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How to Read an ECG - Complete Guide

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Part 1 - Understanding the ECG Paper

• Small box = 1 mm wide = 0.04 seconds horizontally, 0.1 mV vertically
• Large box = 5 mm wide = 0.2 seconds horizontally, 0.5 mV vertically
• Standard paper speed = 25 mm/second
• Standard calibration = 10 mm = 1 mV (the calibration box appears at the start of a strip)

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Part 2 - The ECG Leads

Lead Placement

• RA = right arm/wrist, LA = left arm/wrist, LL = left leg/ankle, RL = right leg (ground)
• Generate leads I, II, III (bipolar) and aVR, aVL, aVF (augmented unipolar)

What Each Lead "Sees"

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Part 3 - The Normal ECG Waveform

        R
        |
   P    |       T
   /\   |      /\
--/  \--|--S--/  \--U--
        Q

The Waves, Intervals and Segments

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Part 4 - The Systematic 7-Step Approach

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Step 1 - Rate

• 1 box = 300 bpm
• 2 boxes = 150 bpm
• 3 boxes = 100 bpm
• 4 boxes = 75 bpm
• 5 boxes = 60 bpm
• 6 boxes = 50 bpm

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Step 2 - Rhythm

• Is the rhythm regular (R-R intervals equal) or irregular?
• Is there a P wave before every QRS?
• Is there a QRS after every P wave?
• Are the P waves upright in lead II (normal sinus origin)?

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Step 3 - Axis

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Step 4 - P Waves

• Present? Upright in I and II? Rounded? One P per QRS?
• Wide (>120 ms) or notched = left atrial enlargement
• Tall peaked (>2.5 mm in lead II) = right atrial enlargement
• Absent + irregular rhythm = think atrial fibrillation

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Step 5 - PR Interval

• Short (<120 ms) = pre-excitation (WPW) or junctional rhythm
• Long (>200 ms) = 1st degree AV block (or worse)
• Progressively getting longer until a QRS drops = Mobitz I (Wenckebach)
• Constant PR with random dropped QRS = Mobitz II
• P and QRS completely independent = 3rd degree (complete) heart block

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Step 6 - QRS Complex

• Wide QRS (>120 ms): bundle branch block, ventricular rhythm, WPW, hyperkalemia
• RBBB pattern: RSR' (bunny ears) in V1, wide S in I and V6 - "WiLLiaM MaRRoW" (LBBB: W in V1, M in V6)
• LBBB pattern: Broad notched R in I, V5, V6; QS in V1
• Pathological Q waves (>25% of R height or >40 ms wide): old MI in that territory
• Low voltage (<5 mm in limb leads, <10 mm in precordial): pericardial effusion, emphysema, hypothyroidism

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Step 7 - ST Segment and T Waves

• ST elevation (>1 mm in 2 contiguous limb leads, >2 mm in 2 chest leads): STEMI until proven otherwise
• ST depression (≥0.5 mm in ≥2 contiguous leads): ischemia, NSTEMI, digoxin
• T wave inversion: ischemia, LVH strain, RBBB, PE, intracranial hemorrhage
• Tall peaked T waves: hyperkalemia (early sign)
• Prolonged QT: risk of torsades de pointes - drugs, hypokalemia, hypocalcemia

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Part 5 - Normal 12-Lead ECG (Reference)

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Part 6 - Common ECG Abnormalities

Arrhythmia/Rhythm Overview Chart

Killer Rhythms and Special Patterns

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1. Sinus Tachycardia

• Rate >100 bpm
• Normal P waves before every QRS
• Regular rhythm

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2. Sinus Bradycardia

• Rate <60 bpm
• Normal P waves, regular
• Prominent U waves possible

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3. Atrial Fibrillation (AF)

• Absent P waves - replaced by irregular fibrillatory baseline
• Irregularly irregular ventricular rhythm
• Normal or narrow QRS (unless aberrant conduction)

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4. Atrial Flutter

• Sawtooth flutter waves at 250-300 bpm
• Regular atrial activity
• Usually 2:1, 3:1 or 4:1 block giving regular ventricular rates of 150, 100 or 75 bpm

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5. 1st Degree AV Block

• PR interval >200 ms (>5 small boxes)
• Every P followed by a QRS (no dropped beats)

