The diastolic flow-pressure gradient relation in coronary stenoses in humans

A spatiotemporal analysis of the left coronary artery biomechanics using fluid-structure interaction models

Biomechanics plays a critical role in coronary artery disease development. FSI simulation is commonly used to understand the hemodynamics and mechanical environment associated with atherosclerosis pathology. To provide a comprehensive characterization of patient-specific coronary biomechanics, an analysis of FSI simulation in the spatial and temporal domains was performed. In the current study, a three-dimensional FSI model of the LAD coronary artery was built based on a patient-specific geometry using COMSOL Multiphysics. The effect of myocardial bridging was simulated. Wall shear stress and its derivatives including time-averaged wall shear stress, wall shear stress gradient, and OSI were calculated across the cardiac cycle in multiple locations. Arterial wall strain (radial, circumferential, and longitudinal) and von Mises stress were calculated. To assess perfusion, vFFR was calculated. The results demonstrated the FSI model could identify regional and transient differences in biomechanical parameters within the coronary artery. The addition of myocardial bridging caused a notable change in von Mises stress and an increase in arterial strain during systole. The analysis performed in this manner takes greater advantage of the information provided in the space and time domains and can potentially assist clinical evaluation.

Simultaneous evaluation of plaque stability and ischemic potential of coronary lesions in a fluid-structure interaction analysis

The measurement of fractional flow reserve (FFR) and superficial wall stress (SWS) identifies inducible myocardial ischemia and plaque vulnerability, respectively. A simultaneous evaluation of both FFR and SWS is still lacking, while it may have a major impact on therapy. A new computational model of one-way fluid-structure interaction (FSI) was implemented and used to perform a total of 54 analyses in virtual coronary lesion models, based on plaque compositions, arterial remodeling patterns, and stenosis morphologies under physiological conditions. Due to a greater lumen dilation and more induced strain, FFR in the lipid-rich lesions (0.81 ± 0.15) was higher than that in fibrous lesions (0.79 ± 0.16, P = 0.001) and calcified lesions (0.79 ± 0.16, P = 0.001). Four types of lesions were further defined, based on the combination of cutoff values for FFR (0.80) and maximum relative SWS (30 kPa): The level of risk increased from (1) plaques with mild-to-moderate stenosis but negative remodeling for lipid-rich (Type A: non-ischemic, stable) to (2) lipid-rich plaques with mild-to-moderate stenosis and without-to-positive remodeling (Type B: non-ischemic, unstable) or plaques with severe stenosis but negative remodeling for lipid-rich (Type C: ischemic, stable) to (3) lipid-rich plaques with severe stenosis and without-to-positive remodeling (Type D: ischemic, unstable). The analysis of FSI to simultaneously evaluate inducible myocardial ischemia and plaque stability may be useful to identify coronary lesions at a high risk and to ultimately optimize treatment. Further research is warranted to assess whether a more aggressive treatment may improve the prognosis of patients with non-ischemic, intermediate, and unstable lesions.

Discordance between pressure drift after wire pullback and intracoronary distal pressure offset affects stenosis physiology appraisal

BACKGROUND

Drift is a well-known issue affecting intracoronary pressure measurements. A small pressure offset at the end of the procedure is generally considered acceptable, while repeat assessment is advised for drift exceeding ±2 mmHg. This practice implies that drift assessed after wire pullback equals that at the time of stenosis appraisal, but this assumption has not been systematically investigated. Our aim was to compare intra-and post-procedural pressure sensor drift and assess benefits of correction for intra-procedural drift and its effect on diagnostic classification.

METHODS

In 70 patients we compared intra- and post-procedural pressure drift for 120 hemodynamic tracings obtained at baseline and throughout the hyperemic response to intracoronary adenosine. Intra-procedural drift was derived from the intercept of the stenosis pressure gradient-velocity relationship. Diagnostic reclassification after correction for intra-procedural drift was assessed for the mean distal-to-aortic pressure ratio at baseline (Pd/Pa) and hyperemia (fractional flow reserve, FFR), and corresponding stenosis resistances.

