Quantification of coronary artery stenosis in vivo

Exhaustion of coronary vasodilatory reserve in the resting state: Clinical characteristics and long-term outcomes after intervention

OBJECTIVES/BACKGROUND

Resting coronary blood flow approximates flow with maximal vasodilation in very severe coronary stenosis. We studied the incidence of exhausted vasodilatory reserve by FFR, its clinical characteristics and long-term prognosis after FFR guided percutaneous coronary intervention (PCI).

METHODS

Consecutive patients undergoing FFR-guided PCI for coronary stenosis with reduced resting blood flow (baseline Pd/Pa < 0.8) were included. Basal maximal vasodilation (BMV) was defined as less than 5% difference between resting Pd/Pa and FFR, that is, FFR-baseline Pd/Pa < 0.05.

RESULTS

Of 658 vessels that underwent FFR-guided PCI in 602 patients, 151 vessels had resting blood flow in the ischemic range (baseline Pd/Pa ≤ 0.8) and were included in the analysis. Of these, 28 lesions in 28 patients met criteria for BMV (4.25% of the entire registry and 18.5% of those with the reduced basal coronary flow). Stenosis severity was a significant predictor of the presence of BMV. In long term follow-up (median 106 ± 3.6 months), BMV was not associated with increased target vessel revascularization (TVR) or major adverse cardiac event compared to non-BMV(OR 1.9, 95% CI 0.7-4.8, p-value 0.2 and OR 1.3, 95% CI 0.75-2.5, p = 0.3, respectively).

CONCLUSION

Low baseline Pd/Pa that approximates fractional flow reserve (exhausted vasodilatory reserve) defines a subgroup of patients with severe coronary artery stenosis. Prognosis, when treated with PCI along with medical therapy, appears similar to those with intact vasodilatory reserve.

Comparison of stenosis models for usage in the estimation of pressure gradient across aortic coarctation

Non-invasive estimation of the pressure gradient in cardiovascular stenosis has much clinical importance in assisting the diagnosis and treatment of stenotic diseases. In this research, a systematic comparison is conducted to investigate the accuracy of a group of stenosis models against the MRI- and catheter-measured patient data under the aortic coarctation condition. Eight analytical stenosis models, including six from the literature and two proposed in this study, are investigated to examine their prediction accuracy against the clinical data. The two improved models proposed in this study consider comprehensively the Poiseuille loss, the Bernoulli loss in its exact form, and the entrance effect, of the blood flow. Comparison of the results shows that one of the proposed models demonstrates a cycle-averaged mean prediction error of -0.15 ± 3.03 mmHg, a peak-to-peak prediction error of -1.8 ± 6.89 mmHg, which is the best among the models studied.

Noninvasive Estimation of Pressure Changes Using 2-D Vector Velocity Ultrasound: An Experimental Study With In Vivo Examples

A noninvasive method for estimating intravascular pressure changes using 2-D vector velocity is presented. The method was first validated on computational fluid dynamic (CFD) data and with catheter measurements on phantoms. Hereafter, the method was tested in vivo at the carotid bifurcation and at the aortic valve of two healthy volunteers. Ultrasound measurements were performed using the experimental scanner SARUS, in combination with an 8 MHz linear array transducer for experimental scans and a carotid scan, whereas a 3.5-MHz phased array probe was employed for a scan of an aortic valve. Measured 2-D fields of angle-independent vector velocities were obtained using synthetic aperture imaging. Pressure drops from simulated steady flow through six vessel geometries spanning different degrees of diameter narrowing, running from 20%-70%, showed relative biases from 0.35% to 12.06%, depending on the degree of constriction. Phantom measurements were performed on a vessel with the same geometry as the 70% constricted CFD model. The derived pressure drops were compared to pressure drops measured by a clinically used 4F catheter and to a finite-element model. The proposed method showed peak systolic pressure drops of -3 kPa ± 57 Pa, while the catheter and the simulation model showed -5.4 kPa ± 52 Pa and -2.9 kPa, respectively. An in vivo acquisition of 10 s was made at the carotid bifurcation. This produced eight cardiac cycles from where pressure gradients of -227 ± 15 Pa were found. Finally, the aortic valve measurement showed a peak pressure drop of -2.1 kPa over one cardiac cycle. In conclusion, pressure gradients from convective flow changes are detectable using 2-D vector velocity ultrasound.

Absolute quantification of myocardial blood flow

With the increasing availability of positron emission tomography (PET) myocardial perfusion imaging, the absolute quantification of myocardial blood flow (MBF) has become popular in clinical settings. Quantitative MBF provides an important additional diagnostic or prognostic information over conventional visual assessment. The success of MBF quantification using PET/computed tomography (CT) has increased the demand for this quantitative diagnostic approach to be more accessible. In this regard, MBF quantification approaches have been developed using several other diagnostic imaging modalities including single-photon emission computed tomography, CT, and cardiac magnetic resonance. This review will address the clinical aspects of PET MBF quantification and the new approaches to MBF quantification.

