Approach to ECG Interpretation

• Patient name, date, time of recording
• Paper speed: 25 mm/s
• Calibration: 10 mm/mV
• Check lead placement and rule out artifacts

• Normal: 50–100 bpm
• Regular rhythm
  • use the 300 rule: 300 / number of large squares between two R waves
• Irregular rhythm
  • count the number of R waves in a 10-second strip and multiply by 6
• Escape rhythms
  • atrial: 60–80 bpm
  • junctional: 40–60 bpm
  • ventricular: 20–40 bpm

• Regular vs. irregular vs. irregularly irregular
• Identify the P wave before every QRS and the QRS after every P wave
• Sinus rhythm: P upright in leads I, II, and aVF + rate 50–100 bpm
• Irregularly irregular = think AFib

• Look at leads I and aVF:
  • lead I +, aVF + → Normal axis
  • lead I +, aVF − → Left axis deviation (LAD)
  • lead I −, aVF + → Right axis deviation (RAD)
  • lead I −, aVF − → Extreme axis
• Normal axis: −30° to +90°

• PR interval: 120–200 ms (3–5 small squares)
• QRS duration: ≤120 ms
• QT interval:
  • use Bazett’s formula for QTc
  • normal: <440 ms (men), <460 ms (women)

• Left atrial enlargement (LAE)
  • bifid P wave in lead II
  • biphasic P in V1 (terminal negative portion >1 mm)
• Right atrial enlargement (RAE)
  • tall, peaked P waves in II and V1 (>2.5 mm)

• Bundle branch blocks (BBB)
  • LBBB: broad, notched R in V5/V6; deep S in V1
  • RBBB: rsR’ in V1; wide S in V6
• LVH criteria
  • S in V1 + R in V5 or V6 >35 mm
• RVH criteria
  • RAD + R/S >1 in V1

• Ischemia: ST depression, T wave inversion (V1–V6)
• Infarction: ST elevation in the affected area
• Reciprocal changes in opposite leads
• T wave changes: hyperacute (early), inverted (later)

figures drawn for MOC. waveforms are schematic, not patient recordings.

• 1 small square wide
• 25% of R wave height
• Present in ≥2 contiguous leads

• Electrolytes
  • hyperK: tall T waves → sine wave
  • hypoK: U waves, ST depression, long QT
  • hypoCa: long QT; hyperCa: short QT
• Drugs: digoxin (scooped ST), antipsychotics (QT prolongation)
• Pre-excitation (WPW)
  • short PR <120 ms + delta wave (slurred QRS upstroke), broad QRS
  • irregular broad-complex tachycardia in WPW = pre-excited AF: avoid AV-nodal blockers (adenosine, verapamil, diltiazem, beta-blockers, digoxin), see palpitations
• Others
  • Osborne J waves (hypothermia)
  • diffuse ST elevation + PR depression (pericarditis)