Two terms you’ve probably heard mentioned are STEMI and NSTEMI, and you may be wondering what the difference between these two cardiac events is. 

STEMI refers to ST-elevation myocardial infarction and NSTEMI refers to non ST-elevation myocardial infarction. Both of these involve evaluating the ST segment on the ECG…so let’s start with that. 

The ST segment is a flat, isoelectric section that begins at the end of the S wave (also known as the J point) and extends to the beginning of the T wave. The ST segment represents the interval between ventricular polarization and repolarization. While there are many causes for ST segment elevation, the most critically important is myocardial infarction or ischemia. Other potential causes for ST elevation include pericarditis, coronary vasospasm and left ventricular hypertrophy (among others!). 

ST-elevation myocardial infarction (STEMI)

A STEMI occurs when there is a complete and persistent occlusion of a coronary artery. It is seen on the ECG as an elevated ST segment. For an ST segment to be considered pathologically elevatedl, it must be elevated in at least two contiguous leads and be greater than 2mm in V1, V2 or V3, or greater than 1mm in other leads. According to, a new left bundle branch block combined with signs of acute coronary syndrome is also suggestive of STEMI.

What do we mean by contiguous leads? These are leads that show electrical activity at adjoining areas of cardiac tissue. Since these are not necessarily adjoining areas on the ECG, one of the easiest ways to see which leads are contiguous is by using a badge card reference or a graphic such as this. 

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Non ST-elevation myocardial infarction (NSTEMI)

An NSTEMI usually results from significant coronary narrowing, microemboli, or a short-term occlusion. As the name suggests, there is no ST elevation on the ECG, which can make an NSTEMI more difficult to recognize. Instead, it is diagnosed based on clinical symptoms and the presence of elevated cardiac enzymes. Note that an NSTEMI can cause ECG changes, but since they are not always present, their absence does not rule out an infarction. Some changes that  may be seen on the ECG include ST depression, transient ST-elevation, and new-onset T-wave inversion.

What are the signs and symptoms of a myocardial infarction?

The classic signs of a myocardial infarction are chest pain often described as crushing or squeezing, shortness of breath, nausea or indigestion, vomiting, cool and clammy skin, dizziness, and fatigue. 

Note that not everyone who has a myocardial infarction experiences chest pain. For example, patients with diabetes will often not experience chest pain due to the damage that the disease causes the nerves (a condition called neuropathy). Also, studies show that women can experience MI differently than men and may be more likely to report symptoms of shortness of breath, back pain, jaw pain and nausea/vomiting. And what about heart transplant patients? Studies show that  many patients with heart transplant have SOB and weakness as their main symptoms. When an MI occurs without chest pain, it’s often referred to as a “silent MI.” 

Tests for myocardial infarction

In addition to evaluating the 12-lead ECG, other tests the MD may order include an echocardiogram and cardiac enzymes. The key cardiac enzyme is troponin I, because it is the most sensitive and specific marker for myocardial injury. Troponins typically increase three to four hours after the infarction occurs and are re-checked every six or so hours while the patient is being evaluated for a myocardial infarction.

How are STEMI and NSTEMI treated?

When cardiac muscle is compromised, time is critical! The patient must be assessed quickly and interventions started promptly. Many emergency departments have a goal of completing a 12 lead ECG within 10 minutes of arrival.

Initial interventions for both STEMI and NSTEMI:

  • Supplemental oxygen as needed to maintain SpO2 > 90%.
  • Establish IV access.
  • Treat arrhythmias as needed to maintain hemodynamics.
  • Administer 325 mg nonenteric coated aspirin to be chewed and swallowed. If unable to give orally, administer as a suppository.
  • Administer nitroglycerin sublingually for patients with persistent chest pain. Nitroglycerin is contraindicated in hypotension, concurrent use of phosphodiesterase inhibitors and right ventricular infarction. Giving nitroglycerin in these situations could cause significant and life-threatening hypotension.
  • Administer a beta blocker such as 25 mg metoprolol to patients without heart failure or risk for heart failure, hemodynamic compromise, bradycardia, or reactive airway disease.
  • Provide analgesia with morphine if chest pain persists after nitroglycerin administration.
  • If the patient is not taking a statin, initiate statin therapy as soon as possible. The recommendation is 80 mg atorvastatin.
  • Draw labs for cardiac biomarkers.

STEMI treatment

The gold standard treatment for STEMI is reperfusion therapy, with the preferred method being catheterization and PCI (percutaneous intervention). The goal for many hospitals is a “door to balloon time” of 90 minutes, meaning the patient goes to the cath lab and has an intervention to open the blocked vessel within 90 minutes of arrival to the ED. If PCI is not available, fibrinolytics are recommended within 120 minutes of of arrival, provided there are no contraindications. Other therapies include:

  • Antiplatelet therapy – The dose and medication utilized will depend on several factors including the patient’s age, whether they have received fibrinolytics, and whether they have undergone PCI. Medications recommended for antiplatelet therapy are clopidogrel, ticagrelor or prasugrel. 
  • Anticoagulation therapy – The preferred medication for anticoagulation is heparin, though bivalirudin may also be utilized. Patients who have been treated with a fibrinolytic may receive enoxaparin or fondaparinux instead.

NSTEMI treatment

  • Antiplatelet therapy – Just as with STEMI treatment, the dose and medication utilized depends on a few different variables such as age, weight, past medical history, and whether or not the patient had an invasive procedure.
  • Anticoagulation therapy – Heparin or bivalirudin are utilized for patients undergoing catheterization within four hours or angiography within 4 to 48 hours. Patients not undergoing an invasive procedure may be anticoagulated with enoxaparin or fondaparinux.

What’s the prognosis for STEMI and NSTEMI?

Overall, patients who suffer from STEMI have poorer short-term prognosis at 28-days than those with NSTEMI. However, studies show that at the two year mark and onward, patients have an annual mortality rate of about 2% regardless of which type of infarction they initially had.

Some complications for myocardial infarction include heart failure, cardiogenic shock, pericarditis, mitral regurgitation, ventricular septal rupture, cardiac tamponade, arrhythmias, and heart blocks.

Did you find this article helpful? Explore more cardiovascular topics here.

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