Endoscopic Ultrasound Features of Multiple Endocrine Neoplasia Type 1-Related versus Sporadic Pancreatic Neuroendocrine Tumors: A Single-Center Retrospective Study

AIM

Pancreatic neuroendocrine tumors (pNETs) can occur in patients with a familial syndrome either as multiple endocrine neoplasia type 1 (MEN-1) or as sporadic tumors. Endoscopic ultrasound (EUS) has become one of the first-line investigations for pNET characterization. The ultrasonographic features of pNETs may differ depending on the familial versus sporadic pathogenesis of the tumor. Therefore, the EUS findings could help and direct the definition of a pNET with an impact on the most appropriate diagnostic and therapeutic patient management.

METHODS

In this single-center retrospective study, we reviewed the EUS features of 94 pNETs from 37 MEN-1 patients and 15 pNETs from 11 sporadic disease patients at the time of their first EUS assessment. We analyzed the most relevant morphological and ultrasonographic characteristics of the tumors and compared the findings between the 2 patient groups.

RESULTS

Patients with MEN-1 more likely present with multiple pNETs than patients with sporadic disease. Sporadic pNETs are usually much bigger than those due to MEN-1. Moreover, pNETs are more heterogeneous in patients with sporadic disease than in those with MEN-1. No statistical difference with regard to definition of the margins, morphology, and vascularization of the pNETs appears between the 2 groups.

CONCLUSIONS

Patients with sporadic disease usually present with bigger and more heterogeneous pNETs than patients with MEN-1, who tend to present with a higher number of lesions. EUS can facilitate the precise characterization of a pNET, and the ultrasonographic features of the lesion can help and distinguish MEN-1-related versus sporadic disease.

Outcomes of adrenal-sparing surgery or total adrenalectomy in phaeochromocytoma associated with multiple endocrine neoplasia type 2: an international retrospective population-based study

BACKGROUND

The prevention of medullary thyroid cancer in patients with multiple endocrine neoplasia type 2 syndrome has demonstrated the ability of molecular diagnosis and prophylactic surgery to improve patient outcomes. However, the other major neoplasia associated with multiple endocrine neoplasia type 2, phaeochromocytoma, is not as well characterised in terms of occurrence and treatment outcomes. In this study, we aimed to systematically characterise the outcomes of management of phaeochromocytoma associated with multiple endocrine neoplasia type 2.

METHODS

This multinational observational retrospective population-based study compiled data on patients with multiple endocrine neoplasia type 2 from 30 academic medical centres across Europe, the Americas, and Asia. Patients were included if they were carriers of germline pathogenic mutations of the RET gene, or were first-degree relatives with histologically proven medullary thyroid cancer and phaeochromocytoma. We gathered clinical information about patients'RET genotype, type of treatment for phaeochromocytoma (ie, unilateral or bilateral operations as adrenalectomy or adrenal-sparing surgery, and as open or endoscopic operations), and postoperative outcomes (adrenal function, malignancy, and death). The type of surgery was decided by each investigator and the timing of surgery was patient driven. The primary aim of our analysis was to compare disease-free survival after either adrenal-sparing surgery or adrenalectomy.

FINDINGS

1210 patients with multiple endocrine neoplasia type 2 were included in our database, 563 of whom had phaeochromocytoma. Treatment was adrenalectomy in 438 (79%) of 552 operated patients, and adrenal-sparing surgery in 114 (21%). Phaeochromocytoma recurrence occurred in four (3%) of 153 of the operated glands after adrenal-sparing surgery after 6-13 years, compared with 11 (2%) of 717 glands operated by adrenalectomy (p=0.57). Postoperative adrenal insufficiency or steroid dependency developed in 292 (86%) of 339 patients with bilateral phaeochromocytoma who underwent surgery. However, 47 (57%) of 82 patients with bilateral phaeochromocytoma who underwent adrenal-sparing surgery did not become steroid dependent.

INTERPRETATION

The treatment of multiple endocrine neoplasia type 2-related phaeochromocytoma continues to rely on adrenalectomies with their associated Addisonian-like complications and consequent lifelong dependency on steroids. Adrenal-sparing surgery, a highly successful treatment option in experienced centres, should be the surgical approach of choice to reduce these complications.

Characterization of altered myocardial fatty acid metabolism in patients with inherited cardiomyopathy

Inherited defects in myocardial long-chain fatty acid metabolism are increasingly recognized as a cause of cardiomyopathy and sudden death in children. To evaluate whether the phenotypic expression of these genetic diseases could be delineated using positron emission tomography (PET), 11 patients with inherited defects in fatty acid metabolism were evaluated and results were compared with those of 6 nonaffected siblings. Myocardial perfusion, myocardial oxygen consumption (MVO2), and long-chain fatty acid metabolism were determined noninvasively with PET using quantitative mathematical models. There were no differences in haemodynamics, perfusion, MVO2 or plasma substrate levels between groups. Patients with defects in enzymes of fatty acid beta-oxidation (acyl-CoA dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase deficiencies) (n = 5) had diminished myocardial palmitate oxidation compared with healthy siblings (3.2 +/- 3.0 vs. 13.0 +/- 5.6 nmol/g per min, p < 0.03) and a decrease in the percentage of MVO2 accounted for by palmitate (2% +/- 3% vs. 9% +/- 5%, p < 0.04). In these patients, extracted palmitate was shunted into a slow-turnover compartment (predominantly reflecting esterification to triglycerides) with expansion of palmitate in that pool (185 +/- 246 compared with 27 +/- 67 nmol/g in healthy siblings,p < 0.02). In contrast, myocardium of patients with carnitine deficiency (n = 6) (all on oral carnitine therapy) had normal palmitate extraction but expansion of the interstitial/cytosolic fatty acid pool (617 +/- 399 vs. 261 +/- 73 nmol/g in healthy siblings, p < 0.04), suggesting different mechanisms for handling upstream fatty acyl intermediates. Thus, PET can be used to noninvasively assess abnormal myocardial handling of fatty acids in patients with inherited defects of metabolism. This approach should be useful in the assessment of altered myocardial fatty acid metabolism associated with cardiomyopathy as well as for evaluating the efficacy of therapeutic interventions in affected patients.

Enalaprilat attenuates ischemic rises in intracellular sodium in the isolated rat heart via the bradykinin receptor

PURPOSE

Angiotensin-converting enzyme (ACE) inhibitors have been shown to have beneficial effects on ischemic myocardium. We examined whether the ACE inhibitor, enalaprilat (EN), improves intracellular sodium homeostasis during myocardial ischemia and the relationship of this effect to bradykinin.

METHODS

EN (3.2 nM) was administered to isolated rat hearts that were subjected to ischemia and reperfusion. Intracellular sodium and pH were monitored using magnetic resonance spectroscopy (MRS). The specific bradykinin B2 receptor antagonist, HOE 140 (10 nM), was administered with EN in some hearts to determine the effect of bradykinin blockade on EN-mediated effects.