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6. 2nd Degree AV Block - Mobitz I (Wenckebach)

• PR interval progressively lengthens until a QRS is dropped
• Cycle repeats
• PR shortest after the dropped beat

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7. 2nd Degree AV Block - Mobitz II

• Constant PR interval with suddenly dropped QRS (no warning)
• More serious than Mobitz I - can progress to complete block
• Often needs pacemaker

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8. 3rd Degree (Complete) AV Block

• P waves and QRS complexes completely independent (no relationship)
• Atrial rate > ventricular rate (escape rhythm)
• Wide, slow QRS if ventricular escape; narrow if junctional escape
• Requires emergency pacing if symptomatic

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9. STEMI (ST-Elevation Myocardial Infarction)

• ST elevation >1 mm in ≥2 contiguous limb leads, or >2 mm in ≥2 chest leads
• Later: pathological Q waves develop (infarction)
• Territory localization (inferior = II/III/aVF; anterior = V1-V4; lateral = I/aVL/V5-V6)

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10. NSTEMI / Ischemia

• ST depression ≥0.5 mm in ≥2 contiguous leads
• T-wave inversion in ischemic territory
• No ST elevation

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11. Left Bundle Branch Block (LBBB)

• QRS >120 ms (broad)
• M-shaped (notched) R wave in I, V5, V6
• QS pattern in V1
• New LBBB in chest pain = treat as STEMI equivalent

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12. Right Bundle Branch Block (RBBB)

• QRS >120 ms
• RSR' (rabbit ears / M pattern) in V1
• Wide slurred S wave in I and V6
• Remember: WiLLiaM (LBBB) and MaRRoW (RBBB) mnemonic

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13. Wolff-Parkinson-White (WPW) Syndrome

• Short PR interval (<120 ms)
• Delta wave - slurred, slow upstroke of QRS
• Wide QRS
• Risk of SVT and, in AF, rapid conduction causing VF

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14. Ventricular Tachycardia (VT)

• Rate 100-250 bpm, regular
• Wide, bizarre QRS complexes (>120 ms)
• P waves absent or dissociated from QRS
• Life-threatening - do not mistake for SVT with aberrancy

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15. Ventricular Fibrillation (VF)

• Chaotic, rapid, irregular deflections
• No organized P waves, QRS, or T waves
• Pulseless cardiac arrest - immediate defibrillation required

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16. Premature Ventricular Complexes (PVCs)

• Wide, bizarre QRS appearing early (no preceding P wave)
• Full compensatory pause follows
• Concerning if: frequent (>10/hour), runs of 3+ (= non-sustained VT), R-on-T phenomenon

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17. Hyperkalemia

1. Tall, tented (peaked) T waves (early - K+ ~5.5-6.5 mmol/L)
2. Widening QRS + flattened P waves (K+ ~6.5-7.0)
3. Sine-wave pattern
4. VF / asystole (severe)

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18. Hypokalemia

• Flattened or inverted T waves
• Prominent U waves (may appear as QT prolongation)
• Risk of torsades de pointes

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19. Prolonged QT / Torsades de Pointes

• QTc >440 ms in men or >460 ms in women
• Risk of self-terminating runs of torsades de pointes ("twisting of the points" - QRS complexes twist around the baseline)
• Can degenerate to VF

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20. Pericardial Effusion / Cardiac Tamponade

1. Sinus tachycardia
2. Low voltage QRS (<5 mm in limb leads)
3. Electrical alternans - QRS height alternates beat-to-beat (from the heart swinging in fluid)

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21. Brugada Syndrome

• Coved ST elevation >2 mm in V1-V3
• Followed by a negative T wave
• Right bundle branch block-like morphology
• Can occur in young, otherwise healthy people - especially at rest/sleep

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22. Pulmonary Embolism (PE)

• Sinus tachycardia (most common)
• Classic but uncommon: S1Q3T3 pattern (S wave in I, Q wave + T wave inversion in III)
• Right axis deviation
• RBBB (new)
• T wave inversions in V1-V4 (right heart strain)

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Part 7 - Full ECG Abnormalities Summary Chart

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Quick-Reference Cheat Sheet

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