RESULTS

Post- and intra-procedural drift exceeding the tolerated threshold was observed in 73% and 64% of the hemodynamic tracings, respectively. Discordance in terms of acceptable drift level was present for 42% of the tracings, with avoidable repeat physiological assessment in 25% and unacceptable intra-procedural drift unrecognized at final drift check in 17% of the tracings. Correction for intra-procedural drift caused higher reclassification rates for baseline than hyperemic functional indices.

CONCLUSIONS

Post-procedural pressure drift frequently does not match drift during physiological assessment. Tracing-specific correction for intra-procedural drift can potentially lower the risk of inadvertent diagnostic misclassification and prevent unnecessary repeats.

Numerical analysis of the impact of flow rate, heart rate, vessel geometry, and degree of stenosis on coronary hemodynamic indices

Background

The stenosis of the coronary arteries is usually caused by atherosclerosis. Hemodynamic significance of patient-specific coronary stenoses and the risk of its progression may be assessed by comparing the hemodynamic effects induced by flow disorders. The present study shows how stenosis degree and variable flow conditions in coronary artery affect the oscillating shear index, residence time index, pressure drop coefficient and fractional flow reserve. We assume that changes in the hemodynamic indices in relation to variable flow conditions and geometries evaluated using the computational fluid dynamics may be an additional factor for a non-invasive assessment of the coronary stenosis detected on multi-slice computed tomography.

Methods

The local-parametrised models of basic shapes of the vessels, such as straight section, bend, and bifurcation as well as the global-patient-specific models of left coronary artery were used for numerical simulation of flow in virtually reconstructed stenotic vessels. Calculations were carried out for vessels both without stenosis, and vessels of 10 to 95% stenosis. The flow rate varied within the range of 20 to 1000 ml/min, and heart rate frequency within the range of 30 to 210 cycles/min.

The computational fluid dynamics based on the finite elements method verified by the experimental measurements of the velocity profiles was used to analyse blood flow in the coronary arteries.

Results

The results confirm our preliminary assumptions. There is significant variation in the coronary hemodynamic indices value caused by disturbed flow through stenosis in relation to variable flow conditions and geometry of vessels.

Conclusion

Variations of selected hemodynamic indexes induced by change of flow rate, heart rate and vessel geometry, obtained during a non-invasive study, may assist in evaluating the risk of stenosis progression and in carrying out the assessment of the hemodynamic significance of coronary stenosis. However, for a more accurate assessment of the variability of indices and coronary stenosis severity both local (near the narrowing) and global (in side branches) studies should be used.

Effect of distal thickening and stiffening of plaque cap on arterial wall mechanics

To investigate the effect of longitudinal variations of cap thickness and tissue properties on wall stresses and strains along the atherosclerotic stenosis, stenotic plaque models (uniformly thick, distally thickened, homogenous, and distally stiffened) were constructed and subjected to computational stress analyses with due consideration of fluid-structure interactions (FSI). The analysis considered three different cap thicknesses-45, 65, and 200 μm-and tissue properties-soft, fibrous, and hard. The maximum peak cap stress (PCS) and strain were observed in the upstream throat section and demonstrated increases of the order of 345 and 190%, respectively, as the cap thickness was reduced from 200 to 45 μm in uniformly thick models. Distal stiffening increased PCS in the downstream region; however, the overall effect of this increase was rather small. Distal thickening did not affect maximum PCS and strain values for cap thicknesses exceeding 65 μm; however, a noticeable increase in maximum PCS and corresponding longitudinal variation (or spatial gradient) in stress was observed in the very thin (45-μm-thick) cap. It was, therefore, inferred that existence of a rather thin upstream cap demonstrating distal cap thickening indicates an increased risk of plaque progression and rupture. Graphical Abstract ᅟ.

