Fractional flow reserve: a review: invasive imaging

The effect of aortic wall and aortic leaflet stiffening on coronary hemodynamic: a fluid-structure interaction study

Pathologies of the aortic valve such as aortic sclerosis are thought to impact coronary blood flow. Recent clinical investigations have observed simultaneous structural and hemodynamic variations in the aortic valve and coronary arteries due to regional pathologies of the aortic valve. The goal of the present study is to elucidate this observed and yet unexplained phenomenon, in which a local pathology in the aortic valve region could potentially lead to the initiation or progression of coronary artery disease. Results revealed a considerable impact on the coronary flow, velocity profile, and consequently shear stress due to an increase in the aortic wall or aortic leaflet stiffness and thickness which concur with clinical observations. The cutoff value of 0.75 for fractional flow reserve was reached when the values of leaflet thickness and aortic wall stiffness were approximately twice and three times their normal value, respectively. Variations observed in coronary velocity profiles as well as wall shear stress suggest a possible link for the initiation of coronary artery disease.

Quantification of absolute coronary flow reserve and relative fractional flow reserve in a swine animal model using angiographic image data

Coronary flow reserve (CFR) and fractional flow reserve (FFR) are important physiological indexes for coronary disease. The purpose of this study was to validate the CFR and FFR measurement techniques using only angiographic image data. Fifteen swine were instrumented with an ultrasound flow probe on the left anterior descending artery (LAD). Microspheres were gradually injected into the LAD to create microvascular disruption. An occluder was used to produce stenosis. Contrast material injections were made into the left coronary artery during image acquisition. Volumetric blood flow from the flow probe (Q(q)) was continuously recorded. Angiography-based blood flow (Q(a)) was calculated by using a time-density curve based on the first-pass analysis technique. Flow probe-based CFR (CFR(q)) and angiography-based CFR (CFR(a)) were calculated as the ratio of hyperemic to baseline flow using Q(q) and Q(a), respectively. Relative angiographic FFR (relative FFR(a)) was calculated as the ratio of the normalized Q(a) in LAD to the left circumflex artery (LC(X)) during hyperemia. Flow probe-based FFR (FFR(q)) was measured from the ratio of hyperemic flow with and without disease. CFR(a) showed a strong correlation with the gold standard CFR(q) (CFR(a) = 0.91 CFR(q) + 0.30; r = 0.90; P < 0.0001). Relative FFR(a) correlated linearly with FFR(q) (relative FFR(a) = 0.86 FFR(q) + 0.05; r = 0.90; P < 0.0001). The quantification of CFR and relative FFR(a) using angiographic image data was validated in a swine model. This angiographic technique can potentially be used for coronary physiological assessment during routine cardiac catheterization.

Minimal effect of collateral flow on coronary microvascular resistance in the presence of intermediate and noncritical coronary stenoses

Depending on stenosis severity, collateral flow can be a confounding factor in the determination of coronary hyperemic microvascular resistance (HMR). Under certain assumptions, the calculation of HMR can be corrected for collateral flow by incorporating the wedge pressure (P(w)) in the calculation. However, although P(w) > 25 mmHg is indicative of collateral flow, P(w) does in part also reflect myocardial wall stress neglected in the assumptions. Therefore, the aim of this study was to establish whether adjusting HMR by P(w) is pertinent for a diagnostically relevant range of stenosis severities as expressed by fractional flow reserve (FFR). Accordingly, intracoronary pressure and Doppler flow velocity were measured a total of 95 times in 29 patients distal to a coronary stenosis before and after stepwise percutaneous coronary intervention. HMR was calculated without (HMR) and with P(w)-based adjustment for collateral flow (HMR(C)). FFR ranged from 0.3 to 1. HMR varied between 1 and 5 and HMR(C) between 0.5 and 4.2 mmHg·cm(-1)·s. HMR was about 37% higher than HMR(C) for stenoses with FFR < 0.6, but for FFR > 0.8, the relative difference was reduced to 4.4 ± 3.4%. In the diagnostically relevant range of FFR between 0.6 and 0.8, this difference was 16.5 ± 10.4%. In conclusion, P(w)-based adjustment likely overestimates the effect of potential collateral flow and is not needed for the assessment of coronary HMR in the presence of a flow-limiting stenosis characterized by FFR between 0.6 and 0.8 or for nonsignificant lesions.

