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Approach to the ECG

Melis Sahinoz

Method = mastery, approach each EKG the same.


  • Regular rhythms = “Rule of 300” = 300 ÷ (large boxes between QRS complexes)
  • 1 box = 300 bpm, 2 boxes = 150 bpm, 3 boxes = 100 bpm, 4 boxes = 75 bpm, 5 boxes = 60 bpm, 6 boxes = 50 bpm
  • 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, PE


  • 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: 60 – 100 ms (normal)
    • If 100-120 ms: Incomplete BBB or non-specific intraventricular conduction delay (IVCD)
    • If > 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)
    • Quick estimate: normal QT is less than half the preceding RR interval
    • QTc estimates the QT interval at a HR of 60 bpm (to allow for comparison across HRs)
    • A couple of formulas exist to calculate QTc:
      • QTcB= most commonly used due to simplicity, most accurate HR of 60
      • QTcF= more accurate when HR is outside the range of 60-100
    • Clinically significant when generally QTc > 500 ms
    • Causes of Prolonged QTc: hereditary, medication-induced (anti-emetics, ABX, psychiatric meds), hypokalemia, hypomagnesemia, hypocalcemia, ischemia


  • P wave: P waves in limb leads should be ≤2.5 small box high and ≤2.5 small box wide
    • Right Atrial Enlargement: Peaked P Wave 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
    • If ≥ 3 different P wave morphologies in same lead: wandering atrial pacemaker (HR < 100) or multifocal atrial tachycardia (HR > 100)
  • 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 exist
        • Sokolow-Lyon criteria is a common example: S in V1 + R in V5 or V6 >35mm, R in aVL ≥ 1.1 mV
  • Conduction delays
    • RBBB: Wide QRS and RSR’ in V1 or V2; deep broad S In lateral leads
    • LBBB: Wide QRS, large S in V1, broad monophasic R wave in lateral leads (I, aVL, V6)
  • R wave progression
    • R wave normally gets progressively larger from V1 to V6
    • If the transition does not occur by V4, this is called "poor R wave progression". This is seen in chronic lung disease, LVH, left anterior fascicular block, and anterior MI.
  • Q-wave: 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
  • ST Segment
    • 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
    • See ACS section for STEMI criteria, Wellens Syndrome
  • T wave
    • Normal T waves are upright in all leads except aVR and V1
    • Inverted T Waves
      • Acute ischemia (if present in contiguous leads), LBBB (in lateral leads), RBBB (V1-V3), LVH (‘strain’ pattern similar to LBBB), RVH (RV ‘strain’ in V1-V3 or inferior leads), PE (right heart strain or part of S1,Q3,T3), intracranial pathology
    • Peaked T Waves
      • Hyperkalemia vs ‘hyperacute’ T waves that precede ST elevation and Q waves in STEMI