Physiology of Angina and Its Alleviation With Nitroglycerin: Insights From Invasive Catheter Laboratory Measurements During Exercise

BACKGROUND

The mechanisms governing exercise-induced angina and its alleviation by the most commonly used antianginal drug, nitroglycerin, are incompletely understood. The purpose of this study was to develop a method by which the effects of antianginal drugs could be evaluated invasively during physiological exercise to gain further understanding of the clinical impact of angina and nitroglycerin.

METHODS

Forty patients (mean age, 65.2±7.6 years) with exertional angina and coronary artery disease underwent cardiac catheterization via radial access and performed incremental exercise using a supine cycle ergometer. As they developed limiting angina, sublingual nitroglycerin was administered to half the patients, and all patients continued to exercise for 2 minutes at the same workload. Throughout exercise, distal coronary pressure and flow velocity and central aortic pressure were recorded with sensor wires.

RESULTS

Patients continued to exercise after nitroglycerin administration with less ST-segment depression (P=0.003) and therefore myocardial ischemia. Significant reductions in afterload (aortic pressure, P=0.030) and myocardial oxygen demand were seen (tension-time index, P=0.024; rate-pressure product, P=0.046), as well as an increase in myocardial oxygen supply (Buckberg index, P=0.017). Exercise reduced peripheral arterial wave reflection (P<0.05), which was not further augmented by the administration of nitroglycerin (P=0.648). The observed increases in coronary pressure gradient, stenosis resistance, and flow velocity did not reach statistical significance; however, the diastolic velocity-pressure gradient relation was consistent with a significant increase in relative stenosis severity (k coefficient, P<0.0001), in keeping with exercise-induced vasoconstriction of stenosed epicardial segments and dilatation of normal segments, with trends toward reversal with nitroglycerin.

CONCLUSIONS

The catheterization laboratory protocol provides a model to study myocardial ischemia and the actions of novel and established antianginal drugs. Administration of nitroglycerin causes changes in the systemic and coronary circulation that combine to reduce myocardial oxygen demand and to increase supply, thereby attenuating exercise-induced ischemia. Designing antianginal therapies that exploit these mechanisms may provide new therapeutic strategies.

Temporal dissociation between the minimal distal-to-aortic pressure ratio and peak hyperemia during intravenous adenosine infusion

The present study sought to compare the temporal relation between maximal coronary flow (peak hyperemia) and minimal coronary-to-aortic pressure ratio (Pd/Pa) for intracoronary (IC) and intravenous (IV) adenosine administration. Peak hyperemia is assumed to coincide with the minimal Pd/Pa value. However, this has not been confirmed for systemic hemodynamic variations during IV adenosine infusion. Hemodynamic responses to IV and IC adenosine administration were obtained in 12 patients (14 lesions) using combined IC pressure and flow velocity measurements. A fluid dynamic model was used to predict the change in Pd/Pa for different stenosis severities and varying Pa. Hemodynamic variability during IV adenosine hyperemia was greater than during IC adenosine, as assessed by the coefficient of variation. During IV adenosine, flow velocity peaked 28 ± 4 (SE) s after the onset of hyperemia, while Pd/Pa reached a minimum (0.82 ± 0.01) 22 ± 7 s later ( P < 0.05), when Pa had declined by 6.1% and hyperemic velocity by 4.5% ( P < 0.01). Model outcomes corroborated the role of variable Pa in this dissociation. In contrast, maximal flow and minimal Pd/Pa coincided for IC adenosine, with IV-equivalent peak velocities and a higher Pd/Pa ratio (0.86 ± 0.01, P < 0.01). Hemodynamic variability during continuous IV adenosine infusion can lead to temporal dissociation of minimal Pd/Pa and peak hyperemia, in contrast to IC adenosine injection, where maximal velocity and minimal Pd/Pa coincide. Despite this variability, stenosis hemodynamics remained stable with both ways of adenosine administration. Our findings suggest advantages of IC over IV adenosine to identify maximal hyperemia from pressure-only measurements.