RESULTS

EN blunted the rise in ischemic intracellular sodium, measured using MRS. With reperfusion, EN-treated hearts recovered 80% of their preischemic ventricular function, compared with negligible recover, in controls. These beneficial effects of EN were blocked when the bradykinin receptor antagonist, HOE 140, was coadministered with EN. HOE 140 also blocked EN-mediated attenuation of ischemic intracellular acidosis.

CONCLUSIONS

These results suggest that EN exerts beneficial effects on ischemic intracellular sodium and pH homeostasis via the bradykinin receptor. These effects of EN may provide a mechanism for the beneficial actions of this agent during ischemia.

Incidence of major cardiovascular events in black patients with normal myocardial stress perfusion study results

BACKGROUND

Previous studies have shown that the risk of major cardiovascular events at 1 year is less than 1% in patients with normal myocardial stress perfusion study results. However, the racial distribution of patients enrolled in these studies is not known. Hence, the prognostic value of normal stress perfusion study results in black patients is not well established. Our objective was to determine the incidence of major cardiovascular events in black patients with normal stress perfusion study results over a 12-month period.

METHODS AND RESULTS

We searched the nuclear cardiology database at our institution for all black patients who had normal stress perfusion study results between January 1990 and December 1996. We excluded patients with a history of coronary revascularization, valvular heart disease, cardiomyopathy, congenital heart disease, left bundle branch block, or pre-excitation syndrome. Patients were followed up for at least 12 months from the time of inclusion. A total of 592 patients were enrolled and were followed up for 18 +/- 6 months (mean +/- SD). Of these, 388 underwent treadmill exercise testing, 155 underwent dipyridamole stress testing, and the remainder underwent dobutamine stress testing. Perfusion studies were performed in all patients with thallium 201 single photon emission computed tomography imaging. During the follow-up period, 11 cardiac deaths and 7 myocardial infarctions (MIs) occurred. The incidence of cardiac deaths was 1.2% per year, and that of nonfatal MIs was 0.8% per year. The total incidence of major cardiovascular events was 2% per year. In patients who underwent treadmill exercise testing, the incidence of major cardiovascular events was 1% per year. Performance of a pharmacologic stress test and a prior MI were significantly associated with death or nonfatal MI (P <.05).

CONCLUSIONS

The overall incidence of major cardiovascular events in black patients after normal exercise perfusion study results were obtained was low (1%). However, black patients who had normal perfusion study results but underwent pharmacologic stress testing or had a history of MI were at intermediate risk. These patients require close surveillance for major cardiovascular events.

Protection of ischemic myocardium in diabetics by inhibition of electroneutral Na+-K+-2Cl- cotransporter

Diabetes increases both the incidence of cardiovascular disease and complications of myocardial infarction and heart failure. Studies using diabetic animals have shown that changes in myocardial sodium transporters result in alterations in intracellular sodium (Na(i)) homeostasis. Because the changes in sodium homeostasis can be due to increased entry of Na+ via the electroneutral Na+-K+-2Cl- cotransporter (NKCC), we conducted experiments in acute diabetic hearts to determine if 1) net inward cation flux via NKCC is increased, 2) this cotransporter contributes to a greater increase in Na(i) during ischemia, and 3) inhibition of NKCC limits injury and improves function after ischemia-reperfusion. These issues were investigated in perfused type I diabetic and nondiabetic rat hearts subjected to ischemia and 60 min of reperfusion. A group of diabetic and nondiabetic hearts was perfused with 5 microM of bumetanide, an inhibitor of NKCC. Flux via NKCC, Na(i), and ATP was measured in each group with the use of radiotracer 86Rb, 23Na, and 31P nuclear magnetic resonance spectroscopy, respectively, whereas ischemic injury was assessed by measuring creatine kinase release on reperfusion. Cation flux via NKCC, as measured by 86Rb uptake, was significantly increased in diabetic hearts. Inhibition of NKCC significantly reduced ischemic injury in diabetic hearts, improved functional recovery on reperfusion, attenuated the ischemic rise in Na(i), and conserved ATP during ischemia-reperfusion. Parallel studies in nondiabetic hearts showed that NKCC inhibition was not cardioprotective. These findings demonstrate that flux via NKCC is increased in type I diabetic hearts and that inhibition with bumetanide attenuates changes in Na(i) and ATP during ischemia and protects against ischemic injury. The data suggest a therapeutic role for pharmacological agents that inhibit flux via NKCC in diabetic patients with myocardial ischemia.

Protection of ischemic hearts perfused with an anion exchange inhibitor, DIDS, is associated with beneficial changes in substrate metabolism

OBJECTIVE

Metabolic interventions that promote glucose use during ischemia have been shown to protect the myocardium and improve functional recovery on reperfusion. In this study we evaluated if cardioprotection can be accomplished by inhibiting fatty acid uptake, which would be expected to increase glycolytic metabolism.

METHODS

Diisothiocyanostilbene sulfonic acid (DIDS), commonly used to inhibit Band-3 mediated anion exchanger, and has also been demonstrated to inhibit fatty acid transport in adipocytes, was used to inhibit fatty acid uptake prior to ischemia. Isolated rat hearts were perfused with buffer containing 5 mM glucose, 70 mU/l insulin, 0.4 mM palmitate, and 0.4 mM albumin, paced at 300 beats/min, and subjected to 50 min of low-flow ischemia followed by 60 min of reperfusion.

RESULTS

Ischemic injury, as assessed by creatine kinase release, was diminished in hearts perfused with DIDS (334+/-72 in DIDS vs. 565+/-314 IU/g dry wt in controls, P<0.04). Increases in LVEDP during ischemia were attenuated (8+/-3 mmHg in DIDS vs. 15+/-18 mmHg in controls, P<0.03) and the % recovery of LV function with reperfusion was enhanced in DIDS-treated hearts (78+/-10% of baseline in DIDS vs. 62+/-19% of baseline in controls, P<0.04). These beneficial effects of DIDS were associated with increased glucose metabolism and ATP content during ischemia and reperfusion. Furthermore, treatment with DIDS lowered the accumulation of long chain acyl carnitines.

CONCLUSIONS

This study demonstrates that DIDS protects ischemic myocardium, and is associated with inhibition of fatty acid uptake, improved glucose metabolism, and enhanced functional recovery on reperfusion. The data presented here suggest a potential role for therapeutic agents that lower fatty acid uptake as a metabolic adjunct in the treatment of myocardial ischemia.

