Quantitative assessment of prolonged metabolic abnormalities in reperfused canine myocardium

Quantifying the Area at Risk in Reperfused ST-Segment-Elevation Myocardial Infarction Patients Using Hybrid Cardiac Positron Emission Tomography-Magnetic Resonance Imaging

BACKGROUND

Hybrid positron emission tomography and magnetic resonance allows the advantages of magnetic resonance in tissue characterizing the myocardium to be combined with the unique metabolic insights of positron emission tomography. We hypothesized that the area of reduced myocardial glucose uptake would closely match the area at risk delineated by T2 mapping in ST-segment-elevation myocardial infarction patients.

METHODS AND RESULTS

Hybrid positron emission tomography and magnetic resonance using (18)F-fluorodeoxyglucose (FDG) for glucose uptake was performed in 21 ST-segment-elevation myocardial infarction patients at a median of 5 days. Follow-up scans were performed in a subset of patients 12 months later. The area of reduced FDG uptake was significantly larger than the infarct size quantified by late gadolinium enhancement (37.2±11.6% versus 22.3±11.7%; P<0.001) and closely matched the area at risk by T2 mapping (37.2±11.6% versus 36.3±12.2%; P=0.10, R=0.98, bias 0.9±4.4%). On the follow-up scans, the area of reduced FDG uptake was significantly smaller in size when compared with the acute scans (19.5 [6.3%-31.8%] versus 44.0 [21.3%-55.3%]; P=0.002) and closely correlated with the areas of late gadolinium enhancement (R 0.98) with a small bias of 2.0±5.6%. An FDG uptake of ≥45% on the acute scans could predict viable myocardium on the follow-up scan. Both transmural extent of late gadolinium enhancement and FDG uptake on the acute scan performed equally well to predict segmental wall motion recovery.

CONCLUSIONS

Hybrid positron emission tomography and magnetic resonance in the reperfused ST-segment-elevation myocardial infarction patients showed reduced myocardial glucose uptake within the area at risk and closely matched the area at risk delineated by T2 mapping. FDG uptake, as well as transmural extent of late gadolinium enhancement, acutely can identify viable myocardial segments.

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.

Imaging myocardial metabolic remodeling

Myocardial metabolic remodeling is the process in which the heart loses its ability to utilize different substrates, becoming dependent primarily on the metabolism of a single substrate such as glucose or fatty acids for energy production. Myocardial metabolic remodeling is central to the pathogenesis of a variety of cardiac disease processes such as left ventricular hypertrophy, myocardial ischemia, and diabetic cardiomyopathy. As a consequence, there is a growing demand for accurate noninvasive imaging approaches of various aspects of myocardial substrate metabolism that can be performed in both humans and small-animal models of disease, facilitating the crosstalk between the bedside and the bench and leading to improved patient management paradigms. SPECT, PET, and MR spectroscopy are the most commonly used imaging techniques. Discussed in this review are the strengths and weaknesses of these various imaging methods and how they are furthering our understanding of the role of myocardial remodeling in cardiovascular disease. In addition, the role of ultrasound to detect the inflammatory response to myocardial ischemia will be discussed.

Combined thallium-201 and dynamic iodine-123 iodophenylpentadecanoic acid single-photon emission computed tomography in patients after acute myocardial infarction with effective reperfusion

BACKGROUND

Considerable derangements of energy metabolism are to be expected during ischemia and reperfusion. In ischemic myocardium, the oxidative degradation of carbohydrates is shifted toward the anaerobic production of lactate and the oxidation of fatty acids is suppressed.

HYPOTHESIS

The aim of this study was to examine the uptake and metabolism of iodine-123 (123I) iodophenylpentadecanoic acid (IPPA) in stunned myocardium.

METHODS

In 15 patients, SPECT with 201Tl and 123I IPPA as well as echocardiography with low-dose dobutamine stimulation were performed 12 +/- 5 days after myocardial infarction with reperfusion. Follow-up echocardiography was carried out 24 +/- 8 days later for documentation of functional improvement. Uptake of 201Tl and 123I IPPA were obtained in five left ventricular segments, and dynamic SPECT imaging was used for calculation of the fast and the slow components of the biexponential myocardial 123I IPPA clearance.