NEW & NOTEWORTHY Systemic hemodynamic variability during intravenous adenosine infusion produces substantial temporal dissociation between peak hyperemia appraised by coronary flow velocity and the minimal distal-to-aortic pressure ratio commonly used to determine functional stenosis severity. This dissociation was absent for intracoronary adenosine administration and tended to be mitigated in patients receiving Ca2+ antagonists.

Can Resting Indices Obviate the Need for Hyperemia and Promote the Routine Use of Physiologically Guided Revascularization?

This article assesses the data from contemporary human studies to address some of the common assumptions regarding hyperemic and baseline physiology in the context of the baseline pressure-derived index of instant wave-free ratio and the hyperemic index of fractional flow reserve. The article aims to determine if the available evidence supports the continued investigation, development, and use of baseline indices.

The haemodynamic effects of collateral donation to a chronic total occlusion: Implications for patient management

Physiological lesion assessment in the form of Fractional Flow Reserve (FFR) is now well established for the purpose of guiding multi-vessel revascularization. Chronic total coronary occlusions are frequently associated with multi-vessel disease and the collateral dependent myocardium distal to the occlusion is often supplied by a collateral supply from another epicardial coronary artery. The haemodynamic effect of collateral donation upon collateral donor vessel flow may have important implications for the vessel's FFR; rendering it unreliable at predicting ischaemia should the CTO be revascularized. As a consequence, in the setting of multi-vessel disease, optimal revascularization strategy might be altered. There is a paucity of work in the medical literature directly examining this phenomenon. We endeavoured to review the existing literature related to it, to summarise from current knowledge of coronary physiology what is known about the potential effects of CTO revascularization on both collateral flow and collateral donor vessel physiology, and to highlight where further studies might inform practice.

Flow restoration post revascularization predicted by stenosis indexes: sensitivity to hemodynamic variability

The expected blood flow improvement following a coronary intervention is inversely related to the stenotic-to-normal flow ratio Qs/Qn. Since Qn cannot be measured prior to intervention, treatment decisions rely on stenosis-severity indexes, e.g., area stenosis (%AS), hyperemic stenosis resistance (HSR), and fractional flow reserve (FFR), where treatment cut-off levels have been established for each index based on presence of inducible ischemia. Here, we studied the dependence of these indexes-predicted Qs/Qn under physiological perturbations of stenosis features and of hemodynamic and mechanical conditions. Dynamic coronary flow was simulated based on measured coronary morphometric data and a physics-based computational model. Simulations were used to evaluate the relationship between each index level and Qs/Qn. Under each perturbation, an independence measure (IM) was calculated for each index based on the relative change in Qs/Qn associated with each perturbation. The results show that while %AS prediction of Qs/Qn is largely independent (IM > 90%) of physiological changes in heart rate, venous pressure, and lesion length and location on the epicardial tree, HSR is also independent of changes in left ventricle pressure. FFR-predicted Qs/Qn is also independent of changes in aortic pressure, blood hematocrit, and stenotic vessel stiffness. Nevertheless, independence of all indexes is substantially compromised (IM < 70%) under changes in vasculature stiffness. Specifically, a physiological stiffening elevates Qs/Qn value by 21% at the FFR cut-off value (0.75). These findings suggest that the current FFR cut-off value for treatment of stenotic lesions overestimates the benefit of coronary intervention in patients with a stiffer coronary vasculature (e.g., diabetics, hypertensives).

Is discordance of coronary flow reserve and fractional flow reserve due to methodology or clinically relevant coronary pathophysiology?

OBJECTIVES

The purpose of this study was to determine whether observed discordance between coronary flow reserve (CFR) and fractional flow reserve (FFR) is due to methodology or reflects basic coronary pathophysiology.

BACKGROUND

Despite the clinical importance of coronary physiological assessment, relationships between its 2 most common tools, CFR and FFR, remain poorly defined.

