Isorhythmic atrioventricular dissociation in an alert normothermic cat

An 11-year-old male neutered Birman cat was referred for investigation of pyelonephritis. On physical examination, gallop sounds and bradycardia were noted. There was mild azotaemia and marked neutrophilia, attributed to the pyelonephritis. Transthoracic echocardiography revealed a non-specific cardiomyopathy phenotype with biatrial enlargement. Electrocardiography showed wide QRS complexes, bradycardia and synchronous dissociation between atrial and ventricular activity. During the first examination, the synchronisation appeared mostly fixed, with consistent PR intervals; on subsequent evaluations, the PR intervals appeared to vary on a beat-to-beat basis. The diagnosis was of isorhythmic atrioventricular dissociation type I, with an initial long phase of fixed synchronisation, mimicking an isorhythmic atrioventricular dissociation type II, and concomitant ventricular depolarisation with a left bundle branch block morphology.