A 2-year-old 30-kg (66-lb) sexually intact male Golden Retriever was referred to the Clinica Veterinaria Malpensa because of exercise intolerance and episodic weakness of 1 month' s duration. During 1 episode of weakness, an ECG examination performed by the primary veterinarian revealed a narrow-QRS complex tachycardia with a cycle length of 200 milliseconds. The patient was treated with sotalol a (2 mg/kg [0.9 mg/lb], PO, q 12 h) to control episodic supraventricular tachycardia and to reduce clinical signs.At the initial evaluation, a physical examination revealed no abnormalities. Findings of survey thoracic radiography and transthoracic echocardiography were considered normal. An initial 12-lead ECG revealed short PQ intervals, slurring of the onset of the QRS complexes (delta wave), and alteration of the QRS-ST segment morphology. These findings were consistent with ventricular pre-excitation.The presence of an accessory pathway (AP)-mediated tachycardia was strongly suspected, and an electrophysiological assessment was performed. Sotalol administration was discontinued for 24 hours (ie, a period equivalent to 5 half-lives of the drug) before the procedure. The patient was anesthetized and placed in dorsal recumbency during the entire interventional procedure. Multiple access sites on jugular and femoral veins were used to allow introduction of endocardial catheters by use of the Seldinger technique. The electrophysiological assessment was conducted in accordance with standard techniques. 1,2 A decapolar catheter b was introduced at the level of the coronary sinus and positioned in contact with the epicardal surface, and a quadripolar catheter c was inserted near the bundle of His. An ablation catheter d was used to perform programmed atrial and ven- Figure 1-Surface ECG tracings (6 leads of a 12-lead ECG) obtained during electrophysiological assessment of a dog that was evaluated because of exercise intolerance and episodic weakness of 1 month's duration. On the basis of initial ECG findings, an accessory pathway (AP) was suspected. The electrophysiological examination revealed the presence of an AP that was subsequently ablated by use of a radiofrequency catheter. During the procedure and before the ablation, the dog developed a paroxysm of sustained atrial fibrillation (AF). The first 6 beats of the ECG tracings have features typical of AF with orthodromic conduction through the His-Purkinje system, including absence of the P wave, narrow QRS complex (duration, 60 milliseconds; reference range, < 70 milliseconds), and irregular R-R intervals. From the seventh beat onward, antegrade activation of the AP with signs of ventricular pre-excitation is evident. Wide QRS complexes (duration, 100 milliseconds), the presence of a delta wave preceding each R wave, and persistence of irregular R-R intervals are indicative of the occurrence of pre-excited AF. Paper speed = 50 mm/s; 1 cm = 1 mV.

A 14-year-old spayed female mixed-breed dog was referred to the Clinica Veterinaria Malpensa because of frequent episodes of syncope (frequency, 1 to 5 episodes/d). At the initial evaluation, the dog appeared mildly depressed. The femoral pulse was bradyarrhythmic with a mean heart rate of 40 beats/min. Results of auscultation of the heart and lungs were considered normal, and no other clinical abnormalities were detected. Thoracic radiographic and echocardiographic findings were also considered normal. Twelve-lead surface ECG (6 peripheral standard leads and 6 precordial leads as previously described 1 ; Figure 1) was performed with the dog placed in right lateral recumbency. ECG InterpretationThe 12-lead surface ECG tracing ( Figure 1) revealed a severe bradyarrhythmia with a mean ventricular rate of 40 beats/min. The P-wave duration was 40 milliseconds (reference range, < 40 milliseconds) and amplitude was 0.4 mV (reference range, < 0.4 mV), with a sinus rate of 140 beats/ min and a normal P-wave axis on the frontal plane (+80°; reference range, -18° to +90°). An atrioventricular (AV) conduction disturbance with lack of concordance between atrial and ventricular depolarization rate was evident. Only a few P waves were followed by ventricular QRS complexes with an AV conduction ratio of 2:1 (ie, 2 P waves [1 unconducted and 1 conducted] for 1 QRS complex) that sometimes evolved into advanced (or complete) AV block (AV conduction ratios of 3:1 and 4:1). The PQ interval for the conducted beats was 120 milliseconds (reference range, 60 to 130 milliseconds).These features were consistent with a second-degree (2:1 type) AV block with episodes of advanced AV block. Moreover, intraventricular conduction disturbance was evident because of a prolongation of the QRS complexes (duration, 80 milliseconds; reference range, < 70 milliseconds) and tall, slurred, and delayed R waves in the inferior leads (II, III, and aVF) and left precordial lead tracings. The ratecorrected QT interval (QTc) was slightly prolonged (duration, 249 milliseconds; reference range, 150 to 240 milliseconds). The mean electrical axis of the QRS complex was +82°, which was within reference limits (+40° to +100°);