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Here is a comprehensive, step-by-step guide to reading an ECG report:

How to Read an ECG Report

The ECG Grid

Each small square = 0.04 sec (horizontal) and 0.1 mV (vertical) Each large square = 0.20 sec (5 small squares)

Step 1 — Check Heart Rate

Fastest method (regular rhythms):
300 ÷ number of large squares between two R peaks
Large squares between R-RHeart rate
1300 bpm
2150 bpm
3100 bpm
475 bpm
560 bpm
650 bpm
For irregular rhythms: Count QRS complexes in a 10-second rhythm strip × 6.
  • Normal: 60–100 bpm
  • Bradycardia: < 60 bpm
  • Tachycardia: > 100 bpm

Step 2 — Assess Rhythm

Ask: Is there a P wave before every QRS? Is the R-R interval regular?
FindingLikely Rhythm
Regular, P before every QRSNormal Sinus Rhythm
Irregular, no P wavesAtrial Fibrillation
Regularly irregular2nd degree AV block / Wenckebach
Absent/buried P wavesJunctional or Ventricular rhythm

Step 3 — Determine Electrical Axis

Look at Lead I and aVF:
Lead IaVFAxis
↑ Positive↑ PositiveNormal (−30° to +90°)
↑ Positive↓ NegativeLeft Axis Deviation (LAD)
↓ Negative↑ PositiveRight Axis Deviation (RAD)
↓ Negative↓ NegativeExtreme / Northwest axis

Step 4 — Measure Intervals

ECG intervals diagram
IntervalNormal RangeAbnormal Meaning
PR interval0.12–0.20 sec (3–5 small squares)Prolonged → AV block; Short → pre-excitation (WPW)
QRS duration< 0.12 sec (< 3 small squares)Wide → Bundle branch block or ventricular beat
QT/QTc< 0.44 sec (men), < 0.46 sec (women)Prolonged → risk of Torsades de Pointes

Step 5 — Examine Individual Waves

P Wave (Atrial Depolarization)

  • Normal: < 0.12 sec wide, < 2.5 mm tall, upright in Lead II
  • Tall peaked P in II → Right atrial enlargement
  • Broad notched P in II → Left atrial enlargement

QRS Complex (Ventricular Depolarization)

  • Normal: narrow (< 0.12 sec), Q waves < 1 mm wide and < 25% of R-wave height
  • Wide QRS → Bundle branch block (LBBB/RBBB) or ventricular origin
  • Pathological Q waves → Prior myocardial infarction

T Wave (Ventricular Repolarization)

  • Normal: same direction as QRS
  • Inverted T → Ischemia, strain, or infarction
  • Peaked tall T → Hyperkalemia or early STEMI (hyperacute T)

Step 6 — Analyze ST Segment

This is the most critical step for detecting heart attack:
FindingSignificance
ST elevation ≥ 1 mm in ≥ 2 contiguous leadsSTEMI (acute MI) — emergency!
ST depressionIschemia, NSTEMI, or digoxin effect
Flat/isoelectric STNormal

Localizing the Infarct by Leads:

Leads with changesTerritory
II, III, aVFInferior (RCA)
I, aVL, V5, V6Lateral (LCx)
V1–V4Anterior (LAD)
V1–V2Septal

Step 7 — Look for Special Patterns

PatternECG Finding
Right Bundle Branch Block (RBBB)"M" shape (rSR') in V1; wide S in V6 — mnemonic: MaRRoW
Left Bundle Branch Block (LBBB)Broad notched R in V5/V6; QS in V1 — mnemonic: WiLLiaM
WPW (pre-excitation)Short PR + delta wave
Atrial flutterSawtooth P waves, ~300 bpm atrial rate
1st degree AV blockPR > 0.20 sec
2nd degree Mobitz I (Wenckebach)Progressively longer PR until a QRS drops
2nd degree Mobitz IIConstant PR, then sudden dropped QRS
3rd degree (complete) AV blockP waves and QRS are completely dissociated
HyperkalemiaPeaked T → wide QRS → sine wave pattern
HypokalemiaFlat T, prominent U wave

Quick Systematic Checklist

  1. ✅ Rate (bradycardia / normal / tachycardia)
  2. ✅ Rhythm (regular? P before every QRS?)
  3. ✅ Axis (Lead I + aVF)
  4. ✅ Intervals (PR, QRS, QT)
  5. ✅ P wave morphology
  6. ✅ QRS morphology (Q waves, bundle branch block)
  7. ✅ ST segment (elevation or depression?)
  8. ✅ T wave changes
  9. ✅ Compare with previous ECG if available
Sources: Braunwald's Heart Disease, 12e; Miller's Anesthesia, 10e; UNC School of Medicine EKG Interpretation
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