Normal adult 12-lead ECG

The diagnosis of the normal electrocardiogram is made by excluding any recognised abnormality. It's description is therefore quite lengthy.

• normal sinus rhythm
  • each P wave is followed by a QRS
  • P waves normal for the subject
  • P wave rate 60 - 100 bpm with <10% variation
    • rate <60 = sinus bradycardia
    • rate >100 = sinus tachycardia
    • variation >10% = sinus arrhythmia
• normal QRS axis
• normal P waves
  • height < 2.5 mm in lead II
  • width < 0.11 s in lead II
    • for abnormal P waves see right atrial hypertrophy, left atrial hypertrophy, atrial premature beat, hyperkalaemia
• normal PR interval
  • 0.12 to 0.20 s (3 - 5 small squares)
    • for short PR segment consider Wolff-Parkinson-White syndrome or Lown-Ganong-Levine syndrome (other causes - Duchenne muscular dystrophy, type II glycogen storage disease (Pompe's), HOCM)
    • for long PR interval see first degree heart block and 'trifasicular' block
• normal QRS complex
  • < 0.12 s duration (3 small squares)
    • for abnormally wide QRS consider right or left bundle branch block, ventricular rhythm, hyperkalaemia, etc.
  • no pathological Q waves
  • no evidence of left or right ventricular hypertrophy
• normal QT interval
  • Calculate the corrected QT interval (QTc) by dividing the QT interval by the square root of the preceeding R - R interval. Normal = 0.42 s.
  • Causes of long QT interval
    • myocardial infarction, myocarditis, diffuse myocardial disease
    • hypocalcaemia, hypothyrodism
    • subarachnoid haemorrhage, intracerebral haemorrhage
    • drugs (e.g. sotalol, amiodarone)
    • hereditary
      • Romano Ward syndrome (autosomal dominant)
      • Jervill + Lange Nielson syndrome (autosomal recessive) associated with sensorineural deafness
• normal ST segment
  • no elevation or depression
    • causes of elevation include acute MI (e.g. anterior, inferior), left bundle branch block, normal variants (e.g. athletic heart, Edeiken pattern, high-take off), acute pericarditis
    • causes of depression include myocardial ischaemia, digoxin effect, ventricular hypertrophy, acute posterior MI, pulmonary embolus, left bundle branch block
• normal T wave
  • causes of tall T waves include hyperkalaemia, hyperacute myocardial infarction and left bundle branch block
  • causes of small, flattened or inverted T waves are numerous and include ischaemia, age, race, hyperventilation, anxiety, drinking iced water, LVH, drugs (e.g. digoxin), pericarditis, PE, intraventricular conduction delay (e.g. RBBB)and electrolyte disturbance.
• normal U wave 

Source : http://ecglibrary.com/norm.html

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