Protection of ischemic hearts by high glucose is mediated, in part, by GLUT-4

Metabolic interventions that promote glucose use during ischemia have been shown to protect ischemic myocardium and improve functional recovery on reperfusion. We evaluated whether the cardioprotection afforded by high glucose during low-flow ischemia is associated with changes in the sarcolemmal content of glucose transporters, specifically GLUT-4. Isolated rat hearts were paced at 300 beats/min and perfused under normal glucose (5 mM) or high glucose (10 mM) conditions in buffer containing 0.4 mM albumin, 0.4 mM palmitate, and 70 mU/l insulin and subjected to 50 min of low-flow ischemia and 60 min of reperfusion. To determine the importance of insulin-sensitive glucose transporters in mediating cardioprotection, a separate group of hearts were perfused in the presence of cytochalasin B (10 microM), a preferential inhibitor of insulin-sensitive glucose transporters. Ischemic contracture during low-flow ischemia and creatine kinase release on reperfusion was decreased, and the percent recovery of left ventricular function with reperfusion was enhanced in hearts perfused with high glucose (P < 0.03). Hearts perfused with high glucose exhibited increased GLUT-4 protein expression in the sarcolemmal membrane compared with control hearts under baseline conditions, and these changes were additive with low-flow ischemia. In addition, high glucose did not affect the baseline distribution of sarcolemmal GLUT-1 and blunted any changes with low-flow ischemia. These salutary effects were abolished when glucose transporters are blocked with cytochalasin B. These data demonstrate that protection of ischemic myocardium by high glucose is associated with increased sarcolemmal content of the insulin-sensitive GLUT-4 and suggest a target for the protection of jeopardized myocardium.

Failure of right precordial electrocardiography during stress testing to identify coronary artery disease

BACKGROUND

It has been reported that the use of right precordial leads results in the same diagnostic accuracy as thallium-201 exercise scintigraphy for the detection of coronary artery disease (CAD). The aim of this study was to evaluate the utility of right precordial leads in the detection of CAD.

METHODS AND RESULTS

We evaluated 900 consecutive patients (514 men, 386 women) ranging in age from 39 to 84 years (mean +/- SD, 64 +/- 11 years). Seven hundred forty patients underwent treadmill exercise testing, and 160 underwent pharmacologic stress testing for the diagnosis of chest pain or dyspnea. All received either Tl-201 or technetium-99m sestamibi during stress. During stress testing, the ECG was recorded every minute with 12 limb and left precordial leads and 3 right precordial leads (V(3)R, V(4)R, and V(5)R). The electrocardiogram was considered positive when the ST segment was either elevated or depressed by at least 0.1 mV at 80 ms after the J point, and results were also compared with single photon emission computed tomography myocardial perfusion imaging results. Of the 900 patients, 158 had significant positive changes in the limb or left precordial leads. Only 4 patients had positive changes in the right precordial leads (Fisher exact test, P <.001). Of the patients who had positive electrocardiographic changes, 95 (60%) had abnormal myocardial perfusion scans, with 91 in patients with normal right precordial leads. All 4 patients with ischemic changes in the right precordial leads had abnormal scans, but the left leads were also positive. Three hundred seventy-three of 900 patients (41%) had abnormal scans with no electrocardiographic evidence of ischemia.

CONCLUSIONS

Our experience is far different than that published and suggests that the use of right precordial leads during stress testing fails to provide the same diagnostic accuracy as either the standard left-sided electrocardiography or myocardial perfusion imaging for the detection of CAD.

Use of wavelet transforms in analysis of time-activity data from cardiac PET

Because of its intrinsic quantitative properties, PET permits measurement of myocardial perfusion and metabolism in absolute terms (i.e., mL/g/min). However, quantification has been limited by errors produced in image acquisition, selection of regions of interest, and data analysis. The goal of this study was to evaluate a newly developed, novel, wavelet-based noise-reduction approach that can objectively extract biologic signals hidden within dynamic PET data.

METHODS

Quantification of myocardial perfusion using dynamic PET imaging with 82Rb, H2(15)O, and 13NH3 was selected to evaluate the effects of the wavelet-based noise-reduction protocol. Dynamic PET data were fitted to appropriate mathematic models before and after wavelet-based noise reduction to get flow estimates. Time-activity curves, precision, accuracy, and differentiating capacity derived from the wavelet protocol were compared with those obtained from unmodified data processing. A total of 84 human studies was analyzed, including 43 at rest (18 82Rb scans, 18 H2(15)O scans, and 7 13NH3 scans) and 41 after coronary hyperemia with dipyridamole (17 82Rb scans, 17 H2(15)O scans, and 7 13NH3 scans).

RESULTS

For every tracer tested under all conditions, the wavelet method improved the shape of blood and tissue time-activity curves, increased estimate-to-error ratios, and maintained fidelity of flow in regions as small as 0.85 cm3. It also improved the accuracy of flow estimates derived from 82Rb to the level of that achieved with H2(15)O, which was not affected markedly by the wavelet process. In studies of patients with coronary disease, regional heterogeneity of myocardial perfusion was preserved and flow estimates in infarcted regions were differentiated more easily from normal regions.

CONCLUSION

The wavelet-based noise-reduction method effectively and objectively extracted tracer time-activity curves from data with low signal-to-noise ratios and improved the accuracy and precision of measurements with all tracer techniques studied. The approach should be generalizable to other image modalities such as functional MRI and CT and, therefore, improve the ability to quantify dynamic physiologic processes.

Quantification of myocardial perfusion in human subjects using 82Rb and wavelet-based noise reduction

Quantification of myocardial perfusion with 82Rb has been difficult to achieve because of the low signal-to-noise ratio of the dynamic data curves. This study evaluated the accuracy of flow estimates after the application of a novel multidimensional wavelet-based noise-reduction protocol.

METHODS

Myocardial perfusion was estimated using 82Rb and a two-compartment model from dynamic PET scans on 11 healthy volunteers at rest and after hyperemic stress with dipyridamole. Midventricular planes were divided into eight regions of interest, and a wavelet transform protocol was applied to images and time-activity curves. Flow estimates without and with the wavelet approach were compared with those obtained using H2(15)O.

RESULTS

Over a wide flow range (0.45-2.75 mL/g/min), flow achieved with the wavelet approach correlated extremely closely with values obtained with H2(15)O (y = 1.03 x -0.12; n = 23 studies, r = 0.94, P < 0.001). If the wavelet noise-reduction technique was not used, the correlation was less strong (y = 1.11 x + 0.24; n = 23 studies, r = 0.79, P < 0.001). In addition, the wavelet approach reduced the regional variation from 75% to 12% and from 62% to 11% (P < 0.001 for each comparison) for resting and stress studies, respectively.

CONCLUSION

The use of a wavelet protocol allows near-optimal noise reduction, markedly enhances the physiologic flow signal within the PET images, and enables accurate measurement of myocardial perfusion with 82Rb in human subjects over a wide range of flows.

Improving PET-based physiological quantification through methods of wavelet denoising

The goal of this study was to evaluate methods of multidimensional wavelet denoising on restoring the fidelity of biological signals hidden within dynamic positron emission tomography (PET) images. A reduction of noise within pixels, between adjacent regions, and time-serial frames was achieved via redundant multiscale representations. In analyzing dynamic PET data of healthy volunteers, a multiscale method improved the estimate-to-error ratio of flows fivefold without loss of detail. This technique also maintained accuracy of flow estimates in comparison with the "gold standard," using dynamic PET with O15-water. In addition, in studies of coronary disease patients, flow patterns were preserved and infarcted regions were well differentiated from normal regions. The results show that a wavelet-based noise-suppression method produced reliable approximations of salient underlying signals and led to an accurate quantification of myocardial perfusion. The described protocol can be generalized to other temporal biomedical imaging modalities including functional magnetic resonance imaging and ultrasound.

