OBJECTIVES: Impairment of coronary flow reserve (CFR) precedes preclinical atherosclerosis. However, data are lacking regarding its prognostic utility using SPECT imaging. Thus, this study aimed to determine the clinical utility of CFR by sestamibi imaging in predicting future cardiac events in patients with normal and abnormal myocardial perfusion scan (MPS).

METHODS: This was a prospective cohort study of 54 consecutive adult patients with suspected coronary artery disease referred to Nuclear Medicine Division, Philippine Heart Center for dipyridamole technetium-99m sestamibi SPECT MPS from August 2012 to September 2013. Patients with normal (summed stress score, SSS <3) and abnormal (SSS> 4) perfusion scans were further subdivided based on their CFR whether normal (CFR> 2) or abnormal (CFR < 2). Using a validated 2-day protocol, estimated CFR was computed from the ratio of dipyridamole myocardial blood flow (MBE) to resting MBF. MBF was obtained by dividing the global myocardial counts with the arterial input function. Global myocardial counts were derived from the average of the mean counts/pixel of two representative short-axis tomograms at the mediobasal and medioapical views. For the arterial input function, first-pass analysis of the pulmonary artery was performed to generate the time-activity curve (TAC). The mean counts/pixel of the area under the curve represented the input junction. Patients were monitored for major adverse cardiac events (MACE) through phone contact by patient and by review of hospital and physician's record six months after scintigraphy (mean follow-up 9+3 months). Major cardiac events include cardiac death, acute coronary syndrome, unstable angina, and revascularization.

RESULTS: A prospective cohort of 54 consecutive patients with no known CAD, were enrolled in the study. Abnormal MPI revealed significantly lower CFR (1.64 + 0.47 vs. 1.19 + 0.36, p=0.005). The annual cardiac event rate increased in the presence of reduced CFR in spite of a normal MPI (from 0% to 6.9%), and was even higher when both MPI and CFR were abnormal (from 0% to 34.7%). In Kaplan-Meier analysis, patients with abnormal perfusion revealed-significantly higher incidence of cardiac events compared with normal perfusion (chi-square 4.93, p=0.027). There was a trend towards increased incidence of cardiac events in patients with abnormal CFR; however, this did not reach statistical significance (chi-square 0.61, p=0.434).

CONCLUSION: A low CFR was associated with an increased incidence of MACE, particularly in the presence of abnormal perfusion findings.