Functional recovery after coronary revascularization for chronic coronary artery disease is dependent on maintenance of oxidative metabolism

Gated metabolic myocardial imaging, a surrogate for dual perfusion-metabolism imaging by positron emission tomography

Objective

Perfusion-metabolism mismatch pattern on positron emission tomography (PET) predicts hibernating myocardium. We assess the ECG-gated metabolic PET as a surrogate for the perfusion-metabolism mismatch pattern on PET imaging.

Methods

¹³N-Ammonia (NH₃) and ¹⁸F-fluorodeoxyglucose (FDG) are respectively perfusion and metabolism PET tracers. We used ECG gating to acquire FDG-PET to collect wall thickening (mechanical) data. These allow detection of metabolic activity in regions with reduced contraction (metabolism-mechanical mismatch pattern). We had two data sets on each patient: perfusion-metabolism and metabolism-mechanical data sets. We tested the hypothesis that metabolism-mechanical pattern on PET could predict perfusion-metabolism mismatch pattern.

Results

We studied 55 patients (48 males), mean age 62 years. All were in sinus rhythm, and had impaired left ventricular contraction. Perfusion-metabolism mismatch pattern was found in 26 patients. Metabolism-mechanical mismatch pattern was found in 25 patients. The results were concordant in 52 patients (95%). As a surrogate for perfusion-metabolism mismatch pattern, demonstration of metabolism-mechanical mismatch pattern is highly sensitive (92%) and specific (97%). In this cohort, the positive and negative predictive accuracy of the new method are 96% and 93%, respectively.

Conclusion

Metabolism-mechanical mismatch pattern could predict perfusion-metabolism mismatch pattern in patients with myocardial viability criteria on PET. Prospective validation against the gold standard of improved myocardial contraction after revascularisation is needed.

Evaluation of ECG-gated [(11)C]acetate PET for measuring left ventricular volumes, mass, and myocardial external efficiency

BACKGROUND

Noninvasive estimation of myocardial external efficiency (MEE) requires measurements of left ventricular (LV) oxygen consumption with [(11)C]acetate PET in addition to LV stroke volume and mass with cardiovascular magnetic resonance (CMR). Measuring LV geometry directly from ECG-gated [(11)C]acetate PET might enable MEE evaluation from a single PET scan. Therefore, we sought to establish the accuracy of measuring LV volumes, mass, and MEE directly from ECG-gated [(11)C]acetate PET.

METHODS

Thirty-five subjects with aortic valve stenosis underwent ECG-gated [(11)C]acetate PET and CMR. List mode PET data were rebinned into 16-bin ECG-gated uptake images before measuring LV volumes and mass using commercial software and compared to CMR. Dynamic datasets were used for calculation of mean LV oxygen consumption and MEE.

RESULTS

LV mass, volumes, and ejection fraction measured by CMR and PET correlated strongly (r = 0.86-0.92, P < .001 for all), but were underestimated by PET (P < .001 for all except ESV P = .79). PET-based MEE, corrected for bias, correlated fairly with PET/CMR-based MEE (r = 0.60, P < .001, bias -3 ± 21%, P = .56). PET-based MEE bias was strongly associated with LV wall thickness.

CONCLUSIONS

Although analysis-related improvements in accuracy are recommended, LV geometry estimated from ECG-gated [(11)C]acetate PET correlate excellently with CMR and can indeed be used to evaluate MEE.

Imaging of Myocardial Oxidative Metabolism in Heart Failure

Metabolic imaging has a potential for better understanding of pathophysiology of heart failure. C-11 acetate is taken up by the heart, rapidly converted to acetylCoA and readily metabolized to C-11 CO₂ through TCA cycle with oxidative phosphorylation. Thus, the myocardial turnover rate of this tracer is tightly correlated with its clearance of C-11 CO₂, reflecting overall oxidative metabolism. The heart relies on aerobic oxidative substrate for the generation of ATP, which is required to maintain its contractile function. The progression to heart failure is associated with a gradual decline in the activity of mitochondrial respiratory pathways, leading to diminished capacity for ATP production. The work metabolic index can also be estimated by the combination of C-11 acetate PET and hemodynamics by echocardiography, the metabolic index is a significant marker to understand the pathophysiology of heart failure as well as myocardial oxidative metabolism.

Targeted metabolic imaging to improve the management of heart disease

Tracer techniques are powerful methods for assessing rates of biological processes in vivo. A case in point is intermediary metabolism of energy providing substrates, a central feature of every living cell. In the heart, the tight coupling between metabolism and contractile function offers an opportunity for the simultaneous assessment of cardiac performance at different levels in vivo: coronary flow, myocardial perfusion, oxygen delivery, metabolism, and contraction. Noninvasive imaging techniques used to identify the metabolic footprints of either normal or perturbed cardiac function are discussed.

Could 13C MRI assist clinical decision-making for patients with heart disease?

