12 Lead ECGs

  • Check name, date, time of EKG
  • Note if it was taken during episode of chest pain
  • Rate
    • Many methods exist for calculating rate
    • If regular rhythm use the counting down method, assuming 25mm/sec stri
      • Find and R wave on the rhythm strip that lands on the line for one of the larger boxes (0.2sec) and count each larger box line until you arrive at the next R wave -> 300bpm,150bpm, 100bpm, 75bpm, 60bpm, 50bpm
      • Approximate if the second R wave is not situated exactly on the line
    • If irregular count the number of complexes over 6 seconds and multiply by 10, or determine the rates for the closest complexes and the furthest complexes and divide by 2
  • Rhythm
    • Sinus if there is a P wave before every QRS and a QRS after every P wave, if the P waves are upright in leads II, III, aVF
    • Atrial fibrillation if there are no p waves
    • Atrial flutter at 2:1 block, look for the saw tooth pattern
    • Blocks
      • 10 – PR interval is longer than 0.20 seconds
      • 20
        • 1. Mobitz type 1 (Wenchebach): progressively elongated PR interval eventually resulting in complete failure to conduct and a lack of ventricular contraction
        • 2. Mobitz type 2: unexpected nonconducted atrial impulses
      • 30 – complete dissociation of atrial and ventricular impulses



  • Axis
    • Look at QRS complexes in leads I and aVF
      • Both upright – normal
      • If lead I is downwards – right deviation
      • If lead aVF is downward look at lead II
        • 1. Upright – normal
        • 2. Downwards – left deviation
  • Intervals
    • PR 0.12-0.2 seconds
    • QRS ≤ 0.10 seconds
    • QTc ≤ 0.44 seconds → calculation: QT Interval / √ (RR interval)
  • P wave morphology
    • Look at lead II
      • Right atrial enlargement if p wave initial component is enlarged taller than 2.5m
    • Look at lead V1
      • Left atrial enlargement if p wave has a downward deflection of terminal componen
  • QRS complex  morphology
    • Right ventricular hypertrophy if tall R waves in V1 and V2, deep S wave in V6 and right axis deviation
    • Left ventricular hypertrophy is tall R waves in V6, deep S wave in V1, AND one of:
      • R in V5 or V6 ≥ 35mm
      • R in aVL > 11mm
      • R in I > 15m
    • Incomplete BBB – QRS 0.10-0.12 seconds
      • LAFB: Q wave in I, aVL; initial R wave in II, III, aVF
      • LPFB: Q in II, III, aVF; initial R wave in I, aV
    • Complete BBB – QRS > 0.12 seconds
      • RBBB: R in V1 (bunny ear appearance), and S in V6
      • LBBB: absent normal R in V1 and Q in V; terminal R’ in V6 and downward deflection in V1 (W appearance
    • Q waves – normal in V6 and aVL; pathologic if >0.04 seconds or depth >25% QRS height
  • ST changes – ST segment and T waves
    • MI
      • ST elevation – returns to baseline in days
      • T wave inversion – weeks to months
      • Q waves – persists
      • ST segment may remain elevated if fibrotic scar develop
    • Wellen’s syndrome – proximal LAD stenosis
      • Biphasic T waves in V2 and V3
    • Brugada syndrome
      • Persistent ST elevations in V1-V3
      • RBBB appearance without the terminal S waves in the lateral lead
    • Pericarditis
      • Diffuse ST elevation
      • PR depressio
    • Hyperkalemia
      • Tall peaked T waves
      • Flat P waves
      • Wide QR
    • Hypokalemia
      • U wave
      • ST depression
      • Flat T wave

Further Reading:

  1. How to Read an EKG (Source: South Sudan Medical Journal)
  2. How to Read an ECG (Source: Geeky Medics)

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