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.

Myocardial flow reserve (MFR) with positron emission tomography (PET)/computed tomography (CT): clinical impact in diagnosis and prognosis

In recent years, radionuclide myocardial perfusion imaging (MPI) using positron emission tomography/computed tomography (PET/CT) has emerged as a robust tool for the diagnosis, risk stratification and management of patients with known or established coronary artery disease (CAD). Cardiac PET/CT imaging affords key advantages compared to single photon emission computed tomography (SPECT) that encompass: (I) improved diagnostic accuracy; (II) decreased radiation exposure due to the utilization of short-lived radiopharmaceuticals, and importantly; (III) the ability to quantify noninvasively myocardial blood flow (MBF) in absolute terms, that is in ml per minute per gram of tissue. Quantitative approaches that measure MBF with PET can facilitate the diagnosis of multivessel CAD and offer the opportunity to monitor responses to lifestyle and/or risk factor modification and to therapeutic interventions. The aim of this review is to focus on the potential clinical utility of MBF and will discuss: (I) basics aspects of PET clinical perfusion tracers and flow quantification parameters; (II) limitations of relative MPI, (III) summarize a classification of diseases where flow quantification may be of use; (IV) specifically, review data on the diagnosis and prognostic value of flow quantification.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

3-D Vector Flow Estimation With Row-Column-Addressed Arrays

Simulation and experimental results from 3-D vector flow estimations for a 62 + 62 2-D row-column (RC) array with integrated apodization are presented. A method for implementing a 3-D transverse oscillation (TO) velocity estimator on a 3-MHz RC array is developed and validated. First, a parametric simulation study is conducted, where flow direction, ensemble length, number of pulse cycles, steering angles, transmit/receive apodization, and TO apodization profiles and spacing are varied, to find the optimal parameter configuration. The performance of the estimator is evaluated with respect to relative mean bias ~B and mean standard deviation ~σ . Second, the optimal parameter configuration is implemented on the prototype RC probe connected to the experimental ultrasound scanner SARUS. Results from measurements conducted in a flow-rig system containing a constant laminar flow and a straight-vessel phantom with a pulsating flow are presented. Both an M-mode and a steered transmit sequence are applied. The 3-D vector flow is estimated in the flow rig for four representative flow directions. In the setup with 90° beam-to-flow angle, the relative mean bias across the entire velocity profile is (-4.7, -0.9, 0.4)% with a relative standard deviation of (8.7, 5.1, 0.8)% for ( v_x, v_y, v_z ). The estimated peak velocity is 48.5 ± 3 cm/s giving a -3% bias. The out-of-plane velocity component perpendicular to the cross section is used to estimate volumetric flow rates in the flow rig at a 90° beam-to-flow angle. The estimated mean flow rate in this setup is 91.2 ± 3.1 L/h corresponding to a bias of -11.1%. In a pulsating flow setup, flow rate measured during five cycles is 2.3 ± 0.1 mL/stroke giving a negative 9.7% bias. It is concluded that accurate 3-D vector flow estimation can be obtained using a 2-D RC-addressed array.

Changes in Atheroma Volume Estimated from Digitized Femoral Arteriograms

To evaluate the effects of treatment in patients with cardiovascular risk factors, valid and reproducible methods for assessing changes in atheroma volume are required. We postulated that these changes could be accurately estimated by repeat measurement of the lumen volume of the artery to be studied. With a computer-based technique, the lumen volume of a 20 cm segment of the femoral artery was measured in arteriograms from 107 patients with hypercholesterolemia. Films were digitized with use of a high-resolution scanner, cross-sectional areas were calculated with a slice thickness of 150 μm and the lumen volume was obtained by their integration. The validity of the method was demonstrated in model experiments. An automatic algorithm to correct for changes due to patient positioning was developed and validated in a model experiment. With repeat measurement 10 min and 11 to 13 months apart the coefficients of variation were 2.9 per cent (N = 107) and 6.1 per cent (N = 29), respectively.

