Use of Myocardial Perfusion Imaging in the Emergency Department for Risk Stratification
Ethan J. Spiegler, MD, FACPAssistant Professor of Radiology
Instructor of Medicine
Johns Hopkins Hospital
Director, Nuclear Medicine
St. Agnes HealthCare
Baltimore, MD
Quick Overview
Currently, the management of patients with chest pain of suspected cardiac origin involves admission to a monitored bed for precautionary reasons. Typically, the hospital stay lasts 24 to 48 hours, followed by a provocative stress test prior to discharge. This strategy poorly utilizes limited medical resources, as at least one third of these patients have not experienced acute myocardial infarction (AMI). Despite the low threshold for admitting patients with chest pain, as many as 5% to 8% of patients discharged from the ED are found to have suffered an AMI. (2-4) In fact, failure to diagnose and treat AMI patients in the ED accounts for the largest settlements of malpractice lawsuits. (5)
Ideally, stratification of patients into risk categories would allow for appropriate disposition. For example, low-risk stratification would facilitate early ED discharge, while the accurate identification of a high-risk patient subgroup would allow early triage of patients to a monitored setting and prompt initiation of appropriate therapy.
Radionuclide Perfusion Imaging
Nuclear medicine/nuclear cardiology laboratories have used thallium-201 to assess myocardial perfusion in patients with known or suspected coronary artery disease (CAD). The use of thallium-201 has a number of disadvantages including a relatively long half-life, which limits the amount of radioactivity used, as well as the emission of a low-energy photon, which may be easily attenuated by soft tissues, such as diaphragm or breast. Thallium-201 also has the tendency to redistribute, rendering its use impractical in evaluating patients with acute chest pain syndromes in the ED. (6,7)Sestamibi and, more recently, tetrofosmin are rapidly taken up by myocardial cells in proportion to blood flow. (8) Sestamibi and tetrofosmin are labeled with technetium (Tc) 99m, and have physical characteristics ideally suited for typical gamma camera systems. Tc-99m has a short half-life, allowing the safe use of approximately ten times the dose of radioactivity used in thallium-201 imaging. In addition, the higher energy photon is less likely to be attenuated by soft tissues. These tracers are ideally suited for single photon emission computed tomography (SPECT) imaging. The SPECT study may also be gated to the ECG to evaluate regional wall motion and wall thickening of each tomographic slice. Most importantly, these agents undergo no significant redistribution following injection and, therefore, reflect myocardial perfusion at the time of injection, even when imaging is delayed for several hours. These characteristics make the Tc-99m agents ideally suited for imaging patients with acute chest pain syndromes.
Multiple studies have confirmed the utility of early imaging of ED patients who present with chest pain and nondiagnostic ECGs. (9-13) Varetto et al (12) studied 274 consecutive patients presenting to the ED with chest pain of suspected cardiac origin. To be included in the study, patients had to have chest pain lasting more than 30 minutes and occurring within 12 hours of ED presentation, a nondiagnostic ECG, and symptoms not readily explained by noncardiac disease. Patients with a history of previous AMI, over 70 years of age, or pain lasting over 12 hours were excluded. In total, 64 patients (24%) met the inclusion criteria and underwent SPECT Tc-99m sestamibi studies. Scans were considered positive if there was a perfusion defect, and were considered negative if perfusion was normal. The scans showed a sensitivity of 100%, a specificity of 92%, and an accuracy rate of 90% for detecting those patients with the final diagnosis of CAD. All patients with negative scans remained free of cardiac events up to 18 months. Although this study is limited by the small number of patients enrolled, it suggests that imaging with Tc-99m sestamibi may have diagnostic as well as prognostic value.
Quick Overview
Hilton et al (10) studied 102 patients with typical chest pain and nondiagnostic ECGs. Patients were stratified into three risk groups based upon the number of coronary risk factors (< 3 versus > 3) and ECG results (normal versus abnormal but nondiagnostic). The follow-up cardiac event rate for these groups-low, intermediate, and high-were 6%, 11%, and 35%, respectively.
The Tc-99m sestamibi scans of these patients displayed a much higher sensitivity, specificity, and accuracy for predicting cardiac events: 94%, 83%, and 85%, respectively. Of 70 patients with normal scans, only 1 suffered a cardiac event. The authors were further able to stratify most patients with chest pain and nondiagnostic ECGs into either a very low (< 2%) or very high (> 70%) risk group for developing acute cardiac events.