RESULTS

Wall motion improved in 14 of 26 dysfunctional segments (54%). Stunned segments were characterized by a reduced 123I IPPA extraction, a shorter half-life of the fast, and a longer half-life of the slow clearance component. All parameters of the combined 201Tl/123I IPPA study predicted functional recovery with similar accuracies (area under the receiver operator characteristic curves between 0.68 and 0.76; p = NS). Analysis of 201Tl uptake alone could not predict functional recovery in this study.

CONCLUSIONS

Stunned myocardium is characterized by a disturbance of fatty acid metabolism. For prediction of functional improvement, 123I IPPA imaging added significant diagnostic information.

Independency of myocardial stunning of endothelial stunning?

BACKGROUND AND OBJECTIVE

Vascular endothelial cells play an important role in the control of vascular tone. The reasons for coronary endothelial dysfunction are complex and may involve ischemia/reperfusion injury. We investigated whether endothelial, smooth muscle, and myocardial dysfunction are independent phenomena.

METHODS

Rabbit hearts were rapidly excised without intermittent ischemia, connected to a modified Langendorff apparatus, and perfused with a modified Krebs-Henseleit solution containing bovine erythrocytes. Normoxic control hearts (n = 16) were perfused for 125 min. Postischemic hearts (n = 15) were perfused for 45 min, submitted to global ischemia (20 min) and reperfused (60 min). Both the normoxic and the postischemic hearts were divided into three groups that received either 0.9% NaCl (placebo), or 3-morpholinosydnonimine (SIN-1; 100 microM),or substance P (SP; 5 nM).

RESULTS

After SIN-1, CBF in the normoxic hearts was increased by maximum 63% and after SP by 62%. 60 min after the onset of reperfusion, the postischemic hearts of both groups had recovered to 95% LVP(max). In the postischemic hearts, SIN-1 increased CBF still by 58%, while the endothelium-dependent vasomotion was impaired: SP improved CBF by only 9%.

SUMMARY AND CONCLUSIONS

The particular protocol permitted differentiation between myocardial and vascular stunning. The results show that, while myocardial function has already recovered, endothelial cells are more severely impaired than smooth muscle cells, and that this injury persists beyond myocardial stunning. Thus, endothelial-dependent dysfunction can still impair vasodilatation while ventricular dysfunction has already resolved.

Comparative assessment of18F-fluorodeoxyglucose PET and99mTc-tetrofosmin SPECT for the prediction of functional recovery in patients with reperfused acute myocardial infarction

PURPOSE

Although preserved glucose metabolism is considered to be a marker of myocardial viability in the chronic stage, it has not been fully elucidated whether this is also true with regard to reperfused acute myocardial infarction (AMI). The aim of this study was to compare the diagnostic performance of(99m)Tc-tetrofosmin SPECT and(18)F-fluorodeoxyglucose (FDG) PET for the prediction of functional recovery in reperfused AMI.

METHODS

The study population comprised 28 patients. Both tetrofosmin SPECT and FDG PET were performed in all 28 patients at ca. 2 weeks and in 23 at 6 months. The tetrofosmin and FDG findings in infarct-related segments were compared with the regional wall motion score assessed by left ventriculography over 6 months to determine the predictive value for functional recovery.

RESULTS

Of 120 infarct-related segments, 83 had preserved flow (tetrofosmin uptake >/=50%) and 81 had preserved glucose metabolism (FDG uptake >/=40%). The sensitivity and specificity of tetrofosmin SPECT for the prediction of functional recovery tended to be superior to those of FDG PET (90.0% and 72.5% vs 85.0% and 67.5%, respectively). Thirteen segments with preserved flow and decreased glucose metabolism demonstrated marked recovery of contractile function from 2.5+/-1.0 to 1.4+/-1.4 (p<0.01), with restoration of glucose metabolism at 6 months. In contrast, 11 segments with decreased flow and preserved glucose metabolism demonstrated incomplete functional improvement from 3.0+/-0.8 to 2.2+/-1.2.

