Intracoronary pressure and flow velocity for hemodynamic evaluation of coronary stenoses

Pre-angioplasty instantaneous wave-free ratio pullback predicts hemodynamic outcome in diffuse coronary artery disease

Background: Serial stenoses or diffuse vessel narrowing hamper pressure wire–guided management of coronary stenoses. Characterization of functional relevance of individual stenoses or narrowed segments constitutes an unmet need in ischemia-driven percutaneous revascularization. Aim of the Study: To perform hemodynamic mapping of the entire vessel using pullback technique of a pressure guidewire with continuous instantaneous wave-free ratio (iFR) measurement compared coronary angiography aiming to minimize the procedure, decrease number and length of stents used. Materials and Methods: This study was conducted on 40 patients presented with diffuse coronary artery disease and undergoing elective PCI. Diagnostic coronary angiography using the routine angiographic projections was done with assessment of non-obstructive coronary lesions by 2D quantitative coronary angiography and iFR pullback measurement Results: Percentage of difference between probable sig lesions via pullback technique and No of actual sig lesions for studied group, was (59.5%). The difference was statistically highly significant p=0.0001. Conclusion: Compared with angiography alone, availability of iFR pullback data significantly decreased the number and length of hemodynamically significant lesions identified for revascularization.

The role of coronary collaterals in chronic total occlusions

A chronic total occlusion (CTO) describes a completely occluded coronary artery. This type of lesion is found in about 18% of all significant lesions in patients with coronary artery disease. A system of collateral connections are observed in almost all of these lesions, which have the capacity to prevent myocardial necrosis and may even uphold metabolic supply to the territory distal to an occlusion to maintain full contractile capacity. During exercise these collaterals are limited in their functional reserve, and more than 90% of patients with a well collateralized occlusion will experience ischemia. in the absence of ideal animal models that mimic the human collateral circulation, we need to rely on studies in man. The knowledge of collateral physiology in man has increased considerably over the past two decades with the advent of intracoronary sensors of coronary pressure and flow velocity. A number of basic physiologic questions have been answered by these studies. The blood supply through coronary arteries depends on a complex array of in general serial resistance systems, with an additional array of multiple parallel resistances on the collateral level. There seems to be a great interindividual variability in the ability to recruit preformed collateral connections in the case of an epicardial occlusion. Collateral function can develop to a similar functional level in patients post myocardial infarction with large akinetic territories as it does in patients with normal preserved regional function. The presence of viability is thus not a prerequisite for collateral development. The question of practical relevance in the era of interventional therapy of chronic occlusions is also, whether a patient with coronary artery disease will remain protected by collaterals after removing the obstruction in the collateralized artery, or whether collaterals regress and lose their functional capacity. Both developments are observed again mainly depending of individual predisposition.

The Instantaneous wave-Free Ratio (iFR) pullback: a novel innovation using baseline physiology to optimise coronary angioplasty in tandem lesions

Coronary intervention is increasingly performed in complex disease with tandem and diffuse disease. Pressure wires enable detailed assessment of the physiological significance of a stenosis but in the presence of tandem disease, predicting the impact of a stenting a given stenosis can be difficult and is impeded by flow interaction between stenoses under hyperemia. In this review, we consider the physiological difficulties posed by flow interaction under hyperemia and consider alternative approaches such as assessment under baseline conditions. Specifically we consider the potential value of the iFR-Pullback approach and its capacity to enable Virtual-PCI, which may assist in planning intervention.

Diagnostic classification of the instantaneous wave-free ratio is equivalent to fractional flow reserve and is not improved with adenosine administration. Results of CLARIFY (Classification Accuracy of Pressure-Only Ratios Against Indices Using Flow Study)

OBJECTIVES

This study sought to determine if adenosine administration is required for the pressure-only assessment of coronary stenoses.

BACKGROUND

The instantaneous wave-free ratio (iFR) is a vasodilator-free pressure-only measure of the hemodynamic severity of a coronary stenosis comparable to fractional flow reserve (FFR) in diagnostic categorization. In this study, we used hyperemic stenosis resistance (HSR), a combined pressure-and-flow index, as an arbiter to determine when iFR and FFR disagree which index is most representative of the hemodynamic significance of the stenosis. We then test whether administering adenosine significantly improves diagnostic performance of iFR.

