Acute coronary Syndrome

aCute Coronary syndrome

ACS is a term that describes a spectrum of conditions compatible with acute myocardial ischaemia and/or infarction due to an acute reduction in coronary artery blood flow. These include:

• NSTEACS (Non-ST Elevation ACS). This incorporates unstable angina and Non-ST

Elevation Myocardial Infarction (NSTEMI) • STEACS (ST-Elevation ACS). This term is used interchangeably with ST Elevation

Myocardial Infarction (STEMI) The SNAPSHOT ACS study in 2012 looked at 4,398 patients hospitalised with suspected or confirmed ACS across 286 sites in Australia and New Zealand. The graph below summarises the diagnosed cause of the ACS symptoms.1

Figure 3.2: Cause of ACS Symptoms (SNAPSHOT ACS Study 2012) Of interest is the finding that while 33% of the patients were diagnosed with acute myocardial infarction (AMI), only a third of these had ST segment changes consistent with STEMI. In other words, two-thirds of the patients with AMI did not have an ECG that met STEMI criteria. While hospital investigations include bio-marker assays to identify NSTEMI, pre-hospital clinicians are typically reliant on patient presentation alone. As such, paramedics need to maintain a high level of suspicion for AMI in the face of on-going ACS symptoms and a non-specific 12-Lead ECG.

NSTEACS

Unstable Angina

Unstable angina is considered to be an ACS in which there is no discernible release of enzymes or biomarkers indicative of myocardial infarction. Where it differs from stable angina is that the pain is often more intense, is more easily provoked, and ST segment depression or elevation may be present on ECG.

The symptoms of unstable angina can be similar to myocardial infarction making it difficult to distinguish between the two without serial ECGs and biomarker assays. Click on the icons below to view articles containing more information.

NSTEMI

NSTEMI is distinguished from unstable angina through the elevated levels of cardiac enzymes and biomarkers indicative of myocyte necrosis. The diagnosis is based on the presence of abnormal biomarker assays at 8-12 hours after the onset of chest pain1. It is thought that the lack of ST segment seen in NSTEMI is because the infarct doesn’t involve the full thickness of the myocardium (transmural). While STEMI typically results from a persistent occlusion of a large epicardial coronary artery, NSTEMI may result from a number of states that reduce coronary flow without complete occlusion. These include distal embolisation of thrombotic material accompanied by coronary spasm, atherosclerotic changes, and plaque rupture2 .

ECG Changes

The main ECG abnormalities seen in NSTEACS are:

• ST depression • T Wave flattening or inversion Other patterns suggestive of ischaemia include:

• Hyper acute T Waves • U Wave inversion

STEACS

STEACS, commonly known as STEMI or AMI, is a clinical syndrome characterised by symptoms of myocardial ischaemia in association with persistent ECG ST segment elevation and subsequent release of biomarkers of myocardial necrosis.3

STEMI with Normal Coronary Arteries

While it is accepted that the irreversible myocardial necrosis seen in STEMI is caused by a plaque rupture with thrombus formation, STEMI can also occur in patients with normal coronary arteries. On a coronary angiogram, it has been suggested that seven percent of patients with acute STEMI myocardial infarction don’t have a significant coronary artery lesion. This phenomenon is usually seen in younger patients and in women.

1 Burns, E. (2014). Myocardial Ischaemia. Life in the Fast Lane. Retrieved from http://lifeinthefastlane.com/ecg-library/myocardialischaemia/ 2 De Winter, R.J. & Tijissen, J.G. (2012). Non-ST-Segment Elevation Myocardial Infarction. Revascularization for Everyone?. J Am Coll Cardiol Intv. 2012;5(9):903-905. doi:10.1016/j.jcin.2012.07.001 3 O’Gara, P.T., Kushner, F.G., Ascheim, D.D., Casey, D.E., Chung, M. K., de Lemos, J.A., Zhao, D.X. (2013). 2013 ACCF/ AHA Guideline for the Management of ST-Elevation Myocardial Infarction. A Report of the American College of Cardiology Foundation/ American Heart Association Task Force on Practice Guidelines. Circulation. 2013; 127:e362-e425.

Potential mechanisms identified in these patients include:

• Coronary artery spasm • Coagulopathy • Toxins (including cocaine) • Collagen vascular disease • Emboli (secondary to cholesterol, air and sepsis) • Coronary trauma • Arteritis • Myocarditis • Factors that increase oxygen demand or decrease oxygen delivery • Microvascular disease • Congenital defects. Takotsubo cardiomyopathy (stress induced cardiomyopathy) is typically characterised by transient systolic dysfunction of the left ventricle that mimics AMI but in the absence of significant coronary artery disease. In a study of 323 women aged 45 years and older with AMI (including elevated troponin), 5.9% met the criteria for stress induced cardiomyopathy.1

STEMI Diagnostic Criteria

The first critical step in STEMI management is early recognition. Patients presenting with non-traumatic chest pain suspicious of ACS require a 12-Lead ECG as part of the initial assessment. ECG changes are often seen early in the onset of myocardial infarction, and can confirm the presence of STEMI. As biomarkers may be within normal levels initially, an early biomarker assay isn’t always helpful or definitive.

A diagnosis of STEMI can be made if at least two of the following three criteria are met:

1. Patient’s clinical presentation consistent with ACS 2. 12-Lead ECG characteristics meet STEMI criteria

3. Biomarker assay indicative of myocardial infarction Given that cardiac biomarker sensitivity is low in the first six hours of symptom onset, timely STEMI recognition and management relies heavily on patient presentation and ECG features.2 Reperfusion therapy often occurs without the results of cardiac biomarkers being available.

1 Reeder, G.S., Kennedy, H.L, & Rosenson, R.S. (2015). Overview of the Acute Management of ST Elevation Myocardial Infarction. UpToDate. Retrieved from http://www.uptodate.com/contents/overview-of-the-acute-management-of-st-elevation-myocardialinfarction 2 Schreiber, D. (2014). Cardiac Markers. Medscape. Retrieved from http://emedicine.medscape.com/article/811905-overview