E-Journal ~ Spring 2001 ~ Society of Chest Pain Centers

Acute coronary syndrome care: emergency department strategies
Alan Storrow, M.D.

Dr. Storrow is assistant professor of medicine and director, Heart ER Program, University of Cincinnati.

I. Introduction

A. The diagnosis of myocardial ischemia and acute myocardial infarction (AMI) is extremely challenging for the practicing emergency physician.

Evaluation of patients with chest discomfort is currently performed through a careful history, physical exam, and electrocardiogram (ECG).

Nearly 50% of patients with AMI present to the emergency department (ED) with a nondiagnostic ECG.

Despite a vigorous effort to identify patients with myocardial disease, 25% of patients presenting to the ED with AMI and chest pain are inadvertently released to home.

Twenty percent of the dollars lost in malpractice litigation for emergency physicians are related to the diagnosis and treatment of myocardial ischemia and AMI.

B. Factors influencing the development of chest pain emergency units in the United States

Rapid treatment with thrombolytic therapy saves lives in AMI (eg, MITI Trial, Weaver et al, JAMA, 1993).

National Heart Attack Alert Program: Thirty minutes to treat AMI in the ED

ED evaluation of patients with chest pain is the most cost-effective.

C. Development of rapid diagnostic and treatment centers in the ED for chest pain

1981 - First center opens, St. Agnes Hospital, Baltimore, MD.

1994 - Estimated 500 chest pain emergency centers in the United States.

2000 - Estimated 3,000 chest pain emergency centers in the United States.

D. Traditional evaluation techniques

A careful history for myocardial ischemia and AMI includes a discussion of chest pain quality, radiation of pain, and associated symptoms such as nausea, vomiting, and diaphoresis. Cardiac risk factors should also be ascertained and documented (Figure 1).

Younger patients, particularly women, may have atypical presentations of acute myocardial syndromes. Dismissing such patients because of their age can be problematic. Any young patient with chest pain should be questioned regarding cocaine use.

The fastest growing group in the United States has been reported to be the octogenarian population. Elderly patients may have blunted autonomic responses minimizing associated symptoms such as nausea, vomiting, and diaphoresis. Pain sensation from the heart may also be decreased. Older patients often pre-sent with neurologic sequelae of AMI including stroke or a change in mental status.

Serial examina- tions of the patient by the emer- gency physician, while in the ED, may improve diagnostic accuracy in these patients.

II. Advances in the diagnosis of myocardial ischemia and AMI The development of thrombolytic therapy and PTCA for AMI has intensified the interest of emergency physicians in the early diagnosis and treatment of patients with AMI. As therapy which reperfuses infarcting myocardium is time-dependent, diagnostic regimens for acute ischemic syndromes have become accelerated and more specific.

A. The 12-lead ECG and ST-segment trend monitoring Approximately 50% of patients with AMI present to the ED with a nondiagnostic 12-lead ECG. The ECG will provide a specific diagnosis in about 5% of ED patients with chest pain. The presence of ST-segment elevation or new left bundle-branch block is highly reliable in identifying patients with transmural AMI; approximately 95% of patients with these findings will develop infarction. ST-segment depression or T-wave inversion are nonspecific and evolve into non – Q wave MI in <15% and represent ua in another 20%. the remainders have stable angina or noncardiac chest pain. significant research efforts have been made to improve the diagnostic capability of the 12-lead ecg.

serial 12-lead ecgs should be ordered every 30 minutes for any patient with ongoing ischemic chest pain in the ed. st-segment elevation can then be treated with reperfusion therapy.

serial electrocardiography is utilized in chest pain centers (cpcs) to detect rest ischemia. st-segment trend monitoring detects st-segment displacement and t-wave amplitude changes. theoretically, this fully programmable technology may detect dysrhythmias or ischemic changes before they become clinically detectable. they may aid in faster identification of candidates for fibrinolytics or angioplasty whose initial ecg is nondiapostic. furthermore, it may aid in the detection of st-segment changes suggestive of ua or non Ð q wave mi, and, therefore, may provide the earliest clues of acute coronary syndrome (acs). data from large randomized studies confirming its diagnostic benefit or cost-effectiveness in the cpc setting have not been reported.

