Diagnostics & Procedures · Topic 1 of 16

Systematic 12-Lead ECG Interpretation

Diagnostics · 12-lead ECG

Systematic 12-Lead ECG Interpretation

Read every ECG in the same order — calibration, rate, rhythm, intervals, axis, then morphology — so a long PR, wide QRS or long QTc is never missed.

Key numbers

Normal PR interval 120–200 msPR 120–200 ms

QRS ≥ 120 ms: complete blockQRS ≥ 120 ms

QTc > 500 ms = torsades riskQTc > 500 ms = high risk

Normal axis −30° to +90°axis −30° to +90°

LVH voltage: Sokolow-Lyon ≥ 35 mmSokolow-Lyon ≥ 35 mm

01 · The numberNormal PR interval 120–200 ms

Normal PR interval is 120–200 ms (3–5 small boxes): PR > 200 ms is first-degree AV block; PR < 120 ms with a delta wave suggests pre-excitation.

02 · Defined byQRS ≥ 120 ms: complete block

Normal QRS duration is 70–100 ms; 100–110 ms is an incomplete bundle branch block or nonspecific conduction delay, and 120 ms or more is a complete bundle branch block or intraventricular conduction delay.

03 · The numberQTc > 500 ms = torsades risk

Correct the QT for rate: normal QTc is about 350–450 ms in men and 360–460 ms in women; a QTc above 500 ms in either sex carries a high torsades risk.

Causes & risk factors

Why it happens

ST elevation has many non-ischemic causesST elevation is not always infarction: early repolarization, LVH, LBBB, LV aneurysm, pericarditis, hyperkalemia, hypothermia and pulmonary embolism also raise the ST segment — read it with the clinical picture (see ECG Ischemia sheet).

Electrolyte clues: T waves and U wavesTall peaked T waves suggest hyperkalemia; flat T waves with prominent U waves suggest hypokalemia or hypomagnesemia; hypocalcemia lengthens and hypercalcemia shortens the QT interval.

Presentation

What you'll see

Atrial enlargement: P mitrale and P pulmonaleLeft atrial abnormality: notched P ≥ 120 ms in II, or terminal negative P in V1 ≥ 1 mm deep and 40 ms wide. Right atrial abnormality: peaked P in II, III, aVF (P pulmonale).

Pathologic Q waves: past or current infarctionNormal Q waves last under 0.05 s; any Q wave in V1–V3, or a wider Q elsewhere, suggests past or current infarction.

Diagnosis & monitoring

Work-up

Normal axis −30° to +90° must knowNormal QRS axis is −30° to +90°; left axis deviation is −30° to −90° (left anterior fascicular block, inferior MI), right axis deviation +90° to +180° (RV hypertrophy, acute PE, pulmonary hypertension).

LBBB: wide QRS, notched R in I, V5–V6 must knowLBBB = QRS ≥ 120 ms with a broad notched or slurred R and no Q waves in I, aVL, V5 and V6, plus discordant ST–T changes; it masks the usual ECG signs of MI.

Check calibration: 25 mm/s, 10 mm/mVConfirm standardization first: paper speed 25 mm/s (small box 0.04 s, large box 0.20 s) and a 10-mm calibration pulse for 1 mV; read half-standard (5 mm/mV) or 50 mm/s tracings accordingly.

Rate: 300 ÷ large boxesFor a regular rhythm, heart rate = 300 ÷ the number of large boxes between consecutive R waves; a complex every 4 large boxes is 75 bpm.

Sinus rhythm: upright P in I and IISinus rhythm = rate 60–100 bpm, a P wave before every QRS, and P waves upright in leads I and II and inverted in aVR; otherwise name the arrhythmia.

RBBB: rsR′ in V1, wide S in V6RBBB = QRS ≥ 120 ms with an rsr′, rsR′ or rSR′ pattern in V1–V2 and a wide S wave in I and V6; secondary ST depression and T inversion in V1 are expected.

Fascicular blocks: left axis −45°, right axis +120°Left anterior fascicular block: axis more negative than −45° with qR in aVL. Left posterior fascicular block: axis beyond +120° with rS in aVL, qR in II and aVF. RBBB plus either is bifascicular.

LVH voltage: Sokolow-Lyon ≥ 35 mmSokolow-Lyon LVH: S V1 + R V5/V6 ≥ 35 mm or R aVL ≥ 11 mm. Cornell: R aVL + S V3 > 28 mm (men), > 20 mm (women). Echo confirms.

Management

Orders

Quick axis check with leads I and IIQuick axis: lead I and II both positive = normal; I positive, II negative = left axis deviation; I negative, aVF positive = right axis deviation; I and aVF both negative = extreme axis.

RBBB or LBBB?

Commonly confused

QRS duration	≥ 120 ms
Key leads	rsR′ in V1–V2; wide S in I and V6
Link to heart disease	Often none; isolated RBBB needs no therapy
MI diagnosis on ECG	Usual criteria still usable

More detail

Reference

Show 6 reference facts

A quick bedside screen for a long QT: the QT should be less than half of the preceding R-R interval at normal heart rates.

Romhilt-Estes point score for LVH: 5 points = definite LVH, 4 points = probable; points come from voltage, typical ST–T strain, left atrial abnormality, left axis deviation and QRS prolongation.

RBBB + fascicular block + first-degree AV block is not true trifascicular block (first-degree block is usually nodal); true trifascicular block — alternating bundle branch blocks — needs immediate, then permanent pacing.

A nonspecific intraventricular conduction delay (QRS > 120 ms without a typical LBBB or RBBB pattern) carries a risk of cardiac events similar to LBBB, so look for structural heart disease.

The 2018 ACC/AHA/HRS guideline defines sinus bradycardia as a rate below 50 bpm; many texts and the standard sinus-rhythm definition still use 60–100 bpm as the normal range.

Right-sided leads (V4R most sensitive) detect right ventricular infarction; posterior leads V7–V9 in the 5th intercostal space detect posterior infarction.

Pearl

Same order, every tracing: calibration, rate, rhythm, intervals, axis, morphology — then the interpretation.

Sources · Merck Manual Professional — Electrocardiography (May 2026) · Merck Manual Professional — Bundle Branch Block and Fascicular Block (cites the AHA/ACCF/HRS 2009 intraventricular conduction recommendations and 2018 ACC/AHA/HRS bradycardia guideline) · Essential Electrocardiography (University of Miami Gordon Center) — Heart rate · Essential Electrocardiography (University of Miami Gordon Center) — Planes and axis · Merck Manual Professional — Long QT Interval Syndromes · Essential Electrocardiography (University of Miami Gordon Center) — Standardization · Essential Electrocardiography (University of Miami Gordon Center) — Rhythm · Essential Electrocardiography (University of Miami Gordon Center) — Morphology (P-wave criteria) · Merck Manual Professional — Hyperkalemia · Essential Electrocardiography (University of Miami Gordon Center) — Intervals · Kusumoto FM et al. 2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay (open copy) · Drew BJ et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF scientific statement. Circulation 2010;121:1047–1060 — PMC · Merck Manual Professional — Wolff-Parkinson-White Syndrome · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).