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Here is a complete guide to ECG basics and how to read one, drawn from Medical Physiology (Boron & Boulpaep) and Braunwald's Heart Disease.

What is an ECG?

An ECG (electrocardiogram) records the time-dependent electrical activity of the heart from electrodes on the body surface. It gives direct information about:
  • Rate - how fast the heart is beating
  • Rhythm - where the impulse originates and how it conducts
  • Axis - the direction of the electrical vector
  • Waveform morphology - signs of ischemia, hypertrophy, or conduction problems

The ECG Paper

  • Vertical axis: 0.1 mV per small box (1 mm)
  • Horizontal axis: 0.04 sec per small box (1 mm); 0.2 sec per large box (5 mm)
  • 5 large boxes = 1.0 second

The 12 Leads

A standard ECG has 12 leads - 6 limb leads and 6 chest (precordial) leads:
GroupLeadsView
Limb leadsI, II, IIIFrontal plane
Augmented limb leadsaVR, aVL, aVFFrontal plane
Precordial leadsV1 - V6Horizontal plane
Here is what a normal 12-lead ECG looks like:
Normal 12-lead ECG showing all leads - I, II, III, aVR, aVL, aVF, V1 through V6

The ECG Waveforms

Each heartbeat produces a characteristic sequence of waves:
Wave / IntervalWhat it represents
P waveAtrial depolarization
PR intervalTime from atrial to ventricular activation (through AV node)
QRS complexVentricular depolarization
ST segmentPlateau phase of ventricular action potential (between QRS and T)
T waveVentricular repolarization
QT intervalTotal ventricular electrical activity (depolarization + repolarization)
U waveSmall wave after T wave (cause debated; seen in hypokalemia)

QRS Wave Naming Convention

  • Q = first negative deflection before an R wave
  • R = first positive deflection
  • S = negative deflection after an R wave
  • R' = second positive deflection (e.g., in RBBB)
  • Capital letters (Q, R, S) = large amplitude; lowercase (q, r, s) = small amplitude

Normal Interval Values

IntervalNormal Duration
PR interval0.12 - 0.20 sec (3-5 small boxes)
QRS duration< 0.12 sec (< 3 small boxes)
QT intervalRate-dependent; corrected QTc < 0.44 sec (men), < 0.46 sec (women)

How to Measure Heart Rate

Direct method: Rate = 60 / (R-R interval in seconds)
Quick method (count large boxes between two R waves):
  • 1 large box = 300 bpm
  • 2 boxes = 150
  • 3 boxes = 100
  • 4 boxes = 75
  • 5 boxes = 60
  • 6 boxes = 50
Memorize: 300 - 150 - 100 - 75 - 60 - 50

Systematic Approach to Reading an ECG

Use this 8-step method every time:
  1. Rate - Calculate using R-R interval (normal: 60-100 bpm)
  2. Rhythm - Regular or irregular? Is every QRS preceded by a P wave?
  3. P waves - Present? Upright in II? One per QRS?
  4. PR interval - Normal (0.12-0.20 sec)? Short (WPW)? Prolonged (heart block)?
  5. QRS duration - Narrow (< 0.12 sec) or wide (bundle branch block)?
  6. Axis - Normal axis is -30° to +90° in the frontal plane
  7. ST segment & T waves - Elevation (STEMI, pericarditis)? Depression (ischemia)?
  8. QT interval - Prolonged (risk of torsades)?

Normal Sinus Rhythm Criteria

  • Rate 60-100 bpm
  • Regular rhythm
  • Upright P wave in lead II (negative in aVR)
  • Each P wave followed by a QRS
  • Normal PR interval (0.12-0.20 sec)
  • Narrow QRS (< 0.12 sec)

Electrical Axis

The normal QRS axis is between -30° and +90° in the frontal plane.
  • Left axis deviation (< -30°): left anterior fascicular block, inferior MI, LVH
  • Right axis deviation (> +90°): right ventricular hypertrophy, left posterior fascicular block, PE
Quick axis check: Look at leads I and aVF:
  • Both positive = normal axis
  • I positive, aVF negative = possible left axis deviation
  • I negative, aVF positive = right axis deviation
This is the foundation. Once you are comfortable with these basics, the next steps are recognizing specific patterns: bundle branch blocks, ST changes, hypertrophy criteria, and arrhythmias. Let me know which area you'd like to go into next.
  • Medical Physiology (Boron & Boulpaep), Box 21-2, pp. 731-734
  • Braunwald's Heart Disease, 12th Ed.
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