Atrial standstill is the temporary or permanent lack of atrial activity resulting from a failure of atrial depolarisation. The ventricles still function normally.
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.
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 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.
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.
Treat the underlying cause. If this fails to resolve the condition or persistent atrial standstill is diagnosed a permanent ventricular pacemaker should be implanted.
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