Measurement of myocardial blood flow with PET using 1-11C-acetate

11C-acetate has been used extensively for the noninvasive assessment of myocardial oxygen consumption and viability with PET. The use of early uptake of acetate by the heart to measure myocardial perfusion has been proposed. This study evaluated the application of 11C-acetate for absolute measurement of myocardial blood flow using a simple compartmental model that does not require blood sampling.

METHODS

Eight healthy volunteers and 13 subjects with concentric left ventricular hypertrophy were studied under resting conditions with both 11Cacetate and 15O-water. Myocardial blood flow with 11C-acetate was obtained by fitting the first 3 min of the blood and tissue tracer activity curves to a two-compartment model. Flows obtained were compared with a validated approach using 15O-water.

RESULTS

In healthy volunteers, regional myocardial perfusion at rest estimated with 11C-acetate was comparable with values obtained with 15O-water (1.06 +/- 0.25 and 0.96 +/- 0.12 mL/g/min, respectively). Perfusion in subjects with left ventricular hypertrophy was also comparable if the recovery coefficient (FMM) used was corrected for ventricular mass. If a fixed FMM was used, flow was greatly overestimated. FMM could be estimated from left ventricular mass (FMM = 0.46 + 0.002 x mass, r = 0.86, P < 0.0001).

CONCLUSION

The results of this study suggest that 11C-acetate can be applied to quantitatively estimate myocardial perfusion under resting conditions using a two-compartment model without the need for blood sampling, provided that an appropriate FMM is chosen. This approach should increase the usefulness of this tracer and obviate administration of a separate tracer to independently measure perfusion.

Absolute quantitation of coronary steal induced by intravenous dipyridamole

OBJECTIVE

The study was done to determine whether coronary steal (defined as an absolute decrease in perfusion from resting blood flow) is induced by intravenous (IV) dipyridamole in patients with severe coronary artery disease (CAD).

BACKGROUND

Myocardial ischemia during coronary vasodilation is usually attributed to coronary steal. However, there is limited data on the absolute magnitude of coronary steal in humans.

METHODS

Eighteen patients with multivessel CAD underwent dynamic positron emission tomography (PET) imaging with 13NH3 at rest and after infusion of IV dipyridamole. Eight myocardial sectors were analyzed per short axis slice and myocardial blood flow calculated with a two-compartment model in absolute terms.

RESULTS

Coronary steal occurred in 8 of the 18 patients. In the 8 patients with coronary steal, myocardial blood flow decreased from 90 +/- 18 ml/100 g/min at rest to 68 +/- 27 ml/100 g/min following dipyridamole in the segments with steal, and increased from 87 +/- 19 to 138 +/- 16 ml/100 g/min following dipyridamole in the segments without steal. Significant clinical correlates of coronary steal were either ST elevation or the combination of ST depression and angina.

CONCLUSIONS

Coronary vasodilation with IV dipyridamole is associated with significant reductions in blood flow to collateral-dependent myocardium consistent with coronary steal in about 45% of patients with severe CAD.

Normal limits for left ventricular ejection fraction and volumes estimated with gated myocardial perfusion imaging in patients with normal exercise test results: influence of tracer, gender, and acquisition camera

BACKGROUND

Myocardial imaging with tracers such as technetium-99m sestamibi or thallium-201 is extensively used as a means of measuring myocardial perfusion. With gated acquisition, these tracers can also be used as a means of measuring left ventricular ejection fraction (EF) and end diastolic and end systolic volumes (EDV and ESV, respectively). The objective of this study was to determine the normal range of EF, EDV, and ESV and to evaluate differences caused by either the tracer used, the gender of the patient, or the acquisition camera used.

METHODS AND RESULTS

A total of 1513 consecutive patients (mean age, 60+/-12 years [SD]) who had normal results on Bruce exercise tests had either Tc-99m sestamibi (n = 884) or Tl-201 (n = 629) injected at peak stress. Although all patients were referred for the evaluation of chest pain or dyspnea and many had cardiac risk factors, all had normal exercise capacity corrected for age, no electrocardiographic signs of ischemia, normal results on perfusion scans, and normal wall motion determined by means of quantitated gated single photon emission computed tomography (QGS). Scans were acquired on 1 of 3 different cameras. The mean EF for all patients who had gated Tc-99m sestamibi scans was 63% +/- 9%, not different from patients who had gated Tl-201 scans (63% +/- 9%). However, when the gender of the patient was considered, the mean EF for women was 66% +/- 8% with Tc-99m sestamibi (n = 519), higher than the mean EF for men (58% +/- 8%, n = 365, P<.0001). Similarly, the mean EF for women studied with Tl-201 (67% +/- 8%, n = 326) was higher than that of men (59% +/- 7%, n = 303,P<.0001). Patients with diabetes mellitus (n = 153) had a slightly reduced EF (62% +/- 10%, P<.001). In a subset of 240 patients, 140 patients studied with Tc-99m sestamibi and 100 studied with Tl-201, the EDV and ESV for women (n = 124) was estimated by means of QGS to be lower (57 +/- 17 mL and 19 +/- 11 mL, respectively) than those for men (74 +/- 22 mL-and 29 +/- 13 mL, respectively; n = 116; P<.001 for each comparison). No clinically significant differences in EF or volumes were noted based on tracers used or acquisition camera. For patients with normal results on exercise treadmill tests and perfusion imaging, the lower limit of normal for EF with gated perfusion imaging with QGS was 50% for women and 43% for men. For EDV and ESV, the upper limit of normal was 91 mL and 40 mL, respectively, for women and 119 mL and 55 mL, respectively, for men.

CONCLUSIONS

No significant differences related to either tracer or acquisition camera used were noted for EF, suggesting equivalency for clinical trials for patients with normal results on exercise tests. However, EF, EDV, and ESV determined by means of gated perfusion imaging need to be corrected for gender.

Tracer kinetic modeling in nuclear cardiology

The introduction of tracer kinetic modeling techniques in conjunction with nuclear imaging has allowed the assessment of physiologic processes in the myocardium in a noninvasive and quantitative manner. Alongside the development of novel radiopharmaceuticals for both positron emission tomography and single photon emission computed tomography is the clarification of their pharmacology, pharmacokinetics, and modeling strategies for assessment of physiologic rates from imaging data. Image analysis and tracer kinetic modeling techniques used in nuclear cardiology must address unique considerations related to the heart. The most commonly used tracers and modeling techniques are presently discussed, with particular attention given to methods that allow absolute quantitation of physiologic processes. The applications of these techniques are obvious in research protocols and may find more use in future clinical studies.

