL model. The fluctuation range of the pressures is much higher in ML model than those in PL model. The velocity flow was more steady and lower in the ML model. For the PL model with 75% artery stenosis the FFR was 0.776, while for the ML model with the same stenosis, the FFR was 0.813.

CONCLUSIONS

This study provides evidence that FFR increased in the presentation of ML condition. There is a strong hemodynamic effect of microcirculation on coronary artery stenosis.

Diagnosis of coronary microvascular dysfunction in the clinic

The coronary microcirculation plays a pivotal role in the regulation of coronary blood flow and cardiac metabolism. It can adapt to acute and chronic pathologic conditions such as coronary thrombosis or long-standing hypertension. Due to the fact that the coronary microcirculation cannot be visualized in human beings in vivo, its assessment remains challenging. Thus, the clinical importance of the coronary microcirculation is still often underestimated or even neglected. Depending on the clinical condition of the respective patient, several non-invasive (e.g. transthoracic Doppler-echocardiography assessing coronary flow velocity reserve, cardiac magnetic resonance imaging, positron emission tomography) and invasive methods (e.g. assessment of coronary flow reserve (CFR) and microvascular resistance (MVR) using adenosine, microvascular coronary spasm with acetylcholine) have been established for the assessment of coronary microvascular function. Individual patient characteristics, but certainly also local availability, methodical expertise and costs will influence which methods are being used for the diagnostic work-up (non-invasive and/or invasive assessment) in a patient with recurrent symptoms and suspected coronary microvascular dysfunction. Recently, the combined invasive assessment of coronary vasoconstrictor as well as vasodilator abnormalities has been titled interventional diagnostic procedure (IDP). It involves intracoronary acetylcholine testing for the detection of coronary spasm as well as CFR and MVR assessment in response to adenosine using a dedicated wire. Currently, the IDP represents the most comprehensive coronary vasomotor assessment. Studies using the IDP to better characterize the endotypes observed will hopefully facilitate development of tailored and effective treatments.

Clinical Outcomes of Deferred Revascularisation Using Fractional Flow Reserve in Diabetic Patients

BACKGROUND

Fractional flow reserve (FFR) is used to assess the functional significance of coronary artery lesions. Diabetic patients are associated with high burden of atherosclerosis and microvascular dysfunction. We studied the clinical outcomes of diabetic patients who underwent FFR-guided deferred revascularisation.

METHODS

Consecutive patients from a single large volume centre who underwent FFR assessment were included. Clinical endpoints were prospectively collected using the national electronic care records system. The primary endpoint was defined as the four-year risk of the vessel-oriented composite outcome of cardiac death, vessel-related myocardial infarction (VMI), and vessel-related urgent revascularisation (VUR). Absolute FFR values groups (0.81 to 0.85; 0.86 to 0.90; and >0.90) were used to further stratify patient outcomes.

RESULTS

FFR-guided deferred revascularisation occurred in 860 patients (63%), of whom 159 were diabetic. The primary endpoint was significantly higher in the diabetic compared to the non-diabetic group [HR 1.76 (95%CI 1.08 to 2.88), P = 0.024]. The difference was driven from cardiac death (6.3% vs. 3.0%, P = 0.044) and VMI (5.0% vs. 1.7%, P = 0.012) but not VUR (8.8% vs. 5.1%, P = 0.07). There was a significant decrease in the incidence of the primary endpoint in the diabetic group according to FFR groups (23.6%, 12.3%, 2.4%, P = 0.001) with comparable clinical outcomes in the non-diabetic group (11.8%, 6.4%, 7.4%, P = 0.085).

CONCLUSIONS

Our study demonstrated an increased risk of death and target vessel MI in diabetic patients undergoing FFR-guided deferred revascularisation compared to non-diabetic group. Nonetheless, FFR remained a useful tool to identify those at future risk, mainly in diabetic patients.

Accuracy of 3-dimensional and 2-dimensional quantitative coronary angiography for predicting physiological significance of coronary stenosis: a FAVOR II substudy

Background

Three-dimensional quantitative coronary angiography (3D-QCA) enables reconstruction of a coronary artery in 3D from two angiographic image projections. This study compared the diagnostic accuracy of 3D-QCA vs. 2-dimensional (2D) QCA in predicting physiologically significant coronary stenosis, using fractional flow reserve (FFR) as the reference standard.

Methods

All interrogated vessels in the FAVOR II China study and the FAVOR II Europe-Japan study were assessed by 2D-QCA and 3D-QCA according to standard operating procedures in core laboratories. QCA analysts were blinded to the corresponding FFR values.