Early changes in myocardial microcirculation in asymptomatic hypercholesterolemic subjects: as detected by perfusion CT

Intramyocardial microvessels show functional changes in early stages of atherosclerosis prior to epicardial coronary artery stenosis. However, clinical CT does not have adequate spatial resolution to resolve the microvessels. To clinically detect changes in the function of the intramyocardial microcirculation, the spatial heterogeneity of the distribution of myocardial perfusion (F) and intramyocardial microcirculatory blood volume (Bv) was determined by perfusion CT. Two human subject groups were studied: (i) a "Control" group (24) with no risk factors nor evidence of coronary artery disease (CAD), and (ii) an "At-Risk" group (24) with hypercholesterolemia, but no evidence of CAD. In the perfusion CT image, a region of interest (ROI) covering the left ventricular myocardium was subdivided into multiple nested ROI (nROI) of equal size and used to compute F and Bv for each nROI. No significant differences between the groups were demonstrable in overall myocardial F, or Bv. The nROI data showed significantly increased spatial heterogeneity in the "At Risk" group when compared to "Control" subjects. This study demonstrates that subresolution distribution at the microcirculatory level can be quantified with myocardial perfusion CT and significant changes in these parameters occur in hypercholesterolemic subjects before they have developed significant changes in conventional perfusion parameters.

Sensitivity analysis and parameter estimation of a coronary circulation model for triple-vessel disease

Mathematical models of the coronary circulation have been shown to provide useful information for the analysis of intracoronary blood flow and pressure measurements acquired during coronary artery bypass graft (CABG) surgery. Although some efforts towards the patient-specific estimation of model parameters have been presented in this context, they are based on simplifying hypotheses about the collateral circulation and do not take advantage of the whole set of data acquired during CABG. In order to overcome these limitations, this paper presents an exhaustive parameter sensitivity analysis and a multiobjective patient-specific parameter estimation method, applied to a model of the coronary circulation of patients with triple vessel disease. The results of the sensitivity analysis highlighted the importance of capillary and collateral development. On the other hand, the estimation method was applied to intraoperative clinical data from ten patients obtained during CABG, which permitted to assess patient-specific collateral vessel situations. These approaches provide new insights regarding the heterogeneous configuration of the collateral circulation.

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).

Nuclear perfusion imaging for functional evaluation of patients with known or suspected coronary artery disease: the future is now

Nuclear imaging, with both single-photon emission computed tomography and PET, has a well-established role in the assessment of patients with known or suspected coronary artery disease. There is a large body of evidence regarding the diagnostic accuracy and prognostic value of these modalities, however, they continue to evolve rapidly with advances in camera and tracer technology, as well as changes in imaging protocols to increase lab efficiency, improve image quality and to decrease radiation exposure to patients. Nuclear imaging also provides insights into atherogenesis at a molecular level and can be combined with other imaging modalities, providing both functional and structural data and complimentary information on the presence of coronary disease and its functional implications.

Coronary pressure-flow relations as basis for the understanding of coronary physiology

Recent technological advancements in the area of intracoronary physiology, as well as non-invasive contrast perfusion imaging, allow to make clinical decisions with respect to percutaneous coronary interventions and to identify microcirculatory coronary pathophysiology. The basic characteristics of coronary hemodynamics, as described by pressure-flow relations in the normal and diseased heart, need to be understood for a proper interpretation of these physiological measurements. Especially the hyperemic coronary pressure-flow relation, as well as the influence of cardiac function on it, bears great clinical significance. The interaction of a coronary stenosis with the coronary pressure-flow relation can be understood from the stenosis pressure drop-flow velocity relationship. Based on these relationships the clinically applied concepts of coronary flow velocity reserve, fractional flow reserve, stenosis resistance and microvascular resistance are discussed. Attention is further paid to the heterogeneous nature of myocardial perfusion, the vulnerability of the subendocardium and the role of collateral flow on hyperemic coronary pressure-flow relations. This article is part of a Special Issue entitled "Coronary Blood Flow".

Theoretical models for coronary vascular biomechanics: progress & challenges

A key aim of the cardiac Physiome Project is to develop theoretical models to simulate the functional behaviour of the heart under physiological and pathophysiological conditions. Heart function is critically dependent on the delivery of an adequate blood supply to the myocardium via the coronary vasculature. Key to this critical function of the coronary vasculature is system dynamics that emerge via the interactions of the numerous constituent components at a range of spatial and temporal scales. Here, we focus on several components for which theoretical approaches can be applied, including vascular structure and mechanics, blood flow and mass transport, flow regulation, angiogenesis and vascular remodelling, and vascular cellular mechanics. For each component, we summarise the current state of the art in model development, and discuss areas requiring further research. We highlight the major challenges associated with integrating the component models to develop a computational tool that can ultimately be used to simulate the responses of the coronary vascular system to changing demands and to diseases and therapies.

Assessment of advanced coronary artery disease: advantages of quantitative cardiac magnetic resonance perfusion analysis

OBJECTIVES

The purpose of this paper was to compare quantitative cardiac magnetic resonance (CMR) first-pass contrast-enhanced perfusion imaging to qualitative interpretation for determining the presence and severity of coronary artery disease (CAD).

BACKGROUND

Adenosine CMR can detect CAD by measuring perfusion reserve (PR) or by qualitative interpretation (QI).