Fructose-2,6-bisphosphate, a potent stimulator of phosphofructokinase, is increased by high exogenous glucose perfusion

BACKGROUND

We have previously demonstrated that perfusion of isolated hearts with high concentrations of glucose results in increased glycolysis during ischemia, diminished ischemic injury, and improved functional recovery with reperfusion.

OBJECTIVE

To evaluate a possible mechanism by which glucose conferred this protection. We examined the hypothesis that increased exogenous glucose concentrations results in increased concentrations of fructose-2,6-bisphosphate, a potent activator of phosphofructokinase-1, and thus increases glycolysis.

METHODS

Perfused rabbit hearts were subjected to 60 min of low-flow ischemia. Control hearts were perfused with buffer containing 0.4 mmol/l palmitate, 5 mmol/l glucose, and 70 mU/l insulin, and treated hearts were perfused with buffer containing 0.4 mmol/l palmitate, 15 mmol/l glucose and 210 mU/l insulin.

RESULTS

Ischemic contracture was attenuated by perfusion of high concentrations of glucose (high glucose) (P < 0.05 compared with control). Glucose uptake and lactate production were greater in hearts perfused with high glucose, as was the ATP concentration at the end of ischemia (P < 0.05 compared with controls). Exogenous glucose uptake and lactate production correlated well with fructose-2,6-bisphosphate content (P = 0.007).

CONCLUSIONS

Enhancement of glycolysis in hearts perfused with high glucose may be the result of stimulation of phosphofructokinase-1 by fructose-2,6-bisphosphate. Accordingly, this may serve as an important mechanism by which cardioprotection may be achieved.

Usefulness of positron emission tomography in defining myocardial viability in patients referred for cardiac transplantation

Positron emission tomography imaging detected evidence of viability in 17 of 33 patients with evidence of infarction only on their thallium scans who were referred for transplant evaluation. Eleven of these 17 underwent revascularization with excellent outcome.

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.

Clinical applications of myocardial perfusion assessments made with oxygen-15 water and positron emission tomography

Positron emission tomography is an intrinsically quantitative tool which permits the quantitative assessment (i.e., in ml/g/min) of regional myocardial perfusion. Oxygen-15-labeled water is the only positron-emitting flow tracer which is freely diffusible within the myocardium. It has been used extensively to define normal myocardial perfusion physiology, as well as to delineate the effects of numerous cardiac diseases on myocardial perfusion and to assess the efficacy of therapeutic interventions. This brief review summarizes the major observations that have been made in humans using oxygen-15 water and positron emission tomography for quantitative estimation of regional myocardial perfusion.

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.

Assessment of blood flow distal to coronary artery stenoses. Correlations between myocardial positron emission tomography and poststenotic intracoronary Doppler flow reserve

BACKGROUND

Previous studies have correlated quantitative coronary angiographic stenosis severity with positron emission tomography (PET) myocardial perfusion and proximal measurements of intracoronary flow velocities in normal and diseased coronary arteries. The aim of this study was to correlate regional myocardial blood flow (RMBF) derived from [15O]H2O PET with directly measured poststenotic intracoronary Doppler flow velocity data acquired under basal conditions and dipyridamole-induced hyperemia.

METHODS AND RESULTS

Eleven consecutive patients 53 +/- 13 years old with ischemic chest pain and isolated proximal left coronary artery stenoses (left anterior descending, 9; left circumflex, 2; mean, 59 +/- 23% diameter stenosis) underwent [15O]H2O myocardial PET and intracoronary Doppler flow velocity studies within 1 week. PET RMBF (mL.g-1.min-1) and myocardial perfusion reserve (MPR) were calculated in poststenotic and normal reference vascular beds. Poststenotic Doppler average peak flow velocities (APV; cm/s) and coronary flow velocity reserve (CFR) were compared with corresponding PET data and quantitative angiographic lesional parameters. PET RMBF and Doppler APV were linearly correlated (r = .60; P < .001), as were poststenotic PET MPR and Doppler CFR (r = .76; P < .0002). Relative coronary flow velocity and MPR ratios between poststenotic and angiographically normal vascular beds were comparably reduced (0.83 +/- 0.25 versus 0.86 +/- 0.21, respectively; P = NS).

CONCLUSIONS

Intracoronary Doppler flow velocities acquired distal to isolated left coronary artery stenoses correlated with [15O]H2O PET regional myocardial perfusion and are useful for assessment of the physiological significance of coronary stenoses in humans.

Detection of cardiac transplant rejection with 111In-labeled lymphocytes and gamma scintigraphy

BACKGROUND

Cardiac transplantation is an increasingly important treatment for patients with end-stage heart failure. Rejection is one of the major limitations, and currently, serial endomyocardial biopsies are required to diagnose rejection. In the year after transplantation, patients routinely undergo 12, 14, or more biopsies. Infiltration of lymphocytes into the graft is a central feature of rejection. Previous studies from our laboratory have demonstrated the feasibility of detecting early rejection noninvasively with gamma scintigraphy after administration of autologous lymphocytes labeled with 111In.

METHODS AND RESULTS

Eight patients were studied at the time of routine biopsy an average of 4.5 months after cardiac transplantation. Autologous lymphocytes were isolated and labeled with 111In. Forty-eight to 72 hours later, patients underwent planar scintigraphic imaging. Myocardial accumulation of labeled lymphocytes was quantified (indium excess, IE) with a previously described and validated technique. Animal studies have shown that an IE > or = 0.07 is associated with rejection. Two of four patients with biopsy grade 0 or 1A rejection had no excess accumulation of labeled lymphocytes. The other two patients with biopsy grade 0 or 1A had an average IE of 0.13 +/- 0.04 (SD), which may actually represent the higher sensitivity of the scintigraphic approach, since the whole myocardium is interrograted. All four patients with biopsy grade 1B rejection had increased accumulation of labeled lymphocytes (IE = 0.18 +/- 0.06, P = .06 compared with all patients with grade 0 or 1A biopsies).

CONCLUSIONS

The development of a sensitive, specific, and noninvasive method of diagnosing cardiac allograft rejection in humans might obviate the need for endomyocardial biopsy as well as improve the accuracy of diagnosis. The results suggest that scintigraphic detection of labeled lymphocytes is a promising approach for the noninvasive detection of cardiac transplant rejection. In addition, the approach should permit the assessment of the efficacy of antirejection therapy.

Quantitation of myocardial fatty acid metabolism using PET

Abnormalities of fatty acid metabolism in the heart presage contractile dysfunction and arrhythmias. This study was performed to determine whether myocardial fatty acid metabolism could be quantified noninvasively using PET and 1-(11)C-palmitate.

METHODS

Anesthetized dogs were studied during control conditions; during administration of dobutamine; after oxfenicine; and during infusion of glucose. Dynamic PET data after administration of 1-(11)C-palmitate were fitted to a four-compartment mathematical model.

