Third-degree atrioventricular block with narrow QRS complexes in a case of hypoadrenocorticism in a dog

A 4-year-old dog presented with lethargy and bradycardia (heart rate 40 bpm). Electrocardiogram diagnosed third-degree atrioventricular block with narrow QRS complexes. An atropine response test did not result in a change of the heart rate. Venous blood gas documented moderate hyperkalaemia and an adrenocorticotrophic hormone stimulation test was consistent with hypoadrenocorticism. The patient repeatedly converted to sinus rhythm with normalisation of serum potassium levels following medical treatment. This is the first report of third-degree atrioventricular block in a patient with hypoadrenocorticism that was not vagally mediated and did not require pacemaker implantation, with conversion to sinus rhythm following treatment of the hyperkalaemia.