Atrial Standstill

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Introduction

Atrial standstill is the temporary or permanent lack of atrial activity resulting from a failure of atrial depolarisation. The ventricles still function normally.

Aetiology

The atrial muscle fails to depolarise, despite the production of an impulse from the sinoatrial node. So instead impulses pass from the sinoatrial node to the atrioventricular node by internodal pathways. This produces a sinoventricular rhythm. This can be caused by electrolyte abnormalities (especially hyperkalaemia - which can develop secondary to a number of conditions including Addison’s disease, oliguric renal failure and urethral obstruction), cardiomyopathies, muscular dystrophy (causing persistent atrial standstill - most commonly seen in the Springer Spaniel), and drug toxicity.

Clinical Signs

Heart sounds are normal on auscultation. As ventricular depolarisation occurs a normal pulse can be felt. It is common for the heart rate to be slow, at less than 60 beats a minute in small animals.

Diagnosis

Diagnosis can be confirmed by ECG or fluoroscopy. With persistent atrial standstill cases, the heart rate will not increase upon administration of atropine. Underlying causes should be investigated, electrolyte abnormalities in particular.

ECG

An ECG should show an absence of P waves. Heart rate is regular but normally slow, due to the presence of an escape rhythm. The QRS complexes are normal or slightly wide.

It is important to have an artefact-free ECG of diagnostic quality to confirm atrial standstill.

Treatment

Treat the underlying cause. If this fails to resolve the condition or persistent atrial standstill is diagnosed a permanent ventricular pacemaker should be implanted.


Atrial Standstill Learning Resources
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Feline Medicine Q&A 01
Small Animal Soft Tissue Surgery Q&A 08


References

Gilson, SD (1998) Self-Assessment Colour Review Small Animal Soft Tissue Surgery Manson

Martin, M (2002) ECG interpretation in small animals : 2. Abnormalities in the conduction system In Practice 2002 24: 194-20

Sparks, AH & Caney, SMA (2005) Self-Assessment Colour Review Feline Medicine Manson




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