METHODS

Forty-one patients with an abnormal nuclear stress scheduled for X-ray angiography underwent dual-bolus adenosine CMR. Segmental myocardial perfusion analyzed using both QI and PR by Fermi function deconvolution was compared to quantitative coronary angiography.

RESULTS

In the 30 patients with complete quantitative data, PR (mean +/- SD) decreased stepwise as coronary artery stenosis (CAS) severity increased: 2.42 +/- 0.94 for <50%, 2.14 +/- 0.87 for 50% to 70%, and 1.85 +/- 0.77 for >70% (p < 0.001). The PR and QI had similar diagnostic accuracies for detection of CAS >50% (83% vs. 80%), and CAS >70% (77% vs. 67%). Agreement between observers was higher for quantitative analysis than for qualitative analysis. Using PR, patients with triple-vessel CAD had a higher burden of detectable ischemia than patients with single-vessel CAD (60% vs. 25%; p = 0.02), whereas no difference was detected by QI (31% vs. 21%; p = 0.26). In segments with myocardial scar (n = 64), PR was 3.10 +/- 1.34 for patients with CAS <50% (n = 18) and 1.91 +/- 0.96 for CAS >50% (p < 0.0001).

CONCLUSIONS

Quantitative PR by CMR differentiates moderate from severe stenoses in patients with known or suspected CAD. The PR analysis differentiates triple- from single-vessel CAD, whereas QI does not, and determines the severity of CAS subtending myocardial scar. This has important implications for assessment of prognosis and therapeutic decision making.

Intra- and inter-operator repeatability of myocardial blood flow and myocardial flow reserve measurements using rubidium-82 pet and a highly automated analysis program

BACKGROUND

Changes in myocardial blood flow between rest and stress states are commonly used to diagnose coronary artery disease. Relative myocardial perfusion imaging (MPI) is used routinely while myocardial blood flow quantification (MBF) may improve the sensitivity for detection of early disease. The ratio of flow at stress and rest (S/R) and their difference (S-R) have both been proposed as a means to detect regions with reduced myocardial flow reserve (MFR). In this study, we describe a highly automated method to calculate regional and global rest, stress, S/R, and S-R polar maps of the left ventricle myocardium.

METHODS

We measured the inter- and intra-operator variability using two randomized datasets (n = 30 each) for each of two operators (novice and expert) with correlation and Bland-Altman reproducibility coefficient (RPC%) analyses.

RESULTS

S-R MBF had less inter-operator dependent variability than S/R (RPC% = 5.0% vs 12.6%, P < .001). While there was no difference in intra-operator variability with S-R MBF (novice vs expert RPC% = 6.4% vs 5.9%, P = ns), variability was higher in the novice-operator for S/R (RPC% = 16.8% vs 8.5% respectively, P < .001), suggesting that S-R may be preferred for detecting small changes in MFR. The novice operator's intervention pattern became more similar to that of the expert in the later dataset, emphasizing the need for adequate training and quality assurance.

CONCLUSION

The proposed method results in low operator-dependent variability, suitable for routine use.

Flat panel volume computed tomography of the coronary arteries

RATIONALE AND OBJECTIVES

Multidetector-row computed tomography (MDCT) has evolved into a sensitive diagnostic tool for the noninvasive detection of coronary artery stenosis, but remains limited by spatial resolution. Flat panel volume computed tomography (fpVCT) offers a higher spatial resolution. In a postmortem investigation of autopsy specimens, the accuracies of fpVCT for measuring the severity of coronary artery stenosis and the size of atherosclerotic plaque components were determined.

METHODS AND MATERIALS

In 25 autopsy cases, hearts were isolated, the left anterior descending coronary arteries filled with contrast agent, and depicted with a prototype fpVCT unit with a slice thickness of 0.25 mm. Transections of the left anterior descending coronary arteries were reconstructed and compared with histopathologic sections using light microscopy.

RESULTS

FpVCT measurements of luminal stenosis (r = 0.81), total plaque area (r = 0.88), calcified plaque area (r = 0.92), noncalcified plaque area (r = 0.83), and lipid core size (r = 0.67; P < .02) correlated well with histopathology (P < .0001). The limits of agreement for measuring any plaque component were three times smaller than those reported for MDCT.

CONCLUSIONS

Postmortem coronary fpVCT provides an accurate and reproducible method for the quantitative assessment of both luminal stenosis and atherosclerotic plaque size. Because of its high spatial resolution, the method should be sufficiently accurate to reliably detect the lipid pools of vulnerable plaques.

Chronic systolic heart failure, guideline-directed medical therapy, and systemic hypotension-less pressure but maybe more risk (does this clinical scenario need more discussion?)

