Electrocardiogram (ECG) — A Complete Guide

1. What Is an ECG?

2. The Cardiac Conduction System

3. The ECG Waveforms

P Wave

• Represents atrial depolarization
• Generated by spread of the action potential from the SA node across both atria
• Triggers atrial contraction (systole)
• Normal duration: <0.09 sec (2.5 small boxes); amplitude <3 mm
• Should be upright in leads I and II in sinus rhythm

PR Interval

• Measured from the onset of P wave to onset of QRS complex
• Normal: 0.12–0.20 sec (3–5 small boxes)
• Represents the physiologic delay at the AV node (plus conduction through atria and His-Purkinje system)
• >0.20 sec = first-degree AV block (prolonged AV nodal conduction)
• Short PR (<0.12 sec) → consider pre-excitation (Wolff-Parkinson-White) or junctional rhythm

QRS Complex

• Represents ventricular depolarization
• Composed of:
  • Q wave: initial negative deflection (septal depolarization, left to right)
  • R wave: first positive deflection
  • S wave: negative deflection following R
• Capital letters (Q, R, S) = deflection ≥5 mm; lowercase (q, r, s) = <5 mm
• Normal duration: 0.06–0.10 sec (upper limit of normal ~0.11 sec = 3 small boxes)
• Prolonged QRS → bundle branch block or intraventricular conduction delay
• Pathological Q waves (≥0.04 sec wide, ≥25% of R height) may indicate prior MI

ST Segment

• From end of QRS (J point) to start of T wave
• Normally isoelectric (flat, on the baseline)
• ST elevation → acute myocardial injury/infarction, pericarditis, vasospasm
• ST depression → myocardial ischemia, digitalis effect, posterior MI

T Wave

• Represents ventricular repolarization
• Occurs 0.25–0.35 sec after depolarization
• Should be upright in I, II, V3–V6; inverted in aVR (normal)
• Peaked T waves → hyperkalemia
• Flattened/inverted T waves → ischemia, electrolyte disturbance, LVH

QT Interval

• From onset of QRS to end of T wave
• Reflects total ventricular depolarization + repolarization time
• Rate-corrected using Bazett's formula: QTc = QT / √RR
• Normal QTc: ≤0.44 sec (men), ≤0.46 sec (women)
• Prolonged QTc → risk of Torsades de Pointes (dangerous polymorphic VT)

U Wave

• Small deflection after T wave (not always present)
• Represents late phases of ventricular repolarization
• Prominent U waves → hypokalemia, bradycardia

4. Paper Speed & Measurements

• Method 1: HR = 60 ÷ (R-R interval in seconds)
• Method 2 (fast): HR = 300 ÷ (number of large boxes between two R waves)
  • 1 box = 300 bpm | 2 boxes = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50

5. The 12-Lead System

Limb Leads (Frontal Plane)

Precordial Leads (Horizontal Plane)

6. QRS Axis

• I (+), aVF (+) → Normal axis
• I (+), aVF (−) → LAD
• I (−), aVF (+) → RAD

7. Systematic Approach to ECG Interpretation

1. Rate — Calculate heart rate
2. Rhythm — Regular vs. irregular? P waves present?
3. P wave morphology — Normal sinus vs. ectopic?
4. P-to-QRS relationship — 1:1 conduction? PR interval normal?
5. Intervals — PR, QRS duration, QT/QTc
6. QRS axis — Normal, LAD, or RAD?
7. QRS morphology — Q waves? Bundle branch block? Voltage?
8. ST segments — Elevation or depression? Which leads?
9. T wave morphology — Upright, inverted, peaked?
10. U waves — Present?

8. Normal Sinus Rhythm Criteria

• Rate 60–100 bpm
• Every QRS is preceded by a P wave
• P wave is upright in leads I and II (confirming SA nodal origin)
• Regular R-R intervals (minor variation = sinus arrhythmia — normal)
• PR interval 0.12–0.20 sec
• QRS ≤0.10 sec

9. Example ECG

10. Key Clinical Patterns to Know