Even at this early stage of development, it is clear that the imaging of hyperpolarized (13)C-enriched molecules and their metabolic products offers a new approach to the study of the physiology and disease of the heart. The technology is practical in humans and, for this reason, we consider whether a role in clinical decision-making should motivate further development. The range of interventions available to treat coronary and valvular heart disease is already extensive, and new options are imminent. Yet the appropriate management of patients with left ventricular dysfunction can be challenging because the mechanism of reduced function may be unclear and the ability of the ventricle to respond to therapy may be difficult to predict. Pyruvate is a promising early target for development as a diagnostic agent because it lies at a critical branch point in cardiac biochemistry. The rate of metabolism of hyperpolarized pyruvate to CO(2) relative to lactate may prove to be a useful indicator of preserved mitochondrial function, and therefore provide a specific signal of viable myocardium. Other species including physiological substrates and nonphysiological molecules may provide additional information. Once suitable technology becomes available, it is likely that clinical research will progress quickly. The ability to monitor directly specific metabolic pathways may lead to an improvement in the selection of patients who will benefit from interventions, pharmacologic or otherwise.

The role of cardiac PET in translating basic science into the clinical arena

Non-invasive imaging has become fundamental in translating findings from basic science research into clinical applications. In this aspect, positron-emission tomography (PET) offers important advantages over other common imaging modalities like single-photon emission computed tomography, computed tomography, and magnetic resonance imaging (MRI), since PET provides superior detection sensitivity in the evaluation of different cardiovascular targets and pathways at the cellular and subcellular level, and because it is a well-established technique for absolute image quantification. The development and the introduction of dedicated small animal PET systems have greatly facilitated and contributed to advancements in the translation of novel radio-labeled compounds from experimental to clinical practice. The scope of the present article is to review the most relevant and successful PET applications in cardiovascular translational research.

Feasibility study of myocardial perfusion and oxygenation by noncontrast MRI: comparison with PET study in a canine model

The purpose of this study was to examine the feasibility of quantifying myocardial blood flow (MBF) and rate of myocardial oxygen consumption (MVO(2)) during pharmacologically induced stress without using a contrast agent. The former was measured by the arterial spin labeling (ASL) method and the latter was obtained by measuring the oxygen extraction fraction (OEF) with the magnetic resonance imaging (MRI) blood oxygenation level-dependent effect and Fick's law. The MRI results were compared with the established positron emission tomography (PET) methods. Six mongrel dogs with induced acute moderate left coronary artery stenosis were scanned using a clinical PET and a 1.5-T MRI system, in the same day. Regional MBF, myocardial OEF and MVO(2) were measured with both imaging modalities. Correlation coefficients (R(2)) of the three myocardial indexes (MBF, OEF and MVO(2)) between MRI and PET methods ranged from 0.70 to 0.93. Bland-Altman statistics demonstrated that the estimated precision of the limits of agreement between MRI and PET measurements varied from 18% (OEF) to 37% (MBF) and 45% (MVO(2)). The detected changes in these indexes, at rest and during dobutamine stress, were similar between two image modalities. The proposed noncontrast MRI technique is a promising method to quantitatively assess myocardial perfusion and oxygenation.

Metabolic imaging using PET

INTRODUCTION

There is growing evidence that myocardial metabolism plays a key role not only in ischaemic heart disease but also in a variety of diseases which involve myocardium globally, such as heart failure and diabetes mellitus. Understanding myocardial metabolism in such diseases helps to elucidate the pathophysiology and assists in making therapeutic decisions.

MEASUREMENT

As well as providing information on regional changes, PET can deliver quantitative information about both regional and global changes in metabolism. This capability of quantitative measurement is one of the major advantages of PET along with physiological positron tracers, especially relevant in evaluating diseases which involve the whole myocardium.

DISCUSSION

This review discusses major PET tracers for metabolic imaging and their clinical applications and contributions to research regarding ischaemic heart disease and other diseases such as heart failure and diabetic heart disease. Future applications of positron metabolic tracers for the detection of vulnerable plaque are also highlighted briefly.

Imaging myocardial metabolism

The prevalence of coronary artery disease and heart failure is increasing in modern industrialized countries, fueling the search for novel therapies. Because metabolism and function in the heart are inextricably linked, energy substrate metabolism has provided a potential target for novel therapies and the development of technologies to image myocardial metabolism has been crucial in establishing new therapeutic approaches. Nuclear imaging probes have been used to successfully evaluate aerobic fatty acid metabolism, anaerobic glucose metabolism, and oxidative metabolism and can be used for the accurate, sensitive, and physiological evaluation of therapeutic effects. More recently, with the advent of stem-cell technologies, imaging approaches have been employed to track the fate of stem cells and to monitor the success of these treatments. In the future, our ability to image myocardial metabolism is likely to assist the development of other new therapies to improve the function of the failing heart.

Mechanisms leading to reversible mechanical dysfunction in severe CAD: alternatives to myocardial stunning

Patients with severe chronic coronary artery disease (CAD) exhibit a highly altered myocardial pattern of perfusion, metabolism, and mechanical performance. In this context, the diagnosis of stunning remains elusive not only because of methodological and logistic considerations, but also because of the pathophysiological characteristics of the myocardium of these patients. In addition, a number of alternative pathophysiological mechanisms may act by mimicking the functional manifestations usually attributed to stunning. The present review describes three mechanisms that could theoretically lead to reversible mechanical dysfunction in these patients: myocardial wall stress, the tethering effect, and myocardial expression and release of auto- and paracrine agents. Attention is focused on the role of these mechanisms in scintigraphically “normal” regions (i.e., regions usually showing normal perfusion, glucose metabolism, and cellular integrity as assessed by nuclear imaging techniques), in which stunning is usually considered, but these mechanisms could also operate throughout the viable myocardium. We hypothesize that reversion of these three mechanisms could partially explain the unexpected functional benefit after reperfusion recently highlighted by high-spatial-resolution imaging techniques.

