12-Lead ECG Interpretation - A Complete Guide for Intermediate Learners

The Systematic Approach - Never Skip Steps

Step 1: Rate

• Regular rhythm quick method: Divide 300 by the number of large boxes between R-R intervals.
  • 1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50
• Irregular rhythm: Count QRS complexes in a 6-second strip and multiply by 10.
• Normal: 60-100 bpm. Under 60 = bradycardia. Over 100 = tachycardia.

Step 2: Rhythm

• Is every QRS preceded by a P wave? Is every P wave followed by a QRS?
• Are P waves identical in morphology? Changing P wave shape = wandering pacemaker or ectopic atrial activity.
• Best leads for P waves: V1 and II (V1 is most sensitive, II shows morphology best).
  • As noted in Roberts and Hedges' Clinical Procedures in Emergency Medicine: "Lead V1 is generally considered the most appropriate lead for detecting the P wave, followed by lead II. In a study of 62 measurements, lead V1 demonstrated the tallest P wave 53% of the time."

Step 3: Axis

Quick axis determination using leads I and aVF:

Clinical significance:

• LAD - Left anterior fascicular block, LVH, inferior MI, WPW
• RAD - Right ventricular hypertrophy, left posterior fascicular block, lateral MI, pulmonary embolism, WPW
• Extreme axis - Ventricular tachycardia, severe RVH, or technical lead reversal

Step 4: Intervals

PR Interval (normal: 120-200 ms = 3-5 small boxes)

• Short PR (<120 ms): WPW syndrome or LGL syndrome (pre-excitation)
• Long PR (>200 ms): 1st degree AV block
• Progressive lengthening then dropped beat: Mobitz I (Wenckebach)
• Fixed PR with dropped beats: Mobitz II
• No relationship between P and QRS: 3rd degree (complete) heart block

QRS Duration (normal: <120 ms = <3 small boxes)

• Narrow QRS = supraventricular origin
• Wide QRS (>120 ms) = bundle branch block, ventricular rhythm, hyperkalemia, or sodium channel toxicity

QT Interval (corrected QTc, normal: <440 ms men, <460 ms women)

• Long QTc: risk for Torsades de Pointes (congenital, drug-induced, hypokalemia, hypomagnesemia)
• Short QTc (<350 ms): hypercalcemia, digoxin effect, rare channelopathy
• Quick rule: QT should be less than half the R-R interval at normal rates

Step 5: Bundle Branch Blocks (the key width patterns)

Right Bundle Branch Block (RBBB)

• Wide QRS (>120 ms)
• rSR' ("rabbit ears") in V1 - the most classic pattern
• Wide, slurred S wave in I and V6
• Memory trick: "WiLLiaM MaRRoW" - in LBBB, W in V1 and M in V6; in RBBB, M in V1 and W in V6

Left Bundle Branch Block (LBBB)

• Wide QRS (>120 ms)
• Broad monophasic R in I, aVL, V5, V6 (no Q, no S)
• Deep S or QS in V1
• Clinical pearl: New LBBB in a patient with chest pain is treated like a STEMI until proven otherwise. According to Roberts and Hedges': "Because of the increased risk, consider pacing for the following conduction blocks: new-onset LBBB, RBBB with left axis deviation or other bifascicular block."

Step 6: ST and T Wave Changes - The Most Important Part

ST Elevation (STEMI criteria)

• ≥1 mm in 2 or more contiguous limb leads
• ≥2 mm in 2 or more contiguous precordial leads (V1-V4)
• ≥1.5 mm in V4-V6 for women

Localizing the infarct by leads:

• Inferior STEMI (II, III, aVF elevation) → reciprocal depression in I, aVL
• Anterior STEMI → reciprocal depression in inferior leads

STEMI equivalents (don't miss these):

• Posterior MI: ST depression in V1-V3 + tall R wave in V1 (mirror image). Do a posterior ECG (V7-V9) to confirm.
• De Winter T-waves: ST depression with tall, symmetric T waves in V1-V6 = LAD occlusion (no elevation!)
• Wellens syndrome: Biphasic or deeply inverted T waves in V2-V3 in a pain-free patient = critical LAD stenosis, do NOT stress test

ST Depression patterns:

• Horizontal or downsloping = ischemia (high specificity)
• Upsloping = less specific, may be rate-related
• Widespread ST depression + ST elevation in aVR = left main or proximal LAD occlusion (very high-risk)

T wave changes:

• Hyperacute T waves (tall, broad, peaked): earliest sign of STEMI, often missed
• Symmetric deep T inversions: ischemia, PE (right heart strain pattern), Takotsubo
• Peaked narrow T waves in V1-V4: hyperkalemia (early sign)
• Flat T waves: hypokalemia, digitalis effect

Step 7: Hypertrophy Patterns

Left Ventricular Hypertrophy (LVH)

• Sokolow-Lyon: S in V1 + R in V5 or V6 ≥35 mm
• Cornell: R in aVL ≥11 mm (simple, fast)
• Associated with ST depression and T inversion in lateral leads = "strain pattern"

Right Ventricular Hypertrophy (RVH)

• Tall R in V1 (R > S in V1)
• Right axis deviation
• ST depression in V1-V3
• Seen in: pulmonary hypertension, chronic PE, severe COPD

Common High-Yield Patterns to Master Next

Practical Learning Progression

• Intro to EKG: Systematic Approach - excellent free YouTube lecture covering exactly this framework
• Ninja Nerd 12-Lead ECG - 2.3M views, step-by-step visual guide
• Repetition apps like ECG Guru and Cardio Cases (free ECG libraries with hundreds of tracings)

The One Rule That Separates Good ECG Readers