CONCLUSION

In the subacute phase, preserved myocardial blood flow is more reliable than glucose metabolism in predicting functional recovery in reperfused myocardium.

Influence of .BETA.-Adrenoceptor Blockade on the Myocardial Accumulation of Fatty Acid Tracer and Its Intracellular Metabolism in the Heart After Ischemia-Reperfusion Injury

BACKGROUND

Increases in sympathetic nerve activity during ischemia may increase intracellular fatty acid (FA) accumulation via enhanced FA uptake and inhibition of beta-oxidation. Therefore, the beneficial effects of beta-adrenoceptor blockade on myocardial ischemic injury might result from the suppression of FA accumulation.

METHODS AND RESULTS

Carvedilol (1 mg/kg) or propranolol (1 mg/kg) was injected 10 min before 15-min occlusion of coronary artery in rats. Myocardial FA accumulation and intracellular metabolites of FA tracer were determined 3 days after reperfusion using (125)I-and (131)I-9-metylpentadecanoic acid (9MPA). Carvedilol significantly decreased 9MPA accumulation in both the ischemic region (IR) and non-IR, as compared with vehicle, and increased its clearance. However, the non-metabolized 9MPA fraction was not different between carvedilol- and vehicle-treated rats. Consequently, the amount of non-metabolized 9MPA in the myocardium was lower in rats treated with carvedilol than in those given vehicle. These effects of carvedilol were not different from those of propranolol.

CONCLUSION

Beta-adrenoceptor blockade did not affect a visual assessment of the autoradiographic image of 9MPA in hearts subjected to ischemia-reperfusion, but it accelerated the clearance of 9MPA in both the IR and non-IR. The administration of beta-blockade before ischemia could accelerate the recovery from ischemia-reperfusion injury by inhibiting myocardial FA accumulation before beta-oxidation.

18F-deoxyglucose and the assessment of myocardial viability

The glucose analogue 18F-deoxyglucose allowed for the first time the ability to noninvasively probe and characterize the regional metabolism of glucose as a major fuel substrate of the human heart. Used with positron emission tomography, it became the tool for demonstrating the operation of metabolic processes, long before established in invasive or destructive experiments in animals, directly in the human myocardium. Clinical investigations with 18F-deoxy-glucose, combined with other radiotracers of the myocardium's substrate metabolism, showed the dependency of the heart's substrate selection on circulating levels of glucose, free fatty acid and insulin, and the operation of Randle's cycle in the human myocardium. Regional responses in substrate metabolism to the myocardial ischemia were now visualized entirely noninvasively as, for example, decreases in fatty acid usage and oxidation and oxygen consumption, but foremost as an increase in glucose use. Regional 18F-deoxyglucose uptake markedly in excess of myocardial blood flow in dysfunctional myocardium of patients after a myocardial infarction, with chronic coronary artery disease or with ischemic cardiomyopathy, soon became recognized as a hallmark of myocardial viability or potentially reversible contractile dysfunction. Defined as blood flow metabolism mismatch, this particular regional glucose uptake pattern identifies patients to be at high risk for cardiac events and, at the same time, to benefit most from surgical revascularization. The patterns predict a postrevascularization improvement in global left ventricular function and, even more important, in symptoms related to congestive heart failure and in long-term survival. 18F-deoxyglucose is now widely used with positron emission tomography and, more recently, with single photon emission computed tomography and radiotracers of myocardial perfursion for stratifying ischemic cardiomyopathy patients to the most efficacious treatment.

