The historical and conceptual evolution of radionuclide assessment of myocardial viability

Single-photon emission computed tomography after nitrate administration predicts cardiac events in patients with previous myocardial infarction and left ventricular dysfunction

BACKGROUND

We determined the impact of viability assessment by nitrate single-photon emission computed tomography (SPECT) on cardiac events during long-term follow-up in patients with previous myocardial infarction, impaired left ventricular (LV) function, and no evidence of inducible ischemia.

METHODS AND RESULTS

Sestamibi SPECT after nitrate was performed in 93 medically treated patients with previous myocardial infarction and LV dysfunction (ejection fraction < 40%). Viability was defined as > or = 2 dysfunctional segments with preserved tracer uptake (> or = 55% of peak activity). Cardiac events were cardiac death, myocardial infarction and late revascularization. Sixty-five (70%) patients had evidence of viability at SPECT, whereas 28 (30%) did not. During 43 +/- 24 months of follow-up, cardiac events occurred in 32 (49%) of the 65 patients with viability and in 5 (18%) of the 28 patients without (P < .001). At Cox analysis, only the extent of viability predicted cardiac events (hazards ratio 1.6, 95% CI 1.3-2.0, global chi-square 14.3, P < .0001). Cumulative probability of event-free survival was 22% in patients with viability and 81% in those without (P < .001).

CONCLUSIONS

The presence of viable myocardium at nitrate SPECT imaging predicts major cardiac events at long-term follow-up and the risk increases with the extent of viability.

Metabolic imaging using SPECT

INTRODUCTION

In normal condition, the heart obtains more than two-thirds of its energy from the oxidative metabolism of long chain fatty acids, although a wide variety of substrates such as glucose, lactate, ketone bodies and amino acids are also utilised. In ischaemic myocardium, on the other hand, oxidative metabolism of free fatty acid is suppressed and anaerobic glucose metabolism plays a major role in residual oxidative metabolism. Therefore, metabolic imaging can be an important technique for the assessment of various cardiac diseases and conditions.

MATERIALS AND METHODS

In SPECT, several iodinated fatty acid traces have been introduced and studied. Of these, (123)I-labelled 15-(p-iodophenyl)3-R, S-methylpentadecanoic acid (BMIPP) has been the most commonly used tracer in clinical studies, especially in some of the European countries and Japan.

RESULTS AND DISCUSSION

In this review article, several fatty acid tracers for SPECT are characterised, and the mechanism of uptake and clinical utility of BMIPP are discussed in detail.

Assessment of myocardial viability by nuclear imaging techniques

The assessment of myocardial viability has become important in the diagnostic and prognostic work up of patients with ischemic cardiomyopathy. Patients with viable myocardium may benefit from revascularization in terms of improvement of function, symptoms, and prognosis. In contrast, patients without viable myocardium do not benefit and should be treated conservatively. Various nuclear imaging techniques are available.

Tagged MRI and PET in severe CAD: discrepancy between preoperative inotropic reserve and intramyocardial functional outcome after revascularization

In severe coronary artery disease (CAD), it has been shown that intramyocardial inotropic reserve as assessed with tagged magnetic resonance imaging (MRI) is uniformly distributed among positron emission tomography (PET) patterns reflecting normal or concomitant reductions in perfusion and glucose metabolism. This preliminary study aimed to delineate the relationship between preoperative values of intramyocardial inotropic reserve (in different PET patterns of perfusion and glucose uptake) and intramyocardial functional outcome after surgical revascularization in severe CAD. Twelve patients underwent preoperative tagged MRI (baseline, 10 μg·kg−1·min−1 of dobutamine), H215O/[18F]fluorodeoxyglucose PET imaging, and postoperative resting tagged MRI. Regional midmyocardial circumferential shortening ( Ecc, in %) and PET patterns (normal, match viable, mismatch viable, and infarcted) were assessed in three tagged MRI/PET short-axis slices. Ecc at baseline ranged from 12 ± 6 to 8 ± 5 and 4 ± 4% in normal, match-viable, and infarcted regions, respectively ( P < 0.05) and was 8 ± 5% in mismatch-viable regions. Of the 429 regions studied, 187 showed preoperative inotropic reserve with dobutamine, but 238 showed postoperative functional improvement. Postoperative functional improvement was less common in infarcted regions (41 vs. ∼60% in the other PET patterns), but the extent of improvement was similar among PET patterns (∼6%). Postoperative functional improvement occurred in 53% of all (normal, match viable, and mismatch viable) regions without inotropic reserve. In severe CAD, revascularization affords greater intramyocardial functional benefit than expected from the evaluation of intramyocardial inotropic reserve with low-dose dobutamine. Postoperative functional improvement in PET-viable regions without inotropic reserve suggests that factors other than regionally enhanced perfusion contribute to such functional improvement.