ECG of the Month

Abstract

The 6-lead ECG tracing revealed that the underlying rhythm was regularly irregular (Figure 1). Initially, there were periods of tachycardia with a heart rate of approximately 180 beats/min; heart rate then slowed to approximately 60 beats/min with evidence of sinus arrest. The P-wave morphology varied, suggestive of a wandering pacemaker. There was no evidence of any APCs on the ECG tracings obtained at the referral hos-pital. Echocardiography was performed, which identified a 2.23 X 1.27-cm hypoechoic solitary mass associated with the free wall of the right atrium and involving the tricuspid valve apparatus. In addition, there was hyperechoic material oscillating between the right atrium and right ventricle that did not appear attached to the mass. The echocardiogram was assessed as most consistent with a right atrial hemangiosarcoma and intracardiac thrombus. Owing to the suspected intracardiac thrombus, an atropine response test was not performed. DiscussionA wandering pacemaker is defined as variability in morphology and amplitude of the P waves over time. 1,2 This is a common ECG finding in dogs and results from a shift in the site of depolarization from the sinus node along the terminal crest of the right atrium toward or occasionally to the atrioventricular node. 2 This causes alteration of the depolarization pathways through the atria and resultant changes in P-wave morphology. 2 A wandering pacemaker is often evident with respiratory sinus arrhythmia, and both phenomena are associated with an increase in vagal tone. 3 Atrial premature contractions are premature supraventricular impulses initiated from sites outside the sinus node. 1,2 In dogs, APCs usually occur secondary to atrial enlargement from myxomatous mitral valve disease or dilated cardiomyopathy. 1,2 In rare instances, APCs can occur secondary to myocardial damage induced by a primary cardiac neoplasm, as was suspected in the dog of the present report. Figure 1-Six-lead ECG tracing obtained from a dog that was referred for evaluation because of an arrhythmia recently identified prior to a left cranial cruciate ligament rupture repair. The dog had no known clinical signs of cardiac abnormalities. Notice the mild variation in P-wave morphology and the accelerating and decelerating heart rate suggestive of a sinus arrhythmia. The second pause exceeds 2 R-R intervals combined, which is consistent with a period of sinus arrest. Paper speed = 50 mm/s; 1 cm = 1 mV.