Role of F-18 FDG positron emission tomography (PET) in the assessment of myocardial viability

Positron emission tomography (PET) is a functional imaging technique with important clinical applications in cardiology, oncology, and neurology. In cardiac imaging, its role has been extensively evaluated in the noninvasive diagnosis of coronary artery disease and in the determination of prognosis. Additionally, cardiac PET with F-18 fluorodeoxyglucose (FDG) is very helpful in selection of patients with coronary artery disease and left ventricular dysfunction who would benefit from coronary artery revascularization. Cardiac PET is arguably considered by many as a gold standard in this particular application. F-18, unlike other positron emitters, has a reasonably long physical half-life, which permits its distribution through commercial radiopharmacies. This is further facilitated by increasing popularity of FDG PET in oncology, which makes cardiac FDG PET a practical option for hospitals and outpatient centers equipped with PET scanners. In addition, gamma camera single photon emission computed tomography (SPECT) systems, routinely used in nuclear medicine departments, can be equipped with coincidence circuit or high-energy 511 KeV collimators, providing a cost-effective means of FDG cardiac imaging. Myocardial utilization of glucose as a substrate is variable, depending, among other factors, on serum levels of glucose and insulin. Therefore, patient preparation is important in obtaining good-quality images and in turn allowing for accurate interpretation of myocardial viability. There are various protocols to choose from that provide diagnostic image quality in both diabetic and nondiabetic patients. Mismatch between blood flow and FDG metabolism, an indicator of viable, jeopardized myocardium, can predict postrevascularization improvement in left ventricular function, symptomatic relief, and long-term survival.

Assessment of myocardial viability

The prevalence of left ventricular (LV) dysfunction and resultant congestive heart failure is increasing. Patients with this condition are at high risk for cardiac death and usually have significant limitations in their lifestyles. Although there have been advances in medical therapy resulting in improved survival and well being, the best and most definitive therapy, when appropriate, is revascularization. In the setting of coronary artery disease, accounting for approximately two thirds of cases of congestive heart failure, LV dysfunction often is not the result of irreversible scar but rather caused by impairment in function and energy use of still viable-myocytes, with the opportunity for improved function if coronary blood flow is restored. Patients with LV dysfunction who have viable myocardium are the patients at highest risk because of the potential for ischemia but at the same time benefit most from revascularization. It is important to identify viable myocardium in these patients, and radionuclide myocardial scintigraphy is an excellent tool for this. Single-photon emission computed tomography perfusion scintigraphy, whether using thallium-201, Tc-99m sestamibi, or Tc-99m tetrofosmin, in stress and/or rest protocols, has consistently been shown to be an effective modality for identifying myocardial viability and guiding appropriate management. Metabolic imaging with positron emission tomography radiotracers frequently adds additional information and is a powerful tool for predicting which patients will have an improved outcome from revascularization, including some patients referred instead for cardiac transplantation. Other noninvasive modalities, such as stress echocardiography, also facilitate the assessment of myocardial viability, but there are advantages and disadvantages compared with the nuclear techniques. Nuclear imaging appears to require fewer viable cells for detection, resulting in a higher sensitivity but a lower specificity than stress echocardiography for predicting post-revascularization improvement of ventricular function. Nevertheless, it appears that LV functional improvement may not always be necessary for clinical improvement. Future directions include use of magnetic resonance imaging, as well as larger, multicenter trials of radionuclide techniques. The increasing population of patients with LV dysfunction, and the increased benefit afforded by newer therapies, will make assessment of myocardial viability even more essential for proper patient management.

Myocardial viability assessment using nuclear imaging

Myocardial assessment continues to be an issue in patients with coronary artery disease and left ventricular dysfunction. Nuclear imaging has long played an important role in this field. In particular, PET imaging using 18F-fluorodeoxyglucose is regarded as the metabolic gold standard of tissue viability, which has been supported by a wide clinical experience. Viability assessment using SPECT techniques has gained more wide-spread clinical acceptance than PET, because it is more widely available at lower cost. Moreover, technical advances in SPECT technology such as gated-SPECT further improve the diagnostic accuracy of the test. However, other imaging techniques such as dobutamine echocardiography have recently emerged as competitors to nuclear imaging. It is also important to note that they sometimes may work in a complementary fashion to nuclear imaging, indicating that an appropriate use of these techniques may significantly improve their overall accuracy. In keeping these circumstances in mind, further efforts are necessary to further improve the diagnostic performance of nuclear imaging as a reliable viability test.

