Part of the ICU Emergencies Hub — browse every related guide in one place.
A 12-lead EKG records cardiac electrical activity from 12 different perspectives (leads) simultaneously. Ten electrodes on the patient's body — four limb leads and six precordial leads — generate 12 distinct lead views of the heart's electrical activity. Each lead looks at the heart from a specific angle, illuminating specific regions of myocardium.
| Lead Group | Leads | Heart Territory | Typical Coronary Artery |
|---|---|---|---|
| Inferior | II, III, aVF | Inferior (bottom) wall of left ventricle; diaphragmatic surface | Right coronary artery (RCA) in most people |
| Lateral | I, aVL, V5, V6 | Lateral wall of left ventricle | Circumflex (LCx) or diagonal branch of LAD |
| Anterior (septal) | V1, V2 | Interventricular septum and anterior RV | Left anterior descending (LAD), proximal |
| Anterior | V3, V4 | Anterior wall of left ventricle | Left anterior descending (LAD) |
| aVR | aVR | Looks at the cavity of the left ventricle from the right — often used for global ischemia and left main/proximal LAD occlusion pattern | ST elevation in aVR + global ST depression = left main occlusion pattern |
Reading a 12-lead EKG systematically — rather than scanning for abnormalities — prevents missed findings. A consistent six-step approach:
Step 1 — Rate: Count the number of QRS complexes in the 10-second rhythm strip, multiply by 6. Or, for regular rhythms: divide 300 by the number of large boxes between R waves. Normal rate: 60–100 bpm.
Step 2 — Rhythm: Regular or irregular? Are P waves present before every QRS? Do all P waves look the same? A completely irregular rhythm without clear P waves = atrial fibrillation until proven otherwise.
Step 3 — P wave: Upright in leads I and II (confirms sinus rhythm); P wave duration less than 120 ms; P wave morphology changes suggest atrial pathology.
Step 4 — PR interval: Normal: 120–200 ms (3–5 small boxes). Prolonged PR = first-degree AV block (consider medication effects, ischemia). Varying PR = second-degree AV block.
Step 5 — QRS complex: Normal duration below 120 ms. Wide QRS (above 120 ms) = bundle branch block, ventricular rhythm, or hyperkalemia. Assess for Q waves (pathological Q wave: wider than 40 ms or deeper than 1/3 of QRS height = old infarction).
Step 6 — ST segment and T waves: This is the most clinically critical step. ST elevation = injury (acute MI or variant angina until proven otherwise). ST depression = ischemia or reciprocal change. T wave inversion = ischemia or strain. QT interval (normal below 440ms in men, below 460ms in women) — prolonged QT = risk for torsades de pointes.
| STEMI Location | ST Elevation in Leads | Reciprocal Depression in | Clinical Concern |
|---|---|---|---|
| Inferior STEMI | II, III, aVF | I, aVL | RCA occlusion; watch for right ventricular infarction (give IV fluids, NOT nitroglycerin in confirmed RV infarct — nitro causes severe hypotension) |
| Anterior STEMI | V1–V4 (anterior); V1–V6 (anterior + lateral = "wraparound") | II, III, aVF | LAD occlusion; large territory at risk; highest mortality STEMI; may present with complete AV block |
| Lateral STEMI | I, aVL, V5, V6 | V1–V3 | LCx or diagonal branch occlusion; may be missed if only looking at inferior leads |
| Posterior STEMI | Tall R waves and ST depression in V1–V3 (posterior infarction "mirrors" through the back of the chest) | ST elevation in V7–V9 (posterior leads, not on standard 12-lead) | Often missed on standard 12-lead; suspect in any patient with ST depression in V1–V3 without anterior ischemia explanation; requires posterior leads (V7–V9) for confirmation |
| Global ischemia / Left main occlusion | ST elevation in aVR + diffuse ST depression in multiple leads | Multiple leads | Left main coronary or proximal LAD occlusion — very high mortality; requires emergent intervention |
Right Bundle Branch Block (RBBB): QRS duration above 120 ms; classic RSR' pattern ("rabbit ears") in V1; wide S waves in I, V5, V6. RBBB is often a normal variant in young people or can indicate right heart strain (pulmonary hypertension, PE). New RBBB in an acute MI increases risk of complete AV block. RBBB does not significantly confound STEMI recognition.
Left Bundle Branch Block (LBBB): QRS duration above 120 ms; broad R waves in I, V5, V6; QS or rS pattern in V1 (no initial R wave); broad notched S wave in V1. LBBB is a significant finding because it distorts ST segments, making STEMI recognition very difficult on the standard EKG. New LBBB in the setting of chest pain was historically considered a STEMI-equivalent (activate cath lab). Current evidence is more nuanced — the Sgarbossa criteria (specific ST changes even within LBBB that suggest occlusion) are used to identify MI within LBBB.
Certain 12-lead EKG findings require immediate provider notification regardless of other clinical findings — they represent either acute cardiac emergencies or high-risk electrical instability:
ST elevation in two or more contiguous leads (possible STEMI — activate cardiac emergency protocol per institution); new wide QRS (new bundle branch block in the setting of symptoms); extremely prolonged QTc (above 500–550 ms — risk for torsades de pointes, especially if any electrolyte abnormality or QT-prolonging medications are present); peaked T waves across precordial leads (possible hyperkalemia — check potassium level); ST changes in an established STEMI who was improving (suggests reocclusion); PR prolongation progressing to dropped QRS beats (second-degree AV block type II — high risk for complete AV block).
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