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ECG basic interpretation waveforms P QRS T

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ECG Basic Interpretation — A Systematic Guide

1. What the ECG Records

The ECG records the electrical activity of the heart from the body surface. The heart's conduction system generates electrical signals that cause depolarization (contraction) and repolarization (recovery) of cardiac muscle. Each electrical event produces a characteristic waveform on the ECG.
The cardiac conduction pathway:
Cardiac conduction system — SA node → AV node → Bundle of His → Bundle branches → Purkinje fibers
The sequence: SA node (right atrium) → spreads through both atria → AV node (physiological delay) → Bundle of His → right and left bundle branches → Purkinje fibers → ventricular muscle. (Goldman-Cecil Medicine, Cardiac Conduction System)

2. The ECG Waveforms

Wave / IntervalWhat it representsNormal duration
P waveAtrial depolarization< 120 ms, amplitude < 2.5 mm
PR intervalAtrial-to-ventricular conduction (AV node + His-Purkinje)120–200 ms (3–5 small boxes)
QRS complexVentricular depolarization< 120 ms (< 3 small boxes)
ST segmentEarly ventricular repolarizationIsoelectric (flat, at baseline)
T waveVentricular repolarizationUpright in most leads; inverted in aVR normally
QT intervalTotal ventricular electrical activity< 440 ms (men), < 460 ms (women)
U waveAfterdepolarizations (late repolarization)Rarely seen; most prominent in V2–V3
Key point: The P wave and QRS complex are depolarization waves. The T wave is a repolarization wave. (Guyton & Hall Medical Physiology)

3. The ECG Paper

  • Paper speed: 25 mm/sec (standard)
  • Small box (1 mm): 0.04 sec (40 ms) horizontally; 0.1 mV vertically
  • Large box (5 mm): 0.20 sec (200 ms) horizontally; 0.5 mV vertically
  • Calibration: 1 mV = 10 mm (two large boxes) height
(Goldman-Cecil Medicine)

4. The 12 Leads

A standard ECG uses 12 leads that view the heart from different angles:
Limb leads (frontal plane):
  • I, II, III — Standard bipolar leads
  • aVR, aVL, aVF — Augmented unipolar leads
Chest (precordial) leads (horizontal plane):
Precordial lead placement V1–V6 on the chest wall
  • V1 — Right sternal border, 4th intercostal space
  • V2 — Left sternal border, 4th ICS
  • V3 — Between V2 and V4
  • V4 — Left midclavicular line, 5th ICS
  • V5 — Anterior axillary line
  • V6 — Midaxillary line
(Harrison's Principles of Internal Medicine, 22nd Ed.)

5. QRS Complex in the Chest Leads: R-wave Progression

Ventricular depolarization phases and corresponding QRS morphology in V1 vs V6
Ventricular depolarization has two phases:
  1. Septal depolarization (left → right): produces a small r in V1 and small q in V6
  2. Main ventricular depolarization (left ventricle dominant, pointing left/posteriorly): produces a deep S in V1 and tall R in V6
R waves should progressively increase from V1 → V4/V5 ("R-wave progression"). The transition zone (where R = S) is normally at V3 or V4.
(Harrison's Principles of Internal Medicine, 22nd Ed.)

6. Systematic 14-Step Approach to Every ECG

Harrison's recommends analyzing these parameters on every ECG:
  1. Standardization & technical features — Is calibration 10 mm/mV? Are all leads present? Any artifact?
  2. Rhythm — Sinus? Regular? Is there a P before every QRS?
  3. Heart rate — Count RR intervals (300 ÷ large boxes between R waves, or 1500 ÷ small boxes)
  4. PR interval — Normal 120–200 ms? Short (pre-excitation) or long (AV block)?
  5. QRS duration — Normal < 120 ms? Wide QRS = BBB or ventricular rhythm
  6. QT/QTc interval — Prolonged QTc raises risk of torsades de pointes
  7. Mean QRS axis — Normal −30° to +100°
  8. P waves — Morphology, upright in II, biphasic in V1
  9. QRS voltages — High = ventricular hypertrophy; low = effusion, COPD, obesity
  10. R-wave progression — V1 → V6 as above
  11. Abnormal Q waves — Pathologic if ≥ 40 ms wide or ≥ 25% of R amplitude → suggests prior MI
  12. ST segments — Elevation (injury/STEMI) or depression (ischemia/strain)
  13. T waves — Inversion, peaking, symmetry
  14. U waves — Prominent U waves → hypokalemia
Always compare with a prior ECG if available.
(Harrison's Principles of Internal Medicine, 22nd Ed.)

7. Quick Heart Rate Calculation

Large boxes between R wavesHeart rate (bpm)
1300
2150
3100
475
560
650
Mnemonic: 300 – 150 – 100 – 75 – 60 – 50

8. Normal ECG Summary

A normal ECG shows:
  • Sinus rhythm: regular P waves, each followed by a QRS
  • HR: 60–100 bpm
  • PR: 120–200 ms
  • QRS: < 120 ms, normal axis, normal R-wave progression
  • ST segments: isoelectric
  • T waves: upright in I, II, V4–V6; inverted in aVR (normal); may be inverted in V1 normally
  • Small septal q waves in V5–V6 are normal
  • Sinus arrhythmia (slight RR variation with breathing) is normal, especially in young people
(Goldman-Cecil Medicine)

Key Reference Works

  • Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 247
  • Goldman-Cecil Medicine, Chapter 42
  • Guyton and Hall Textbook of Medical Physiology, Chapter 11
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