Incremental prognostic value of coronary flow reserve assessed with single-photon emission computed tomography

Abstract

The diagnostic and prognostic values of stress myocardial perfusion imaging (MPI) is well established with SPECT and more recently with PET or PET-CT imaging. Conventional MPI identifies local reductions in perfusion relative to the best ventricular wall, where the maximum value is presumed to be normal. However, this technique is known to underestimate the extent or severity of multi-vessel coronary disease, when the maximum value does not represent normally perfused myocardium. In contrast, myocardial perfusion measured on an absolute scale (mL/min/g) permits the assessment of stress-to-rest perfusion or vasodilator reserve without the assumption of a normal reference region, first demonstrated with PET imaging by Gould et al In this way, the stress perfusion scan is interpreted relative to the individual’s normal resting state, using the patient as their own control. The clinical value of absolute perfusion or myocardial blood flow (MBF) measurements has been reported extensively using dynamic PET imaging with O-water and Nammonia in patient populations such as diabetes, hypertension, hyperlipidemia, smoking, syndrome X, non-compaction, takotsubo, hypertrophic ,and dilated cardiomyopathies. The impact of invasive fractional flow reserve (FFR) imaging to direct PCI and improve patient outcome has recently been reported using FFR measurements in a prospective randomized controlled trial. However, there is limited data available on the value of non-invasive myocardial flow reserve (MFR) imaging to improve patient management, or predict future clinical events, compared to conventional MPI. This is partly due to the limited availability of Nammonia and O-water PET, which require a local cyclotron for tracer production. Tio et al recently reported on the prognostic value of N-ammonia PET flow reserve imaging in patients with known coronary artery disease and LV dysfunction who did not undergo revascularization. This study showed that MFR was a stronger predictor of cardiac death compared to LVEF in this population (HR 4.1 vs 2.8; 7 years median followup), but did not assess the incremental value compared to standard MPI. Herzog demonstrated similar findings that impaired MFR (ratio 2) was an independent predictor of cardiac death (HR 2.9; 5.5 years mean follow-up) using N-ammonia PET in a wider unselected population. This study also showed the incremental prognostic value of MFR over standard MPI in patients with known or suspected CAD, more typical of the clinical population referred for assessment of myocardial ischemia. In patients with normal MPI, impaired MFR was an independent predictor of cardiac death (3.1 vs 0.5%) and major adverse cardiac events (MACE; 6.3 vs 1.4%) within a period of 3 years. In patients with abnormal MPI, impaired MFR also predicted an increased rate of disease progression, even beyond 3 years. A larger patient series, followed for a shorter time, has recently confirmed the incremental prognostic value of generator-produced Rb PET MFR to predict death or myocardial infarction (death ? MI; HR C 3.3) and MACE (HR C 2.4; 1-year median follow-up) in patients with normal or abnormal stress MPI. The current study by Stephania et al builds on these previous findings using Tc-SPECT, which is more widely available as the clinical standard for MPI. The authors use first-pass planar imaging followed by conventional SPECT MPI to estimate a retention index of MBF and corresponding global flow reserve. They From the Division of Cardiology, Cardiac Imaging, The National Cardiac PET Centre, University of Ottawa Heart Institute, Ottawa, ON, Canada. Reprint requests: Robert A. deKemp, PhD, Cardiac Imaging, The National Cardiac PET Centre, University of Ottawa Heart Institute, 40 Ruskin Street, Ottawa, ON K1Y 4W7, Canada; radekemp@ ottawaheart.ca. J Nucl Cardiol 2011;18:541–3. 1071-3581/$34.00 Copyright 2011 American Society of Nuclear Cardiology. doi:10.1007/s12350-011-9408-6