Role of FDG PET scan in evaluating the extent of viability using quantitative method in addition to routine visual method, and its impact on management and clinical outcome.

Introduction: Coronary artery disease (CAD) constitutes one third of all deaths worldwide, making it the leading cause of mortality.Left ventricular dysfunction (LVD) is considered as one of the best prognostic markers in patients with CAD.It has been established that some of the involved tissue may remain viable and revascularization may restore its function and contractility. Thus, the role of viability testing has become crucial. Amongst all modalities, FDG-PET stands superior, and in addition, is the modality of choice where dobutamine stress echo and cardiac MRI face challenges. Here, we assess the role of FDG PET scan in evaluating the extent of viability using not only visual but quantitative method too and its impact on management and subsequent clinical outcome.

Methods: In this retrospective study of 10 years (2011-2021), we included patients of LVD (EF<=35 %) who were found to have perfusion defect size of moderate to large/extensive size (i.e.perfusion defects of >10-20% or >20-40% of the total left ventricular volume respectively). Our study included total of 51 such patients, who were further divided into two groups. Group A constituting those patients whose affected myocardium was predominantly viable, and group B constituting those with predominantly scarred myocardium. Scans were evaluated using both the quantitative (with threshold as 50% as per literature) and visual method where matched perfusion-metabolic defects suggested scar and mismatched images i.e. perfusion defect with presence of metabolism suggested viable myocardium.Patients underwent subsequent surgical revascularisation procedures or were kept on medical management and followed up. The outcome was assessed based on change in ejection fraction (EF) of at least >=5% pre and post intervention (>=6 months) and presence or absence of cardiac event free survival.

Results: Of the total 51 patients, group A comprised of 26 patients(50.9%) with predominantly viable myocardium.18 of these 26 patients(69.7%) further underwent revascularisation, of whom most (94.4%) showed improved or stable EF. Amongst rest of the 8 of 26 patients(30.7%) with viable myocardium kept on medical management , majority i.e. 7 (87.5%) patients showed either fall in EF or developed cardiac events. Group B comprised of remaining 25 of total 51 patients (49.1%) with predominantly scarred myocardium,of which 6/25 patients (24%) were kept on medical management, and they showed improvemed or stable EF in all 6 cases. The remaining 19/25 (76%) patients with predominantly scarred myocardium underwent revascularisation, and (11/19) 57.9% of these patients showed improvement in EF which could be attributed to the presence of additional though smaller areas of viable myocardium in all these 11 patients (demonstrating either viability of at least 7% of total left ventricular volume or viability upto 30% of affected myocardium ) or presence of ischemic areas. The rest 8 of 19 patients (42.1%) showed a fall in EF or developed cardiac events (dyspnoea on exertion, ventricular tachycardia, cardiac tamponade, syncope etc.) in <= 6 months post revascularisation.

Conclusions: Majority of patients with predominantly viable myocardium as demonstrated on FDG PET showed improvement in LVEF post revascularisation. Nearly half of the patients with predominantly scarred myocardium who underwent revascularisation showed either improved or stable EF. These were the patients who demonstrated presence of additional viable myocardium of at least 7% of total left ventricular volume, or viability upto 30% of affected myocardium. This probably explains their favourable outcome. Rest of the patients with predominantly scarred myocardium who demonstrated either negligible presence of viable myocardium or myocardial viability was less than 7 % of total left ventricular volume, showed a fall in EF or cardiac events post revascularisation.Thus, extent of PET determined viable myocardium is a good prognostic factor in determining the clinical outcome post intervention in terms of LVEF improvement and occurrence of cardiac events.