b. early serum markers of ami and prolonged unstable angina early serum markers of ami have proved effective in the ed setting, particularly using serial blood draws.

rapid immunochemical testing for creatine kinase-mb (ck-mb) allows the rapid detection of patients with ami within 4 to 5 hours after symptoms onset. studies have demonstrated serial ck-mb sampling to have a sensitivity and specificity of greater than 90% within 3 hours after ed presentation. the sensitivity for detecting all patients with ami approaches 100% within 10 to 11 hours after symptom onset.

Recent studies have demonstrated that CK-MB elevation is associated with in-hospital complications. Triage of patients with CK-MB elevation, and a nondiagnostic 12-lead ECG, to intensive care settings is warranted.

Other serum cardiac markers including myoglobin, cardiac myosin light chains, and the troponins (T and I) may substantially improve the sensitivity and specificity for detecting myocardial injury. Troponin T has proved effective in detecting patients with severe, prolonged unstable angina, including the identification of patients likely to have ischemic complications including AMI and death.

Cardiac troponin T (cTnT) and cardiac troponin I (cTnI) are sensitive and specific markers for myocardial cell damage. They have been suggested to be more sensitive than CK-MB for myocardial necrosis, though it appears that kinetics of these two markers are very similar. The greatest impact of the troponins is their use for prognosis in patients with ACS. The cardiac troponins stay elevated after myocardial necrosis for at least several days. Specificity for AMI in published series varies from 46% to 96%. As CK-MB is usually held as the gold standard for the diagnosis of AMI in these studies, the troponins increased sensitivity for detecting myocardial necrosis adversely impacts its specificity (Figure 2).

Elevated cardiac troponins with normal CK-MB indicate myocardial damage not detected conventionally. There is substantial evidence that 30% of patients with UA have circulating troponins, indicating cell damage. Increasing troponin levels have been correlated with a higher risk of future cardiac events. While a single level at presentation is not adequate for decision making, two negative values, one obtained at least 6 hours after symptoms, has been suggested to allow safe ED discharge. This study was based on AMI and death within 30 days and included those patients with negative tests who were admitted. Of 20 deaths during the study period, 4 and 1 were in patients with negative troponin T and I values, respectively. It is unclear what the natural history of UA is in patients with negative troponin testing. It may be prudent to limit ED triage decisions based on troponin to the lower-risk subgroup.

Point-of-care testing for cardiac markers can assist in the earlier diagnosis of AMI, prediction of complications, and prevention of inadvertent ED release of AMI patients. The impact of such tests will depend on how quickly quantitative values are available; cost-effectiveness will be affected by volume of testing and decrease in the necessity for hospitalization. Further investigation into their impact in realtime decision making and cost comparisons with traditional strategies is warranted. The increased utility of markers to detect myocardial necrosis in the continuum of ACS predict outcome, and potentially direct therapy is causing a reappraisal of their use in the ED. They have also become an important component of serial assessment in ED-based CPCs for patients at low to moderate risk for ACS.

The National Academy of Clinical Biochemistry (NACB) has prepared recommendations on the use of cardiac markers for patient triage, which has substantial implications for institutions with CPCs. They recommend that those involved in the evaluation of patients with possible ACS should collectively develop an accelerated protocol for cardiac marker use. This should include an early marker which is reliably increased in the blood within 6 hours of symptoms, and a definitive marker that is increased in the blood after 6 to 9 hours with a high sensitivity and specificity for myocardial injury, and remaining abnormal for several days after onset. Two markers are recommended because of the varying interval between chest pain onset and ED presentation (Figure 3).

Early marker considerations include myoglobin, CK-MB mass, and CK-MB isoforms. While there is some debate whether highly sensitive CK-MB mass assays are detectable as early as myoglobin, the preponderance of evidence is in myoglobinÕs favor. A few institutions use CK-MB isoforms as an alternative. Although myoglobin has significant limitations in specificity, it seems a reasonable choice for an early marker as it is widely recognized by clinicians as an early marker for myocardial injury. The NACB has suggested the use of a cardiac troponin as the definitive marker.

