Myocardial perfusion imaging in the acute care setting: Does it still have a role?

Patients with possible acute coronary syndrome (ACS) are a common Emergency Department (ED) problem, accounting for approximately 10% of all ED visits. Initial risk stratification hinges on the initial ECG, clinical presentation, and a set of cardiac biomarkers that includes troponin. In patients who are hemodynamically stable, the presence of ischemic ECG changes, either elevation or depression, identifies a highrisk group who require admission and early aggressive care with anti-platelet and anti-thrombotic treatment, with the addition of early reperfusion with those who have ST elevation. In the absence of ischemic ECG changes, patients with a history of coronary disease represent an intermittent risk group. The group of patients who remain, who have a non-ischemic ECG and no history of coronary disease, make up the majority of all ED chest pain patients. Although risk of an ACS is relatively low, it is not negligible. Many of the risk stratification protocols and strategies that have been developed are targeted to accurately identify those few high-risk patients in this group. To accelerate the evaluation process, numerous studies have incorporated various types of imaging into chest pain evaluation protocols. Imaging has various potential advantages when applied to this population. By accurately identifying high-risk patients, initiation of appropriate pharmacotherapy and admission is accelerated. In low-risk patients, the frequency of unnecessary admissions is reduced. By accurately differentiating between highand low-risk patients, the probability of inappropriate disposition is decreased. As a result, costs should be reduced through increased overall efficiency. Utilization of acute imaging for identifying ED patients who have ACS is not a new concept. In the 1970s, Wackers et al used thallium-201 imaging to evaluate 203 patients who presented with presumed ACS. All patients who had myocardial infarction (MI) had abnormal MPI, whereas the majority of those who had atypical chest pain or stable angina had normal MPI, with a minority having equivocal MPI; none had rest MPI interpreted as abnormal. Despite this and other promising studies, the use of acute thallium-201 MPI for evaluating chest pain patients was not adapted widely for various reasons. First, acute imaging with thallium-201 is logistically difficult. Thallium-201 begins to redistribute shortly after myocardial uptake; therefore, imaging must be performed shortly after injection to obtain images reflecting the acute presentation. The isotope characteristics of thallium-201 in combination with the lower sensitivity of planar imaging have limited image quality. In the 1990s, the development of the Tc isotopes, sestamibi, and tetrofosmin led to a renewed interest in using imaging for acute evaluation of chest pain patients. Technetium isotopes have the advantage in that they do not redistribute. Therefore, patients can be injected during symptoms and imaged later after stabilization. The images that are obtained at that time reflect the ischemic state and coronary blood flow at the time of isotope injection. The favorable energy and dosimetry of Tc also allows gated acquisition and reconstruction of dynamic functional images, thereby providing simultaneous assessment of both regional and global ventricular functions that can be correlated with perfusion defects. By using computer algorithms, ejection fraction can be quantitatively obtained. The value of incorporating gating into acute MPI interpretation has been under-recognized. Kontos et al analyzed outcomes from 2,826 consecutive patients admitted after undergoing acute rest MPI. Overall images were interpreted as abnormal in 40% and normal in 32%. Patients who had perfusion defects but with normal wall motion (27%) were considered negative for ACS and classified as non-acute MPI. Event rates were similar and not significantly different for those with From the Departments of Internal Medicine, Emergency Medicine and Radiology, VCU Medical Center, Virginia Commonwealth University, Richmond, VA. Adapted from a presentation at American Society of Nuclear Cardiology, Philadelphia, PA, September 2010. Reprint requests: Michael C. Kontos, MD, FACC, FAHA, Departments of Internal Medicine, Emergency Medicine and Radiology, VCU Medical Center, Virginia Commonwealth University, Room 285 Gateway Building, Second Floor, 1200 E Marshall St., P.O. Box 980051, Richmond, VA 23298-0051; mkontos@mcvh-vcu.edu, mckontos@vcu.edu. J Nucl Cardiol 2011;18:342–50. 1071-3581/$34.00 Copyright 2011 American Society of Nuclear Cardiology. doi:10.1007/s12350-011-9349-0