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ECG Basics

The PR Interval

February 15th, 2021


The PR interval is the time between atrial depolarisation and ventricular depolarisation. It is an indicator of atrioventricular conduction time, involving conduction from the atria, through the AV node and down the His-Purkinje system.
    • Normal Duration

    • 120-200ms (3-5mm)
  • Look For

  • The distance between the start of the P wave and the start of the R wave.

Prolonged PR Interval

Also known as first degree heart block; a prolonged PR interval suggests delayed conduction between the atria and the ventricles.
  • Look For

  • PR interval >200ms (5mm).
  • Prolonged PR Interval
  • Causes of First Degree AV Block

  • Intrinsic (AV Nodal)

  • Idiopathic degeneration
  • Myocardial infarction
  • Congenital - congenital heart disease, neonatal lupus
  • Infiltrative disease - sarcoidosis, amyloidosis, haemochromatosis
  • Cardiac procedures - valvular surgery, correction of congenital heart disease, catheter ablation
  • Systemic lupus erythematosus
  • Myocarditis
  • Extrinsic

  • Physiologic - sleep, athletes
  • Autonomic - carotid sinus massage, carotid sinus hypersensitivity
  • Drugs - beta blockers, calcium channel blockers, digoxin, adenosine, amiodarone
  • Hyperkalaemia
  • Hypothyroidism

Short PR Interval

  • Look For

  • PR interval <120ms (3mm).
  • Causes of Short PR Interval

  • Normal variant (accelerated conduction through the AV node)
  • Preexcitation (accessory pathway) - Wolff-Parkinson-White syndrome, Lown-Ganong-Levine syndrome
  • Example

  • Short PR interval in the context of Wolff-Parkinson-White syndrome, with visible delta wave:
  • Short PR Interval

PR Depression

  • Look For

  • Deviation of the PR interval below baseline.
  • PR Depression
  • Causes of PR Depression

  • Normal variant
  • Pericarditis
  • Atrial infarction
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