Many clinical trials have demonstrated the survival benefit of medication regimens that modulate the neurohormonal activation that occurs with chronic heart failure (HF). These medications, however, also commonly lower systemic blood pressure (BP). Low arterial BP in patients with chronic HF has been shown to be an independent predictor of increased mortality. Given this apparent paradox in therapeutic goals (treat aggressively but keep BP from going too low), how low should we allow systemic BP to go as a result of our medication regimens before we compromise the proven benefits of such drug therapy? Or is the association between the BP-lowering effects of standard therapy and outcomes in HF even meaningful clinically? It is from this perspective that the merits, potential clinical implications, and the relevant published literature pertaining to this patient and practice management issue will be discussed.

Accurate geometric calibration in stepping-table digital subtraction angiography

Accurate measurements of vessel dimensions are desirable in many clinical applications. This work uses the known relative motion between X-ray source and the patient in stepping-table digital subtraction angiography (DSA) to provide an accurate geometric calibration for quantitative measurements. The method results in a calibration factor that converts the size of the object measured in pixels on the image to its size in millimetres. The main sources of error relate to: (i) the assessment of relative displacement of a structure in a series of images; (ii) patient motion throughout data acquisition; and (iii) image distortion. Error was evaluated both with a test object consisting of a large grid of ball bearings (2x2 cm spaced) and, in vivo, in five renal DSA examinations performed with identical catheters of known diameter. The calibration factor was calculated with 0.1% accuracy for the test object and at least 2% accuracy in vivo, even with breath holding and pulsatile motion. This demonstrates that the calculation of the calibration factor can be very accurate, and that the method we propose is capable of the submillimetre accuracy required for clinical studies if used in conjunction with an accurate measurement of the vessel size in pixels. In conclusion, accurate geometric measurements can be performed in stepping-table DSA, without the need for external reference objects.

Will 3-dimensional PET-CT enable the routine quantification of myocardial blood flow?

Quantification of myocardial blood flow (MBF) and flow reserve has been used extensively with positron emission tomography (PET) to investigate the functional significance of coronary artery disease. Increasingly, flow quantification is being applied to investigations of microvascular dysfunction in early atherosclerosis and in nonatherosclerotic microvascular disease associated with primary and secondary cardiomyopathies. Fully three-dimensional (3D) acquisition is becoming the standard imaging mode on new equipment, bringing with it certain challenges for cardiac PET, but also the potential for MBF to be measured simultaneously with routine electrocardiography (ECG)-gated perfusion imaging. Existing 3D versus 2D comparative studies support the use of 3D cardiac PET for flow quantification, and these protocols can be translated to PET-CT, which offers a virtually noise-free attenuation correction. This technology combines the strengths of cardiac CT for evaluation of anatomy with cardiac PET for quantification of the hemodynamic impact on the myocardium. High throughput clinical imaging protocols are needed to evaluate the incremental diagnostic and prognostic value of this technology.

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.

Effects of distension of urinary bladder on coronary conduit and resistance vessels in hyperlipidemic patients

BACKGROUND

Distension of the urinary bladder reflexly causes a change of coronary vasomotor response. The effect of such distension on the coronary circulation in hyperlipidemic patients, a condition with impaired endothelial function, remains unknown.

HYPOTHESIS

We tested the hypothesis whether urinary bladder distension caused an exaggerated vasomotor response of epicardial and resistance vasoconstriction in hyperlipidemic patients.

METHODS

Thirty patients with early atherosclerosis (< 50% diameter stenosis) were divided into three groups: Group 1 (n = 10): hyperlipidemia without doxazosin administration; Group 2 (n = 10): hyperlipidemia with pretreatment of alpha1-adrenergic receptor blocker (oral doxazosin 2 mg); and Group 3 (n = 10): normolipidemia. A prospective analysis of the results of quantitative angiograms, intracoronary Doppler flow, and lactate concentrations from aortic root and coronary sinus was performed during distension of urinary bladder.

RESULTS

Bladder distension significantly decreased coronary diameter at the stenotic segments (p = 0.004), coronary blood flow (p = 0.05), and increased coronary resistance (p = 0.006) compared with baseline values, in Group 1 patients. In Group 2 patients during bladder distension, coronary diameter, coronary blood flow, and coronary resistance showed no significant changes compared with baseline values. There were significant differences of stenotic coronary diameter (p = 0.01) between Groups 1 and 3 during bladder distension despite similar changes in rate-pressure product. No significant differences were noted among the groups in the responses of coronary diameter, coronary blood flow, and coronary resistance after nitroglycerin administration.

CONCLUSIONS

The present study showed that urinary bladder distension caused an abnormal vasomotor response of epicardial vasoconstriction and that a concomitant increased coronary resistance involved mechanisms related to alpha1-adrenoceptors. Hyperlipidemia may further impair the response. Pretreated administration of doxazosin had reversed the changes toward baseline. Vasoconstriction during bladder distension can be relieved after nitroglycerin administration, suggesting an unchanged responsiveness of vascular smooth muscle cells to such distension.