Robotic preparation of Sodium Acetate C 11 Injection for use in clinical PET

Sodium Acetate C 11 Injection is a radiopharmaceutical commonly used for clinical studies with positron emission tomography (PET). We have designed a fully-automated robotic system for the compounding of this 20-minute half-lived tracer in the clinical setting. The system is comprised of five modular workstations that are configured in a flexible manner to accommodate all of the steps in the production of the labeled drug. The Trapping Station isolates cyclotron-produced [11C]CO(2) gas from the target and directs carbonation of methylmagnesium Grignard in diethyl ether. The Heating Station hydrolyzes the intermediate, and removes ether and unreacted [11C]CO(2) from the mixture. The Extraction Station removes ionic and organic contaminants from the drug using solid-phase extraction (AG 11A8 and C18 resin). The Filtration Station sterilizes the radiopharmaceutical, and tests membrane patency post filtration. The Assay Station measures the weight and radioactivity content of the Final Product Container, facilitating calculation of activity concentration in a remote manner. Rapid quality control methodology has also been developed that is suitable for pre-release analysis of the drug product. For a 7.5 min irradiation with a 40 microA proton beam, 223-300 mCi of Acetate C 11 Injection with purity meeting USP standards is produced within 23 min. This robotic system is a reliable method for producing Sodium Acetate C 11 Injection, USP in the clinical setting with minimal personnel exposure. Moreover, its design flexibility permits synthesis of additional (11)C- or (18)F-labeled PET tracers within the same shielded hot cell.

The pathophysiology of myocardial hibernation: current controversies and future directions

It is now widely accepted that patients with chronic coronary artery disease can experience prolonged regional ischemic dysfunction that does not necessarily arise from irreversible tissue damage and, to some extent, can be reversed by restoration of blood flow. Recent clinical and experimental data suggest that this form of chronic but reversible left ventricular dysfunction represents a complex, progressive, and dynamic phenomenon. The initial stages of dysfunction are probably caused by chronic stunning. They are characterized by normal resting perfusion but reduced flow reserve, mild myocyte alterations, maintained membrane integrity (allowing the transport of both thallium and glucose), preserved capacity to respond to an inotropic stimulus, and no or little tissue fibrosis. After revascularization, functional recovery will probably be rapid and complete. On the other hand, the more advanced stages of dysfunction likely correspond to chronic hibernation. They usually are associated with reduced rest perfusion; increased tissue fibrosis; more severe myocyte alterations (degeneration[?], apoptosis); and a decreased ability to respond to inotropic stimuli. Nonetheless, membrane function and glucose metabolism may long remain preserved. After revascularization, functional recovery, if any, will probably be quite delayed and mostly incomplete.

Energy stores and metabolites in chronic reversibly and irreversibly dysfunctional myocardium in humans

OBJECTIVES

Our goal was to study metabolic energy stores and lactate content in chronic reversibly and irreversibly dysfunctional myocardium.

BACKGROUND

It is unknown whether metabolism is deranged in chronic reversibly and irreversibly dysfunctional myocardium in humans. Semiquantitative histological examinations have shown altered mitochondrial morphology and glycogen accumulation in dysfunctional regions.

METHODS

We studied 25 patients with a mean ejection fraction of 38 +/- 9% scheduled for coronary artery bypass surgery. Regional perfusion and metabolism were assessed by positron emission tomography, and regional function was assessed by echocardiography. Perioperative myocardial biopsies were obtained from a control region and from a dysfunctional region. We analyzed biopsies for contents of noncollagen protein (NCP), ATP, ADP, AMP, glycogen and lactate. Six months after surgery we assessed wall motion by echocardiography to group patients in those with (n = 11) and without (n = 14) functional improvement.

RESULTS

Reversibly dysfunctional myocardium had reduced perfusion (0.59 +/- 0.16 vs. 0.69 +/- 0.20 ml/g/min, p < 0.05), similar glucose-tracer uptake (92 +/- 12 and 95 +/- 14%), ATP/ADP ratio (2.4 +/- 1.1 and 2.4 +/- 0.7), glycogen content (631 +/- 174 and 632 +/- 148 nmol/microg NCP) and lactate levels (59 +/- 27 and 52 +/- 29 nmol/microg NCP) compared with control regions. Irreversibly dysfunctional regions (n = 14) had severely reduced perfusion (0.48 +/- 0.15 vs. 0.72 +/- 0.12 ml/g/min, p < 0.001) and glucose-tracer uptake (52 +/- 16 vs. 94 +/- 15%, p < 0.001), reduced ATP/ADP ratio (1.5 +/- 0.9 vs. 2.3 +/- 0.9, p < 0.05), similar glycogen content (579 +/- 265 vs. 593 +/- 127 nmol/microg NCP) and increased lactate levels (114 +/- 52 vs. 89 +/- 24 nmol/microg NCP, p < 0.01) compared with control regions.

CONCLUSIONS

Contents of metabolic energy stores and lactate in chronic reversibly dysfunctional myocardium were preserved. In contrast, energy stores were depleted in myocardium without functional recovery after revascularization.

Noninvasive characterization of stunned, hibernating, remodeled and nonviable myocardium in ischemic cardiomyopathy

OBJECTIVES

We evaluated a novel protocol of dual-isotope, gated single-photon emission computed tomographic (SPECT) imaging combined with low and high dose dobutamine as a single test for the characterization of various types of altered myocardial dysfunction.

BACKGROUND

Myocardial perfusion tomography and echocardiography have been used separately for the assessment of myocardial viability. However, it is possible to assess perfusion, function and contractile reserve using gated SPECT imaging.

