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Here is a thorough guide to ECG basics and interpretation, drawn from Harrison's Principles of Internal Medicine, Braunwald's Heart Disease, Ganong's Medical Physiology, and Medical Physiology.

ECG Basics & Interpretation

What is an ECG?

An ECG (electrocardiogram) records the time-dependent electrical vectors of the heart from body surface electrodes. It gives direct information about rate, rhythm, the origin of the cardiac action potential, and its conduction through the heart. - Harrison's Principles of Internal Medicine 22E, p. 1912

The ECG Paper

  • Vertical axis: amplitude - calibrated at 0.1 mV per mm
  • Horizontal axis: time
    • 1 small box = 1 mm = 0.04 seconds
    • 1 large box = 5 mm = 0.20 seconds
    • 5 large boxes = 1.0 second

The 12 Leads

A standard ECG uses 12 leads, each viewing the heart from a different angle:
Limb leads (frontal plane):
  • Bipolar: I, II, III (Einthoven triangle)
  • Augmented unipolar: aVR, aVL, aVF
Precordial leads (horizontal/transverse plane):
Precordial lead placement V1-V6 on the chest
  • V1: 4th intercostal space, right sternal border
  • V2: 4th intercostal space, left sternal border
  • V3: Between V2 and V4
  • V4: 5th intercostal space, midclavicular line
  • V5: Anterior axillary line
  • V6: Midaxillary line

The Waves, Intervals & Segments

ComponentWhat it RepresentsNormal Value
P waveAtrial depolarization (SA node → atria)Duration < 120 ms; positive in II, negative in aVR
PR intervalAV node conduction time (P onset to QRS onset)120-200 ms
QRS complexVentricular depolarizationDuration < 120 ms
ST segmentVentricular plateau (early repolarization)Isoelectric (no deviation)
T waveVentricular repolarizationUpright in most leads
QT intervalTotal ventricular electrical systoleVaries with rate; QTc < 440-450 ms
  • Medical Physiology, p. 731

Step 1: Calculate the Heart Rate

Quick method: Count the large boxes between two R waves and use:
Rate = 300 ÷ (number of large boxes between R-R)
R-R (large boxes)Rate (bpm)
1300
2150
3100
475
560
650
Normal sinus rate: 60-100 bpm

Step 2: Assess the Rhythm

Ask four questions:
  1. Is there a P wave before every QRS?
  2. Is the PR interval constant?
  3. Are the R-R intervals regular?
  4. Is the P wave upright in lead II and inverted in aVR? (confirms sinus origin)
Normal sinus rhythm = SA node pacemaker, regular rate 60-100 bpm, P wave positive in II/negative in aVR, constant PR interval.

Step 3: Assess the P Wave

  • Normal: positive in II, biphasic in V1 (small positive then small negative component)
  • Negative in II + positive in aVR = ectopic/retrograde atrial activation
  • Tall, peaked P in II = right atrial enlargement ("P pulmonale")
  • Broad, notched P in II + prominent negative terminal component in V1 = left atrial abnormality ("P mitrale")

Step 4: Measure the PR Interval

  • Short PR (< 120 ms): pre-excitation (WPW), junctional rhythm
  • Long PR (> 200 ms): first-degree AV block
  • Progressive lengthening then dropped beat: Wenckebach (2nd degree type I)
  • Fixed dropped beats: Mobitz II / complete heart block

Step 5: Analyze the QRS Complex

The QRS reflects two sequential phases of ventricular depolarization:
Ventricular depolarization phases - V1 shows rS, V6 shows qR
  • Phase 1: Septal depolarization - left to right → small r in V1, small q in V6
  • Phase 2: LV dominates, vector left and posterior → large S in V1, tall R in V6
Normal QRS duration: < 120 ms. If > 120 ms, think bundle branch block or pre-excitation.
R-wave progression: R wave should increase in amplitude from V1 to V5. Poor R-wave progression suggests anterior infarction or LVH.

Step 6: Determine the QRS Axis

Normal axis: -30° to +90°
Quick method using leads I and aVF:
Lead IaVFAxis
++Normal (0 to +90°)
+-Left axis deviation (<-30°)
-+Right axis deviation (>+90°)
--Extreme axis (northwest)
Left axis deviation: LBBB, left anterior fascicular block, inferior MI, LVH Right axis deviation: RBBB, left posterior fascicular block, RVH, pulmonary embolism

Step 7: Assess the ST Segment & T Wave

  • ST elevation: transmural ischemia/STEMI (current of injury - epicardial), pericarditis (diffuse), normal variant (early repolarization)
  • ST depression: subendocardial ischemia/NSTEMI, digoxin effect, LVH strain
  • T wave inversion: ischemia, ventricular hypertrophy, bundle branch block, Wellens syndrome
  • Tall, peaked T waves: hyperkalemia, hyperacute MI

Step 8: Measure the QT Interval

The QT shortens at faster heart rates. Use the corrected QT (QTc):
Bazett's formula: QTc = QT ÷ √(R-R in seconds)
  • QTc > 450 ms (men) / > 460 ms (women) = prolonged
  • Prolonged QTc risks Torsades de Pointes (polymorphic VT)

A Normal 12-Lead ECG

Normal 12-lead ECG from Harrison's - sinus rhythm, HR 75, PR 160ms, QRS 80ms, QT 360ms, axis +70°
Normal ECG: sinus rhythm, HR 75 bpm, PR 160 ms, QRS 80 ms, QT 360 ms, QTc ~390 ms, mean QRS axis +70°. Normal R-wave progression with transition zone in V3-V4. - Harrison's, p. 1913

Systematic Approach Summary (8-Step Checklist)

  1. Rate - calculate from R-R interval
  2. Rhythm - sinus vs. non-sinus; regular vs. irregular
  3. P wave - morphology, axis, duration
  4. PR interval - normal, short, or prolonged
  5. QRS duration - narrow (<120 ms) vs. wide (>120 ms)
  6. QRS axis - normal, LAD, or RAD
  7. ST-T changes - elevation, depression, inversion
  8. QT interval - calculate QTc
Want me to go deeper into any specific area - arrhythmias, bundle branch blocks, STEMI localization, or a specific pattern?
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