Results

A total of 645 vessels from 576 patients with 3D-QCA, 2D-QCA, and FFR were analyzed. Using the conventional cut-off value of 50% for percent diameter stenosis (DS%), 3D-QCA was more accurate in predicting FFR ≤0.80 than 2D-QCA [accuracy 74.0% (95% CI: 69.9-77.7%) vs. 64.9% (95% CI: 61.3-68.7%), difference: 9.1%, P<0.001]. Sensitivity was higher by 3D-QCA compared with 2D-QCA [69.1% (95% CI: 63.0-75.1%) vs. 47.1% (95% CI: 40.5-53.6%), difference: 22.0%, P<0.001] and specificity was similar [76.5% (95% CI: 72.5-80.6%) vs. 74.4% (95% CI: 70.2-78.6%), difference: 2.1%, P=0.40]. Area under the receiver operating characteristic curve was significantly higher for 3D-QCA than for 2D-QCA [0.81 (95% CI: 0.77-0.84) vs. 0.66 (95% CI: 0.62-0.71), P<0.001].

Conclusions

3D-QCA demonstrated better diagnostic performance in predicting physiologically significant coronary stenosis compared with 2D-QCA, when FFR was used as the reference standard.

Pressure- and Velocity-Based Physiological Assessment of Stenotic Lesions at Hyperemia in Superficial Femoral Artery Disease: Importance of Hyperemic Stenosis Resistance

Background: In superficial femoral artery (SFA) stenosis, stenosis resistance may increase, but the relationship between stenosis resistance and stenotic severity remains to be seen. This study aimed to investigate the physiological response, through a hyperemic condition, and the pathophysiological significance of Doppler flow and stenosis resistance in SFA. Methods: Twenty-four limbs with focal stenosis of the SFA were analyzed. We assessed the fractional flow reserve (FFR), hyperemic stenosis resistance (h-SR), and vascular flow reserve (VFR) of the SFA with a pressure/Doppler flow sensor-tipped combination guidewire before and after endovascular therapy (EVT). Results: FFR, h-SR, and VFR changed significantly after EVT. h-SR was more strongly correlated with % area stenosis, measured by intravascular ultrasound than FFR (FFR: r=-0.716, h-SR: r=0.741, p<0.0001, respectively). However, VFR was not associated with % area stenosis. A receiver operating characteristic curve showed cut-offs h-SR >0.36 mmHg·sec/cm, and FFR <0.88 predicted >75% area stenosis with area under curves of 0.883 and 0.828, respectively. Conclusion: h-SR can indicate stenotic severity in an SFA focal lesion more prominently than FFR and may be a new physiological index to determine indication for EVT. VFR was not feasible for assessment in SFA focal stenosis.

Assessing Coronary Microvascular Dysfunction in Ischaemic Heart Disease: Little Things Can Make a Big Difference

The role of coronary microvascular dysfunction (CMD) in the pathogenesis of ischaemic heart disease and in determining long-term prognosis is increasingly recognised. In selected patients, a comprehensive coronary assessment including an assessment of microvascular function may help refine risk stratification and improve patient outcomes. Various non-invasive and invasive techniques have been developed to assess the coronary microcirculation. Many of these tests utilise the indicator-dilution principle to determine coronary or myocardial blood flow. However, these techniques are often limited by their variability and lack of specificity for the coronary microvasculature. Consequently, there is still paucity of data on targeted therapies for CMD and their implications on long-term clinical outcomes, particularly in the setting of non-ST elevation acute coronary syndromes. Recent technical advancements, such as the index of microcirculatory resistance, have largely overcome these limitations and are able to provide novel insights into the assessment and treatment of CMD. This review summarises the currently available techniques for the assessment of CMD and provides an overview of its clinical implications.

Shock-Wave Therapy Improves Myocardial Blood Flow Reserve in Patients with Refractory Angina: Evaluation by Real-Time Myocardial Perfusion Echocardiography

BACKGROUND

Cardiac shock-wave therapy (CSWT) has been demonstrated as an option for the treatment of patients with refractory angina (RA), promoting immediate vasodilatory effects and, in the long-term, neoangiogenic effects that would be responsible for reducing the myocardial ischemic load. The aim of this study was to determine the effects of CSWT on myocardial blood flow reserve (MBFR) assessed by quantitative real-time myocardial perfusion echocardiography in patients with RA.

METHODS

Fifteen patients (mean age 61.5 ± 12.8 years) with RA who underwent CSWT during nine sessions, over 3 months of treatment, were prospectively studied. A total of 32 myocardial segments with ischemia were treated, while another 31 did not receive therapy because of technical limitations. Myocardial perfusion was evaluated at rest and after dipyridamole stress (0.84 mg/kg) before and 6 months after CSWT, using quantitative real-time myocardial perfusion echocardiography. Clinical effects were evaluated using Canadian Cardiovascular Society grading of angina and the Seattle Angina Questionnaire.

