Single photon emission computed tomography (SPECT) techniques for resynchronization: Phase analysis and equilibrium radionuclide angiocardiography

Although cardiac resynchronization therapy (CRT) has been shown to improve heart failure (HF) symptoms and survival in selected patients, questions remain about optimal patient selection and implementation of this therapy. Efforts to improve patient selection have focused on the cardiac substrate for resynchronization, including assessment of mechanical discoordination, or dyssynchrony, of the left ventricle (LV) and other structural characteristics such as scar or volumes that may be associated with the likelihood of response. This issue of the Journal contains papers on two distinct applications of nuclear imaging for patients who may be candidates for CRT or who have received CRT. In the study by Samad et al, phase analysis was performed retrospectively in a series of patients with LVEF B 0.35 who had undergone single photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) at a single center for evaluation of coronary heart disease (CHD), whereas the study by Clements et al presents an analysis of right ventricular (RV) and LV structure and function in a series of patients referred for biventricular pacemaker upgrades. The interesting study by Samad et al provides needed insights into baseline predictors of an abnormal SPECT phase analysis based on a retrospective series of 260 patients with SPECT MPI. Patients in this study were not referred for CRT, and no data on CRT response are available based on the study design. The authors found that significant multivariate predictors of phase SD C 43 included greater QRS duration (QRSd), lower LVEF, and greater summed rest perfusion score (SRS). The influence of resting perfusion abnormalities on phase SD and the distribution of abnormal phase SD based on QRSd are of particular interest and will be explored further in this editorial. Overall, abnormal phase SD was present in 39% with QRS \ 100 ms, 41% with QRS 100-119 ms, 61% with QRSd 120-149 ms, and 80% with QRSd greater than 150 ms. The principle of phase analysis, as pioneered by Chen and colleagues, is that the phase extracted from the regional LV count changes throughout the cardiac cycle is related to regional variability in the time to onset of mechanical contraction. The standard deviation of the phase distribution may then be calculated as the phase SD (the more commonly used parameter), while the phase bandwidth represents the phase range during which 95% of the LV is initiating contraction. From a physiological standpoint, the phase SD, with its characterization of regional timing in onset of LV strain, contrasts with other dyssynchrony parameters based on regional differences in time to peak strain, although a good correlation (r = 0.80, P \ .0001) has been found between phase SD and difference in time to peak velocity in opposing walls by echocardiographic tissue Doppler imaging (TDI). Of note, quantification of timing either to peak onset or to peak strain is difficult and less meaningful in regions with akinesis due to myocardial infarction since strain remains close to zero throughout systole. Of note, this highlights a role for other types of metrics in these patients. The rationale for using phase SD in patients referred for CRT is based largely on the well-designed but relatively small prospective study by Henneman et al. The 42 patients included had New York Heart Association (NYHA) Class III-IV HF, LVEF \ 0.35, and QRSd [ 120 ms. Overall, the mean QRSd, LV end-diastolic volume (LVEDV), LVEF, and NYHA Class were 153 ± 32 ms, 251 ± 87 mL, 24 ± 7%, and 2.9 ± 0.4, respectively. The overall response rate to CRT, defined as an improvement of at least one NYHA class, was 71% at From the Department of Medicine, University of Virginia Health System, Charlottesville, VA. Reprint requests: Kenneth C. Bilchick, MD, MS, Department of Medicine, University of Virginia Health System, P.O. Box 800158, Charlottesville, VA 22901; bilchick@virginia.edu. J Nucl Cardiol 2011;18:16–20. 1071-3581/$34.00 Copyright 2010 American Society of Nuclear Cardiology. doi:10.1007/s12350-010-9312-5