Glucose uptake and glycogen levels are increased in pig heart after repetitive ischemia

Repetitive myocardial ischemia increases glucose uptake, but the effect on glycogen is unclear. Thirteen swine instrumented with a hydraulic occluder on the circumflex (Cx) artery underwent 10-min occlusions twice per day for 4 days. After 24 h postfinal ischemia and in the fasted state, echocardiogram and positron emission tomography imaging for blood flow ([(13)N]-ammonia) and 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG) uptake were obtained. Tissue was then collected for ATP, creatine phosphate (CP), glycogen, and glucose transporter-4 content, and hexokinase activity. After reperfusion, regional function and CP-to-ATP ratios in the Cx and remote regions were similar. Despite the absence of stunning, the Cx region demonstrated higher glycogen levels (33 +/- 11 vs. 24 +/- 11 micromol/g; P < 0.05), and this increase correlated well with the increase in FDG uptake (r(2) = 0.78; P < 0.01). Hexokinase activity was also increased relative to remote regions (0.62 +/- 0.29 vs. 0.37 +/- 0.19 IU/g; P < 0.05), with no difference in GLUT-4 content. In summary, 24 h after repetitive ischemia, glucose uptake and glycogen levels are increased at a time that functional and bioenergetic markers of stunning have recovered. The significant correlation between glycogen content and FDG accumulation in the postischemic region suggests that increased rates of glucose transport and/or phosphorylation are linked to increased glycogen levels in hearts subjected to repetitive bouts of ischemia.

Dynamic changes in cardiac fatty acid metabolism in the stunned human myocardium

BACKGROUND

The chronological changes or mechanisms in cardiac fatty acid metabolism under clinical conditions of hypoxia and ischemia have not been fully elucidated. 123-15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid (BMIPP) can be used with single photon emission computed tomography (SPECT) to evaluate myocardial fatty acid metabolism. We investigated chronological changes in energy metabolism in the stunned human myocardium by means of 123I-BMIPP myocardial SPECT.

METHODS AND RESULTS

We conducted 123I-BMIPP myocardial SPECT in 10 patients with stunned myocardium during the acute, subacute and chronic phases after onset. The left ventricle was divided into 9 regions on SPECT, and the degree of abnormalities in each region was scored in four grades from normal (0) to defect (4). We also examined wash-out rates on BMIPP images. The scores on early BMIPP images in the acute, subacute and chronic phases were 5.6 +/- 1.8, 13.4 +/- 3.5 and 2.5 +/- 1.1, respectively, and the score was highest in the subacute phase (p < 0.001). Similarly, scores on the late images were 2.3 +/- 1.7, 18.3 +/- 4.5 and 4.7 +/- 2.6, respectively, and highest in the subacute phase (p < 0.001). The wash-out rates (normal: 18.2 +/- 2.1%) in the acute, subacute and chronic phases were 12.1 +/- 4.8%, 44.9 +/- 10.0% and 23.1 +/- 4.6%, respectively, with the value being lowest during the acute phase (p < 0.05), and highest during the subacute phase (p < 0.001).

CONCLUSION

These results suggested that fatty acid metabolism in the stunned human myocardium changes dynamically over time.

Postrevascularization recovery of fatty acid utilization in ischemic myocardium: a randomized clinical trial of potassium channel opener

BACKGROUND

Abnormal fatty acid metabolism persists in hibernating myocardium, even after reperfusion. This study was designed to determine whether the K+ channel opener, nicorandil, improves fatty acid utilization after percutaneous transluminal coronary angioplasty (PTCA).

METHODS

Patients undergoing elective PTCA were randomly assigned to treatment (group N, n = 26) or control groups (group C, n = 22). Group N received intracoronary and intravenous nicorandil during PTCA. Myocardial fatty acid use and perfusion were quantitatively evaluated by means of iodine-123-beta-methyl-p-iodophenyl-pentadecanoic acid single photon emission computed tomography (I-123 BMIPP SPECT) and thallium-201 (Tl-201) imaging before PTCA, 72 hours after PTCA, and 3 months after PTCA. Left ventricular function was also evaluated by means of contrast ventriculography before and 3 to 6 months after PTCA.

RESULTS

The 1-123 BMIPP defect score in group N significantly decreased, from 28%+/-13% to 20%+/-20% after PTCA and to 18%+/-17% 3 months later. In contrast, the I-123 BMIPP defect score in group C increased from 28%+/-20% to 36%+/-15% (P<.05 versus group N) after PTCA, then returned to 28%+/-17% (P<.05 versus group N) 3 months after PTCA. Recovery of left ventricular function paralleled the recovery of I-123 BMIPP uptake.