RESULTS

The ischemic segments treated with CSWT had increased MBFR (from 1.33 ± 0.22 to 1.74 ± 0.29, P < .001), a benefit that was not observed in untreated ischemic segments (1.51 ± 0.29 vs 1.54 ± 0.28, P = .47). Patients demonstrated increased global MBFR (from 1.78 ± 0.54 to 1.89 ± 0.49, P = .017). Semiquantitative single-photon emission computed tomographic analysis of the treated ischemic segments revealed a score reduction from 2.10 ± 0.87 to 1.68 ± 1.19 (P = .024). There was improvement in Canadian Cardiovascular Society score (from 3.20 ± 0.56 to 1.93 ± 0.70, P < .05) and in Seattle Angina Questionnaire score (from 42.3 ± 12.99 to 71.2 ± 14.29, P < .05). No major cardiovascular events were recorded during follow-up.

CONCLUSIONS

CSWT improved MBFR in ischemic segments, as demonstrated by quantitative real-time myocardial perfusion echocardiography. These results suggest that CSWT has the potential to increase myocardial blood flow, with an impact on symptoms and quality of life in patients with RA.

Physiological Consequences of Coronary Arteriolar Dysfunction and Its Influence on Cardiovascular Disease

To date, the major focus of diagnostic modalities and interventions to treat coronary artery disease has been the large epicardial vessels. Despite substantial data showing that microcirculatory dysfunction is a strong predictor of future adverse cardiovascular events, very little research has gone into developing techniques for in vivo diagnosis and therapeutic interventions to improve microcirculatory function. In this review, we will discuss the pathophysiology of coronary arteriolar dysfunction, define its prognostic implications, evaluate the diagnostic modalities available, and provide speculation on current and potential therapeutic opportunities.

Relationship between FFR, CFR and coronary microvascular resistance - Practical implications for FFR-guided percutaneous coronary intervention

Objective

The aim was threefold: 1) expound the independent physiological parameters that drive FFR, 2) elucidate contradictory conclusions between fractional flow reserve (FFR) and coronary flow reserve (CFR), and 3) highlight the need of both FFR and CFR in clinical decision making. Simple explicit theoretical models were supported by coronary data analyzed retrospectively.

Methodology

FFR was expressed as a function of pressure loss coefficient, aortic pressure and hyperemic coronary microvascular resistance. The FFR-CFR relationship was also demonstrated mathematically and was shown to be exclusively dependent upon the coronary microvascular resistances. The equations were validated in a first series of 199 lesions whose pressures and distal velocities were monitored. A second dataset of 75 lesions with pre- and post-PCI measures of FFR and CFR was also analyzed to investigate the clinical impact of our hemodynamic reasoning.

Results

Hyperemic coronary microvascular resistance and pressure loss coefficient had comparable impacts (45% and 49%) on FFR. There was a good concordance (y = 0.96 x − 0.02, r² = 0.97) between measured CFR and CFR predicted by FFR and coronary resistances. In patients with CFR < 2 and CFR/FFR ≥ 2, post-PCI CFR was significantly >2 (p < 0.001), whereas it was not (p = 0.94) in patients with CFR < 2 and CFR/FFR < 2.

Conclusion

The FFR behavior and FFR-CFR relationship are predictable from basic hemodynamics. Conflicting conclusions between FFR and CFR are explained from coronary vascular resistances. As confirmed by our results, FFR and CFR are complementary; they could jointly contribute to better PCI guidance through the CFR-to-FFR ratio in patients with coronary artery disease.

Doppler Flow Velocity and Thermodilution to Assess Coronary Flow Reserve: A Head-to-Head Comparison With [15O]H2O PET

OBJECTIVES

This study sought to compare Doppler flow velocity reserve (CFR_Doppl) and thermodilution-derived coronary flow reserve (CFR_thermo) head-to-head with the gold standard for quantification of myocardial perfusion, [¹⁵O]H₂O positron emission tomography (PET).

BACKGROUND

Coronary flow reserve (CFR) is an important parameter for assessing coronary vascular function. To date, 2 techniques are available for invasive assessment of CFR: Doppler flow velocity and thermodilution. Although these techniques have been compared with each other, neither has been compared with [¹⁵O]H₂O PET perfusion imaging.

METHODS

CFR was assessed in 98 vessels of 40 consecutive stable patients with suspected coronary artery disease. Patients underwent [¹⁵O]H₂O PET, followed by invasive angiography in conjunction with simultaneous measurements of fractional flow reserve, CFR_Doppl, and CFR_thermo. Both normal and obstructed arteries were included.

RESULTS

The quality of Doppler flow velocity traces was significantly lower than that of thermodilution curves (p < 0.001). A moderate correlation was observed between CFR_Doppl and CFR_thermo (r = 0.59; p < 0.001). CFR_Doppl correlated well with PET-derived CFR (CFR_PET) (r = 0.82; p < 0.001). In contrast, the correlation between CFR_thermo and CFR_PET was only modest (r = 0.55; p < 0.001). This difference in correlation with CFR_PET was significant (t = 4.9; df = 95; p < 0.001). Bland-Altman analysis revealed a tendency of CFR_thermo to overestimate flow reserve at higher values.

CONCLUSIONS

Coronary flow reserve, determined using Doppler flow velocity, has superior agreement with [¹⁵O]H₂O PET in comparison with CFR_thermo.

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.