Bradyarrhythmias
 

Sinoatrial Exit Block

 
 
Bookmark

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
       
 
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Want more info like this?
  • Your electronic clinical medicine handbook
  • Guides to help pass your exams
  • Tools every medical student needs
  • Quick diagrams to have the answers, fast
  • Quizzes to test your knowledge
Explore
   
 
 

Read More...

Adan V, Crown LA. Diagnosis and treatment of sick sinus syndrome. American family physician. 2003 Apr;67(8):1725-32.
 Mangrum JM, DiMarco JP. The evaluation and management of bradycardia. New England Journal of Medicine. 2000 Mar 9;342(10):703-9.
Feedback