Coronary wave intensity during the Valsalva manoeuvre in humans reflects altered intramural vessel compression responsible for extravascular resistance

Our aim was to investigate the effect of altered cardiac-coronary interaction during the Valsalva manoeuvre (VM) on coronary wave intensity and the response of coronary microvascular resistance. In 13 patients, left ventricular (P(LV)) and aortic pressure were measured during catheterization, together with intracoronary pressure and blood flow velocity (U) via a dual-sensor guide wire advanced into an angiographically normal coronary artery. Signals were analysed for the following phases of VM: baseline (B1), onset of strain (S1), sustained strain (S2), onset of release (R1), maximal response during recovery (R2), and baseline after VM. The immediate effects of VM were most evident from diastolic P(LV) (LVDP), which increased from 11.0 ± 2.3 to 36.4 ± 2.7 mmHg between B1 and S1 and fell from 28.3 ± 3.4 to 8.3 ± 1.9 mmHg between S2 and R1. Wave intensities and rate pressure product (RPP) were only minimally affected at these transient phases, but coronary wave energies decreased by about 50% and RPP by 38% from S1 to S2, together with a 30% depression of LVdP/dt. All signals were restored to baseline values during the recovery. U did not vary significantly throughout the VM. Despite the depressed cardiac performance during VM strain, microvascular resistance, calculated with LVDP as backpressure, decreased by 31% from B1 to S2, whereas an increase via metabolically induced vasoconstriction was expected. Since coronary U remained essentially constant despite the marked reduction in oxygen consumption, microvascular vasoconstriction must have been compensated by a decrease in the contraction-mediated impediment on coronary blood flow, as confirmed by the reduced coronary wave energies.

The multi-scale modelling of coronary blood flow

Coronary flow is governed by a number of determinants including network anatomy, systemic afterload and the mechanical interaction with the myocardium throughout the cardiac cycle. The range of spatial scales and multi-physics nature of coronary perfusion highlights a need for a multiscale framework that captures the relevant details at each level of the network. The goal of this review is to provide a compact and accessible introduction to the methodology and current state of the art application of the modelling frameworks that have been used to study the coronary circulation. We begin with a brief description of the seminal experimental observations that have motivated the development of mechanistic frameworks for understanding how myocardial mechanics influences coronary flow. These concepts are then linked to an overview of the lumped parameter models employed to test these hypotheses. We then outline the full and reduced-order (3D and 1D) continuum mechanics models based on the Navier–Stokes equations and highlight, with examples, their application regimes. At the smaller spatial scales the case for the importance of addressing the microcirculation is presented, with an emphasis on the poroelastic approach that is well-suited to bridge an existing gap in the development of an integrated whole heart model. Finally, the recent accomplishments of the wave intensity analysis and related approaches are presented and the clinical outlook for coronary flow modelling discussed.

Long-term clinical outcome after fractional flow reserve-guided percutaneous coronary revascularization in patients with small-vessel disease

BACKGROUND

Small coronary vessels supply small myocardial territories. The clinical significance of small-vessel stenoses is therefore questionable. Moreover, percutaneous coronary intervention (PCI) of nonfunctionally significant lesions does not improve clinical outcome and might be associated with potential procedural or stent related risks. The aim of this study was to assess the clinical outcome of fractional flow reserve (FFR)-guided PCI in the treatment of small coronary vessel lesions as compared with an angio-guided PCI.

