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Bradyarrhythmias
 
 

Sinoatrial Exit Block

 
 
 
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Overview

Sinoatrial exit block is a delay in conduction between the sinoatrial node and atrial myocardium.
  • Causes of Sinoatrial Exit Block

  • Intrinsic

  • Idiopathic degeneration
  • Myocardial infarction
  • Infiltrative disease - sarcoidosis, amyloidosis, haemochromatosis
  • Connective tissue disease - SLE, scleroderma, RA
  • Cardiac surgery - valvular surgery, correction of congenital heart disease
  • Infections - Lyme disease, endocarditis
  • Extrinsic

  • Physiologic - sleep, athletes
  • Autonomic - neurocardiogenic syncope, carotid sinus hypersensitivity
  • Drugs - beta blockers, calcium channel blockers, digoxin, adenosine, ivabradine, clonidine, acetylcholinesterase inhibitors
  • Hypothyroidism
  • Hypothermia
  • Hyperkalaemia
  • Obstructive sleep apnoea
  • Raised intracranial pressure

1st Degree SA Exit Block

First degree sinoatrial exit block is due to delayed transmission of an impulse from the sinoatrial node, and is not detectable on ECG.

2nd Degree SA Exit Block: Type I

Also known as Wenkebach sinoatrial exit block.
Type I second degree sinoatrial exit block occurs when there is progressively delayed SA transmission eventually resulting in a missed P wave due to failure of conduction to the atrial myocardium.
  • Look For

  • Progressively shortened PP intervals, followed by a pause.
  • 2nd Degree SA Exit Block: Type I
     

2nd Degree SA Exit Block: Type II

Type II second degree sinoatrial exit block occurs when there is an intermittently missed P wave due to failure of conduction to the atrial myocardium.
  • Look For

  • A constant PP interval, with a pause that is a multiple of the PP interval.
  • 2nd Degree SA Exit Block: Type II
     

3rd Degree SA Exit Block

In third degree sinoatrial exit block, there is complete failure of transmission to the atria - this is indistinguishable from sinus arrest on ECG.
  • Look For

  • Complete lack of P waves
  • An escape rhythm may be present
  • 3rd Degree SA Exit Block
     
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