Transient integral boundary layer method to calculate the translesional pressure drop and the fractional flow reserve in myocardial bridges

Background

The pressure drop – flow relations in myocardial bridges and the assessment of vascular heart disease via fractional flow reserve (FFR) have motivated many researchers the last decades. The aim of this study is to simulate several clinical conditions present in myocardial bridges to determine the flow reserve and consequently the clinical relevance of the disease. From a fluid mechanical point of view the pathophysiological situation in myocardial bridges involves fluid flow in a time dependent flow geometry, caused by contracting cardiac muscles overlying an intramural segment of the coronary artery. These flows mostly involve flow separation and secondary motions, which are difficult to calculate and analyse.

Methods

Because a three dimensional simulation of the haemodynamic conditions in myocardial bridges in a network of coronary arteries is time-consuming, we present a boundary layer model for the calculation of the pressure drop and flow separation. The approach is based on the assumption that the flow can be sufficiently well described by the interaction of an inviscid core and a viscous boundary layer. Under the assumption that the idealised flow through a constriction is given by near-equilibrium velocity profiles of the Falkner-Skan-Cooke (FSC) family, the evolution of the boundary layer is obtained by the simultaneous solution of the Falkner-Skan equation and the transient von-Kármán integral momentum equation.

Results

The model was used to investigate the relative importance of several physical parameters present in myocardial bridges. Results have been obtained for steady and unsteady flow through vessels with 0 – 85% diameter stenosis. We compare two clinical relevant cases of a myocardial bridge in the middle segment of the left anterior descending coronary artery (LAD). The pressure derived FFR of fixed and dynamic lesions has shown that the flow is less affected in the dynamic case, because the distal pressure partially recovers during re-opening of the vessel in diastole. We have further calculated the wall shear stress (WSS) distributions in addition to the location and length of the flow reversal zones in dependence on the severity of the disease.

Conclusion

The described boundary layer method can be used to simulate frictional forces and wall shear stresses in the entrance region of vessels. Earlier models are supplemented by the viscous effects in a quasi three-dimensional vessel geometry with a prescribed wall motion. The results indicate that the translesional pressure drop and the mean FFR compares favourably to clinical findings in the literature. We have further shown that the mean FFR under the assumption of Hagen-Poiseuille flow is overestimated in developing flow conditions.

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.

Myocardial perfusion after angioplasty in patients suspected of having single-vessel coronary artery disease: improvement detected at rest-stress first-pass perfusion MR imaging--initial experience

PURPOSE

To prospectively assess myocardial perfusion before and after successful intervention in patients suspected of having single-vessel coronary artery disease by using a steady-state free precession (SSFP) perfusion magnetic resonance (MR) imaging sequence.

MATERIALS AND METHODS

Local ethics committee approval and informed consent were obtained. Rest-stress perfusion MR imaging studies were performed in 18 patients with coronary artery disease (12 men, six women; mean age, 58.6 years +/- 13.6 [standard deviation]; range, 30-79 years) at 1.5 T with a multisection saturation-recovery SSFP sequence and 0.025 mmol gadopentetate dimeglumine per kilogram of body weight. MR studies were performed before (n = 18), several days after (n = 18), and 8 months after (n = 10) coronary intervention. Nine patients underwent percutaneous transluminal coronary angioplasty (PTCA) alone, and nine patients underwent PTCA with stent placement. Myocardial perfusion reserve index (MPRI) was calculated by dividing results of myocardial perfusion at maximal vasodilation by results at rest. The standard for myocardial perfusion was technetium 99m tetrofosmin single photon emission computed tomography. Statistical significance was tested with univariate variance analysis and Student t tests.

RESULTS

In the area of the stenosed vessel, MPRI was 1.04 +/- 0.24 before treatment and 2.18 +/- 0.57 several days afterward (P < .001). In remote areas, MPRI was 2.42 +/- 0.44. In the stent group, MPRI increased by 156%, from 0.99 +/- 0.20 before stent placement to 2.53 +/- 0.53 after (P < .001). Similarly, in the PTCA only group, MPRI increased by 72%, from 1.08 +/- 0.27 before PTCA to 1.87 +/- 0.39 after (P < .001). At follow-up in patients without recurring chest pain, MPRI was 2.14 +/- 0.37 in the area of the treated artery and 2.29 +/- 0.47 in remote areas (P = .06).

CONCLUSION

The MPRI, derived from rest-stress examinations, can provide information on success of interventional procedures in stenosed coronary arteries.

Potential utility of rubidium 82 PET quantification in patients with 3-vessel coronary artery disease

BACKGROUND

Standard perfusion imaging may underestimate the extent of disease in 3-vessel coronary atherosclerosis. This study determined whether positron emission tomography quantification of perfusion reserve by use of rubidium 82 net retention defined a greater extent of disease than the standard approach in patients with 3-vessel disease.