METHODS

The worst CFR and stress relative uptake were recorded from 1,500 sequential cardiac positron emission tomography cases from our center. From the literature, we assembled all combined, invasive CFR-FFR measurements, including a subset before and after angioplasty. Both datasets were compared with a fluid dynamic model of the coronary circulation predicting relationships between CFR and FFR for variable diffuse and focal narrowing.

RESULTS

A modest but significant linear relationship exists between CFR and FFR both invasively (r = 0.34, p < 0.001) and using positron emission tomography (r = 0.36, p < 0.001). Most clinical patients undergoing CFR or FFR measurements have diffusely reduced CFR consistent with diffuse atherosclerosis or small-vessel disease. The theoretical model predicts linear relationships between CFR and FFR for progressive stenosis with slopes dependent on diffuse narrowing, matching observed data. Reported changes in CFR and FFR with angioplasty agree with model predictions of removing focal stenosis but leaving diffuse disease. Although CFR-FFR concordance is common, discordance is due to dominant or absent diffuse versus focal disease, reflecting basic pathophysiology.

CONCLUSIONS

CFR is linearly related to FFR for progressive stenosis superimposed on diffuse narrowing. The relative contributions of focal and diffuse disease define the slope and values along the linear CFR and FFR relationship. Discordant CFR and FFR values reflect divergent extremes of focal and diffuse disease, not failure of either tool. With such discordance observed by invasive and noninvasive techniques and also fitting fluid dynamic predictions, it reflects clinically relevant basic coronary pathophysiology, not methodology.

Microcalcifications increase coronary vulnerable plaque rupture potential: a patient-based micro-CT fluid-structure interaction study

Asymptomatic vulnerable plaques (VP) in coronary arteries accounts for significant level of morbidity. Their main risk is associated with their rupture which may prompt fatal heart attacks and strokes. The role of microcalcifications (micro-Ca), embedded in the VP fibrous cap, in the plaque rupture mechanics has been recently established. However, their diminutive size offers a major challenge for studying the VP rupture biomechanics on a patient specific basis. In this study, a highly detailed model was reconstructed from a post-mortem coronary specimen of a patient with observed VP, using high resolution micro-CT which captured the microcalcifications embedded in the fibrous cap. Fluid-structure interaction (FSI) simulations were conducted in the reconstructed model to examine the combined effects of micro-Ca, flow phase lag and plaque material properties on plaque burden and vulnerability. This dynamic fibrous cap stress mapping elucidates the contribution of micro-Ca and flow phase lag VP vulnerability independently. Micro-Ca embedded in the fibrous cap produced increased stresses predicted by previously published analytical model, and corroborated our previous studies. The 'micro-CT to FSI' methodology may offer better diagnostic tools for clinicians, while reducing morbidity and mortality rates for patients with vulnerable plaques and ameliorating the ensuing healthcare costs.

Effect of guidewire on contribution of loss due to momentum change and viscous loss to the translesional pressure drop across coronary artery stenosis: an analytical approach

Background

Guidewire (GW) size and stenosis dimensions are the two major factors affecting the translesional pressure drop. Studying the combined effect of these parameters on the mean pressure drop (Δp) across the stenosis is of high practical importance.

Methods

In this study, time averaged mass and momentum conservation equations are solved analytically to obtain pressure drop-flow, Δp-Q, curves for three different percentage area blockages corresponding to moderate (64%), intermediate (80%), and severe (90%) stenoses. Stenosis is considered to be axisymmetric consisting of three different sections namely converging, throat, and diverging regions. Analytical expressions for pressure drop are obtained for each of these regions separately. Using this approach, effects of lesion length and GW insertion on the mean translesional pressure drop and its component (loss due to momentum change and viscous loss) are analyzed.