METHODS

In 51 vessels, intracoronary pressure and flow velocity was measured distal to the stenosis at rest and during adenosine-mediated hyperemia. The iFR (at rest and during adenosine administration [iFRa]), FFR, HSR, baseline, and hyperemic microvascular resistance were calculated using automated algorithms.

RESULTS

When iFR and FFR disagreed (4 cases, or 7.7% of the study population), HSR agreed with iFR in 50% of cases and with FFR in 50% of cases. Differences in magnitude of microvascular resistance did not influence diagnostic categorization; iFR, iFRa, and FFR had equally good diagnostic agreement with HSR (receiver-operating characteristic area under the curve 0.93 iFR vs. 0.94 iFRa and 0.96 FFR, p = 0.48).

CONCLUSIONS

iFR and FFR had equivalent agreement with classification of coronary stenosis severity by HSR. Further reduction in resistance by the administration of adenosine did not improve diagnostic categorization, indicating that iFR can be used as an adenosine-free alternative to FFR.

Diagnostic accuracy of combined intracoronary pressure and flow velocity information during baseline conditions: adenosine-free assessment of functional coronary lesion severity

BACKGROUND

The assessment of functional coronary lesion severity using intracoronary physiological parameters such as coronary flow velocity reserve and the more widely used fractional flow reserve relies critically on the establishment of maximal hyperemia. We evaluated the diagnostic accuracy of the stenosis resistance index during nonhyperemic conditions, baseline stenosis resistance index, compared with established hyperemic intracoronary hemodynamic parameters, because achievement of hyperemia can be cumbersome in daily clinical practice.

METHODS AND RESULTS

A total of 232 patients, including 299 lesions (mean stenosis diameter 55%±11%), underwent myocardial perfusion scintigraphy for documentation of reversible perfusion defects. Distal coronary pressure and flow velocity were assessed with sensor-equipped guidewires during baseline and maximal hyperemia, induced by an intracoronary bolus of adenosine (20-40 µg). We determined stenosis resistance (SR) during baseline and hyperemic conditions as well as fractional flow reserve and coronary flow velocity reserve. The discriminative value for myocardial ischemia on myocardial perfusion scintigraphy of all parameters was compared using receiver-operating-characteristic curves. Baseline SR showed good agreement with myocardial perfusion scintigraphy. The diagnostic performance of baseline SR (area under the curve, 0.77; 95% CI, 0.71-0.83) was as accurate as fractional flow reserve and coronary flow velocity reserve (area under the curve, 0.77; 95% CI, 0.71-0.83 and area under the curve, 0.75; 95% CI, 0.68-0.81 respectively; P>0.05 compared with baseline SR for both). However, hyperemic SR, combining both pressure and flow velocity information during hyperemia, was superior to all other parameters (area under the curve, 0.81; 95% CI, 0.76-0.87; P<0.05 compared with all other parameters).

CONCLUSIONS

Combined pressure and flow velocity measurements during baseline conditions may provide a useful tool for functional lesion severity assessment without the need for potent vasodilators.

Development and validation of a new adenosine-independent index of stenosis severity from coronary wave-intensity analysis: results of the ADVISE (ADenosine Vasodilator Independent Stenosis Evaluation) study

OBJECTIVES

The purpose of this study was to develop an adenosine-independent, pressure-derived index of coronary stenosis severity.

BACKGROUND

Assessment of stenosis severity with fractional flow reserve (FFR) requires that coronary resistance is stable and minimized. This is usually achieved by administration of pharmacological agents such as adenosine. In this 2-part study, we determine whether there is a time when resistance is naturally minimized at rest and assess the diagnostic efficiency, compared with FFR, of a new pressure-derived adenosine-free index of stenosis severity over that time.

METHODS

A total of 157 stenoses were assessed. In part 1 (39 stenoses), intracoronary pressure and flow velocity were measured distal to the stenosis; in part 2 (118 stenoses), intracoronary pressure alone was measured. Measurements were made at baseline and under pharmacologic vasodilation with adenosine.

