Acute Coronary Syndrome (ACS) Symptoms and Diagnosis

Clinical research at GHICL, Lille, France. Cardiology department. Cardiac stress test performed under scan and ECG (electrocardiogram) control.
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If you or a loved one has coronary artery disease (CAD), you may have heard the term “acute coronary syndrome,” also referred to as ACS. ACS is a relatively new term used by cardiologists, and it can be a little confusing. However, because it represents the new way of thinking about CAD, it can be useful to take a few minutes to understand it. 

Acute coronary syndrome is pretty much what it sounds like. It is an urgent condition affecting the coronary arteries; an emergency. It indicates that a person’s CAD has suddenly become unstable, and that permanent cardiac damage is either happening right now or is likely to happen at any time. 


ASC occurs when a blood clot suddenly forms within a coronary artery, usually due to the acute rupture of an atherosclerotic plaque. Plaque rupture can occur at any time, often completely without warning. The blood clot may produce partial or complete blockage of the artery, either way placing the heart muscle supplied by that artery in immediate jeopardy.

Any plaque in any coronary artery is subject to rupture, even small plaques that are usually ignored by cardiologists during cardiac catheterizations. This is why you will often hear of people who have a myocardial infarction (MI, or heart attack), shortly after being told their CAD is "insignificant."

The Three Types of ACS

Cardiologists divide ACS into three distinct clinical patterns. Two of them represent different forms of MI, and one represents a particularly severe form of angina, called "unstable angina." All three are caused by acute blood clots in the coronary arteries.

If the blood clot is large enough and persists for more than just a few minutes, some of the heart muscle cells begin to die. The death of heart muscle is what defines an MI. Two types of MI that can be produced by ACS. 

  1. ST-Elevation myocardial infarction (STEMI), so named because the "ST segment" on the ECG appears "elevated,” occurs when a coronary artery is completely blocked so that a large proportion of the heart muscle being supplied by that artery begins to die. A STEMI is the most severe form of ACS.
  2. Non-ST-Elevation myocardial infarction (NSTEMI), in which the "ST segment" is not elevated, occurs when the blockage in the coronary artery is “only” partial. Enough blockage is occurring to damage some of the heart muscle cells being supplied by the diseased artery, but the damage tends to be less extensive than with a STEMI. One problem with an NSTEMI, however, is that with inadequate treatment the blockage is likely to become complete, and the NSTEMI will become a STEMI.
  3. Sometimes ACS produces a blood clot that is not yet large enough, or does not persist long enough, to produce any permanent heart muscle damage. (The body's protective mechanisms try to dissolve blood clots that form within blood vessels.) When an ACS is producing symptoms without yet causing the heart muscle to die, it is termed unstable angina. People with unstable angina have a high risk of progressing to an NSTEMI or a STEMI.

Both NSTEMI and unstable angina can be considered as “incomplete” heart attacks. These two forms of ACS need similar, aggressive medical management in order to reduce the likelihood that they will progress to a STEMI—which cardiologists often call a “completed” MI.


The most common symptom of ACS is chest pain, pressure, tightness, or discomfort. The quality of the chest discomfort with ACS is generally similar to that experienced with stable angina but is often much more intense, frequent, and persistent. Along with chest discomfort, people with ACS often have other disturbing symptoms such as sweating, dizziness, nausea, extreme anxiety, and what is often described as a "feeling of impending doom." The chest pain may be untouched by nitroglycerin (which usually relieves stable angina). On the other hand, some people who have ACS will have only mild symptoms and may even fail to notice any symptoms at all—at least initially.

Unfortunately, whether or not ACS causes significant symptoms, if left untreated ACS often produces permanent heart damage that, sooner or later, will produce symptoms.

Making the Right Diagnosis

To summarize, once a blood clot forms in a coronary artery, if extensive heart muscle damage has already occurred a STEMI is diagnosed. If a "little" heart muscle damage occurs, an NSTEMI is diagnosed. If no measurable heart muscle damage occurs, unstable angina is diagnosed. 