METHODS

We studied 54 patients with ischemic cardiomyopathy using rest and 4 h redistribution thallium-201 imaging and dobutamine technetium-99m sestamibi SPECT imaging. The sestamibi images were acquired 1 h after infusion of the maximal tolerated dose of dobutamine and again during infusion of dobutamine at a low dose to estimate contractile reserve. Myocardial segments were defined as hibernating, stunned, remodeled or scarred.

RESULTS

Severe regional dysfunction was present in 584 (54%) of 1,080 segments. Based on the combination of function and perfusion characteristics in these 584 segments, 24% (n = 140) were labeled as hibernating; 23% (n = 136) as stunned; 30% (n = 177) as remodeled; and 22% (n = 131) as scarred. Contractile reserve, represented by improvement in wall motion/thickening by low dose dobutamine, was observed in 83% of stunned, 59% of hibernating, 35% of remodeled and 13% of scarred myocardial segments (p<0.05).

CONCLUSIONS

It is possible with this new imaging technique to characterize dysfunctional myocardium as stunned, hibernating, remodeled and nonviable. These subtypes often coexist in the same patient.

Regional myocardial oxygen consumption estimated by carbon-11 acetate and positron emission tomography before and after repetitive ischemia

BACKGROUND

Preserved myocardial oxygen consumption estimated by carbon 11-acetate and positron emission tomography (PET) in myocardial regions with chronic but reversibly depressed contractile function in patients with ischemic heart disease have been suggested to be caused by repeated short episodes of acute myocardial ischemia. To evaluate this hypothesis myocardial 11C-acetate PET imaging was performed before and after acute repetitive myocardial ischemia.

METHODS AND RESULTS

In open chest dogs (n = 8), the left anterior descending coronary artery was occluded 4 times for 5 minutes alternating with 5 minutes of reperfusion. Before and after repetitive coronary occlusions, oxygen 15 water/oxygen 15 carbon monoxide (blood flow), and 11C-acetate (oxygen consumption) PET imaging were performed. Left ventricular regional systolic wall thickening was measured with sonomicrometry. Forty-five minutes after the ischemic episodes, systolic ventricular wall thickening was decreased by 90%, whereas myocardial blood flow was reduced by 21% compared with baseline values (P < .05). Ninety minutes after the ischemic episodes, estimated oxygen consumption was unaltered compared with the baseline level despite a sustained 70% decrease in the regional contractile function (P < .05).

CONCLUSIONS

Oxygen consumption estimated by 11C-acetate PET imaging is preserved after repeated episodes of acute myocardial ischemia despite a severe impairment of contractile function.

[Nuclear cardiology: technical bases and clinical applications]

Although the role of nuclear cardiology is currently well consolidated, the addition of new radiotracers and modern techniques makes it necessary to continuously update the requirements, equipment and clinical applications of these isotopic tests. The characteristics of the radioisotopic drugs and examinations presently used are explained in the first part of this text. In the second, the indications of them in diagnostic and prognostic evaluation of the different coronary diseases are presented.

Blood flow-metabolism imaging with positron emission tomography in patients with diabetes mellitus for the assessment of reversible left ventricular contractile dysfunction

OBJECTIVES

The purpose of this study was to evaluate the predictive accuracy of positron emission tomography (PET) blood flow-F-18 fluorodeoxyglucose (FDG) imaging in coronary artery disease (CAD) patients with diabetes mellitus (DM).

BACKGROUND

Positron emission tomography accurately predicts the postrevascularization improvement in left ventricular dysfunction in unselected patients with CAD. In diabetic patients, however, poor myocardial glucose utilization may limit the accuracy of the approach.

METHODS

Forty patients (64+/-10 years old; 19 with DM = group I; 21 without DM = group II) with reduced left ventricular ejection fraction (LVEF = 29+/-6%) were studied with N-13 ammonia and FDG PET before coronary revascularization. Studies were performed after intravenous injection of regular insulin (group I) or oral glucose administration (group II). Blood flow-FDG mismatches and matches were identified by polar map analysis in the three vascular territories of the left anterior descending, left circumflex and right coronary artery. Wall motion and LVEF were assessed by two-dimensional echocardiography before and 158+/-123 days after revascularization.

RESULTS

Of 107 vascular territories analyzed, 46 were classified as mismatch, 29 as match and 32 as normal. The FDG image quality, assessed by F-18 myocardium to blood pool activity ratios, and the predictive accuracy were similar in both groups; presence of a blood flow/FDG mismatch had a sensitivity of 92% (group I) and 94% (group II) and a specificity of 85% (group I) and 79% (group II) for an improvement in regional left ventricular function. A postrevascularization improvement in global left ventricular function was related to the extent of blood flow/FDG mismatch; LVEF increased from 30+/-7% to 35+/-7% (p = 0.017) in patients with one mismatch and from 27+/-4% to 41+/-7% (p < 0.001) in those with two mismatches.

CONCLUSIONS

The predictive accuracy of blood flow/FDG imaging is maintained in patients with DM when a clinically acceptable study protocol, which guarantees good FDG image quality, is used. The extent of a blood flow/metabolism mismatch is correlated with the magnitude of the postrevascularization improvement in global left ventricular function.