METHODS AND RESULTS

From January 2004 to December 2008, all patients treated with PCI for stable or unstable angina in small native coronary vessels (reference vessel diameter and stent size <3 mm) were retrospectively analyzed. Patients were divided into angio-guided and an FFR-guided PCI groups. A total of 717 patients were enrolled (495 angio-guided, 222 FFR-guided). End points were death, nonfatal myocardial infarction (MI), combined death or nonfatal MI, target vessel revascularization (TVR), and procedure costs. Major adverse cardiac events (MACE) were defined as death, nonfatal MI, and TVR. Clinical follow-up was obtained in 97.5% (median follow-up: 3.3 [from 0.01-5] years) of the patients. Seventy-eight patients (35%) had a significant FFR (<0.80) and underwent PCI. Using a propensity score adjusted Cox analysis, patients treated with FFR-guided PCI had significantly lower combined death or nonfatal MI (hazard ratio [HR], 0.413; 95% confidence interval [CI], 0.227-0.750; P=0.004), nonfatal MI (HR, 0.063; 95% CI, 0.009-0.462; P=0.007), TVR (HR, 0.517; 95% CI, 0.323-0.826; P=0.006), and MACE (HR, 0.458; 95% CI, 0.310-0.679; P<0.001). No difference was observed in mortality alone (HR, 0.684; 95% CI, 0.355-1.316; P=0.255). Procedure costs were also reduced in the FFR guided strategy (3253±102 Euros versus 4714±37 Euros, P<0.0001).

CONCLUSIONS

FFR-guided PCI of small coronary arteries is safe and results in better clinical outcomes when compared with an angio-guided PCI.

Myocardial fractional flow reserve. Its role in guiding PCI in stable coronary artery disease

Revascularization of coronary artery lesions should be based on objective evidence of ischemia, as recommended by the guidelines of the European Society of Cardiology. However, even in the case of stable coronary artery disease and elective percutaneous coronary intervention (PCI), pre-procedural noninvasive stress test results are available in a minority of patients only. It is common practice for physicians to make decisions on revascularization in the catheterization laboratory after a cursory review of the angiogram, despite the well-recognized inaccuracy of such an approach. Myocardial fractional flow reserve (FFR) measured by a coronary pressure wire is a specific index of the functional significance of a coronary lesion, with superior diagnostic accuracy for the detection of ischemia than any noninvasive stress test. FFR trials on patients with single and multivessel disease, such as the DEFER and FAME studies, have demonstrated that the clinical benefit of PCI with respect to patient outcome is greatest when revascularization is limited to lesions inducing ischemia, whereas lesions not inducing ischemia should be treated medically.

Assessment of coronary microcirculation in a swine animal model

Coronary microvascular dysfunction has important prognostic implications. Several hemodynamic indexes, such as coronary flow reserve (CFR), microvascular resistance, and zero-flow pressure (P(zf)), were used to establish the most reliable index to assess coronary microcirculation. Fifteen swine were instrumented with a flow probe, and a pressure wire was advanced into the distal left anterior descending artery. Adenosine was used to produce maximum hyperemia. Microspheres were used to create microvascular dysfunction. An occluder was used to produce stenosis. Blood flow from the probe (Q(p)), aortic pressure, distal coronary pressure, and right atrium pressure were recorded. Angiographic flow (Q(a)) was calculated using a time-density curve. Flow probe-based CFR and angiographic CFR were calculated using Q(p) and Q(a), respectively. Flow probe-based (NMR(qh)) and angiographic normalized microvascular resistance (NMR(ah)) were determined using Q(p) and Q(a), respectively, during hyperemia. P(zf) was calculated using Q(p) and distal coronary pressure. Two series of receiver operating characteristic curves were generated: normal epicardial artery model (N model) and stenosis model (S model). The areas under the receiver operating characteristic curves for flow probe-based CFR, angiographic CFR, NMR(qh), NMR(ah), and P(zf) were 0.855, 0.836, 0.976, 0.956, and 0.855 in N model and 0.737, 0.700, 0.935, 0.889, and 0.698 in S model. Both NMR(qh) and NMR(ah) were significantly more reliable than CFR and P(zf) in detecting the microvascular deterioration. Compared with CFR and P(zf), NMR provided a more accurate assessment of microcirculation. This improved accuracy was more prevalent when stenosis existed. Moreover, NMR(ah) is potentially a less invasive method for assessing coronary microcirculation.

Fractional flow reserve for the assessment of nonculprit coronary artery stenoses in patients with acute myocardial infarction

OBJECTIVES

We investigated the reliability of fractional flow reserve (FFR) of nonculprit coronary stenoses during percutaneous coronary intervention (PCI) in acute myocardial infarction.

BACKGROUND

Assessing the hemodynamic severity of the nonculprit coronary artery stenoses at the acute phase of a myocardial infarction could improve risk stratification and shorten the diagnostic work-up.