CONCLUSIONS

Nicorandil improves the recovery of myocardial fatty acid utilization and cardiac function after PTCA. K(ATP) channel activation may have a protective effect during coronary artery occlusion and improve subsequent recovery.

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.

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.

Superiority of C-11 acetate compared with F-18 fluorodeoxyglucose in predicting myocardial functional recovery by positron emission tomography in patients with acute myocardial infarction

In patients with chronic coronary artery disease, preservation of myocardial oxidative metabolism measured by positron emission tomography (PET) with 11C-acetate is a more accurate predictor of subsequent myocardial functional recovery than is maintenance of glucose metabolism estimated with 18F-fluorodeoxyglucose. However, whether measurements of myocardial oxidative metabolism are more accurate than measurements of glucose metabolism in predicting functional recovery in patients with recent myocardial infarction is unknown. Myocardial oxidative metabolism was measured within 10 days of infarction in 19 patients by analysis of the rate of myocardial clearance of 11C-acetate. Metabolism of glucose was assessed by analysis of the uptake of 18F-fluorodeoxyglucose. Criteria for prediction of the recovery of function based on measurements of oxidative metabolism and glucose metabolism were compared. Threshold criteria with 11C-acetate exhibited superior positive and negative predictive values (89% and 73%, respectively) compared with the criteria of 18F-fluorodeoxyglucose (65% and 57%, respectively) (p <0.025). In addition, the magnitude of functional recovery after revascularization correlated with the severity of the metabolic abnormality present initially. In patients with recent myocardial infarction, the extent of functional recovery can be predicted accurately by measurement of regional oxidative metabolism by PET with 11C-acetate, and these measurements are superior to those of 18-fluorodeoxyglucose.

Analysis of respiratory water--a new method for evaluation of myocardial energy metabolism

Aerobic ATP synthesis via oxidative phosphorylation causes a proportional production of respiratory water. Thus the amount of respiratory water produced at a given time should be a reliable measure of the current ATP demand of the mammalian myocardium. Respiratory water from isolated rabbit hearts was labeled by using the stable oxygen isotope 18O. The hearts were perfused according to the method of Langendorff (O. Langendorff. Pfluegers Arch. 61: 291-332, 1895) with 18O2-equilibrated Krebs-Henseleit solution. Control hearts were exclusively perfused with carbogen-equilibrated Krebs-Henseleit solution. Myocardial tissue was then lyophilized; the extracted water and samples from the coronary venous effluent were converted to CO2 by using the guanidine hydrochloride technique. The delta 18O values within the CO2 samples were determined by mass spectrometry and related to the standard mean ocean water (SMOW) scale. Compared with control hearts, the 18O-labeled hearts exhibited a significant increase of delta 18O values from tissue water (-47.50 +/- 0.64 vs. -40.35 +/- 2.05% SMOW; P < 0.05). The values were also significantly increased in the coronary venous effluent after a perfusion time of only 50 s (-47.50 +/- 0.64 vs. -43.66 +/- 0.91% SMOW; P < 0.05). Thus this first adaptation of the guanidine hydrochloride technique on microliter samples of myocardial tissue water and coronary venous effluent demonstrates that this method can be used to evaluate both respiratory activity and the kinetics of cardiac metabolic processes.

Suppression of TCA cycle activity in the cardiac muscle cell by hydroperoxide-induced oxidant stress

Excess H2O2 contributes to myocardial reperfusion injury. We detail the effect of H2O2-induced oxidant stress on the tricarboxylic acid (TCA) cycle in isolated heart muscle cells. Cardiomyocyte exposure to bolus H2O2 ( > 50 microM) acutely suppressed TCA cycle activity. Loss of cardiomyocyte TCA cycle function on cellular H2O2 exposure was supported by the rapid in situ inactivation of aconitase along with cardiomyocyte membrane peroxidation. Without peroxidation, the loss of aconitase catalysis was itself sufficient to jeopardize TCA cycle activity. Only H2O2 dismutation completely preserved both cardiomyocyte aconitase activity and TCA cycle flux during H2O2 overload. Restoration of aconitase catalysis after alleviation of the oxidant insult was prohibited by cell-permeable metal chelators, and TCA cycle flux could not be reestablished in peroxidized cells, even if aconitase activity had recovered. The characteristics of aconitase inactivation-reactivation observed are consistent with adverse redox changes to the enzyme's (Fe-S) cluster. These data demonstrate that specific aspects of the TCA cycle in heart muscle are sensitive to H2O2-induced oxidative stress and identify a peroxidative component of the injury process.

