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I now have everything needed for a thorough, well-illustrated answer. Here is the complete guide to ECG basics:

ECG: Basic Principles & How to Read

1. What an ECG Records

The electrocardiogram records the electrical activity of the heart from the body surface. Because cardiac depolarization and repolarization wavefronts have both direction and magnitude, they can be represented as vectors, and each ECG lead captures the projection of that vector onto a specific spatial orientation - like different camera angles of the same electrical event.
The impulse originates in the sinoatrial (SA) node, spreads through the atria, delays at the AV node/His bundle (AV junction), then rapidly distributes through the right and left bundle branches via the Purkinje fibers to the ventricular myocardium. Depolarization travels from endocardium to epicardium.
Cardiac conduction system
The cardiac conduction system: SA node → AV junction → His bundle → right/left bundle branches → Purkinje fibers

2. Basic Waveforms and Intervals

Basic ECG waveforms and intervals
The standard ECG complex: P, QRS, ST segment, T wave, U wave, and major intervals (PR, QRS, QT)
Waveform / SegmentRepresentsNormal Value
P waveAtrial depolarization<120 ms duration
PR intervalAtrial depolar. + AV node delay120-200 ms
QRS complexVentricular depolarization≤100-110 ms
J pointJunction of QRS end and ST startAt isoelectric baseline
ST segmentPlateau phase of ventricular action potential (isoelectric)Flat, on baseline
T waveVentricular repolarization (phase 3)Upright in most leads
QT intervalTotal ventricular depolar. + repolar.Rate-corrected (QTc): ≤450 ms men, ≤460 ms women
U wavePossibly Purkinje fiber repolarizationSmall, same polarity as T

Relationship to Action Potential Phases

  • Phase 0 (rapid upstroke, Na+ influx) → onset of QRS
  • Phase 2 (plateau) → isoelectric ST segment
  • Phase 3 (active repolarization) → T wave
Drugs or electrolyte changes affect specific phases - e.g., flecainide widens QRS by slowing phase 0; amiodarone/hypocalcemia prolong QT by lengthening phases 2-3; hypercalcemia/digoxin shorten QT.

3. ECG Paper and Measurements

Standard recording speed is 25 mm/s:
  • Each small box (1 mm) = 40 ms (0.04 s)
  • Each large box (5 mm) = 200 ms (0.20 s)
  • Vertically: 1 mV = 10 mm (standard calibration)
Heart rate calculation:
  • Count large boxes between two R waves → divide 300 by that number
  • Or: count small boxes between two R waves → divide 1500 by that number

4. The 12 Leads

The 12 conventional leads are split into two groups:
GroupLeadsPlane
Limb (extremity) leadsI, II, III, aVR, aVL, aVFFrontal
Precordial (chest) leadsV1, V2, V3, V4, V5, V6Horizontal
  • Limb leads form a hexaxial diagram; each has a specific angular orientation relative to Lead I (0°). The mean electrical axis of the QRS is measured against this system.
  • Precordial leads are placed across the chest (V1 right sternal border → V6 mid-axillary line). They capture the horizontal plane.
  • Each lead is like a different camera angle looking at atrial and ventricular depolarization/repolarization.

5. Systematic Approach to Reading an ECG (14 Steps)

Per Harrison's Principles of Internal Medicine, analyze every ECG in this order:
  1. Standardization & technical features - Is calibration correct? Any artifacts or lead misplacements?
  2. Rhythm - Regular or irregular? What is the pacemaker?
  3. Heart rate - Bradycardia (<60), normal (60-100), or tachycardia (>100)?
  4. PR interval / AV conduction - Normal 120-200 ms; prolonged = AV block
  5. QRS interval - Normal ≤100-110 ms; widened = bundle branch block or ventricular origin
  6. QT / QTc interval - Prolonged QTc increases torsades risk
  7. Mean QRS electrical axis - Normal: -30° to +90° (frontal plane)
  8. P waves - Present? Morphology? One P per QRS?
  9. QRS voltages - High = hypertrophy; low = effusion, obesity, COPD
  10. Precordial R-wave progression - Normal transition V1 to V6 (rS to Rs)
  11. Abnormal Q waves - Pathologic Q (>40 ms or >25% of R height) = old infarction
  12. ST segments - Elevation or depression?
  13. T waves - Inversions, peaked T, or biphasic T?
  14. U waves - Prominent U = hypokalemia; inverted U = ischemia
"Many mistakes in ECG interpretation are errors of omission. Therefore, a systematic approach is essential." - Harrison's Principles of Internal Medicine, 22nd Ed.

6. Normal vs. Abnormal at a Glance

FindingNormalAbnormal (clue to)
PR interval120-200 ms>200 ms → AV block; <120 ms → pre-excitation (WPW)
QRS duration≤110 ms>120 ms → BBB or ventricular rhythm
QTc≤450 ms (M), ≤460 ms (F)Prolonged → drug toxicity, hypokalemia, channelopathy
ST segmentIsoelectricElevation → STEMI, pericarditis; Depression → ischemia, NSTEMI
Axis-30° to +90°Left axis → LAFB, inferior MI; Right axis → RVH, posterior MI
Q wavesSeptal q's in lateral leadsPathologic Q → prior MI
Source: Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 247 - Electrocardiography
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