Results and Conclusion

It is observed that for a given percent area stenosis (AS), increase in the throat length only increases the viscous loss. However, increase in the severity of stenosis and GW insertion increase both loss due to momentum change and viscous loss. GW insertion has greater contribution to the rise in viscous loss (increase by 2.14 and 2.72 times for 64% and 90% AS, respectively) than loss due to momentum change (1.34% increase for 64% AS and 25% decrease for 90% AS). It also alters the hyperemic pressure drop in moderate (48% increase) to intermediate (30% increase) stenoses significantly. However, in severe stenoses GW insertion has a negligible effect (0.5% increase) on hyperemic translesional pressure drop. It is also observed that pressure drop in a severe stenosis is less sensitive to lesion length variation (4% and 14% increase in Δp for without and with GW, respectively) as compared to intermediate (10% and 30% increase in Δp for without and with GW, respectively) and moderate stenoses (22% and 48% increase in Δp for without and with GW, respectively). Based on the contribution of pressure drop components to the total translesional pressure drop, it is found that viscous losses are dominant in moderate stenoses, while in severe stenoses losses due to momentum changes are significant. It is also shown that this simple analytical solution can provide valuable information regarding interpretation of coronary diagnostic parameters such as fractional flow reserve (FFR).

Assessment of coronary flow reserve using single photon emission computed tomography with technetium 99m-labeled tracers

The quantitative assessment of coronary flow reserve (CFR) may be useful for the functional evaluation of coronary artery disease (CAD), allowing judgment of its severity, tracking of disease progression, and evaluation of the anti-ischemic efficacy of therapeutic strategies. Invasive techniques, such as intracoronary Doppler ultrasound and the pressure-derived method, which directly assess CFR velocity and fractional flow reserve, have been used for the evaluation of the physiologic significance of coronary lesions. Considerable progress has been made in the improvement of technologies directed toward the noninvasive quantification of myocardial blood flow and CFR. Positron emission tomography has emerged as an accurate technique to quantify CFR. The absolute measurements obtained with this noninvasive approach have been widely validated. Nevertheless, it has not been applied to routine studies because of its high cost and complexity. On the other hand, technetium 99m-labeled tracers have been largely used for the evaluation of myocardial perfusion with single photon emission computed tomography (SPECT) imaging in patients with suspected or known CAD. Recently, attempts to estimate CFR with SPECT tracers have been made to obtain, with noninvasive methods, data for quantitative functional assessment of CAD. This review analyzes the relative merit and limitations of CFR measurements by cardiac SPECT imaging with Tc-99m-labeled tracers and describes the potential clinical applications of this technique.

Characteristics of the flow velocity-pressure gradient relation in the assessment of stenoses: an in vitro study

OBJECTIVES

The flow velocity-pressure gradient (v-dp) relation is clinically used to assess coronary stenoses. This in vitro study aimed to investigate the ability to determine the impact of each individual stenosis in the setting of two consecutive stenoses, the effect of variable stenosis reference diameters and the impact of one or two wires in a stenosis, on the v-dp relation.

METHODS

The model consisted of a reservoir and different sized tubes and stenoses. Pressure gradient and flow velocity were assessed with a pressure and a Doppler wire. By plotting flow velocity and pressure gradient on an X-Y plot, the v-dp relation was determined.

RESULTS

The v-dp relation of a proximal stenosis was not influenced by a distal stenosis. The diameter of the segment where flow velocity was measured influenced the v-dp relation. This could be corrected by substituting flow velocity with volume flow. The presence of one or two wires in a stenosis made the v-dp relation substantially steeper.

CONCLUSIONS

The v-dp relation can be used to determine the significance of each individual stenosis in arteries with consecutive stenoses, provided that the distance between the stenoses is large enough. The diameter of the segment where flow velocity is measured and the presence of one or two wires substantially affect the v-dp relation. (Neth Heart J 2008;16:156-62.).