METHODS AND RESULTS

Rb-82 net retention was quantified as an estimation of absolute perfusion at rest and with dipyridamole stress by use of dynamic positron emission tomography imaging. The percent of abnormal myocardial sectors, as compared with a normal database, for a standard and quantification approach was determined. Twenty-three patients were evaluated. Defect sizes were larger in patients with 3-vessel disease (n = 13) by use of quantification methods: 44% +/- 18% of the myocardial sectors were abnormal by use of the standard approach versus 69% +/- 24% of sectors when measured by quantification of the stress-rest perfusion difference (P =.008). In patients with single-vessel disease (n = 10), defect sizes were smaller with quantification methods.

CONCLUSIONS

Quantification of Rb-82 net retention to measure the stress-rest perfusion difference in the myocardium defined a greater extent of disease than the standard approach in this group of patients with triple-vessel disease. More accurate measurement of the extent of coronary artery disease could facilitate better risk stratification and identify more high-risk patients in whom aggressive intervention is required.

Vascular reserve in the lower limbs of cirrhotic patients: a duplex Doppler ultrasound study

AIM

To evaluate femoral artery impedance at rest and during reactive hyperaemia.

PATIENTS

Study population comprised 11 cirrhotic patients without ascites, 10 with ascites and 16 age- and sex-matched healthy subjects.

METHODS

Echocardiographic assessment of systemic haemodynamics; duplex Doppler ultrasound measurement of femoral artery pulsatility index and vascular reserve [pulsatility index rest/pulsatility index hyperaemia).

RESULTS

Cirrhotic patients had elevated cardiac index and low systemic vascular resistance. Pulsatility index (right femoral artery) was not statistically different either at rest or after reactive hyperaemia (controls: rest 10.6 +/- 0.4, hyperaemia 2.6 +/- 0.2; compensated cirrhosis: rest 10.1 +/- 0.8, hyperaemia 3.4 +/- 0.4; ascitic cirrhosis: rest 11.4 +/- 1.6, hyperaemia 2.9 +/- 0.4. Vascular reserve was 4.38 +/- 0.35 in controls, 3.33 +/- 0.39 in compensated and 4.70 +/- 0.89 in ascitic cirrhosis (p = not significant). No correlation was found between systemic haemodynamic parameters and either pulsatility index or vascular reserve.

CONCLUSIONS

The lower limb vascular reserve is preserved in cirrhosis.

Hypotension in patients with coronary disease: can profound hypotensive events cause myocardial ischaemic events?

OBJECTIVE

To determine whether anginal episodes might be related to extremes of hypotension in patients with ischaemic heart disease taking drugs to treat angina and heart failure.

DESIGN AND SETTING

Observational study of patients with ischaemic heart disease attending an urban tertiary referral cardiology centre.

INTERVENTIONS AND OUTCOME MEASURES

A selected patient population was enrolled, having: angina on one or more hypotensive cardiovascular medications; hypotension on clinic or ambulatory measurement; and a resting ECG suitable for ambulatory monitoring. Patients had echocardiography, ambulatory blood pressure monitoring, and Holter monitoring. Hypotension induced ischaemic (HII) events were defined as episodes of ST segment ischaemia occurring at least one minute after an ambulatory blood pressure measurement (systolic/diastolic) below 100/65 mm Hg during the day, or 90/50 mm Hg at night.

RESULTS

25 suitable patients were enrolled, and 107 hypotensive events were documented. 40 ST events occurred in 14 patients, of which a quarter were symptomatic. Fourteen HII events occurred in eight patients, with 13 of the 14 preceded by a fall in diastolic pressure (median diastolic pressure 57.5 mm Hg, interquartile range 11, maximum 72 mm Hg, minimum 45 mm Hg), and six preceded by a fall in systolic pressure (chi(2) = 11.9, p < 0.001). ST events were significantly associated with preceding hypotensive events (chi(2) = 40.2, p < 0.0001). Patients with HII events were more frequently taking multiple hypotensive drug regimens (8/8 v 9/17, chi(2) = 5.54, p = 0.022).

CONCLUSIONS

In patients with ischaemic heart disease and hypotension, symptomatic and silent ischaemia occurred in a temporally causal relation with hypotension, particularly for diastolic pressures, suggesting that patients with coronary disease may be susceptible to ischaemic events incurred as a result of low blood pressure caused by excessive hypotensive drug treatment.

Clinical validation of intravascular ultrasound imaging for assessment of coronary stenosis severity: comparison with stress myocardial perfusion imaging

OBJECTIVES

To validate intravascular ultrasound (IVUS) measurements for differentiating functionally significant from nonsignificant coronary stenosis.

