Approach to the ECG – Andy Hughes 

General Schema: Rate Rhythm Axis Intervals Morphology



  • Regular rhythms, “Rule of 300” = 300 ÷ (large boxes between QRS complexes)
    • 300, 150, 100, 75, 60, 50
  • Irregular rhythms or severe bradycardia = (total number of QRS complexes on ECG) x 6



  • Determine regular vs irregular: calipers or march out QRS complexes on paper
  • Criteria for Sinus rhythm: P before every QRS; Upright P in Lead I, II; Negative in aVr



  • Normal: - 30o to + 90o
  • Quick method: Leads I and II
    • Normal Axis: Upright in I and II
    • Left Axis Deviation: Upright in I, down in II
      • Causes: LVH, LBBB, Left anterior fascicular block, prior inferior MI
    • Right Axis Deviation: Down in I, up in II
      • Causes: RVH, RBBB, Left posterior fascicular block, prior lateral MI



  • PR Interval: normal 120 – 200 ms
    • If < 120 ms, consider pre-excitation with accessory pathway (i.e. WPW)
    • If > 200 ms, first degree AV block
  • QRS Complex: normal duration 60 – 100 ms
    • 100-120 ms: Incomplete BBB or non-specific intraventricular conduction delay (IVCD)
    • > 120ms: complete BBB, ventricular tachycardia, hyperkalemia
  • QT interval: Normal duration < 450ms in men and < 460ms in women
    • QT is inversely proportional to HR (QT interval shortens at faster HRs)
    • Clinically significant when generally QTc > 500 ms
    • Quick estimate: normal QT is less than half the preceding RR interval
    • QTc is corrected to estimate QT interval at HR of 60 bpm
    • Causes of Prolonged QTc: hereditary, medication-induced (anti-emetics, ABX, psychiatric meds), hypokalemia, hypomagnesemia, hypocalcemia, ischemia



  • P Wave
    • If 3 different P wave morphologies in same lead: wandering atrial pacemaker (HR < 100) or multifocal atrial tachycardia (HR > 100)
    • Right Atrial Enlargement
      • Peaked P Wave (P Pulmonale) in Lead II that measures >2.5 mm
    • Left Atrial Enlargement
      • Lead II: Bifid P Wave (two humps) with total duration > 110 ms
    • Lead V1: Biphasic P wave, terminal deflection > 1mm wide and deep'
  • Q Waves
    • Small Q waves are normal in most leads (never normal in V1-V3)
    • Pathologic Q Waves: > 1 box wide and 2 boxes deep or > 25% height of R wave
  • QRS Complex
    • Voltage:
      • Low Voltage: QRS amplitude < 5mm in limb leads or < 10mm in precordial
        • Causes: pericardial effusion, infiltrative cardiomyopathy, obesity
      • Right Ventricular Hypertrophy: Tall R Waves in V1 (> 7mm) and Right axis deviation
      • Left Ventricular Hypertrophy: multiple criteria exis
        • Sokolow-Lyon is most common: S wave in V1 + R Wave in V5 or V6 > 35 mm
        • ​​​​​​​​​​​​​​Can be accompanied by ST segment depression and T wave inversion in lateral leads (left ventricular strain pattern)
  • Conduction delays
    • RBBB: Wide QRS and RSR’ in V1 or V2
    • ​​​​​​​LBBB: Wide QRS, large S in V1, broad monophasic R wave in lateral leads (I, aVL, V6)
  • ST Segment Changes
    • ST Elevation:  STEMI, LBBB (ST elevation in leads with deep S waves), LVH, Ventricular paced rhythm, Pericarditis (associated with PR depression), coronary vasospasm, Brugada syndrome
      • ST Depression: ischemia, reciprocal change in STEMI, posterior myocardial infarction (V1-V3), digoxin, hypokalemia
      • Wellens Syndrome:
      • Biphasic T waves or deeply inverted T waves in leads V2-3 in pts with angina
      • Highly specific of critical stenosis of LAD
      • Require coronary angiogram; Increased risk of MI/cardiac arrest with stress test
      • Type A: Biphasic T waves with initial positivity and terminal negativity in V2-V3
      • Type B: Deeply and symmetrically inverted T waves in V2-V3
  • T Wave
    • Normal T waves are upright in all leads except aVR and V1
      • Inverted T Waves
      • DDX: acute ischemia (if present in contiguous leads), LBBB (in lateral leads I, aVL, V5-6), RBBB (V1-V3), LVH (‘strain’ pattern similar to LBBB), RVH (RV ‘strain’ in V1-V3 or inferior leads), Pulmonary Embolism (right heart strain or part of S1,Q3,T3)
          • Peaked T Waves
      • DDX: hyperkalemia vs ‘hyperacute’ T waves that precede ST elevation and Q waves in STEMI