If you are having ACS, usually your symptoms, physical examination, medical history and cardiac risk factors will immediately steer the doctor to strongly suspect the diagnosis. From that point, they will quickly examine your ECG and measure your cardiac enzymes. Cardiac enzymes are released into the bloodstream by dying heart muscle cells, so an elevation in the cardiac enzymes means that heart cell damage is occurring.

Here's the bottom line on diagnosing the type of ACS you are dealing with: The appearance of the ECG (i.e., the presence or absence of "elevation" in the ST segments) will distinguish between STEMI and NSTEMI. And the presence or absence of elevated cardiac enzymes will distinguish between NSTEMI and unstable angina.

The three types of ACS represent the spectrum of the clinical conditions that can occur when a plaque ruptures within a coronary artery. In fact, there is actually no clear line that inherently divides STEMI, NSTEMI, and unstable angina. Where cardiologists draw the line between a STEMI and an NSTEMI, or between an NSTEMI and unstable angina, is a relatively arbitrary decision. Indeed, the definitions of these three types of ACS have changed substantially over the years, as our knowledge—specifically our ability to interpret ECGs and detect heart cell damage with enzyme tests—have improved.


Fundamentally, the treatment of ACS is aimed at relieving the active blockage in the affected coronary artery as quickly as possible, in order to prevent or limit damage to the heart muscle. The specific therapeutic approach that is generally used depends on which of the three forms of ACS you are dealing with.

Treating unstable angina often begins with aggressive medical therapy (using nitrates to relieve chest discomfort, beta blockers to reduce cardiac ischemia, and anti-platelet therapy to stop further propagation of the blood clot). Once the patient is stabilized with medication, the need for invasive therapy (usually, a stent) can be assessed over the next few days. Treatment of an NSTEMI is very similar to treating unstable angina.

Treating a STEMI requires immediate, aggressive therapy aimed at opening the completely blocked artery as rapidly as possible. Today the preferred method for opening the artery in a person with a STEMI is immediate angioplasty and stenting, but if this approach is not feasible, clot-busting drugs can be given to attempt to dissolve the offending clot.

Whichever form of ACS is present, the key to a successful outcome is to receive effective therapy as rapidly as possible. Even a short delay can mean the difference between a complete recovery and lifelong disability, or worse.

This is why anyone who is experiencing symptoms that are consistent with ACS needs to seek medical care immediately.

A Word From Verywell

The important point about ACS is that, in every case, no matter how it is categorized, ACS is a medical emergency and requires immediate medical care. Treatment is meant to accomplish two things: 1) to limit the heart muscle damage being done acutely by the blood clot within the coronary artery, and 2) to limit the possibility that the plaque—which has now shown itself to be unstable and prone to rupture—will rupture again.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Body R, Carley S, Wibberley C, et al. The value of symptoms and signs in the emergent diagnosis of acute coronary syndromes. Resuscitation 2010; 81:281. DOI:10.1016/j.resuscitation.2009.11.014

  2. Amsterdam EA, Wenger NK, Brindis RG, et al. 2014 AHA/ACC guideline for the management of patients with non-ST-elevation acute coronary syndromes: executive summary: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation 2014; 130:2354. DOI:10.1161/CIR.0000000000000133

  3. Roffi M, Patrono C, Collet JP, et al. 2015 ESC Guidelines for the management of acute coronary syndromes in patients presenting without persistent ST-segment elevation: Task Force for the Management of Acute Coronary Syndromes in Patients Presenting without Persistent ST-Segment Elevation of the European Society of Cardiology (ESC). Eur Heart J 2016; 37:267. DOI:10.1093/eurheartj/ehv320

Additional Reading
  • Pollack CV Jr, Diercks DB, Roe MT; Peterson ED. 2004 American College of Cardiology/American Heart Association Guidelines for the Management of Patients With St-Elevation Myocardial Infarction: Implications for Emergency Department Practice. Ann Emerg Med 2005 Apr;45(4):363-76.

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.