Responses of blood flow, oxygen consumption, and contractile function to inotropic stimulation in stunned canine myocardium

To examine the effects of inotropic stimulation on regional myocardial blood flow (MBF), oxidative metabolism, and contractile function in stunned myocardium, nine closed-chest dogs were studied 2 hours postreperfusion after a 25 minute occlusion of the left anterior descending coronary artery (LAD). MBF was determined with microspheres, and regional myocardial oxygen consumption (MVO2) was estimated from the rate constant k1 of the rapid clearance phase of [1-11C] acetate time activity curves, recorded with dynamic positron emission tomography. Myocardium at risk was determined from [13N] ammonia images obtained during occlusion. Wall motion, assessed by two-dimensional echocardiography, was impaired in postischemic myocardium in all dogs 2 hours after reperfusion. Dobutamine infusion increased the rate pressure product by 70% +/- 31% and significantly improved contractile function in the postischemic region in all dogs. In remote myocardium, MVO2 increased from 5.7 +/- 1.2 to 8.6 +/- 1.6 mumol/gm/min, and blood flow from 0.87 +/- 0.16 to 1.52 +/- 0.42 ml/gm/min in response to dobutamine. In reperfused myocardium, MVO2 increased from 3.1 +/- 0.7 to 7.4 +/- 1.5 mumol/gm/min, and blood flow from 0.51 +/- 0.12 to 1.2 +/- 0.4 ml/gm/min. Oxygen extraction increased significantly in reperfused myocardium relative to remote myocardium consistent with a flow-limited response to dobutamine stimulation. The improvement in contractile function failed to correlate significantly with relative increases in MBF or MVO2, suggesting that mechanical function is not as tightly coupled as MBF and MVO2 in postischemic myocardium during inotropic stimulation.

Assessment of left ventricular dysfunction by nuclear cardiology

Nuclear cardiology techniques may be of help in evaluating the patient with symptoms of congestive heart failure and ventricular dysfunction in two respects: quantification of functional parameters by radionuclide angiography, and differentiation of viable from nonviable myocardium by perfusion and metabolic imaging. Left ventricular ejection fraction and volumes can be accurately assessed by equilibrium radionuclide angiography with a count-based method without any geometric assumptions. Indeed, because of its high reproducibility, this method is particularly suited for making sequential measurements in the same patient. The distinction between viable or reversible and scarred or irreversible dysfunctional myocardium can be made on the basis of myocardial perfusion, cell membrane integrity, and metabolic activity. Thallium myocardial imaging is used clinically to assess the first two parameters based on experimental data. Two clinical methods may be applied to the detection of viability: stress-redistribution-reinjection imaging or rest-redistribution imaging. In both of these, the severity of the reduction in thallium activity should be assessed to discriminate viable from nonviable myocardium. Stress-redistribution-reinjection thallium imaging should be the first approach, if possible, because inducible ischemia is a much more significant clinical variable in a patient with ventricular dysfunction in terms of management and risk assessment than is knowledge of myocardial viability. Positron emission tomography (PET) provides enhanced image resolution and correction for body attenuation, thereby overcoming the two major limitations of thallium imaging. In addition, it provides the capacity to quantitate regional blood flow and to assess regional metabolic activity independent of flow. Overall, the accuracies of thallium imaging (around 70%) and PET imaging (around 82%) are similar for the prediction of segmental changes after revascularization. However, in patients with poor global left ventricular function, the accuracy of PET seems to be better. Further studies are needed in a large number of patients evaluated for regional and global function to establish algorithms using thallium and PET imaging in dysfunctional myocardium. Dobutamine echocardiography should also be evaluated in these algorithms.