METHODS

One hundred one patients undergoing PCI for an acute myocardial infarction (n = 75 with ST-segment elevation myocardial infarction [STEMI], and n = 26 with non-ST-segment elevation myocardial infarction) were prospectively recruited. The FFR measurements in 112 nonculprit stenoses were obtained immediately after PCI of the culprit stenosis and were repeated 35 ± 4 days later. In addition, left ventricular ejection fraction, quantitative coronary angiographic measurements of the nonculprit stenoses, Thrombolysis In Myocardial Infarction (TIMI) flow, corrected TIMI frame count (cTFC), and the index of microcirculatory resistance (n = 14) of the nonculprit vessels were assessed in the acute phase and at control angiogram.

RESULTS

The FFR value of the nonculprit stenoses did not change between the acute and follow-up (0.77 ± 0.13 vs. 0.77 ± 0.13, respectively, p = NS). In only 2 patients, the FFR value was higher than 0.8 at the acute phase and lower than 0.75 at follow-up. The TIMI flow, cTFC, percentage diameter stenosis, minimum lumen diameter, and index of microcirculatory resistance did not change. Left ventricular ejection fraction increased significantly in patients with STEMI (from 54 ± 13% to 57 ± 13%, p = 0.03).

CONCLUSIONS

During the acute phase of acute coronary syndromes, the severity of nonculprit coronary artery stenoses can reliably be assessed by FFR. This allows a decision about the need for additional revascularization and might contribute to a better risk stratification.

Quantification of coronary microvascular resistance using angiographic images for volumetric blood flow measurement: in vivo validation

Structural coronary microcirculation abnormalities are important prognostic determinants in clinical settings. However, an assessment of microvascular resistance (MR) requires a velocity wire. A first-pass distribution analysis technique to measure volumetric blood flow has been previously validated. The aim of this study was the in vivo validation of the MR measurement technique using first-pass distribution analysis. Twelve anesthetized swine were instrumented with a transit-time ultrasound flow probe on the proximal segment of the left anterior descending coronary artery (LAD). Microspheres were injected into the LAD to create a model of microvascular dysfunction. Adenosine (400 μg·kg(-1)·min(-1)) was used to produce maximum hyperemia. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. Volumetric blood flow measurements (Q(a)) were made using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. Blood flow from the flow probe (Q(p)), coronary pressure (P(a)), and right atrium pressure (P(v)) were continuously recorded. Flow probe-based normalized MR (NMR(p)) and angiography-based normalized MR (NMR(a)) were calculated using Q(p) and Q(a), respectively. In 258 measurements, Q(a) showed a strong correlation with the gold standard Q(p) (Q(a) = 0.90 Q(p) + 6.6 ml/min, r(2) = 0.91, P < 0.0001). NMR(a) correlated linearly with NMR(p) (NMR(a) = 0.90 NMR(p) + 0.02 mmHg·ml(-1)·min(-1), r(2) = 0.91, P < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between NMR(a) and NMR(p). In conclusion, a technique based on angiographic image data for quantifying NMR was validated using a swine model. This study provides a method to measure NMR without using a velocity wire, which can potentially be used to evaluate microvascular conditions during coronary arteriography.

Effective radiation dose, time, and contrast medium to measure fractional flow reserve

OBJECTIVES

This study sought to define the additional effective radiation dose, procedural time, and contrast medium needed to obtain fractional flow reserve (FFR) measurements after a diagnostic coronary angiogram.

BACKGROUND

The FFR measurements performed at the end of a diagnostic angiogram allow the obtaining of functional information that complements the anatomic findings.

METHODS

In 200 patients (mean age 66 +/- 10 years) undergoing diagnostic coronary angiography, FFR was measured in at least 1 intermediate coronary artery stenosis. Hyperemia was achieved by intracoronary (n = 180) or intravenous (n = 20) adenosine. The radiation dose (mSv), procedural time (min), and contrast medium (ml) needed for diagnostic angiography and FFR were recorded.