Our laboratory has been performing SPECT Tc-99m sestamibi imaging in a 400-bed community hospital in Baltimore, Md. (11) We use similar inclusion and exclusion criteria as the studies noted above. Of the 248 patients studied, we have noted a sensitivity of 94%, a specificity of 85%, and an accuracy rate of 86% for the prediction of acute cardiac events. Examples of normal and abnormal sestamibi SPECT studies are shown in Figures 1 and 2.
Figure 1
Figure 2
Timing of Injection
Of interest, in the study of Varetto et al (12) were a number of patients whose chest pain had resolved up to 12 hours before ED arrival but who had persistent perfusion defects. The authors postulate this may be secondary to myocardial stunning and/or diminished collateral blood flow. These results suggest that there may be a window of opportunity to inject the radiopharmaceutical for a number of hours after the patients' symptoms have subsided. The implication is that one may not require the radiopharmaceutical or staff on site to inject a symptomatic patient. If these results are confirmed, one could call a covering technologist or physician at home to come in to inject and scan a patient in the ED.A recent study by Stowers et al (14) disputes these results. In their study, the scans of patients injected while symptomatic had an overall accuracy rate of 82%. The scans of those injected following the resolution of symptoms (range, 5 to 360 minutes) showed a markedly diminished accuracy rate of 38%. It appears likely that there is a window of opportunity for injecting a patient following resolution of symptoms and still obtaining an accurate test; however, further study will be required to determine this time interval.
Cost-Effectiveness
Two studies have retrospectively reviewed the cost-effectiveness of early imaging of ED chest pain patients. Radensky et al (15) noted that prior to employing an early imaging strategy, the admission rate of ED patients with chest pain and nondiagnostic ECGs at their hospital was 96%. Following the shift to early imaging, the admission rate fell to 60%. They demonstrated a 17% reduction in hospital costs and a $923 savings per patient in the group undergoing scanning. There were no cardiac events noted during the follow-up period in any patient discharged from the ED.Weissman et al (16) noted that for 50 patients with unexplained chest pain, sestamibi imaging altered triage decisions for 34. This allowed either admission to a level of care that was less intense than originally planned or to direct patient discharge. Overall, the group found that screening with sestamibi saved $1,771 per patient and prevented a total of 69.4 unnecessary inpatient hospital days.
Conclusion
Early myocardial perfusion imaging appears to enable physicians to appropriately risk stratify a previously difficult and challenging patient group. Data appear to support the cost-effectiveness and clinical utility of this approach. Although there is great interest in utilizing this procedure, the logistics of setting up an acute myocardial imaging program are not insignificant. To be effective, a true team approach is necessary to bring this to fruition. ED physicians and nurses, cardiologists, nuclear medicine physicians, radiologists, technologists, and referring internists and family practitioners must work synchronously to effect a change in the workup of the low-probability chest pain patient. Twenty-four hour availability of myocardial perfusion imaging must also be provided. Further prospective evaluation of this technique is warranted. By accomplishing these goals we will be able to deliver a cost-effective and clinically effective strategy that will benefit our patients.References
1. Kaul S, Abbott RD. Evaluation of chest pain in the emergency department. Ann Intern Med. 1994;121:976.
2. Lee TH, Rouan GW, Weisberg MC, et al. Clinical characteristics and natural history of patients with acute myocardial infarction sent home from the emergency room. Am J Cardiol. 1987;60:219-224.
3. McCarthy BD, Beshansky JR, Agonstino RB, et al. Missed diagnosis of acute myocardial infarction in the emergency department: results from a multicenter study. Ann Emerg Med. 1993;22:579-582.
4. Lee TH, Cook EF, Weisberg MC, et al. Acute chest pain in the emergency room: identification and examination of low-risk patients. Arch Intern Med. 1985;145:65-69.
5. Rogers JT. Risk Management in Emergency Medicine. Dallas, Tex: American College of Emergency Physicians; 1985:1-36.
6. Wackers FJT, Lie KI, Liem KL, et al. Potential value of thallium-201 scintigraphy as a means of selecting patients for the coronary care unit. Br Heart J. 1979;41:111-117.
7. Mace SE. Thallium myocardial scanning in the emergency department evaluation of chest pain. Am J Emerg Med. 1989;7:321-328.
8. Okada RD, Glover D, Gaffney T, et al. Myocardial kinetics of technetium-99m-hexakis-2-methoxy-2-methylpropyl-isonitrile. Circulation. 1988;77:491-498.