RESULTS

Wave-intensity analysis identified a wave-free period in which intracoronary resistance at rest is similar in variability and magnitude (coefficient of variation: 0.08 ± 0.06 and 284 ± 147 mm Hg s/m) to those during FFR (coefficient of variation: 0.08 ± 0.06 and 302 ± 315 mm Hg s/m; p = NS for both). The resting distal-to-proximal pressure ratio during this period, the instantaneous wave-free ratio (iFR), correlated closely with FFR (r = 0.9, p < 0.001) with excellent diagnostic efficiency (receiver-operating characteristic area under the curve of 93%, at FFR <0.8), specificity, sensitivity, negative and positive predictive values of 91%, 85%, 85%, and 91%, respectively.

CONCLUSIONS

Intracoronary resistance is naturally constant and minimized during the wave-free period. The instantaneous wave-free ratio calculated over this period produces a drug-free index of stenosis severity comparable to FFR. (Vasodilator Free Measure of Fractional Flow Reserve [ADVISE]; NCT01118481).

Functional and anatomical diagnosis of coronary artery stenoses

BACKGROUND

Functional/physiological evaluation of coronary artery stenoses may be more important than anatomical measurements of severity. Optimization of thresholds for stenosis intervention and treatment endpoints depend on coupling functional hemodynamic and anatomical data. We sought to develop a single prognostic parameter correlating stenosis-specific anatomy, pressure gradient, and velocities that could be measured during catheterization.

MATERIALS AND METHODS

In vivo Experiments were performed in six swine (41 +/- 3 kg). The lumen area of the left anterior descending coronary artery was measured with intravascular ultrasound. An angioplasty balloon was inflated to create the desired intraluminal area obstructions. Fractional flow reserve (FFR), coronary flow reserve (CFR), and hyperemic-stenosis-resistance index were measured distal to the balloon at peak hyperemia with 10 mg intracoronary papaverine. A functional index:pressure drop coefficient (CDP) and a combined functional and anatomical index:lesion flow coefficient (LFC) were calculated from measured hyperemic pressure gradient, velocity, and percentage area stenosis. P < 0.05 was considered statistically significant.

RESULTS

The CDP and LFC correlated linearly and significantly with FFR and CFR. The CDP (R(2) = 0.72, P < 0.0001) correlated better than LFC (R(2) = 0.19, P < 0.003) with hyperemic-stenosis-resistance index. When LFC was correlated simultaneously with FFR and CFR, R(2) improved to 0.82 (P < 0.0001). Inclusion of percentage area stenoses concurrently with FFR and CFR marginally improved the correlation with LFC.

CONCLUSIONS

A dimensionless parameter combining measured pressure gradient, velocity, and area reduction data can optimally define the severity of coronary stenoses based on our preliminary results and could prove useful clinically.

Effects of left ventricular unloading by Impella recover LP2.5 on coronary hemodynamics

OBJECTIVES

We studied the effects of LV unloading by the Impella on coronary hemodynamics by simultaneously measuring intracoronary pressure and flow and the derived parameters fractional flow reserve (FFR), coronary flow velocity reserve (CFVR), and coronary microvascular resistance (MR).

BACKGROUND

Patients with compromised left ventricular (LV) function undergoing high-risk percutaneous coronary intervention (PCI) may benefit from LV unloading. Limited information is available on the effects of LV unloading on coronary hemodynamics.

METHODS

Eleven patients (mean LV ejection fraction of 35 +/- 11%) underwent PCI during LV support by the LV unloading device (Impella Recover LP2.5). Intracoronary measurements were performed in a nonstenotic coronary artery after the PCI, before and after adenosine-induced hyperemia at four different support levels (0-2.5 L/min).

RESULTS

Aortic and coronary pressure increased with increasing support levels, whereas FFR remained unchanged. Baseline flow velocity remained unchanged, while hyperemic flow velocity and CFVR increased significantly with increasing support levels (61 +/- 24 to 72 +/- 27 cm/sec, P = 0.001 and 1.88 +/- 0.52 to 2.34 +/- 0.63, P < 0.001 respectively). The difference between baseline MR and hyperemic MR significantly increased with increasing support levels (1.28 +/- 1.32 to 1.89 +/- 1.43 mm Hg cm(-1) sec, P = 0.005).