C. Two-dimensional echocardiography The echocardiogram provides a noninvasive method for detecting myocardial ischemia and AMI in patients presenting to the ED.

Regional wall motion abnormalities and global hypokinesis are easily detected by echocardiography. Detection of impaired myocardial contractility from severe ischemia, without AMI, can make echocardiography a valuable adjunct in the ED setting.

Study quality is related to the experience of the operator, expertise of the reader at the patient bedside, and the inability of the two-dimensional echocardiogram to distinguish between ischemia, AMI, or old infarction.

Echocardiography is typically less expensive than radionuclide ventriculography, offers more anatomic detail, and avoids using radionuclides.

D. Radionuclide scanning Several studies suggest that thallium scintigraphy may be useful in detecting AMI in patients presenting to the ED with chest pain.

Areas of decreased thallium uptake by myocar- dium, seen as a negative scintigraphic image, indi- cate regions of severely ischemic or infarcted myo- cardium evident within the first 6 hours after symptom onset.

Radionuclide studies are difficult to perform in the very early time period after patient presentation to the ED. Thallium may not be immediately available and skilled interpretation of scans is often not present 24 hours a day in many medical centers.

Sestamibi scans, performed from the emer- gency setting, may offer a superior method for detecting ACS in CPC patients (Tatum, 1997).

E. Computer-aided decision making Several studies have evaluated the use of computer algorithms to improve detection of acute ischemic syndromes.

Predictive instruments use data from prior studies to develop sophisticated algorithms for evaluating patients presenting to the ED with chest pain. Typically, the 12-lead ECG is used with certain demographic data and pain descriptions to provide a probability of myocardial ischemia.

Recently, a neural network has been prospec- tively validated in a trial of 331 consecutive ED patients with a chief complaint of anterior chest pain. Thirty-six of these patients were ultimately diagnosed as having AMI. The network performed with a sensitivity of 97.2% and a specificity of 96.2% in this patient cohort. The physicians caring for the study patients performed with a sensitivity of 77.7% and a specificity of 84.7%.

III. University of Cincinnati Heart ER Program At the University of Cincinnati Center for Emergency Care, a program has been developed for evaluating patients presenting to the ED with chest pain and possible acute myocardial ischemia or AMI. Since October 17, 1991, over 2,600 patients have been evaluated in this program (Figure 4).

A. Exclusion criteria

Patients with ST-segment elevation indicative of AMI or ST-segment depression consistent with myocardial ischemia are directly admitted to the hospital after appropriate ED therapy.

Patients with ongoing chest pain and a clinical presentation consistent with unstable angina are admitted to the hospital with intravenous heparin and nitroglycerin.

Hemodynamically unstable patients are admitted to the hospital after evaluation in the catheterization laboratory.

B. Evaluation methods

Serial CK-MB, myoglobin, and cTnT levels are determined at 0, 3, and 6 hours while the patient is in the ED.

Serial ECGs (trends) are obtained every 20 seconds over while the patient is in the ED.

Finally, if all previous tests are normal, the patient undergoes graded exercise testing or radionuclide scanning.

Patients with chest discomfort, having negative evaluations in the Heart ER program, are released to home from the ED (Figure 5).

C. Results
The Heart ER program has proved to be a safe, medically effective, and cost-effective method for evaluating patients presenting to the ED with chest pain and possible acute ischemic syndromes. Of 2,131 patients reviewed in 1998, 1,822 (85.5%) were released home from the ED and 309 (14.5%) required admission. Ninety-four of the admitted patients (30%) were found to have a cardiac cause (including 23 acute MI and 63 angina or unstable angina). Follow-up was obtained in 1,696 (93%) of patients discharged from the Heart ER with nine cardiac events (0.53%, 95% CI 0.24%-1.01%; 7 PTCA; 1 CABG; 1 death).