Prevalence of hibernating myocardium in patients with severely impaired ischaemic left ventricles

OBJECTIVE

Severe impairment of left ventricular (LV) contraction is associated with an adverse prognosis in patients with ischaemic heart disease. Revascularisation may improve the impaired LV contraction if hibernating myocardium is present. The proportion of patients likely to benefit from this intervention is unknown. Therefore, the prevalence of hibernating myocardium in patients with ischaemic heart disease and severe impairment of LV contraction was assessed.

DESIGN

From a consecutive series of patients undergoing coronary angiography for the investigation of chest pain or LV impairment, all patients with ischaemic heart disease and an LV ejection fraction (LVEF) < or = 30% were identified. These patients underwent positron emission tomography (PET) to detect hibernating myocardium, identified by perfusion metabolism mismatch.

SETTING

A teaching hospital directly serving 500,000 people.

RESULTS

Of a total of 301 patients, 36 had ischaemic heart disease and an LVEF < or = 30%. Twenty-seven patients had PET images, while nine patients were not imaged because of emergency revascularisation (three), loss to follow up (one), inability to give consent (four), and age < 50 years (one, ethics committee guidelines). Imaged and non-imaged groups were similar in LV impairment, demographic characteristics, and risk factor profile. Fourteen patients (52% of the imaged or 39% of all patients with ischaemic heart disease and LVEF < or = 30%) had significant areas of hibernating myocardium on PET.

CONCLUSION

It is possible that up to 50% of patients with ischaemic heart disease and severely impaired left ventricles have hibernating myocardium.

Thallium scintigraphy compared with 18F-fluorodeoxyglucose positron emission tomography for assessing myocardial viability in patients with moderate versus severe left ventricular dysfunction

Thallium-201 reinjection imaging and positron emission tomography provide concordant information regarding myocardial viability in many patients with coronary artery disease and left ventricular (LV) dysfunction. It is unclear whether this concordance applies to patients with severe, as well as those with moderate, LV dysfunction. We studied 44 patients with chronic coronary artery disease and LV dysfunction, subgrouped on the basis of severity of dysfunction: 23 patients had moderate and 21 had severe dysfunction (ejection fractions 34 +/- 6% and 19 +/- 6%). Patients underwent exercise thallium single-photon emission computed tomography (SPECT) with 3- to 4-hour redistribution and reinjection imaging, as well as positron emission tomography (PET) imaging with 18fluorodeoxyglucose and 15O-water. Data were analyzed quantitatively in aligned transaxial PET and SPECT tomograms. A myocardial region was considered nonviable by PET if 18fluorodeoxyglucose activity was <50% of that in a normal region, associated with proportional reduction in blood flow. Similarly, regions were considered nonviable by thallium if activity was <50% of activity in normal regions on redistribution and reinjection studies. Thallium SPECT and PET data were concordant regarding viability in 98% and 93% of myocardial regions, respectively, in patients with moderate and with severe LV dysfunction. Lower concordance was observed only when regions with severe irreversible thallium perfusion defects on redistribution images were considered in both groups: 86% and 78%, respectively (p <0.01). Thus, thallium SPECT with reinjection yields information regarding regional myocardial viability that is similar to that provided by PET in patients with severe as well as moderate LV dysfunction. However, there is discordance in >20% of regions manifesting severe irreversible thallium defects in patients with severely reduced LV function.

Cardiac positron emission tomography

Positron emission tomography (PET) is an intrinsically quantitative tool that provides a unique and unparalleled approach for clinicians and researchers to interrogate the heart noninvasively. The ability to label substances of physiological interest with positron-emitting radioisotopes has permitted insight into normal blood flow and metabolism and the alterations that occur with disease states. The efficacies of interventional therapies also have been demonstrated with cardiac PET. PET is unequaled in establishing the presence or absence of coronary artery disease (CAD) as well as for assessment of myocardial viability. Using mathematically and physiologically appropriate models, myocardial blood flow, metabolism, and ligand density and flux can be measured noninvasively, providing physicians and researchers with an exceptional window to the heart. Future advances in both instrumentation as well as radiochemistry and image processing will improve our understanding of the heart under normal conditions as well as with disease and should provide therapeutic approaches to enhancing the treatment of patients with heart disease of diverse etiologies.

Coronary revascularization in the treatment of moderate and severe postischemic left ventricular dysfunction

Chronic postischemic left ventricular (LV) dysfunction can improve following coronary revascularization (hibernating myocardium). However, it is not clear whether the severity of LV dysfunction determines functional outcome after revascularization and the accuracy of tests to predict myocardial viability. We studied 47 patients with coronary artery disease and chronic LV dysfunction. Before coronary bypass, patients underwent (18F)2-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) during euglycemic hyperinsulinemic clamp to assess viability. Global and regional LV function were assessed before and 4 to 6 months after surgery. Patients were arbitrarily divided into 2 groups with moderate and severe LV dysfunction. Group 1 (n = 26) had an ejection fraction (EF) of < or = 30% and group 2 (n = 21) > 30%. After bypass, the EF (22+/-6% vs 31+/-10%; p <0.0001) and global wall motion score (WMS) (2.05+/-0.39 vs 1.56+/-0.34; p <0.001) improved in group 1, whereas the EF (43+/-9% vs 43+/-12%; p = NS) was unchanged in group 2, although WMS tended to improve (1.42+/-0.38 vs 1.32+/-0.39; p = 0.09). The proportion of dysfunctional segments (72% vs 32%; p <0.0001) and FDG uptake in these segments (0.44+/-0.15 vs 0.34+/-0.15 micromol/g/min, p <0.0001) were greater in group 1 than in group 2. The baseline EF influenced the predictive accuracy of PET, with highest positive predictive accuracy in group 2 and highest negative predictive accuracy in group 1. Thus, coronary revascularization has the potential for greatest benefit in patients with the most severe dysfunction, but with evidence of viability, and the entity of LV dysfunction affects the predictive accuracy of viability studies.

