ECG of the Month

Matthew Boothe Blue Pearl Veterinary Partners, 455 Abernathy Rd NE, Sandy Springs, GA 30328.

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Daniel K. Newhard Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.

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Randolph L. Winter Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.

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A 9-year-old 21.7-kg (47.7-lb) spayed female Rottweiler was evaluated because of progressive weakness, lethargy, and diarrhea. Signs of weakness became noticeable approximately 4 weeks prior to evaluation. The dog's weakness progressively worsened, and the diarrhea had been unsuccessfully treated several times during that period. Several days prior to evaluation, the dog developed ascites. The dog was not receiving any medications other than a flea, tick, and heartworm preventative. On physical examination, bradycardia was detected and prompted echocardiographic and ECG examinations to be performed.

ECG Interpretation

An initial lead II ECG tracing revealed an underlying sinus rhythm that was conducted with third-degree atrioventricular (AV) block (Figure 1). The atrial rate was 165 beats/min, whereas the ventricular rate was 32 beats/min. P waves were immediately followed by negative deflections, which were most consistent with atrial repolarization waves or Ta waves. P-wave duration was 50 to 60 milliseconds, and Ta-wave duration was 120 milliseconds. The QRS complexes were 80 to 100 milliseconds in duration and of left bundle block morphology, suggestive of ventricular escape complexes of right ventricular origin. A subsequent ECG tracing obtained several minutes after the initial ECG recording revealed a nonsustained accelerated idioventricular rhythm (AIVR) with a rate of approximately 95 beats/min (Figure 2). At the conclusion of these AIVR beats, a period of 2.9 seconds elapsed before the ventricular escape complexes reappeared, which was consistent with overdrive suppression of the ventricular escape focus by the AIVR focus.

Figure 1—
Figure 1—

Lead II ECG tracing obtained from a 9-year-old dog that was referred for evaluation of progressive weakness, lethargy, and diarrhea. The dog had also developed ascites. Notice the negative deflection (arrows) found immediately after each P wave. These are atrial repolarization waves, also known as Ta waves. The underlying sinus rhythm is conducted with third-degree atrioventricular block. Paper speed = 25 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 250, 12; 10.2460/javma.250.12.1377

Figure 2—
Figure 2—

Lead II ECG tracing obtained from the dog in Figure 1 several minutes after the initial recording. Notice the 2 QRS complex morphologies, both of which originate in the ventricular myocardium and are not initiated by P waves. Ventricular escape QRS complexes (white arrows) occur at a rate of 32 beats/min. The QRS complexes of an accelerated idioventricular rhythm (black arrows) occur at a rate of 95 beats/min. A pause of 2.9 seconds is present after the termination of the accelerated idioventricular rhythm before ventricular escape complexes resume, which is consistent with overdrive suppression. Paper speed = 25 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 250, 12; 10.2460/javma.250.12.1377

Echocardiography revealed a 4.2 × 3.9-cm heterogeneous solitary mass associated with the inter-ventricular septum, interatrial septum, and portions of the right atrial wall near the tricuspid valve apparatus. The echocardiographic findings were assessed as most consistent with a primary, infiltrative neoplastic lesion of unknown cell type.

Discussion

Ta waves are atrial repolarization waves and are not routinely apparent on standard ECG lead tracings. The atria repolarize during the period represented by the PQ segment and QRS complex, which is 1 reason that routine evaluation of Ta waves is difficult during normal sinus rhythm with appropriate AV nodal conduction. However, during instances of AV block, Ta waves may no longer be concealed within QRST complexes and are detectable on an ECG tracing.1–4 High-grade AV block as well as first-degree AV block with severely prolonged P-R intervals may reveal the Ta waves.5 Ta waves are most commonly observed in lead II tracings; almost exclusively, each has a direction opposite to the preceding P wave, indicating that the vector of atrial repolarization is opposite to the vector of atrial depolarization. The identification of a Ta wave has been used as an aid in the diagnosis of atrial enlargement or atrial pathological changes in dogs and humans2,4,6 as well as other cardiovascular diseases more prevalent in humans, such as myocardial infarction.2,7 Additionally, analysis of Ta waves may provide additional information regarding arrhythmogenesis within the atrial myocardium.1,4

A recent study8 evaluated the features of Ta waves in 36 dogs that had third-degree AV block without atrial pathological changes or enlargement. The Ta waves were most often visible in the lead II tracings (32/36 [88.9%] dogs), as was the case for the dog of the present report. The mean ± SD Ta-wave duration in the lead II tracings of the 36 dogs was 140.17 ± 35.19 milliseconds. The dog of the present report had a Ta-wave duration of 120 milliseconds. Similar to findings of other studies,3,4 each Ta wave was found to be longer in duration but smaller in amplitude than the preceding P wave. This relationship between the duration and amplitude of the Ta and P waves was also evident in the ECG tracings for the dog of the present report. In the earlier study,8 the 36 dogs had no structural heart disease, but the dog of the present report had similar Ta-wave characteristics and measurements. Therefore, it is important to be aware that although dogs may have Ta waves with characteristics that are within reference ranges,8 echocardiography is still warranted to rule out atrial, AV nodal, or ventricular structural disease as a cause for the ECG abnormalities.

The large, infiltrative mass identified echocardiographically in the dog of the present report was likely the cause for the ECG changes detected. Specifically, the infiltrative mass likely caused or contributed to AV block as well as the generation of an AIVR through ventricular myocardial involvement. An AIVR is an ectopic rhythm originating in the ventricular myocardium with 3 or more consecutive ventricular beats, which occur at a rate faster than those of ventricular escape rhythms (30 to 40 beats/min) but slower than those of ventricular tachycardias (180 to 200 beats/min).9 Although a definitive link between Ta waves and structural disease has yet to be determined in dogs, atrial pathological changes leading to prominent Ta waves in humans have been reported.2 Such a relationship in dogs has been suggested.4,6 Additionally, some studies10–12 have identified atrial pathological changes in dogs with third-degree AV block. The presence of the Ta waves in the dog of the present report may be considered related to the infiltrative mass in either a primary (ie, atrial pathological changes) or secondary (ie, third-degree AV block) manner.

For the dog of the present report, a diagnosis of third-degree AV block with concurrent episodes of AIVR was made. The severity of AV block can vary in some dogs, and there are reports13,14 of AV conduction acutely worsening. A differential diagnosis for the nonsustained period of relative tachycardia in this dog would be acute improvement in AV conduction (ie, the sinus rhythm was conducted with variable AV nodal conduction and concurrent bundle branch block). The diagnosis of AIVR was considered most likely on the basis of the wide QRS complexes of similar morphology that occurred regularly and the extreme variation in the interval between P waves and onset of QRS complexes. An atropine response test can be used to investigate the relationship between P waves and QRS complexes more fully in some dogs. However, because of the presumed diagnosis of an AIVR in the dog of the present report, an atropine response test was not performed to avoid potential worsening of the AIVR (ie, increase in ventricular ectopic rate). Additionally, there was concern regarding what effect an increased heart rate might have on the dog's cardiac output given the space-occupying nature of the right atrial mass.

Abdominocentesis was performed after echocardiography, and the fluid obtained was consistent with a modified transudate. The cause of the abdominal effusion was assessed to be physical obstruction of the caudal vena cava return by the mass, as well as poor cardiac output associated with the third-degree AV block. Pacemaker implantation and endomyocardial biopsy were offered to the owner, but these were declined. The owner took the dog home to be euthanized by the referring veterinarian the following morning, but the dog died at home a few hours after discharge from the hospital. A necropsy was not performed.

References

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