Acute Coronary Syndrome
Acute Coronary Syndrome is “bread and butter” emergency medicine - Daniel Kreider MD
image by: Armando Hasudungan
Accurate and expeditious diagnosis of acute myocardial infarction or acute coronary syndrome (ACS) is one of the key charges of emergency medicine providers. Chest pain is the second most common reason for emergency department visits in the United States and coronary artery disease is our leading cause of death.
Up until the late 1980s, most patients presenting to the emergency department (ED) with chest pain were simply admitted to a coronary care unit (CCU). Some of these patients had acute myocardial infarction (MI) and were treated accordingly. However, the majority were low risk patients admitted simply to monitor for the development of acute MI with serial ECGs and cardiac enzyme measurement (CK-MB at that time), with over half eventually being “ruled out” for myocardial infarction.
Admission of these “rule out MI” patients to an intensive care unit was costly, leading hospitals to seek ways of distinguishing low-risk patients who could be discharged earlier. Researchers developed decision aids to help identify low risk patients in the ED and attempted to narrow down the shortest amount of time in which MI can be safely ruled out. In the end, several different scores were developed and it was determined that 12-24 hours was an acceptable observation period for ruling out MI. The issue remained, however, that these patients were still being admitted to the CCU for the 24 hour observation period.
In 1991, emergency physicians at Brigham and Women’s Hospital published a study describing a “coronary observation unit” in which patients with low risk chest pain were monitored in an emergency department observation unit rather than being admitted to the CCU (9). They used their own clinical algorithm to determine low risk, which they defined as <10% risk of acute myocardial infarction, and in the observation unit, patients underwent ECG every 12 hours and CK-MB testing every 8 hours for a total of 24 hours. Cardiology consultation was optional.
If serial ECGs and cardiac enzymes remained normal during the observation period, the patient was discharged home. Study investigators found no increase in cardiac events or death between the patients placed in the ED observation unit and the controls who received usual care i.e. admission to the CCU.
Three years later, the same researchers published a follow-up paper from their observation unit, this time with a larger patient cohort. Again they confirmed that ED observation was safe and in this study they found significant cost savings for the hospital, piquing the interest of hospital administrators worldwide.
An important side note from this study: prior to discharge, the patients in the ED observation unit underwent an exercise stress test. The authors admitted that stress testing has poor sensitivity and specificity, but stated it provided a “reasonable estimation of prognosis” for the patients in whom MI had already been ruled out. It is unclear what happened if patients had abnormal stress test results.
Since 1991, observation units have been widely adopted in emergency departments around the country and most of them have a “rule out MI” pathway. The pathways at each hospital differ, but typically include 12 to 24 hours of telemetry monitoring, serial ECGs, and serial cardiac enzyme testing. Most pathways also include an inpatient stress test or one scheduled within 72 hours, as per the 2014 AHA/ACC guideline recommendations. Optional additions to the pathway include cardiology consultation and a rest echocardiogram.
Now that we are 30 years on from the first observation unit, we can re-evaluate the system – does observation for “rule out MI” still make sense with the improved technology and knowledge we have today? The answer, for me at least, is no.
For one, we now have an entirely new cardiac enzyme that is more sensitive and specific for myocardial necrosis than CK-MB – the fabled troponin. Troponin I and troponin T rise within 3-4 hours of myocardial injury and remain elevated for up to 14 days. The enhanced qualities of troponin testing allow us to pick up more MIs, and earlier MIs, than CK-MB ever could.
Recent studies show that many patients who have negative CK-MB measurement will actually have positive troponin levels, highlighting the increased sensitivity with troponin testing. A more sensitive test will decrease the rate of missed MI in the ED, thus obviating the need for further monitoring in an observation unit.
In addition to the new cardiac enzymes, we have improved clinical prediction tools to identify low risk patients. The TIMI score, GRACE score, and more recently, the HEART score are all validated scoring systems. Emergency providers have warmly embraced the HEART score (and the related HEART pathway) given its simplicity, accuracy, and improvement in ED workflow. Through the use of these clinical prediction tools, we can not only avoid CCU admission for all patients with chest pain, we have identified a large subset who are so low risk that they can be safely discharged straight from the ED with no observation whatsoever.
Now that we are identifying most MIs in the ED and safely discharging home a large swath of patients who are low risk, who is left for the observation unit? Are there any patients who still benefit from this practice? Stay tuned for Part 2 and 3 which will attempt to answer these questions.
Source: Dr. Charles Murchison, Does Observation for ACS Makes Sense? Part 1: A History of Observation for Chest Pain, County EM, June 6, 2019.