9. Bilodeau L, Theroux P, Gregoire J, et al. Technetium-99m sestamibi tomography in patients with spontaneous chest pain: correlations with clinical electrocardiographic and angiographic findings. J Am Coll Cardiol. 1991;18:1684-1691.
10. Hilton TC, Thompson RC, Williams HJ, et al. Technetium-99m sestamibi myocardial perfusion imaging in the emergency room evaluation of chest pain. J Am Coll Cardiol. 1994;23:1016-1022.
11. Spiegler EJ, Civelek AC, Bahr R, et al. The use of technetium-99m sestamibi in the emergency room: can it assist in the triage of patients with chest pain? Clin Nucl Med. 1993;18:807. Abstract.
12. Varetto T, Cantalupi D, Altieri A, et al. Emergency room technetium sestamibi imaging to rule out acute myocardial ischemic events in patients with nondiagnostic electrocardiograms. J Am Coll Cardiol. 1993:22:1804-1808.
13. Tatum JL, Ornato JP, Jesse RL, et al. A diagnostic strategy using Tc-99m sestamibi for evaluation of patients with chest pain in the emergency department. Circulation. 1994;90:I-307. Abstract.
14. Stowers SA, Abuan THE, Szymanski TJ, et al. Technetium-99m sestamibi SPECT and technetium-99m tetrofosmin SPECT in prediction of cardiac events in patients injected during chest pain and following resolution of pain. J Nucl Med. 1995;36:88P. Abstract.
15. Radensky PW, Stowers SA, Hilton TC, et al. Cost-effectiveness of acute myocardial infarction perfusion imaging with Tc-99m sestamibi for risk stratification of emergency room patients with acute chest pain. Circulation. 1994;90:I-528. Abstract.
16. Weissman IA, Dickinson CZ, Dworkin HJ, et al. Emergency center myocardial perfusion SPECT - long-term follow-up: cost-effective imaging providing diagnostic and prognostic information. J Nucl Med. 1995;36:88P. Abstract.
Return to Index of Articles for Clinician;
Volume 14.4
- Faculty Listing 3
- Introduction: Community Message in Acute Myocardial Ischemia 4
- Concept of Community Chest Pain Centers in Emergency Departments 5
- Reawakening Awareness of the Importance of Prodromal Symptoms in the Shifting Paradigm to Early Heart Attack Care (EHAC) 7
- The Role of Emergency Medicine in the Future of American Medical Care: A Summary of the Josiah Macy, Jr, Foundation Conference 12
- From ACC/AHA Guidelines to Best Practice: Can "Acute MI Report Cards" Save Lives?Measuring Quality of Care for Patients With Cardiovascular Disease 16
- Improving Lay Responses to Cardiovascular Emergencies and the Potential of Shifting the Paradigm to Early Heart Attack Care 18
- National Survey of Chest Pain Centers Rationale and Potential Results 21
- Outpatient Rule-Out Myocardial Infarction in Observation Units 22
- Reducing Time to Thrombolytic Treatment 24
- Intervention Programs to Reduce Patient Delays in Acute Myocardial Infarction 29
- Public Education Using the Early Heart Attack Care Program Shawnee Mission Medical Center 31
- School Component: Early Heart Attack Care Program Development and Evaluation 33
- Educational Strategies to Prevent Prehospital Delay in Patients at High Risk for Acute Myocardial Infarction 36
- Precipitating Factors in Acute Cardiovascular Disease 41
- Use of Myocardial Perfusion Imaging in the Emergency Department for Risk Stratification 44
- The Evolution of Chest Pain Centers: The Impact of Technological Advances on the Evaluation of Patients With Possible Acute Ischemic Coronary Syndrome 47
- Critical Pathways for Triage and Treatment of Chest Pain Patients in the Emergency Department 53
- Chest Pain Centers: An Emerging Model for the Emergency Evaluation of Chest Pain and Treatment of Acute Myocardial Infarction 56
- The Key to Success in the Chest Pain Emergency Department: The Observational Role of theNurse 59
- Triage in Moderate Risk Patients With Chest Pain: The Emerging Role of Real-Time Continuous ECG Monitoring 61
- Diffusion of Innovations: The Introduction and Impact of the Chest Pain Emergency
- Department and its Comparison to the Coronary Care Unit Development 64
- Economics of Chest Pain Centers: What Really Matters? 68
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