BACKGROUND

To date, there are no validated criteria for the definition of a flow-limiting coronary artery stenosis by IVUS.

METHODS

Preinterventional IVUS imaging (30-MHz imaging catheter) of 70 de novo coronary lesions was performed. The lesion lumen area and three IVUS-derived stenosis indixes comparing lesion lumen area with the lesion external elastic lamina (EEL) area, the mean reference lumen area and the mean reference EEL area were compared with the results of stress myocardial perfusion imaging.

RESULTS

The lesion lumen area and three IVUS-derived stenosis indexes showed sensitivities and specificities ranging between 80% and 90% using stress myocardial perfusion imaging as the gold standard. The lesion lumen area < or =4 mm2 is a simple and highly accurate criterion for significant coronary narrowing.

CONCLUSIONS

Quantitative IVUS indices can be reliably used for identifying significant epicardial coronary artery stenoses.

CT angiography in vascular intervention for steno-occlusive diseases: role of multiplanar reconstruction and source images

The aim of this study was to assess the efficacy of CT angiography for steno-occlusive diseases before and after interventional procedures, focusing on the role of multiplanar reconstruction (MPR) and source images. 17 patients with 20 steno-occlusive lesions underwent CT angiography before and after interventional procedures. For each lesion, the percentage stenosis obtained on CT angiography was compared with that on conventional angiography. In addition, MPR and source images were evaluated for the presence of wall thickening and calcification before interventional procedures, and the presence of dissection and luminal shape after interventional procedures. These findings were compared with those of conventional angiography. Although the percentage stenosis depicted on CT angiography correlated well with that on conventional angiography, MPR and source images clearly demonstrated the effect of intervention and the residual stenosis. MPR and source images clearly depicted wall thickening, wall calcification, the presence of dissection and the luminal shape. CT angiography provides useful information before intervention, while MPR and source images are of value in evaluating arterial wall abnormalities and morphological changes associated with interventional procedures.

Quantitative coronary angiography with deformable spline models

Although current edge-following schemes can be very efficient in determining coronary boundaries, they may fail when the feature to be followed is disconnected (and the scheme is unable to bridge the discontinuity) or branch points exist where the best path to follow is indeterminate. In this paper, we present new deformable spline algorithms for determining vessel boundaries, and enhancing their centerline features. A bank of even and odd S-Gabor filter pairs of different orientations are convolved with vascular images in order to create an external snake energy field. Each filter pair will give maximum response to the segment of vessel having the same orientation as the filters. The resulting responses across filters of different orientations are combined to create an external energy field for snake optimization. Vessels are represented by B-Spline snakes, and are optimized on filter outputs with dynamic programming. The points of minimal constriction and the percent-diameter stenosis are determined from a computed vessel centerline. The system has been statistically validated using fixed stenosis and flexible-tube phantoms. It has also been validated on 20 coronary lesions with two independent operators, and has been tested for interoperator and intraoperator variability and reproducibility. The system has been found to be specially robust in complex images involving vessel branchings and incomplete contrast filling.

Practical coronary physiology. Clinical application of the Doppler flow velocity guide wire

Coronary angiography incompletely delineates the physiologic consequences of many epicardial stenoses. Intracoronary translesional flow velocity measurements using the Doppler flow wire during cardiac catheterization provide immediate data discriminating the physiologic significance of coronary stenoses. The validity and accuracy of the flow wire for analyzing lesion hemodynamic significance have been confirmed in multiple studies. Flow velocity analysis provides objective criteria for refining the selection of cases for revascularization, and prospective clinical data have confirmed the safety of deferring intervention on lesions with normal physiologic assessment. Translesional and distal coronary flow velocity dynamics during procedures also provide immediate data assessing the physiologic adequacy of intervention. Impaired postintervention distal coronary flow velocity and vasodilator reserve can predict subsequent clinical events, and comparisons of flow velocity indices prestenting and poststenting suggest that physiologically inadequate results of angioplasty may be improved by additional intervention. Flow velocity assessment may also have utility in profiling the adequacy of infarct artery reperfusion following acute myocardial infarction. Evidence has been accumulated to support use of Doppler flow velocity analysis as a clinically relevant technique for improving both diagnostic and therapeutic aspects of cardiovascular medicine.

Clinical application of coronary flow reserve using an intracoronary Doppler guide wire

Coronary flow reserve (CFR) is a critical measurement in the assessment of the coronary circulation. The development of this physiologic variable in animal and human studies is reviewed. Human studies documenting the limitations of coronary angiography, especially in the setting of severe diffuse coronary artery disease, are analyzed. Furthermore, the important variables that must be accounted for when CFR is measured are examined. With this background, the application of CFR in a variety of clinical settings and the development and use of the Doppler FloWire for its measurement are discussed.