CONCLUSIONS

Unloading of the LV by the Impella increased aortic and intracoronary pressure, hyperemic flow velocity and CFVR, and decreased MR. The Impella-induced increase in coronary flow, probably results from both an increased perfusion pressure and a decreased LV volume-related intramyocardial resistance.

Characterizing momentum change and viscous loss of a hemodynamic endpoint in assessment of coronary lesions

Myocardial fractional flow reserve (FFR(myo)) and coronary flow reserve (CFR), measured with guidewire, and quantitative angiography (QA) are widely used in combination to distinguish ischemic from non-ischemic coronary stenoses. Recent studies have shown that simultaneous measurements of FFR(myo) and CFR are recommended to dissociate conduit epicardial coronary stenoses from distal resistance microvascular disease. In this study, a more comprehensive diagnostic parameter, named as lesion flow coefficient, c, is proposed. The coefficient, c, which accounts for mean pressure drop, Delta p, mean coronary flow, Q, and percentage area stenosis, can be used to assess the hemodynamic severity of a coronary artery stenoses. Importantly, the contribution of viscous loss and loss due to momentum change for several lesion sizes can be distinguished using c. FFR(myo), CFR and c were calculated for pre-angioplasty, intermediate and post-angioplasty epicardial lesions, without microvascular disease. While hyperemic c decreased from 0.65 for pre-angioplasty to 0.48 for post-angioplasty lesion with guidewire of size 0.35 mm, FFR(myo) increased from 0.52 to 0.87, and CFR increased from 1.72 to 3.45, respectively. Thus, reduced loss produced by momentum change due to lower percentage area stenosis decreased c. For post-angioplasty lesion, c decreased from 0.55 to 0.48 with the insertion of guidewire. Hence, increased viscous loss due to the presence of guidewire decreased c compared with a lesion without guidewire. Further, c showed a linear relationship with FFR(myo), CFR and percentage area stenosis for pre-angioplasty, intermediate and post-angioplasty lesion. These baseline values of c were developed from fluid dynamics fundamentals for focal lesions, and provided a single hemodynamic endpoint to evaluate coronary stenosis severity.

Physiological basis of clinically used coronary hemodynamic indices

In deriving clinically used hemodynamic indices such as fractional flow reserve and coronary flow velocity reserve, simplified models of the coronary circulation are used. In particular, myocardial resistance is assumed to be independent of factors such as heart contraction and driving pressure. These simplifying assumptions are not always justified. In this review we focus on distensibility of resistance vessels, the shape of coronary pressure-flow lines, and the influence of collateral flow on these lines. We show that (1) the coronary system is intrinsically nonlinear because resistance vessels at maximal vasodilation change diameter with pressure and cardiac function; (2) the assumption of collateral flow is not needed to explain the difference between pressure-derived and flow-derived fractional flow reserve; and (3) collateral flow plays a role only at low distal pressures. We conclude that traditional hemodynamic indices are valuable for clinical decision making but that clinical studies of coronary physiology will benefit greatly from combined measurements of coronary flow or velocity and pressure.

Correlation of Hemodynamic Events with Clinical and Pathological Observations

The correlation of hemodynamic events with clinical or pathological observations is represented by a variety of applications reflecting the broad range of this theme. The position paper describes several cases in which benefits of combining imaging information with transport models of contrast material, can cause a gain in hemodynamic information. What appears to be lack of cohesiveness among the cases illustrates the variety in the application of hemodynamic research to the practice of medicine. Some of the contributions presented at the symposium do not directly apply to clinical medicine, but instead described mathematical models or applications to animal physiology or technical advancements in measuring blood rheology. Although related to this theme topic they fall somewhat outside the main scope. The topics summarized below demonstrate four examples in which translation of the research to the clinical arena can be realized in a short period of time. Overall recommendations for priority objectives related to this topic are provided at the end of this position paper.