RESULTS

A total of 296 stenoses (1.5 +/- 0.7 stenoses per patient) were assessed. The additional mean radiation dose, procedural time, and contrast medium needed to obtain FFR expressed as a percentage of the entire procedure were 30 +/- 16% (median 4 mSv, range 2.4 to 6.7 mSv), 26 +/- 13% (median 9 min, range 7 to 13 min), and 31 +/- 16% (median 50 ml, range 30 to 90 ml), respectively. The radiation dose and contrast medium during FFR were similar after intravenous and intracoronary adenosine, though the procedural time was slightly longer with intravenous adenosine (median 11 min, range 10 to 17 min, p = 0.04) than with intracoronary adenosine (median 9 min, range 7 to 13 min). When FFR was measured in 3 or more lesions, radiation dose, procedural time, and contrast medium increased.

CONCLUSIONS

The additional radiation dose, procedural time, and contrast medium to obtain FFR measurement are low as compared to other cardiovascular imaging modalities. Therefore, the combination of diagnostic angiography and FFR measurements is warranted to provide simultaneously anatomic and functional information in patients with coronary artery disease.

Usefulness of the fractional flow reserve derived by intracoronary pressure wire for evaluating angiographically intermediate lesions in acute coronary syndrome

INTRODUCTION AND OBJECTIVES

In contrast to findings in stable ischemic heart disease, in acute coronary syndrome (ACS), measurement of the fractional flow reserve (FFR) using an intracoronary pressure wire has not been shown to be useful for evaluating angiographically equivocal coronary lesions. The aim of this study was to analyze outcomes at 1 year in ACS patients with lesions that were classed as intermediate on coronary angiography and which were not nonrevascularized because of the FFR value determined by intracoronary pressure wire.

METHODS

The observational study involved a cohort of patients admitted for ACS who had intermediate lesions on coronary angiography that were not revascularized because the FFR was >0.75. Functional studies were not carried out if there was angiographic evidence of instability. All-cause mortality, non-fatal myocardial infarction, revascularization of the target lesion and readmission for cardiac causes in the first year of the study were recorded.

RESULTS

The study included 106 patients with 127 lesions that were not revascularized because the FFR was >0.75. Their mean age was 69.9+/-10 years, 92 (86.8%) had non-ST-elevation ACS, the mean angiographic stenosis was 40.5+/-7.8%, and the mean FFR was 0.88+/-0.06. There were no complications during the procedure. The follow-up rate at 1 year was 95.1%. Events observed at 1 year were: 2 deaths (total mortality 1.9%), 0 fatal acute myocardial infarctions, 1 (0.9%) target lesion revascularization and 5 (4.7%) readmissions for cardiac causes.

CONCLUSIONS

Once lesions with clear angiographic signs of instability are excluded, intracoronary pressure wire measurement could be useful in ACS patients for avoiding unnecessary revascularization of angiographically intermediate coronary lesions.

Assessment of the effect of external counterpulsation on myocardial adaptive arteriogenesis by invasive functional measurements--design of the arteriogenesis network trial 2

BACKGROUND

Stimulation of collateral artery growth is a promising therapeutic option for patients with coronary artery disease. External counterpulsation is a non-invasive technique suggested to promote the growth of myocardial collateral arteries via increase of shear stress. The Art.Net.2 Trial tests invasively and functionally for the first time the hypothesis whether a treatment course with external counterpulsation (over 7 weeks) can induce the growth of myocardial collateral arteries.

METHODS

This study is designed as a prospective, controlled, proof-of-concept study. Inclusion criteria are (1) age 40 to 80 years, (2) stable coronary disease, (3) a residual significant stenosis of at least one epicardial artery and (4) a positive ischemic stress-test for the region of interest. As primary endpoint serves the pressure-derived collateral flow index (CFIp), the invasive gold-standard to assess myocardial collateral pathways. CFIp is determined by simultaneous measurement of mean aortic pressure (Pa, mm Hg), distal coronary occlusive (wedge) pressure (Pw, mm Hg) and central venous pressure (Pv, mm Hg). The index is calculated as CFIp=(Pw-Pv)/(Pa-Pv). The pressure derived fractional flow reserve (FFR) and the index of microcirculatory resistance (IMR) are assessed as secondary invasive endpoints to investigate the effect of ECP on the myocardial vasculature. The non-invasive secondary endpoints include symptoms (CCS and NYHA classification), treadmill-testing and analysis of shear-stress related soluble proteins.

CONCLUSIONS

The Art.Net.-2 Trial will report within the next months whether direct evidence can be brought that ECP promotes coronary collateral growth in patients with stable angina pectoris.