RESULTS

Modeled rates of palmitate utilization correlated closely with directly measured myocardial palmitate and total long-chain fatty acid utilization (r = 0.93 and 0.96, respectively, p < 0.001 for each) over a wide range of arterial fatty acid levels and altered patterns of myocardial substrate use (fatty acid extraction fraction ranging from 1% to 56%, glucose extraction fraction from 1% to 16% and myocardial fatty acid utilization from 1 to 484 nmole/g/ min). The percent of fatty acid undergoing oxidation could also be measured.

CONCLUSION

The results demonstrate the ability to quantify myocardial fatty acid utilization with PET. The approach is readily applicable for the determination of fatty acid metabolism noninvasively in patients.

Regional myocardial perfusion assessed with generator-produced copper-62-PTSM and PET

We have previously demonstrated that myocardial perfusion can be estimated accurately in experimental animals with the generator-produced positron-emitting tracer, 62Cu-pyruvaldehyde bis (N4-methylthio- semicarbazone)(62Cu-PTSM) and PET. This study evaluated the feasibility of quantifying regional myocardial blood flow using 62Cu-PTSM and PET in human subjects.

METHODS

Regional perfusion was estimated using a previously described and validated two-compartment model from dynamic PET scans obtained after an intravenous bolus of 62Cu-PTSM in 10 healthy volunteers and in 6 patients with coronary artery disease at rest: and in 9 of the volunteers and 4 of the patients after administration of dipyridamole intravenously. Flow estimates were compared with those obtained using H2(15)O.

RESULTS

Contrast was high between myocardium and blood or lung with 62Cu-PTSM, resulting in high-quality myocardial images. Liver uptake was also high. At flows of up to 1.5 ml/g/min, flow estimated with 62Cu-PTSM correlated closely with estimates obtained with H2(15)O (y = 0.71x .21, n = 169 regional comparisons, r = 0.66, p < 0.55), but this relationship was not maintained at higher flows.

CONCLUSION

The results demonstrate that quantification of myocardial perfusion with 62Cu-PTSM is feasible in human subjects but cannot be used to estimate hyperemic flows due most likely to the strong binding of the tracer to human serum albumin. Copper-62-PTSM congeners with less avidity for human albumin may prove more suitable for evaluation of hyperemic flows.

Delineation of myocardial stunning and hibernation by positron emission tomography in advanced coronary artery disease

With positron emission tomography, the resting flow abnormalities underlying reversible left ventricular dysfunction in 17 patients with chronic coronary artery disease were delineated. The level of flow in reversible dysfuncTional segments (i.e., those demonstrating improvement after revascularization) was markedly variable, ranging from 0.32 to 1.25 ml/gm/min. In 20 of these segments, flow was preserved, whereas in 12 segments, flow was reduced, when compared with that in, age-matched controls. Preservation of flow was associated with preservation of myocardial oxygen consumption and no alterations in myocardial substrate use. In contrast, a reduction in flow resulted in a decrease myocardial oxygen consumption and an increase in myocardial glucose use. Thus resting reversible left ventricular dysfunction in patients with chronic coronary artery disease can reflect a diversity of resting flow abnormalities. Moreover, myocardial perfusion at rest is frequently within normal limits, suggesting that the reversible mechanical dysfunction in these patients is attributable to intermittent myocardial stunning and not hibernation.

Visualization of thrombi in pulmonary arteries with radiolabeled, enzymatically inactivated tissue-type plasminogen activator

BACKGROUND

Despite the high frequency of pulmonary thromboembolism and its significant morbidity and mortality, diagnosis remains suboptimal. We have been developing a method for prompt detection with the use of radiolabeled, inactivated tissue-type plasminogen activator (TPA) and performed the present study to determine whether its use permits rapid scintigraphic visualization of pulmonary thrombi in vivo.

METHODS AND RESULTS

The thrombolytic, but not fibrin-binding, property of TPA was inactivated with a tripeptide chloromethyl ketone (YPACK) that had already been iodinated with 123I to radiolabel the TPA. Pulmonary arterial thrombosis was induced in nine dogs with the use of guide wires modified to provide thrombogenic tips. 123I-YPACK-TPA (1.1 to 7.8 mCi, 0.5 to 7.8 mg) was infused for 5 minutes into either the systemic or the pulmonary circulation. Clearance of radioactivity from the blood was rapid and indistinguishable from that of unlabeled, thrombolytically active TPA, with only 6.7 +/- 1.0% (mean +/- SEM) of peak radioactivity remaining after 60 minutes and minimal release of labeled fragments from the liver during this interval. Thrombi were visualized with single photon emission computed tomography and/or planar imaging 40 to 120 minutes after infusion of tracer in all seven animals given at least 3.7 mCi of 123I-YPACK-TPA. Ratios of radioactivity in thrombus (wet mass, 610 +/- 64 mg) to blood were high (14 +/- 3:1).

CONCLUSIONS

The use of radiolabeled TPA in which thrombolytic activity is inactivated permits prompt scintigraphic detection of thrombi in pulmonary arteries in vivo.

Species-dependent binding of copper(II) bis(thiosemicarbazone) radiopharmaceuticals to serum albumin

Copper-62-labeled pyruvaldehyde bis(N4-methylthiosemicarbazonato)-copper(II) (Cu-PTSM) is a generator-based PET radiopharmaceutical under investigation for use in evaluation of tissue perfusion. Despite promising results from animals, problems have been encountered in the use of 62Cu-PTSM to quantitate myocardial perfusion in humans at high flow rates, possibly due to species-dependent interactions of the tracer with serum albumin.

METHODS

Ultrafiltration and plasma/erythrocyte partitioning studies were performed to assess the protein binding of 67Cu-labeled Cu-PTSM and six related copper(II) bis(thiosemicarbazone) complexes.

RESULTS

These studies reveal significant interspecies variability in the strength of Cu-PTSM binding to serum albumin, with 67Cu-PTSM binding much more strongly to human albumin than to dog albumin. Most of the related Cu(II)-bis(thiosemicarbazone) complexes examined exhibit interspecies variability of albumin binding similar to that observed with Cu-PTSM. Two such complexes, Cu-ETS and Cu-n-PrTS, however, were identified that exhibit no preferential association with human serum albumin.

CONCLUSION

Copper-62-PTSM exhibits substantial interspecies variability in the strength of its binding to serum albumin, which appears to explain the problems encountered in using animal data to predict 62Cu-PTSM behavior in humans. The 62Cu-ETS and 62Cu-n-PrTS complexes may be viable alternatives to 62Cu-PTSM for PET studies to evaluate quantitatively myocardial blood flow in humans.

Heterogeneity of myocardial perfusion provides the physiological basis of perfusable tissue index

Assessment of viable from nonviable myocardium is critical for the care of patients being considered for revascularization procedures. Recently, the perfusable tissue index (PTI) has been proposed as an index of myocardial viability.

