STEMI & NSTEMI: A Nurse's Guide to Reading & Managing Heart Attacks

A STEMI — ST-Segment Elevation Myocardial Infarction — is the most dangerous type of heart attack, and recognizing it fast on the 12-lead can save your patient's heart muscle and their life.

An NSTEMI (Non-STEMI) is still a heart attack, but without ST elevation on the ECG. Ruling out a STEMI is the whole reason 12-leads get ordered, and the sooner one is caught, the better the outcome. Providers should lay eyes on every ECG quickly, but that doesn't always happen on time. The faster a STEMI is identified, the more heart tissue survives — and that recognition often starts with the nurse.

Coronary Artery Anatomy

The coronary arteries lie on the surface of the heart (the epicardium) and deliver the blood and oxygen that keep the myocardium perfused and beating. The three main vessels are the left anterior descending (LAD), the circumflex (Cx), and the right coronary artery (RCA). Knowing what each one feeds tells you which wall is in trouble when you see changes on the ECG.

Acute Coronary Syndrome

Acute coronary syndrome (ACS) is the umbrella term for any condition that drops blood flow to the heart — ischemia. Prolonged ischemia leads to infarction, the death of heart tissue, which releases troponin into the bloodstream.

Most cardiac ischemia is caused by atherosclerosis — plaque buildup in the coronary arteries from factors like diet, obesity, sedentary lifestyle, hyperlipidemia, smoking, and genetics. When that plaque ruptures, it spills its contents, triggers local inflammation and a coagulation cascade, and the resulting clot can completely occlude the artery, causing infarction.

An NSTEMI often shows nonspecific changes — T-wave inversion or ST depression in contiguous leads, with or without inversion — and the ECG can even be completely normal. Troponin is elevated, confirming cell death. A STEMI, by contrast, shows ST elevation in at least 2 contiguous leads, and where that elevation appears tells you which wall is infarcting and which artery is occluded.

Want the lab side of this picture? See Cardiac Lab Interpretation (Troponin, CK, CK-MB, and BNP).

Ischemia & Infarction on the ECG

The ST-segment sits right after the QRS and before the T wave. It represents early ventricular repolarization and should rest at the isoelectric line. Use the TP segment as your baseline, or the PR segment if the TP is hard to see.

ST-Segment Depression

ST depression most often signals cardiac ischemia, and it can appear as a reciprocal change in an acute MI. It can also reflect strain, digitalis effect, hypokalemia, hypomagnesemia, or be rate-related — but those tend to be diffuse rather than localized to 2 contiguous leads.

It's defined as ≥0.5 mm (½ small box) below the isoelectric line, measured 80 ms (2 small boxes) after the J-point. Horizontal and down-sloping depression are more specific for ischemia; up-sloping is less concerning but can still indicate it.

ST-Segment Elevation

ST elevation usually means myocardial infarction when it appears in at least 2 contiguous leads. Other causes exist — coronary vasospasm, pericarditis, benign early repolarization, LBBB, LV hypertrophy, ventricular aneurysm, Brugada, paced rhythms, raised ICP, blunt chest trauma, and hypothermia — so context matters.

It's defined as ≥1 mm (1 small box) above the isoelectric line at the J-point. Leads V2–V3 have higher thresholds: >1.5 mm in women, >2 mm in men ≥40, and >2.5 mm in men <40.

Q Waves

A Q wave is the initial negative deflection of the QRS complex, reflecting septal depolarization. Small Q waves are normal in many leads. Pathologic Q waves point to underlying pathology — usually a current or prior MI.

They're defined as >40 ms wide (1 small box) and >2 mm deep (2 small boxes). Any Q wave in V1–V3 is considered pathologic. Q waves can appear hours to days after an infarct begins and may persist for years.

LBBB or Ventricular Paced Rhythm

Spotting ST changes is tricky in a left bundle branch block (LBBB) or a ventricular paced rhythm, because both normally carry some ST elevation and discordant T waves. To suspect ischemia or infarction in these patients, look for the Sgarbossa-type findings:

New to rhythm strips? Start with How to Read an EKG Rhythm Strip.

STEMI Progression

A STEMI tends to evolve through a recognizable sequence on the ECG. Knowing the stages helps you place where your patient is in the timeline.

STEMI Location

STEMIs are classified by where they sit — which leads are affected on the ECG. Contiguous leads are simply leads that look at the same anatomical region of the heart.

The precordial and lateral leads often light up together, since the infarct area isn't always precise. Here's an inferior wall STEMI as an example:

Acute Management of STEMI

STEMIs are true emergencies. The patient is at high risk for conduction disturbances, arrhythmias, and cardiac arrest, and every minute of delay means more dead heart cells and worse outcomes.

A lot happens fast. Activate a code STEMI (or your facility's equivalent) to alert the interventional cardiologist and cath team immediately. Then work the bedside basics:

Oxygen is given only to keep SpO2 >90% (routine oxygen in non-hypoxic patients isn't beneficial). If the patient has left heart failure, treat with nitro, a loop diuretic like Lasix, and consider BiPAP. Beta-blockers are started within 24 hours unless contraindicated (bradycardia, heart failure, severe reactive airway disease), usually after PCI.

NSTEMI Management

As the name says, an NSTEMI has no ST elevation on the ECG — but it's still a heart attack, marked by an elevated and rising troponin. The ECG may be normal or show nonspecific T-wave changes or ST depression in contiguous leads.

Medication management mirrors a STEMI, with two key differences: NSTEMI patients do not receive fibrinolytics and are not rushed emergently to the cath lab. Instead, medical therapy is maximized, the patient is monitored, and troponins are trended (typically every 6–8 hours). They may or may not get a cardiac cath during the admission.

STEMIs and NSTEMIs are high-stakes events every nurse will encounter. Recognizing a STEMI on the ECG is the first and most important step.