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  • Clinician

Acute coronary syndrome


Acute coronary syndrome (ACS) refers to acute myocardial ischemia and/or infarction due to partial or complete occlusion of a coronary artery. There are three clinical entities grouped under ACS: unstable angina pectoris, non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI). These conditions are often difficult to distinguish from one another based on clinical symptoms alone and require ECG and cardiac biomarker measurement to diagnose. Typical cardiac chest pain is substernal in nature, often described as a feeling of pressure, and is relieved with rest and/or nitrate use. The pain may radiate to the left jaw, neck, epigastrium, upper back, and/or left arm. Additionally, autonomic symptoms such as diaphoresis, nausea, and vomiting are common. ECG and laboratory tests are important diagnostic tools in the initial evaluation. In contrast to angina pectoris, NSTEMI and STEMI are characterized by the destruction of cardiac muscle tissue, which results in elevated cardiac enzymes in the blood (i.e., the elevation of troponin after 3–4 hours). Unlike unstable angina and NSTEMI, STEMI results in specific ECG changes (e.g., ST-segment elevation), which can help to determine the location and stage of the infarct. The need for revascularization with either fibrinolysis or cardiac catheterization should be evaluated immediately, as revascularization significantly affects the prognosis of patients with myocardial infarction. Cardiac catheterization should be performed as soon as possible in STEMI and electively within 2–72 hours in high-risk NSTEMI and/or unstable angina. Medical management of ACS includes anticoagulation, analgesics, and antiplatelet agents. Complications of ACS include congestive heart failure, papillary muscle rupture, arrhythmias, and sudden cardiac death. Subsequent management and secondary prevention of ACS depends on the presence of comorbidities, but most patients should be started on indefinite aspirin and statin therapy.




Unstable angina [5] Non-ST-segment elevation myocardial infarction (NSTEMI) [5] ST-segment elevation myocardial infarction (STEMI) [5]
Clinical presentation
  • Partial occlusion of coronary vessel → decreased blood supply → ischemic symptoms (also during rest)
Cardiac biomarkers
ECG findings
  • Normal

Subtypes of ACS cannot be differentiated based on clinical presentation alone!



Epidemiological data refers to the US, unless otherwise specified.



ACS is most commonly due to unstable plaque formation and subsequent rupture.

Plaque formation and rupture

Coronary artery occlusion


Clinical features

STEMI classically manifests acutely with more severe symptoms, while unstable angina/NSTEMI has a continuous course with milder symptoms.



ECG should be performed immediately once ACS is suspected, followed by measurement of cardiac biomarkers. Further diagnostic workup (e.g., echocardiography) depends on the results of initial evaluation and further risk stratification (e.g., TIMI score).


  • 12-lead ECG is the best initial test if ACS is suspected.
  • Dynamic changes require serial ECG evaluation.
  • Compare to prior ECGs (if available).

ECG changes in STEMI [17]

The sequence of ECG changes over several hours to days: hyperacute T waveST elevation → pathological Q waveT-wave inversionST normalizationT-wave normalization

An acute left bundle branch block accompanied by symptoms of acute coronary syndrome is also considered an ST-elevation myocardial infarction (STEMI) because ST elevations cannot be adequately assessed in the setting of an LBBB.

ECG changes in NSTEMI/unstable angina

  • No ST elevations present
  • Nonspecific changes may be present.

Localization of the myocardial infarct on ECG [17][12][18][19]

ECG leads affected Infarct location Vessel involved [18][19]
  • Extensive anterior (Leads aVL and I can also be affected.)
  • (Antero)septal
  • LAD
  • (Antero)apical
I, aVL
  • Inferior

Infarction of the anterior wall is caused by obstruction of the LAD or its branches. Depending on the extent of anterior wall infarction, it results in ECG changes in the anterior wall leads (V1–6) and/or I and aVL. Infarction of the inferior wall is caused by obstruction of the LCX or RCA or their branches, and ECG changes are seen in leads II, III, and aVF.

To remember the ECG leads with maximal ST elevation in anterior MI, think “SAL”: “Septal (V1–2), Apical (V3–4), Lateral (V5–6).

In severe transmural posterior wall infarction, there may not be any ST elevation on a standard 12-lead ECG.

Laboratory findings

Cardiac biomarkers [20]

Biomarker/enzyme Rise* Maximum* Normalization* Characteristics
Troponin T/I

6-8 h

12–24 h

7–10 days


∼ 1 h

4–12 h

24 h

  • Nonspecific marker that is no longer commonly used
CK-MB ∼ 4–9 h 12–24 h 2–3 days
  • CK-MB is more specific to cardiac tissue than total CK.
  • Can be helpful for evaluating reinfarction because of its short half-life but is no longer commonly used
  • The degree of elevation often correlates with the size of the infarct.
* The values rise, reach a certain maximum, and normalize in the span of hours or days following the onset of myocardial infarction or its symptoms. Values and time references may vary based on the precise laboratory methods employed.

Serum troponin T is the most important cardiac-specific marker and may be measured 3–4 hours after the onset of myocardial infarction. CK-MB values correlate with the size of the infarct, reach a maximum after approximately 12–24 hours, and normalize after only 2–3 days, making CK-MB a good marker for evaluating reinfarction.

Additional findings [22][23]

Coronary angiography

  • Best test for definitive diagnosis of acute coronary occlusion
  • Can be used for concurrent intervention (e.g., PCI; with stent placement)
  • Can identify site and degree of vessel occlusion
  • Indications include

The most commonly occluded coronary arteries (descending order): left anterior descending artery, right coronary artery, circumflex artery.

Additional studies


Risk stratification

TIMI score for unstable angina/NSTEMI [30]

Characteristics Points
Age ≥ 65 years 1

Three or more CAD risk factors (e.g., premature family history, DM, smoking, HTN, hyperlipidemia, PAD, abdominal aortic


Known CAD (prior stenosis > 50%) 1
Two or more episodes of severe angina in the last < 24 hours 1
ASA use in the last 7 days 1
ST deviation (≥ 0.5 mm) 1
Elevated cardiac biomarkers 1
Total points 0–7

References: [30]


Histopathological findings of MI [31]

Time interval post-infarction Histopathological findings
Microscopic Macroscopic

0–24 hrs

  • 0–12 hours: no gross changes
  • 12–24 hours: dark mottling

1–3 days

  • Hyperemia
  • Yellow pallor

3–14 days

  • Hyperemic border
  • Center: yellow-brown, soft

2 weeks to several months

Obstruction of a coronary artery branch due to > 90% stenosis or embolization results in coagulation necrosis of the post-stenotic zone.

Cellular changes



Any patient with ST elevations on ECG requires immediate evaluation for urgent revascularization. The administration of other therapies should not delay care.

All patients [11][5]

Primary interventions of MI treatment include “MONA”: Morphine, Oxygen, Nitroglycerin, and Aspirin. But remember: Morphine, oxygen, and nitroglycerine are not necessarily indicated for every patient (see indications above).

STEMI [11]

Immediate revascularization

Revascularization is the most important step in the management of acute STEMI and initiation of further therapies (e.g., DAPT, anticoagulation) should not delay this step in management.

Medical therapy

"Time is muscle": Revascularization should occur as soon as possible in patients with STEMI!

Unstable angina/NSTEMI [5]

Fibrinolytic treatment is not recommended in patients with unstable angina or NSTEMI.

Subsequent measures



0–24 hours post-infarction

1–3 days post-infarction

3–14 days post-infarction

2 weeks to months post-infarction