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ECG electrocardiogram waves P QRS T basic interpretation

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Here is a comprehensive overview of ECG basic interpretation and waves, sourced from Costanzo Physiology and Goldman-Cecil Medicine.

What Is an ECG?

The electrocardiogram (ECG/EKG) measures tiny potential differences on the body's surface that reflect the heart's electrical activity. Because the myocardium depolarizes and repolarizes in a specific sequence (atria first, then ventricles), potential differences are established between different parts of the heart and detected by surface electrodes.

The Normal ECG Waveform

ECG waveform diagram showing P, Q, R, S, T waves with PR interval, ST segment, and QT interval labeled
Fig. 4.17 — The ECG measured from Lead II (Costanzo Physiology, 7th ed.)

Waves, Intervals & Segments

Waves

WaveRepresentsNotes
P waveAtrial depolarizationWider P wave = slower atrial conduction. Atrial repolarization is buried in the QRS complex
Q waveInitial ventricular depolarization (septal)Small Q waves are normal; pathological Q waves suggest infarction
R waveMain ventricular depolarization (upward deflection)Should show normal progression V1→V6
S waveTerminal ventricular depolarization (downward)—
T waveVentricular repolarizationNormally concordant with QRS direction
U wave(rarely seen) Late ventricular repolarizationProminent in hypokalemia

Intervals & Segments

NameMeasurementNormal ValueMeaning
PR intervalOnset of P wave → onset of QRS0.12–0.20 s (3–5 small boxes)AV node conduction time; prolonged = AV block, shortened = pre-excitation
QRS complexQ onset → S offset<0.12 s (< 3 small boxes)Ventricular depolarization; widened = bundle branch block or ventricular rhythm
ST segmentEnd of QRS (J point) → onset of T waveIsoelectric (flat)Elevated = STEMI, pericarditis; depressed = ischemia, NSTEMI
QT intervalQRS onset → T wave end<0.44 s (varies with HR)Ventricular depolarization + repolarization; prolonged = risk of torsades

ECG Paper / Grid Standards

  • Horizontal axis = time: Each small box (1 mm) = 0.04 s; each large box (5 mm) = 0.20 s
  • Vertical axis = voltage: Each small box = 0.1 mV; 10 mm = 1 mV
  • Standard recording speed: 25 mm/sec over a 10-second period

The Cardiac Conduction System (Basis of ECG)

The electrical sequence generating each ECG complex:
  1. SA node (high right atrium) → spontaneous depolarization → P wave
  2. AV node → physiologic conduction delay → flat PR segment
  3. Bundle of His → Right & Left bundle branches → Purkinje fibers → QRS complex
  4. Ventricular repolarization → ST segment + T wave
The J (junction) point marks the end of the QRS and the start of the ST segment.

A Systematic Approach to ECG Reading

  1. Rate — Count R-R intervals (300 ÷ number of large boxes between R waves)
  2. Rhythm — Regular or irregular? P before every QRS?
  3. Axis — Leads I and aVF: both positive = normal axis (0° to +90°)
  4. P wave — Present, upright in II? Morphology?
  5. PR interval — 0.12–0.20 s?
  6. QRS width — Narrow (<0.12 s) or wide?
  7. ST segment — Elevated, depressed, or isoelectric?
  8. T wave — Upright, inverted, or peaked?
  9. QT interval — Corrected QTc within normal limits?
Sources: Costanzo Physiology, 7th ed. | Goldman-Cecil Medicine, International ed.
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