Effect of simultaneous intracoronary guidewires on the predictive accuracy of functional parameters of coronary lesion severity

The aim of this study was to assess the influence of a second guidewire on the diagnostic accuracy of functional parameters of coronary lesion severity. Sixty-five patients with intermediate coronary lesions underwent myocardial perfusion scintigraphy. Fractional flow reserve (FFR), coronary flow velocity reserve (CFVR), and hyperemic stenosis resistance (HSR) index (HSR = stenosis pressure gradient ÷ velocity) were determined in 77 lesions. Distal pressure and velocity were acquired simultaneously (dual wire) and sequentially (single wire) with two sensor-equipped guidewires. Overall, functional parameters deteriorated from single- to dual-wire assessment. In patients without ischemia, the good diagnostic performance of FFR, CFVR, and HSR deteriorated significantly ( P < 0.001) when assessed by dual wires, with an increase in the number of false-positive results. This trend was more pronounced for HSR, since the presence of a second wire reduced maximal velocity and increased the pressure gradient. The presence of two guidewires, especially across a myocardial perfusion scintigraphy-induced nonsignificant lesion, is associated with overestimation of the hemodynamically assessed lesion severity and, therefore, is likely to have a major impact on clinical decision making. This underscores the advantage of a dual-sensor-equipped guidewire for the evaluation of stenosis severity by combined pressure and velocity measurements.

The diastolic flow velocity-pressure gradient relation and dpv50 to assess the hemodynamic significance of coronary stenoses

To evaluate the hemodynamic impact of coronary stenoses, the fractional (FFR) or coronary flow velocity reserve (CFVR) usually is measured. The combined measurement of instantaneous flow velocity and pressure gradient (v-dp relation) is rarely used in humans. We derived from the v-dp relation a new index, dpv50 (pressure gradient at flow velocity of 50 cm/s), and compared the diagnostic performance of dpv50, CFVR, and FFR. Before coronary angiography was performed, patients underwent noninvasive stress testing. In all coronary vessels with an intermediate or severe stenosis, the flow velocity, aortic, and distal coronary pressure were measured simultaneously with a Doppler and pressure guidewire after induction of hyperemia. After regression analysis of all middiastolic flow velocity and pressure gradient data, the dpv50 was calculated. With the use of the results of noninvasive stress testing, the dpv50 cutoff value was established at 22.4 mmHg. In 77 patients, 124 coronary vessels with a mean 39% (SD 19) diameter stenosis were analyzed. In 43 stenoses, ischemia was detected. We found a sensitivity, specificity, and accuracy of 56%, 86%, and 76% for CFVR; 77%, 99%, and 91% for FFR; and 95%, 95%, and 95% for dpv50. To establish that dpv50 is not dependent on maximal hyperemia, dpv50 was recalculated after omission of the highest quartile of flow velocity data, showing a difference of 3%. We found that dpv50 provided the highest sensitivity and accuracy compared with FFR and CFVR in the assessment of coronary stenoses. In contrast to CFVR and FFR, assessment of dpv50 is not dependent on maximal hyperemia.

Cross-Talk Between Cardiac Muscle and Coronary Vasculature

The cardiac muscle and the coronary vasculature are in close proximity to each other, and a two-way interaction, called cross-talk, exists. Here we focus on the mechanical aspects of cross-talk including the role of the extracellular matrix. Cardiac muscle affects the coronary vasculature. In diastole, the effect of the cardiac muscle on the coronary vasculature depends on the (changes in) muscle length but appears to be small. In systole, coronary artery inflow is impeded, or even reversed, and venous outflow is augmented. These systolic effects are explained by two mechanisms. The waterfall model and the intramyocardial pump model are based on an intramyocardial pressure, assumed to be proportional to ventricular pressure. They explain the global effects of contraction on coronary flow and the effects of contraction in the layers of the heart wall. The varying elastance model, the muscle shortening and thickening model, and the vascular deformation model are based on direct contact between muscles and vessels. They predict global effects as well as differences on flow in layers and flow heterogeneity due to contraction. The relative contributions of these two mechanisms depend on the wall layer (epi- or endocardial) and type of contraction (isovolumic or shortening). Intramyocardial pressure results from (local) muscle contraction and to what extent the interstitial cavity contracts isovolumically. This explains why small arterioles and venules do not collapse in systole. Coronary vasculature affects the cardiac muscle. In diastole, at physiological ventricular volumes, an increase in coronary perfusion pressure increases ventricular stiffness, but the effect is small. In systole, there are two mechanisms by which coronary perfusion affects cardiac contractility. Increased perfusion pressure increases microvascular volume, thereby opening stretch-activated ion channels, resulting in an increased intracellular Ca2+transient, which is followed by an increase in Ca2+sensitivity and higher muscle contractility (Gregg effect). Thickening of the shortening cardiac muscle takes place at the expense of the vascular volume, which causes build-up of intracellular pressure. The intracellular pressure counteracts the tension generated by the contractile apparatus, leading to lower net force. Therefore, cardiac muscle contraction is augmented when vascular emptying is facilitated. During autoregulation, the microvasculature is protected against volume changes, and the Gregg effect is negligible. However, the effect is present in the right ventricle, as well as in pathological conditions with ineffective autoregulation. The beneficial effect of vascular emptying may be reduced in the presence of a stenosis. Thus cardiac contraction affects vascular diameters thereby reducing coronary inflow and enhancing venous outflow. Emptying of the vasculature, however, enhances muscle contraction. The extracellular matrix exerts its effect mainly on cardiac properties rather than on the cross-talk between cardiac muscle and coronary circulation.

