Myocardial Perfusion Imaging in an ED Chest Pain Unit

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Myocardial Perfusion Imaging in an ED Chest Pain Unit

Abstract and Introduction

Abstract


Background: Stress-only myocardial perfusion imaging (MPI) saves time by eliminating rest imaging, which is important for emergency department (ED) throughput but has not been studied in an ED population.
Study Objective: To determine the prognosis of a normal stress-only MPI study compared to a normal rest-stress MPI and establish its effectiveness in an ED setting.
Methods: All patients evaluated in the ED over 6.5 years who underwent a stress-only technetium-99m gated MPI were compared to those who had a rest-stress study. All-cause mortality was determined using the Social Security Death Index. Survival was analyzed in patients with normal and abnormal MPI results.
Results: A total of 4145 studies (2340 stress-only, 1805 rest-stress) were performed. Patients' average age was 57.9 years, 38.5% were male, and most had an intermediate or low pretest risk of coronary artery disease (87.7%). Average follow-up was 35.9 ± 20.9 months. In patients with normal perfusion, at 1 year of follow-up there were 11 deaths in the stress-only group (0.5% 1-year mortality), and 13 deaths in the rest-stress cohort (1.1% 1-year mortality). At the end of follow-up, the stress-only group had a lower all-cause mortality (p < 0.0001) and similar risk adjusted all-cause mortality (p = 0.10) than the rest-stress cohort. Patients with abnormal perfusion demonstrated the expected differential prognosis based on total perfusion deficits in both groups.
Conclusions: A normal stress-only MPI study has a benign 1-year prognosis similar to a rest-stress study when performed in the ED. The ability to triage patients more rapidly and reduce radiation exposure represents an attractive alternative for low-risk patients.

Introduction


Stress single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) plays an important role in the evaluation of patients with suspected coronary artery disease (CAD). The published literature and guidelines support a low event rate after a normal SPECT MPI, which has been found to be < 1% cardiac event rate (cardiac death, myocardial infarction [MI], or coronary revascularization) at 1 year. In addition, the positive predictive value of MPI exceeds that of clinical data and exercise treadmill testing, leaving MPI to play an important role in managing patients. Stress testing with MPI in a protocol-driven chest pain unit (CPU) is used routinely for risk stratification and is a 1B recommendation in the American College of Cardiology (ACC)/American Heart Association (AHA) guidelines for the management of patients with unstable angina in emergency department (ED) practice.

Stress-testing protocols endorsed by the American Society of Nuclear Cardiology (ASNC)/ACC/AHA include 1-day (low dose/high dose) or 2-day (rest-stress/stress-rest) studies (Figure 1). Typical stress protocols employed in CPU patients are rest-stress 1-day protocols, which take 3–4 h to complete. However, if the stress images are normal, then rest imaging is superfluous and does not alter interpretation of perfusion results. A stress-first protocol starts with stress (exercise or pharmacologic) and is followed by stress imaging. When the perfusion images and other parameters (absence of chest pain with exercise, absence of diagnostic electrocardiographic [ECG] changes, and ventricular function) are normal, then the test is complete, taking approximately 90 min. There is no need for the second (rest) tracer injection or rest imaging, making it a stress-only study. The advantages of stress-only imaging include time savings for patients and the laboratory, less radiation exposure for patients, and cost savings for patients and the health care system. Additionally, the greater efficiency provided by a stress-only protocol helps facilitate ED throughput and decreases departmental length of stay. The shorter length of the stress-only MPI study also compares favorably with other modalities used to assess chest pain in the ED, such as stress echocardiography and cardiac computed tomography (CT) angiography. Effective stress-only MPI protocols however, require the use of attenuation correction technology such as radionuclide transmission scanning using a Gd-153 line source or CT imaging to aid in image interpretation.



(Enlarge Image)



Figure 1.



Standard full study (rest-stress) Tc-99m exercise and pharmacologic imaging protocols. (A) Exercise, (B) Adenosine, (C) Dipyridamole.





There is little literature regarding the safety and efficacy of stress-only MPI, and there are no studies assessing the feasibility of stress-only MPI to evaluate patients in an ED setting. An initial clinical study established the feasibility of the stress-only protocol, and two recent series further substantiated the prognostic capabilities of stress-only imaging. A recent clinical update from the ASNC suggested that more study in this area was needed.

The current study was conducted to further explore the feasibility of stress-only MPI in routine clinical practice in an ED CPU. The goal was to determine the prognosis of a stress-only MPI compared to a full rest-stress MPI and establish the effectiveness of the protocol when applied to risk stratification in the ED population.

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