Evaluation of myocardial viability following acute myocardial infarction using 201Tl SPECT after thallium-glucose-insulin infusion--comparison with 18F-FDG positron emission tomography

Radionuclide Imaging of Viable Myocardium: Is it Underutilized?

Coronary artery disease is the major cause of heart failure in North America. Viability assessment is important as it aims to identify patients who stand to benefit from coronary revascularization. Radionuclide modalities currently used in the assessment of viability include (201)Tl SPECT, (99m)Tc-based SPECT imaging, and (18)F-fluorodexoyglucose ((18)F-FDG)-PET imaging. Different advances have been made in the last year to improve the sensitivity and specificity of these modalities. In addition, the optimum amount of viable (yet dysfunctional) myocardium is important to identify in patients, as a risk-benefit ratio must be considered. Patients with predominantly viable/hibernating myocardium can benefit from revascularization from a mortality and morbidity standpoint. However, in patients with minimal viability (predominantly scarred myocardium), revascularization risk may certainly be too high to justify revascularization without expected benefit. Understanding different radionuclide modalities and new developments in the assessment of viability in ischemic heart failure patients is the focus of this discussion.

Effects of GIK (glucose-insulin-potassium) on stress-induced myocardial ischaemia

Despite the evidence in experimental animal models that insulin, or GIK (glucose-insulin-potassium), improves left ventricular function and perfusion during both acute and chronic ischaemia, clinical studies have generated conflicting results. We tested the hypothesis that pretreatment with GIK attenuates the vascular and functional effects of stress-induced myocardial ischaemia in humans. Twenty-two patients with evidence of inducible myocardial ischaemia were enrolled; 11 patients with normal ventricular function underwent two dipyridamole echocardiography tests, and 11 with regional contractility defects from previous myocardial infarction were submitted to two ECG exercise tests combined with 201Tl myocardial perfusion scintigraphy; the tests were preceded by 60 min of either normal saline or an isoglycaemic GIK infusion. On a stress echocardiogram, a 30% reduction in the severity of ischaemia was observed. On ECG ergometry, GIK infusion slightly increased the time to ischaemia (+0.6 min, P=0.07); however, the higher workload (+8%, P=0.07) was achieved at a similar rate-pressure plateau. On scintigraphy, an increase in ischaemic segments (+48%, P<0.001) was imaged mainly at the expense of viable (but non-ischaemic) and non-viable segments, which were reduced by 60%. GIK affected stress-induced left ventricular underperfusion only marginally (GIK: 39.7+/-2.5 compared with saline: 35.4+/-2.2 units, P<0.05), but significantly improved its acute reversibility (-42+/-4 compared with -25+/-4%, P<0.001). We conclude that GIK pretreatment attenuates the effect of ischaemia on myocardial contractility, slightly improves exercise tolerance and causes a more rapid and diffuse recovery of post-ischaemic reperfusion.

Evaluation of myocardial viability with thallium-201 infusion MPSPECT after oral glucose application in patients with chronic coronary artery disease

AIM

The aim of this study was to evaluate the myocardial viability in nondiabetic patients with chronic coronary artery disease (CCAD) or past myocardial infarction (MI), using thallium-201 infusion myocardial perfusion single-photon emission computed tomography (MPSPECT) imaging after oral glucose application (Glu+Tl-infusion).

MATERIALS AND METHODS

In this study, 33 nondiabetic patients (three female, 30 male, mean age: 55.24+/-11 years, range: 33-77 years) with MI history or known CCAD were included. Rest/redistribution/24 h-late-MPSPECT imaging was performed for all patients. In all patients in whom fixed perfusion defect was observed on any wall of the left ventriculi, after 24 h-late-MPSPECT imaging, 75 g oral glucose was given. Thirty minutes later, 1 mCi thallium-201 in 100 ml of physiological saline solution was applied in a period of 20 min by slow infusion. After infusion at the 10th minute, MPSPECT imaging was performed. Perfusion was evaluated visually for a total of 3432 segments with the 26-segment 5-point scoring technique. Scoring measured perfusion as 0 = no perfusion defect, 1 = mildly reduced, 2 = moderately reduced, 3 = severely reduced, and 4 = absent uptake. Scores '0 and 1' were considered normal and scores '2-4' were considered abnormal.

RESULTS

For serum insulin levels measured after glucose application, a significant increase was determined, according to the period before glucose application (P<0.001). When compared with rest MPSPECT images, segmental perfusion improvement both in redistribution and in the 24 h-late-MPSPECT images were 16.3 and 18.3%, respectively. This ratio was found to be 27.2% for Glu+Tl-infusion images. The ratios of segments in which perfusion was worsening were calculated to be 9.4, 14.5, and 7.3%, respectively, for redistribution, 24 h-late-MPSPECT, and Glu+Tl-infusion images. When this evaluation was made for all three vessel areas, again the highest perfusion improvement and the lowest perfusion worsening were detected for Glu+Tl-infusion images. In addition, when this evaluation was made for the three vessel areas according to the coronary narrowing degree, again the highest perfusion improvement was detected for Glu+Tl-infusion images, in segments in the left anterior descending artery, and right coronary artery areas with >/=90% narrowing. In rest images, in segments with segmental scores of 3 and 4, when the total reversibility ratio was evaluated, this ratio was calculated to be 0.7% for redistribution images and 4.5% for 24 h-late-MPSPECT. The highest total reversibility ratio in these segments was detected with Glu+Tl-infusion images to be 10.3%. When we evaluated the patients with respect to the MI history time, the highest segmental perfusion improvement was detected in patients with 0-3 months of MI history.

CONCLUSION

We conclude that in nondiabetic patients who are known to have CCAD or past MI history, Glu+Tl-infusion is an easily applicable method that gives better results for the evaluation of myocardial viability.