Positron emission tomographic measurements of absolute regional myocardial blood flow permits identification of nonviable myocardium in patients with chronic myocardial infarction

OBJECTIVES

This study tested the hypothesis that nonviable myocardium can be identified by quantitative measurements of regional myocardial blood flow obtained using positron emission tomography in conjunction with a mathematical model of nitrogen-13 (N-13) ammonia tracer kinetics.

BACKGROUND

Under steady state basal conditions there is a minimal level of blood flow required to sustain myocardial viability. Therefore, the hypothesis predicts that regions with flow below a certain threshold are likely to be composed primarily of scar.

METHODS

Studies were conducted in 26 patients with chronic myocardial infarction. Positron emission tomographic measurements of basal regional myocardial blood flow (N-13 ammonia) and fluorine-18 (F-18) fluorodeoxyglucose uptake were made and correlated with information about coronary anatomy and regional wall motion to assess myocardial viability.

RESULTS

In patients with chronic myocardial infarction, normal zone blood flow (0.81 +/- 0.32 ml/min per g [mean +/- SD]) was greater (p < 0.02) than that of border zones (0.59 +/- 0.29 ml/min per g), which in turn exceeded (p < 0.001) that of infarct zone flow (0.27 +/- 0.17 ml/min per g). Good correlation was noted between relative F-18 fluorodeoxyglucose uptake and relative regional myocardial blood flow in all zones (r = 0.63, p < 0.001). Mismatch between blood flow and F-18 fluorodeoxyglucose uptake, with a single exception, was not observed in any segment with blood flow < 0.25 ml/min per g. All dyskinetic segments (n = 5) also had blood flow < 0.25 ml/min per g. In contrast, 43 of 45 myocardial segments (23 patients) with normal contraction or only mild hypokinesia had flow > or = 0.39 ml/min per g (average flow 0.78 +/- 0.35 ml/min per g).

CONCLUSIONS

In patients with chronic myocardial infarction, myocardial viability is unlikely when basal regional myocardial blood flow is < 0.25 ml/min per g. Average basal flow in segments with normal or nearly normal wall motion is 0.78 +/- 0.35 ml/min per g. Thus, positron emission tomographic measurement of regional myocardial blood flow is helpful in identifying nonviable myocardium in these patients.

Methodology governing the assessment of myocardial glucose metabolism by positron emission tomography and fluorine 18-labeled fluorodeoxyglucose

Positron emission tomography (PET) with fluorine 18-labeled fluorodeoxyglucose (FDG) has provided important insights into the alterations in myocardial glucose metabolism associated with normal cardiac physiology and pathophysiology. In patients with left ventricular dysfunction caused by coronary artery disease, PET-FDG studies have identified certain patterns of myocardial glucose metabolism relative to flow that are associated with viable myocardium. Consequently, in many centers, PET-FDG imaging is being used to identify patients with left ventricular dysfunction who are likely to benefit from coronary revascularization. However, myocardial glucose metabolism (and thus myocardial uptake of FDG) is markedly dependent on a variety of factors, particularly the substrate environment and level of myocardial perfusion. These and other factors must be considered to obtain and interpret myocardial FDG images accurately with respect to the underlying pathophysiologic process. In this review the impact of these factors on myocardial FDG imaging, as well as the relative advantages and disadvantages of approaches designed to optimize myocardial PET-FDG studies, will be discussed.

The use of carbon 11-labeled acetate for assessment of aerobic metabolism

Carbon 11-labeled acetate has been validated as a tracer of citric acid flux and indirectly of oxidative metabolism. 11C-labeled acetate is predominantly metabolized to 11C-labeled carbon dioxide, which clears from the heart. The myocardial 11CO2 efflux rate that can be estimated by dynamic positron emission tomographic imaging closely correlates with myocardial oxygen consumption over a wide range of flow, substrate use, and metabolic conditions. 11C-labeled acetate clearance rates are indirect indexes of oxidative metabolism. To provide absolute mass fluxes, the 11C-labeled acetate approach would require biochemical validation and configuration of a tracer kinetic model. Clinically, estimates of myocardial oxygen consumption appear to be useful in assessing tissue viability, as shown in patients after acute myocardial infarction or with chronic coronary artery disease. In non-coronary artery disease, 11C-labeled acetate may provide measurement of cardiac efficiency and be useful for monitoring therapy.

