Atrio-Ventricular Block

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Introduction

Atrio-ventricular block is the delay or failure of the atrioventricular node to conduct impulses.

There are three types of AV block:

1st Degree (Partial AV Block) - Delayed conduction through the AV node. Causes include electrolyte imbalance, drug reactions and degenerative changes in the AV node. It may be a permanent or temporary condition.

2nd Degree (Partial AV Block) - Intermittent failure of conduction through the AV node.

Mobitz Type I. = prolonged P-R interval
Mobitz Type II. = no prolongation of the P-R interval.
There are also sub types with Type A: normal QRS length; block is above the bundle of His division (Most commonly seen with Mobitz Type I.)
Type B: longer QRS length; block is below the bundle of His division (Most commonly seen with Mobitz Type II.).
Causes include electrolyte imbalance, drug reactions and degenerative changes in the AV node. The condition may be temporary or permanent.

3rd Degree (Total AV Block) - Total failure of AV conduction causing uncoordinated and independent atrial and ventricular depolarisations. Causes include electrolyte disturbances, drug toxicities (e.g. digitalis), acquired nodal diseases, cardiomyopathies, hypothyroidism, endocarditis, Lyme Disease etc. This is a permanent life threatening condition.


Clinical Signs

Depending on which type of block and severity, clinical signs may vary. Signs may include weakness, syncope, exercise intolerance and pallor.


Diagnosis

Clinical signs plus auscultation of the heart will be indicative of the condition.

An ECG will show characteristic results which will confirm the diagnosis. In 1st degree AV block there will be a longer P-R interval on ECG. In 2nd degree AV block some P waves are not followed by QRS complexes. In 3rd degree AV block there will be independent regular and fast P waves or independent regular and slow QRS waves.


Treatment and Control

1st degree AV block - treat the underlying cause.

2nd degree Av block - Mobitz Type I.: Treat underlying cause if necessary. Mobitz Type II. Treat with atropine etc. or surgical implantation of a pacemaker.

3rd degree AV block needs surgical intervention and the implantation of a cardiac pacemaker.


References

Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition) W.B. Saunders Company

Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2)W.B. Saunders Company

Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition) Mosby Elsevier




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