METHODS

Computer simulations were performed for homogeneously and heterogeneously perfused tissue over a wide range of flows (0.04-6.4 ml/g/min) using both bolus and infusion inputs.

RESULTS

PTI estimated from simulated homogeneously perfused tissue did reflect the amount of tissue being perfused independent of absolute level of flow, type of input or model configuration, whereas PTI obtained from simulated heterogeneously perfused tissue was consistently lower than the simulated "true" PTI and varied with flow, type of input function and model configuration. Flow estimated with 15O-water was not significantly different from that measured with radio labeled microspheres.

CONCLUSION

Oxygen-15-water can diffuse into both acutely and chronically ischemic myocardium irrespective of its functional status. The results suggest that PTI is most likely an index of the heterogeneity of myocardial flow rather than an index of the amount of tissue being perfused. Its utility for delineating myocardial viability is thus related to the amount of tissue perfused that has low absolute levels of perfusion or high degrees of flow heterogeneity.

Rate of glycolysis during ischemia determines extent of ischemic injury and functional recovery after reperfusion

The efficacy of increasing glycolysis during ischemia for enhancing the salutary effects of reperfusion was evaluated in isolated perfused rabbit hearts subjected to low-flow ischemia followed by reperfusion. Control hearts were perfused with buffer containing 0.4 mM palmitate, 5 mM glucose, and 70 mU/l insulin. Additional groups of hearts were perfused with double glucose/insulin and 1 mM dichloroacetate or were subjected to substrate priming to increase preischemic glycogen content. Ischemic contracture was completely prevented in hearts perfused with high glucose/insulin and was delayed markedly by either dichloroacetate or enhanced preischemic glycogen [45 +/- 14 and 31 +/- 20 min, respectively; P < 0.01 each vs. control (11 +/- 10 min)] and inversely related to the rate of lactate production. With reperfusion, recovery of developed pressure was 56 +/- 23% of baseline in control hearts, 90 +/- 8% in hearts receiving high glucose/insulin, 92 +/- 5% in hearts receiving dichloroacetate, and 79 +/- 19% in hearts with increased glycogen (P < 0.05 each vs. control hearts). Creatine kinase release was reduced by > 55% in treated hearts. Thus enhancement of glycolysis by diverse mechanisms during ischemia decreased ischemic damage and improved the recovery of contractile function with reperfusion.

Ischemic preconditioning stimulates anaerobic glycolysis in the isolated rabbit heart

Preconditioning decreases ischemic injury, preserves tissue ATP content, and enhances the salutary effects of reperfusion. To evaluate whether preserved ATP is related to reduced utilization or increased production, 28 paced isolated rabbit hearts, perfused at constant flow, were subjected to 3 min of transient no-flow ischemia followed 12 min later by 1 h of low-flow ischemia and 45 min of reperfusion. Results were compared with those from 34 control hearts subjected to ischemia and reperfusion without preconditioning. Preconditioning delayed the onset of ischemic contracture and decreased its amplitude. At the end of ischemia, tissue ATP content was higher in hearts subjected to preconditioning (9.8 +/- 3.3 vs. 4.5 +/- 1.1 mumol/g dry wt; P < 0.01), accounted for by increased anaerobic ATP production using exogenous glucose. Preconditioning decreased ischemic damage (creatine kinase release 373 +/- 199 vs. 587 +/- 291 U/g dry wt; P < 0.05) and resulted in better functional recovery with reperfusion (74 +/- 11% of baseline developed pressure vs. 60 +/- 23%; P < 0.05). Thus preconditioning appears to protect ischemic myocardium by enhancing anaerobic glycolytic production of ATP using exogenous glucose.

Synthesis and tissue biodistribution of [omega-11C]palmitic acid. A novel PET imaging agent for cardiac fatty acid metabolism

In order to diagnose patients with medium-chain acyl-CoA dehydrogenase deficiency with a noninvasive diagnostic technique such as positron emission tomography, we have developed a synthesis of [omega-11C]palmitic acid. The radiochemical synthesis was achieved by coupling an alkylfuran Grignard reagent (7) with [11C]methyl iodide, followed by rapid oxidative cleavage of the furan ring to the carboxylate using ruthenium tetraoxide. Tissue biodistribution studies in rats comparing [omega-11C]palmitic acid and [1-11C]palmitic acid show that the %ID/g and %ID/organ in the heart tissue after administration of [omega-11C]palmitic acid is approximately 50% greater than after administration of [1-11C]palmitic acid, due to the diminished metabolism of the [omega-11C]palmitic acid. These studies show as well, low uptake in nontarget tissues (blood, lung, kidney, and muscle). PET images of a dog heart obtained after administration of [omega-11C]-and [1-11C]palmitic acid show virtually identical uptake and distribution in the myocardium. The differing cardiac washout of labeled palmitates measured by dynamic PET studies may allow diagnosis of disorders in cardiac fatty acid metabolism.

The super PET 3000-E: a PET scanner designed for high count rate cardiac applications

OBJECTIVE

Mathematical models for the delineation of regional myocardial perfusion and metabolism with PET require faithful reconstruction of arterial and myocardial time-activity curves following administration of radiotracers. High temporal resolution is often required in such measurements. Many commercially available tomographs exhibit long dead times that limit their count rate capabilities. To overcome these limitations, we developed and tested a whole-body tomographic device (Super PET 3000-E) with high count rate capabilities. The use of cesium fluoride scintillation detectors coupled with a one-to-one detector photomultiplier configuration reduces the system resolving and dead times.

MATERIALS AND METHODS

The Super PET 3000-E was subjected to a series of tests with phantoms to determine its resolution, sensitivity, linearity, count rate capabilities, dead time, and random coincidence contribution.

RESULTS

The system sensitivity is 136 kcounts/s/microCi/ml and its transverse and longitudinal resolutions are 8.5 and 10.5 mm full width at half-maximum, respectively. The system can easily record a total event rate of 2.0 Mcounts/s with minimal dead time loss and excellent linearity.

CONCLUSION

The system fulfills its design goals and allows the very high count rate performance needed for the application of the physiological models used in our cardiac studies.

Enhancement of regional myocardial efficiency and persistence of perfusion, oxidative, and functional reserve with paired pacing of stunned myocardium

BACKGROUND

Stunned myocardium reflects postreperfusion dysfunction in myocardium that is destined to ultimately fully recover. Most investigators attribute postreperfusion stunning to a primary defect in excitation-contraction coupling or to an altered sensitivity of the myofilaments to calcium. The aim of the present study was to evaluate the interrelation between myocardial perfusion, oxidative metabolism, and function in an effort to better characterize the phenomenon of myocardial stunning, to define the regional efficiency of stunned myocardium, and to characterize its reserve capacity.