Hydroperoxide-induced oxidative stress alters pyridine nucleotide metabolism in neonatal heart muscle cells

An oxidant burden established by hydrogen peroxide (H2O2) overload may elicit postischemic myocardial damage. We assess herein the influence of H2O2-induced oxidative stress on heart muscle pyridine nucleotide metabolism. Exposure of neonatal rat cardiomyocytes to 50 microM-1.0 mM H2O2 bolus rapidly shifted their pyridine-nucleotide redox balance toward oxidation. At least 30% of the observed NADPH oxidation was independent of glutathione cycle activity and appeared chemical in nature with H2O2 itself, and not a radical metabolite, acting as oxidant. Cell exposure to H2O2 also depleted cardiomyocyte pyridine nucleotides as a consequence of enhanced utilization. The oxidative stress activated one major route of pyridine nucleotide catabolism (i.e., protein ADP-ribosylation) without acute inhibitory effect upon the other (cleavage by NAD glycohydrolase). The limited NAD sparing by metal chelators and inhibitors of ADP-ribosylation reflected pyridine nucleotide utilization for repair of single-strand DNA breaks caused by hydroxyl-like radicals formed intracellularly through iron-dependent H2O2 reduction. Cardiomyocyte NAD depletion during H2O2-induced oxidative stress was independent of cell integrity and lipid peroxidation. The NAD lost after a discrete H2O2 "pulse" was only partly replenished over a 24-h postinjury period. These data demonstrate that cardiomyocyte pyridine nucleotide metabolism is a nonperoxidative injury target that is chronically affected by H2O2 overload. Derangement of myocardial pyridine nucleotide pools due to oxidative stress may contribute to ischemic heart injury in vivo by interfering with cardiac hydrogen metabolism and redox balance.

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

OBJECTIVES

This study was performed to define the importance of maintenance of oxidative metabolism as a descriptor and determinant of functional recovery after revascularization in patients with left ventricular dysfunction attributable to chronic coronary artery disease.

BACKGROUND

Although myocardial accumulation of 18F-fluorodeoxyglucose indicates the presence of tissue that is metabolically active, it may not identify those metabolic processes required for restoration of myocardial contractility. Experimental studies suggest that, under conditions of ischemia and reperfusion, maintenance of myocardial oxidative metabolism is an important metabolic determinant of the capacity for functional recovery.

METHODS

In 16 patients positron emission tomography was performed to characterize myocardial perfusion (with H(2)15O), oxidative metabolism (with 11C-acetate) and utilization of glucose (with 18F-fluorodeoxyglucose). Dysfunctional but viable myocardium was differentiated from nonviable myocardium on the basis of assessments of regional function before and after coronary revascularization. To define the importance of coronary revascularization on myocardial perfusion and metabolism, tomography was repeated in 11 patients after revascularization.

RESULTS

Before revascularization, perfusion in 24 dysfunctional but viable myocardial segments and 29 nonviable segments averaged 79% and 74%, respectively, of that in 42 normal myocardial segments (both p less than 0.01). Dysfunctional but viable myocardium exhibited oxidative metabolism comparable to that in normal myocardium. In contrast, in nonviable myocardium, oxidative metabolism was only 66% of that in normal (p less than 0.01) and 69% of that in reversibly dysfunctional myocardium (p less than 0.003). Regional utilization of glucose normalized to regional perfusion in dysfunctional but viable myocardium was greater than that in normal myocardium (p less than 0.01). However, in both reversibly and persistently dysfunctional myocardium, utilization of glucose normalized to relative perfusion was markedly variable.

CONCLUSIONS

The results indicate that preservation of oxidative metabolism is a necessary condition for recovery of function after coronary recanalization in patients with chronic coronary artery disease. Consequently, approaches that measure myocardial oxygen consumption, such as dynamic positron emission tomography with 11C-acetate, should facilitate the identification of those patients most likely to benefit from coronary revascularization.