History
55-year-old male who has a history of coronary artery disease status post CABG. The rest and stress thallium examination revealed mild apicolateral wall ischemia and transmural scar involving the inferior, inferoapical walls. Evaluation for heart viabil- ity is requested.
Findings
There is hypometabolism seen involving the midportion of the inferior wall consistent with transmural infarction and matching the resting perfusion exam results (Figures 13.1.1 and 13.1.2). There is viability present in the inferoapical and proximal posterior wall (Figures 13.1.3 and 13.1.4). These findings are suggestive of a significant circum- flex artery deficit with the presence of scar in a third of the inferior posterior wall. The remaining two-thirds of the posterior wall is metabolically preserved but compromised.
Impression
Nonviable myocardium in the mid inferior wall.
Pearls and Pitfalls
• FDG will be trapped in the myocardium since the phosphorylated form cannot be further metabolized.4,11
• The myocardium is considered viable if uptake of FDG is present even in the absence of resting perfusion.1,3,4,6,8
• 80% to 85% of the myocardial ischemic patients will improve following revascularization.3,4,11
Discussion
The myocardium preferentially oxidizes the fatty acids and lactate as energy substrates in the fasting state. When perfusion is compromised, this will increase the glycolytic
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Case 13.1
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flux and will increase exogenous glucose utilization. The myocyte can only maintain glycolysis if lactate and hydrogen ions do not accumulate to toxic levels. When the myocardium is still viable but incapable of sustaining mechanical work, it is consid- ered in the state of hibernation. This is reflected in a perfusion-metabolism mismatch where resting perfusion is lost or reduced and glucose uptake is preserved. Contrac- tility will improve with revascularization.
FIGURE13.1.1.
FIGURE13.1.2.
History
73-year-old female who has a history of coronary artery disease status post myocardial infarction involving the anterior and inferior wall. Her most recent thallium perfusion scan demonstrated transmural infarction involving the anteroseptal and anterolateral wall of the apical myocardium. There was mild to moderate ischemia in the infero- posterior wall. Evaluation for viability is requested.
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FIGURE13.1.3.
FIGURE13.1.4.
Case 13.2
13 Heart Viability 149
Findings
There is a significant focal metabolic defect involving the mid anterior wall extending to the apicoseptal wall (Figures 13.2.1 and 13.2.2) similar to the anteroseptal defect seen in the SPECT myocardial perfusion study. There is also a small segment of focal pronounced defect involving the middle third of the inferior wall (Figures 13.2.3 and 13.2.4). These focal abnormalities represent nonviable tissue but only ten percent of FIGURE13.2.1.
FIGURE13.2.2.
the total left ventricular myocardium. The ischemia described on SPECT in the infer- oposterior wall demonstrates preserved metabolism.
Impression
Significant metabolic defect involving the distal anteroseptal wall as well as the middle third of the inferior wall, compatible with infarction. This region is nonviable, but represents about 10% of the total left ventricular myocardium.
150 Part II Clinical Cases
FIGURE13.2.3.
FIGURE13.2.4.
13 Heart Viability 151
Pearls and Pitfalls
• Oral glucose loading and hyperinsulinemic-euglycemic clamping are some of the ways used to optimize the image quality.2,5,8,9,10
• Nonviable tissue is considered a scar when both flow and metabolism demonstrate matched reduction.7
• PET can overestimate the potential for recovery in post-MI patients within one week of the initial event since the function of the myocardium may still be widely variable.1