How to Interpret an ECG

The Cardiac Conduction System (What the ECG Reflects)

ECG Paper & Calibration

• Horizontal axis (time): 1 small box = 0.04 s; 1 large box = 0.2 s; 5 large boxes = 1.0 s
• Vertical axis (voltage): 1 mm = 0.1 mV (standard calibration: 10 mm = 1 mV)

• 1 large box → 300 bpm | 2 → 150 | 3 → 100 | 4 → 75 | 5 → 60 | 6 → 50

The 14-Parameter Systematic Approach (Harrison's)

1. Standardization & Technical Features

• Check calibration (standard = 10 mm/mV)
• Verify lead placement (limb leads: I, II, III, aVR, aVL, aVF; precordial leads: V1–V6)
• Identify artifacts (muscle tremor, lead displacement, electrical interference)
• Always compare with prior ECGs

2. Rhythm

1. Where is the pacemaker? (Normal = SA node)
2. What is the conduction path to the ventricles?
3. Is it regular and at the correct rate?

3. Heart Rate

• Normal: 60–100 bpm
• Bradycardia: < 60 bpm
• Tachycardia: > 100 bpm
• Use the 300 rule (above) for regular rhythms; count complexes in 6 seconds × 10 for irregular rhythms

4. PR Interval (AV Conduction)

• Normal: 0.12–0.20 s (3–5 small boxes)
• Reflects AV node conduction time

5. QRS Duration (Intraventricular Conduction)

• Normal: < 0.10–0.12 s (< 2.5–3 small boxes)
• Narrow QRS = normal ventricular conduction
• Wide QRS (> 0.12 s) = bundle branch block, ventricular rhythm, or pacing

6. QT / QTc Interval

• Measured from start of QRS to end of T wave
• Normal QTc: < 440 ms in men, < 460 ms in women (Bazett formula: QTc = QT ÷ √RR)
• Prolonged QTc: Risk of Torsades de Pointes (TdP) → drugs (quinidine, amiodarone, antihistamines), hypokalaemia, hypomagnesaemia, hypocalcaemia, congenital long QT syndromes
• Short QTc: Hypercalcaemia

7. Mean QRS Electrical Axis

• Normal axis: −30° to +100° (frontal plane)
• Quick method: If QRS is mostly positive in leads I and aVF → normal axis

8. P Waves (Atrial Abnormalities)

• Normal P wave: Upright in I, II, aVF; < 0.12 s duration; < 2.5 mm amplitude
• P mitrale (bifid P in lead II, negative terminal deflection in V1): Left atrial enlargement
• P pulmonale (tall peaked P > 2.5 mm in II, III, aVF): Right atrial enlargement

9. QRS Voltages (Ventricular Hypertrophy)

• S in V1 + R in V5 or V6 > 35 mm (Sokolov-Lyon criterion)
• R in aVL > 11–12 mm
• Often associated with ST-T "strain" changes (downsloping ST depression + T inversion in lateral leads)

• Tall R in V1 (R > S in V1), right axis deviation
• Deep S in V5/V6

10. Precordial R-Wave Progression

• R wave should progressively increase from V1 to V5/V6
• Transition zone (R = S) normally at V3 or V4
• Poor R-wave progression (small R waves through V4): Anterior MI, LBBB, RVH, LVH, technical lead misplacement
• Early transition (tall R in V1/V2): Posterior MI, RVH, WPW

11. Abnormal Q Waves

• Pathological Q waves: ≥ 0.04 s (1 small box) wide AND ≥ 25% of QRS height, or > 1 mm deep
• Indicate prior transmural (Q-wave) MI
• Location of Q waves correlates with territory:

12. ST Segments

• Normal: Isoelectric (flat at baseline), or within ±1 mm of the PR baseline
• ST elevation:
  • STEMI: ≥ 1 mm in ≥ 2 contiguous limb leads, or ≥ 2 mm in ≥ 2 contiguous precordial leads
  • Other causes: Pericarditis (diffuse saddle-shaped elevation), early repolarization, LV aneurysm, Brugada (coved pattern in V1–V2), vasospasm (Prinzmetal's)
• ST depression:
  • Subendocardial ischemia/NSTEMI, LVH strain, digitalis effect (reverse tick/sagging), reciprocal changes in STEMI

13. T Waves

• Normal: Upright in I, II, V3–V6; inverted in aVR; variable in III, V1, V2
• T-wave inversion: Ischemia/infarction, RVH, LVH strain, bundle branch blocks, myocarditis, Wellens' syndrome (deep symmetric T inversions in V2–V3 = critical LAD stenosis)
• Peaked/hyperacute T waves: Early STEMI, hyperkalaemia
• Flat T waves: Hypokalaemia, hypothyroidism

14. U Waves

• Small deflection after the T wave, best seen in V2–V3
• Prominent U waves: Hypokalaemia, bradycardia, drugs (quinidine)
• Inverted U waves: LV volume overload, ischaemia

Summary Mnemonic: "Rate, Rhythm, Axis, P-QRS-T"

Common Patterns at a Glance