Physiological assessment of coronary artery disease in the cardiac catheterization laboratory: a scientific statement from the American Heart Association Committee on Diagnostic and Interventional Cardiac Catheterization, Council on Clinical Cardiology

With advances in technology, the physiological assessment of coronary artery disease in patients in the catheterization laboratory has become increasingly important in both clinical and research applications, but this assessment has evolved without standard nomenclature or techniques of data acquisition and measurement. Some questions regarding the interpretation, application, and outcome related to the results also remain unanswered. Accordingly, this consensus statement was designed to provide the background and evidence about physiological measurements and to describe standard methods for data acquisition and interpretation. The most common uses and support data from numerous clinical studies for the physiological assessment of coronary artery disease in the cardiac catheterization laboratory are reviewed. The goal of this statement is to provide a logical approach to the use of coronary physiological measurements in the catheterization lab to assist both clinicians and investigators in improving patient care.

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.

Single-wire pressure and flow velocity measurement to quantify coronary stenosis hemodynamics and effects of percutaneous interventions

BACKGROUND

Lack of high-fidelity simultaneous measurements of pressure and flow velocity distal to a coronary artery stenosis has hampered the study of stenosis pressure drop-velocity (DeltaP-v) relationships in patients.

METHODS AND RESULTS

A novel 0.014-inch dual-sensor (pressure and Doppler velocity) guidewire was used in 15 coronary lesions to obtain per-beat averages of pressure drop and velocity after an intracoronary bolus of adenosine. DeltaP-v relations from resting to maximal hyperemic velocity were constructed before and after stepwise executed percutaneous coronary intervention (PCI). Before PCI, half of the DeltaP-v relations revealed the presence of a compliant stenosis, which was stabilized by angioplasty. Fractional flow reserve (FFR), coronary flow reserve (CFVR), and velocity-based indices of stenosis resistance (h-SRv) and microvascular resistance (h-MRv) at maximal hyperemia were compared. Stepwise PCI significantly lowered h-SRv, with an initial marked reduction in hyperemic pressure drop followed by further gains in velocity. A concomitant significant reduction of h-MRv accounted for half of the gain in velocity after PCI. The average magnitude of absolute incremental hemodynamic changes was highest for h-SRv (56.8+/-39.2%) compared with CFVR (35.3+/-34.5%, P<0.005) or FFR (19.5+/-25.2%, P<0.0001).

CONCLUSIONS

DeltaP-v relations comprehensively visualize improvements in coronary hemodynamics after PCI. h-SRv is a powerful and sensitive descriptor of the functional gain achieved by PCI, combining information about both pressure gradient and velocity, which are oppositely affected by PCI. Simultaneous assessment of stenosis and microvascular resistance may provide a valuable tool for guidance of PCI.