METHODS AND RESULTS

Regional myocardial perfusion (measured with radiolabeled microspheres), myocardial oxygen consumption (MVO2) (quantified with positron emission tomography using 1-11C-acetate), and myocardial function (assessed with two-dimensional echocardiography) were evaluated in 12 anesthetized, closed-chest dogs subjected to 15 minutes of left anterior descending coronary artery occlusion followed by reperfusion. To evaluate flow, oxidative, and functional reserve after measurements were obtained 1 hour after reperfusion, dogs were subjected to paired pacing (an inotropic stimulus that does not alter systemic hemodynamics), and measurements were repeated. One hour after reperfusion, stunned myocardium was characterized by near-normal levels of myocardial perfusion (0.57 +/- 0.13 mL/g per minute, 81 +/- 13% of that in remote, normal regions) but severe dyskinesis (echo score, 2.6 +/- 0.7; percent wall thickening, 14 +/- 20%). Despite the low level of contractile function, MVO2 averaged 1.72 +/- 0.7 mumol/g per minute, 71 +/- 27% of that observed in remote myocardium. Regional myocardial efficiency (systolic wall thickening divided by MVO2) was markedly diminished. With paired pacing, myocardial perfusion increased proportional to that in remote myocardium, systolic function improved (echo score, 1.4 +/- 0.7; percent wall thickening, 30 +/- 15%), and regional MVO2 nearly doubled (to 3.41 +/- 1.82 mumol/g per minute, P < .05 for each paired measurement). Importantly, with paired pacing, regional myocardial efficiency nearly normalized in reperfused myocardium.

CONCLUSIONS

Stunned myocardium is characterized by near-normal levels of perfusion and oxygen consumption despite marked dyskinesis. Myocardial efficiency is poor. With inotropic stimulation (in the present study, paired pacing), reperfused myocardium demonstrated considerable perfusion, oxidative, and functional reserve and a dramatic improvement in myocardial efficiency. These results may have implications for the treatment of postreperfusion pump failure.

The relative importance of myocardial energy metabolism compared with ischemic contracture in the determination of ischemic injury in isolated perfused rabbit hearts

The mechanical effects of ischemic contracture may be important in the development of irreversible cellular damage as it increases mechanical stress on sarcolemmal membranes and restricts endocardial perfusion. To assess the relative importance of these mechanical effects compared with decreased energy supply in the development of irreversible injury, the effects of inhibiting ischemic contracture with 2,3-butanedione monoxime (BDM), an agent that disrupts excitation-contraction coupling, were delineated in isovolumically contracting isolated rabbit hearts. Administration of 20 mmol/L BDM in 12 hearts subjected to 60 minutes of low-flow ischemia prevented ischemic contracture (left ventricular end-diastolic pressure [LVEDP], 12 +/- 3 compared with 48 +/- 14 mm Hg in 20 control hearts; P < .001), reduced membrane damage (creatine kinase [CK] release, -54% compared with control hearts; P < .05), and enhanced functional recovery during reperfusion (left ventricular developed pressure [LVDP], 86 +/- 10% of baseline compared with 56 +/- 23% in control hearts; P < .01). These observations were not related to increased intracavitary pressure and its effects on flow distribution, since venting the left ventricle in additional hearts did not result in improved function during reperfusion. Although it would be tempting to conclude that BDM protected ischemic myocardium by preventing ischemic contracture, administration of BDM was also associated with reduced depletion of ATP during ischemia, perhaps related to diminished energy demand. To distinguish between the relative importance of inhibiting contracture from provision of adequate energy, the period of ischemia was extended to 120 minutes. BDM still prevented ischemic contracture (LVEDP, 10 +/- 6 mm Hg) and preserved ATP stores, but it did not prevent membrane damage (CK release, 483 +/- 254 U/g dry weight) or contractile failure during reperfusion (LVDP, 68 +/- 7% of baseline). In contrast, increasing the rate of anaerobic glycolysis during ischemia by doubling glucose and insulin in the presence of BDM markedly decreased membrane damage (CK release, 114 +/- 72 U/g dry weight; P < .05) and contractile failure during reperfusion (LVDP, 88 +/- 7% recovery of baseline; P < .01). These results suggest that insufficient energy production is primarily responsible for myocardial ischemic damage, whereas mechanical effects of ischemic contracture appear to play only a minor role.

Use and limitations of metabolic tracers labeled with positron-emitting radionuclides in the identification of viable myocardium

Identification of viable myocardium is crucial in identifying patients who could benefit from interventional therapy such as coronary artery balloon angioplasty or bypass surgery. PET can be used to prospectively identify viable from nonviable myocardium based on the pattern of substrate use in comparison with perfusion. Viable myocardium can be identified with such diverse tracers as 1-11C-palmitate (for assessment of fatty acid metabolism) and 18F-fluorodeoxyglucose (for assessment of the uptake of glucose). Recent studies have suggested that assessment of oxidative metabolism with tracers such as 1-11C-acetate may predict with the greatest accuracy segments of myocardial tissue that will recover after recanalization. Further studies will be necessary to determine whether PET represents a superior technology compared with other more widely available (and less expensive) approaches for identification of jeopardized myocardium. Nonetheless, PET should provide a better understanding of the pathophysiology of myocardial ischemia (since it enables delineation of the biochemical alterations that underlie contractile dysfunction) and of therapeutic strategies likely to be beneficial. It will also be useful for the identification of viable from nonviable myocardium when results of other diagnostic techniques are equivocal, and in patients who are at high risk.

Effects of time discrepancies between input and myocardial time-activity curves on estimates of regional myocardial perfusion with PET

Estimates of myocardial perfusion with PET using kinetic models require faithful recording of radioactivity content in blood and myocardium. Typically the arterial time-activity curve is obtained by placing a region of interest (ROIs) within the left atrial or left ventricular cavity. However, curves generated from these regions appear earlier in time than tissue time-activity curves obtained from ROIs within the myocardial tissue, and such time discrepancies can lead to errors in flow estimates.

METHODS

The magnitude of these time discrepancies and their effect on estimates of regional myocardial perfusion using oxygen-15-water were measured in 30 normal subjects evaluated at rest and again after administration of dipyridamole.

RESULTS

Under baseline conditions, the left atrial curve appeared 0.97 +/- 0.67 (s.d.) before the ascending aorta input curve (p < 0.05) and estimated perfusion decreased from 1.28 +/- 0.28 ml/g/min using the left atrial curve uncorrected for time to 0.98 +/- 0.27 ml/g/min after correction (p < 0.05). After dipyridamole, the left atrial curve appeared 0.68 +/- 0.72 sec before the ascending aorta curve (p < 0.05) and estimated perfusion decreased from 3.60 +/- 1.40 ml/g/min using the left atrial curve uncorrected for time to 3.24 +/- 1.26 ml/g/min using the time-corrected curve (p < 0.05). Because the magnitude of time discrepancies between the left ventricular and ascending aortic curves was less (0.25 +/- 0.34 and 0.19 +/- 0.23 sec at rest and after dipyridamole, respectively), effects on flow estimates were more modest.

CONCLUSIONS

The results of this study demonstrate that time discrepancies between input and tissue time-activity curves can affect estimates of myocardial flow. Correction for this potential source of error is proposed.