Electrocardiographic findings in 45 cats with hyperthyroidism.

Electrocardiographic (ECG) abnormalities were recorded in 36 (80%) of 45 cats with untreated hyperthyroidism caused by hyperfunctioning thyroid adenomas (adenomatous hyperplasia). Tachycardia (greater than or equal to 240/min) and increased R-wave amplitude in lead II (greater than or equal to 0.9 mV) were the most frequent abnormalities recorded (62% and 49%, respectively). Other abnormalities included atrial and ventricular arrhythmias (20%), prolonged QRS duration (16%), shortened Q-T interval (11%), intraventricular conduction disturbances (3%), and ventricular pre-excitation (1%). In 17 cats, repeat ECG were recorded 6 months after hemi- or total thyroidectomy; resolution of tachycardia, increased R-wave amplitudes, shortened Q-T intervals, and atrial and ventricular arrhythmias had occurred in all cats. It was concluded that many of the ECG changes associated with feline hyperthyroidism are similar to those associated with primary myocardial disease in cats; however, the ECG abnormalities and associated cardiovascular signs of hyperthyroidism generally resolve after successful treatment of the hyperthyroid state.