Evaluation of hibernating myocardium in patients with ischemic heart disease

Patients with ischemic heart disease and significant left ventricular dysfunction are often difficult to manage medically. Revascularization procedures may improve left ventricular function and prognosis in this population if hypocontractile yet viable myocardium (hibernating myocardium) is demonstrated. Nuclear cardiology studies (single photon and positron methods), two-dimensional echocardiography, and magnetic resonance imaging studies have been utilized to identify hibernating myocardium. If thallium-201 studies are performed, the use of reinjection of thallium and repeat imaging improves the sensitivity of these studies for the detection of viable myocardium. Dobutamine echocardiographic studies may have a higher specificity and positive predictive value for the subsequent improvement of regional systolic left ventricular function after revascularization than the nuclear techniques. However, thallium studies have an excellent negative predictive value. Positron emission tomography (PET) allows the simultaneous assessment of perfusion and metabolic activity; however, these studies are expensive and not widely available. Functional evaluation with PET is in its infancy. Functional cardiac magnetic resonance imaging (MRI), although not widely available yet, provides the most accurate evaluation of regional ventricular function. MRI spectroscopy may be utilized to assess myocardial viability. As acquisition times improve and "real-time" imaging becomes a reality, MRI and MRI spectroscopy will likely become very accurate tools for assessing functional reserve and metabolic activity. The selection of the most appropriate method for assessment of myocardial viability will include consideration of a patient's characteristics, the presence of coronary arterial tree amenable to revascularization techniques, the techniques available to the clinician to assess viability, and local revascularization experience in this population. The result of an individual patient's evaluation is relevant to the consideration of coronary revascularization, or if this is not possible, cardiac transplantation.

Fatty acid uptake is preserved in chronically dysfunctional but viable myocardium

Glucose uptake appears preserved or even enhanced in the chronically dysfunctional but viable myocardium. However, the use of other fuels such as free fatty acids (FFA) remains unknown. We studied FFA uptake in the chronically dysfunctional but viable myocardium in seven patients with an occluded major coronary artery and a corresponding chronic wall motion abnormality but no previous infarction. Myocardial FFA uptake kinetics in the fasting state were measured with positron emission tomography (PET) and 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA). The FFA uptake index was calculated by multiplying the fractional [18F]FTHA uptake with serum FFA concentration. Myocardial blood flow (MBF) was measured with [15O]H2O and PET. Myocardial viability was confirmed with a static 18F-labeled 2-fluoro-2-deoxy-D-glucose PET imaging and a follow-up echocardiography in the revascularized patients. Regional MBF was slightly but not significantly lower in the dysfunctional compared with normal myocardial segments (0.76 +/- 0.18 vs. 0.81 +/- 0.14 ml.min-1.g-1, means +/- SD; P = 0.16). The fractional [18F]FTHA uptake rates [0.11 +/- 0.03 vs. 0.11 +/- 0.04 ml.g-1.min-1; not significant (NS)], and the FFA uptake indexes (5.8 +/- 1.7 vs. 5.8 +/- 2.1 mumol.100g-1.min-1; NS) were similar in the dysfunctional but viable and in the normal myocardial regions. Thus, in the chronically dysfunctional but viable (collateral-dependent) myocardium, the fatty acid uptake probed by [18F]FTHA appears preserved. Taken together with preserved glucose uptake, the results indicate that there is uncoupling of substrate uptake and mechanical function in the chronically dysfunctional but viable myocardium.

[Clinical usefulness of positron emission tomography (PET) in the evaluation of myocardial viability]

Positron emission tomography (PET) is a radionuclide imaging technique that allows quantitative assessment of regional myocardial function. It is mainly used in clinics to assess viability of dissynergic myocardium, by means of combined images of flow (with ammonia) and metabolism (with fluordeoxyglucose). The mismatch pattern, with an increase in fluordeoxyglucose metabolism in hypoperfused regions, is indicative of viability. The match pattern (a decrease in flow and metabolism in the same areas) is indicative of necrosis. Viability can also be assessed with water or fluordeoxyglucose independently quantified. Other promising methods are based in the study of oxygen consumption with 11C acetate and the study of hypoxia with 18F-misonidazole.

