Sinoatrial Exit Block
On this page:1st Degree SA Exit Block2nd Degree SA Exit Block: Type I2nd Degree SA Exit Block: Type II3rd Degree SA Exit Block
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 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.
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
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