Recent advances in nuclear cardiology in the study of coronary artery disease

A variety of new radiopharmaceutical agents have been introduced to probe myocardial function in vivo. This review will introduce these new techniques which have recently been available in Japan. Tc-99m perfusion imaging agents provide excellent myocardial perfusion images which may enhance diagnostic accuracy in the study of coronary artery disease. In addition, greater photon flux from the tracer permits simultaneous assessment of regional perfusion and function with use of first-pass angiography or ECG-gated acquisition. Positron emission tomography enables metabolic assessment in vivo. Preserved FDG uptake indicates ischemic but viable myocardium which is likely to improve regional dysfunction after revascularization. In addition, FDG-PET seems to be valuable for selecting a high risk subgroup. Recently I-123 BMIPP, a branched fatty acid analog, has been clinically available in Japan. Less uptake of BMIPP than thallium is often observed in the ischemic myocardium. Such perfusion metabolic mismatch which seems to be similarly observed in FDG-PET is identified in the stunned or hibernating myocardium with regional dysfunction. Both of them are likely to recover afterwards. Severe ischemia is identified as reduced BMIPP uptake at rest, suggesting its role as an ischemic memory imaging. I-123 MIBG uptake in the myocardium reflects adrenergic neuronal function in vivo. In the study of coronary artery disease, neuronal denervation is often observed around the infarcted myocardium and post ischemic region as well. More importantly, reduced MIBG uptake in these patients can identify high risk for ventricular arrhythmias and assess severity of congestive heart failure. These new techniques will provide insights into new pathological states in the ischemic heart disease and enable to select optimal treatment in these patients.

Metabolic imaging using F18-fluorodeoxyglucose to assess myocardial viability

Over the past 10 years, F18-fluorodeoxyglucose (FDG) imaging with positron emission tomography (PET) has emerged as an important technique in the delineation of myocardial viability. Using this technique it has become possible to predict recovery of ventricular function after revascularization in patients with chronic coronary artery disease. Data from long-term (although retrospective) follow-up studies have demonstrated that patients with viable myocardium on FDG PET who do not undergo revascularization are prone to cardiac events, including cardiac death and non-fatal infarction. The same studies have pointed out that patients with viable tissue on FDG PET, who do undergo revascularization, improve substantially in symptoms related to congestive heart failure. To allow FDG imaging in centers without PET equipment, recent studies have evaluated the use of FDG imaging with single photon emission computed tomography (SPECT) and 511 keV collimators. Preliminary data using this alternative approach are promising, but need further confirmation. In this review the experience with FDG imaging (using either PET or SPECT) in the assessment of tissue viability in patients with coronary artery disease will be discussed.

Correlation of functional recovery with myocardial blood flow, glucose uptake, and morphologic features in patients with chronic left ventricular ischemic dysfunction undergoing coronary artery bypass grafting

OBJECTIVE

Our objective was to investigate the influence of preoperative myocardial ultrastructure and metabolism on recovery of contractile function after coronary artery bypass grafting in patients with coronary artery disease and left ventricular dysfunction.

METHODS

Dynamic positron emission tomography with 13N-ammonia and 18F-deoxyglucose was used to assess myocardial perfusion and glucose uptake in 53 patients scheduled for coronary revascularization because of coronary artery disease and left ventricular dysfunction. The degree of tissue fibrosis and the presence of potentially reversible alterations of cardiomyocytes (loss of myofilaments and accumulation of glycogen) were quantified from transmural biopsy specimens. These were harvested from the center of the dysfunctional area during the operation and analyzed with a light microscope. The recovery of contractile performance was assessed from the changes in left ventricular function at contrast ventriculography or echocardiography before and 6 months after the operation.

RESULTS

According to postoperative changes in regional wall motion, left ventricular function was considered to have improved in 34 patients, whereas dysfunction persisted in 19 patients. In patients with improved wall motion, ejection fraction rose by 12% and end-systolic volume decreased by 28%. By contrast, in patients with persistent dysfunction, ejection fraction decreased by 6% and end-systolic volume increased by 25%. Before revascularization, myocardium with reversible dysfunction displayed higher levels of absolute myocardial blood flow, higher myocardial glucose uptake, less tissue fibrosis, and more altered cardiomyocytes than myocardium with persistent dysfunction. Significant correlations were found between regional blood flow and the surface of the biopsy specimen covered by fibrosis, as well as between glucose uptake and the density of altered cardiomyocytes.

CONCLUSION

In patients with left ventricular ischemic dysfunction, the recovery of regional and global left ventricular function after surgical revascularization is associated with higher preoperative blood flow and glucose uptake, with less tissue fibrosis and a higher amount of viable cardiomyocytes in the dysfunctional area. The current study thus confirms the value of noninvasive preoperative metabolic imaging for identification of residual viable myocardium and for prediction of the functional outcome after revascularization.

Myocardial viability: methods of assessment and clinical relevance

Myocardial hibernation is hypoperfused dysfunctional myocardium that has the potential to recover function after coronary revascularization. Although recovery of regional function after revascularization is the gold standard for assessing the diagnostic accuracy of various techniques, improvements of EF, symptoms, and survival are fundamental end points. Despite important differences in the markers of viability by positron-emission tomography, single-photon emission tomography, two-dimensional echocardiography, and magnetic resonance imaging, their positive and negative predictive values in nonrandomized studies are fairly comparable. Assessment of myocardial viability may be clinically important in many patients but especially in those with EF < 30% and congestive heart failure. The degree of improvement in EF after coronary revascularization depends on the extent of hibernation, the suitability of coronary structure for revascularization, the lack of perioperative infarction, the completeness of revascularization, and the long-term patency of grafts.