ECG of the Month

Chloë L. Thorn Cardiology Service, Ryan Veterinary Hospital, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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Mark A. Oyama Cardiology Service, Ryan Veterinary Hospital, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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A 10-year-old 3.4-kg (7.48-lb) spayed female domestic shorthair cat was evaluated at the University of Pennsylvania Ryan Veterinary Hospital because of decreased appetite of 1 week's duration and vomiting after each meal of 3 days’ duration. The cat was otherwise well with a normal activity level. Results of clinicopathologic analyses (CBC, serum biochemical analyses, and assessment of serum total thyroxine concentration) provided by the referring veterinarian were all within reference ranges. One day of empirical treatment with subcutaneous fluids, an appetite stimulant, and a gastroprotectant did not improve the clinical signs, and the cat was referred for further diagnostic testing.

On initial examination, the cat was bright and alert. It appeared mildly dehydrated. The cat had a regularly irregular heart rhythm and no murmur; heart rate was 120 beats/min. The examination findings were otherwise unremarkable.

Abdominal ultrasonography revealed diffusely thickened intestinal walls with echogenic mucosa (jejunal wall thickness, ≤ 2.8 mm; ileal wall thickness, ≤ 3.9 mm) and a diffusely enlarged and thickened spleen with a coarse echotexture and several ill-defined, round, hypoechoic nodules (≤ 3.1 mm in diameter). These findings were suggestive of lymphoma of the intestines and spleen, but inflammatory bowel disease and benign changes within the spleen could not entirely be ruled out. Echocardiography was performed and revealed normal cardiac structure and function. Electrocardiography was performed.

ECG Interpretation

An initial 6-lead ECG recording revealed a mean heart rate of 140 beats/min with a regularly irregular rhythm. Heart rate varied from 100 to 158 beats/min (Figure 1), and findings were most consistent with sinus arrhythmia. The morphology of QRS complexes changed during periods of slow heart rate, and the difference was most notable in leads II, III, and aVL. There was slight variation in R-R intervals of the longest cycle durations throughout the recording. Another recording was obtained after a stressful event (ie, a dog entered the room where the cat was being evaluated; Figure 2). In this recording, the mean heart rate increased to 180 beats/min; the rhythm became more regular and was interpreted as sinus rhythm. During the period of sinus rhythm, the QRS complex morphology was uniform.

Figure 1—
Figure 1—

Initial 6-lead ECG recording obtained from a 10-year-old spayed female domestic shorthair cat that was evaluated because of decreased appetite (1 week's duration) and postprandial vomiting (3 days’ duration). A regularly irregular rhythm consistent with sinus arrhythmia is present. The morphology of the QRS complexes is variable particularly after pauses (arrows), and the variability is most notable in leads II, III, and aVL. Paper speed = 25 mm/s; 2 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 247, 11; 10.2460/javma.247.11.1244

Figure 2—
Figure 2—

Another 6-lead ECG recording obtained from the cat in Figure 1 after a stressful event. Sinus rhythm with a heart rate of 180 beats/min is present. The rhythm is more regular, and the variation in QRS complex morphology is no longer present, supporting the conclusion of sinus arrhythmia and aberrant ventricular conduction secondary to high vagal tone. Paper speed = 25 mm/s; 2 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 247, 11; 10.2460/javma.247.11.1244

Discussion

For the cat of the present report, the identified arrhythmia was suspected to be an exaggerated sinus arrhythmia attributable to high vagal tone. Severe abdominal disease may have been responsible for its development. Sinus arrhythmia is a regularly irregular rhythm with cyclic changes in heart rate and is a common finding in healthy dogs.1 Sinus arrhythmia is uncommon and usually an abnormal finding in hospitalized cats, although studies1,2 have indicated that healthy cats in their home environment commonly have periods of sinus arrhythmia and sinus arrhythmia during anesthesia has been reported.3 Pathological causes for high vagal tone in dogs and cats include diseases of the gastrointestinal tract, respiratory system, and CNS.4,5

The changes in QRS complex morphology during periods of slower heart rates in the cat of this report were suspected to be due to aberrant ventricular conduction. A complete bundle branch block was not present because QRS complex duration remained normal. Aberrant ventricular conduction is defined as a transient change in conduction through the ventricles.6 Several mechanisms have been implicated in aberrant ventricular conduction, including phase 3 aberration, retrograde concealed conduction, and phase 4 aberration.6 Phase 4 aberration occurs with bradycardia and is the suspected mechanism for the ECG findings in the case described in the present report. After prolonged diastole, latent pacemakers within the His-Purkinje system begin to spontaneously depolarize toward threshold potential. When the late sinus beat finally reaches the His-Purkinje system, some fibers may not be sufficiently negatively charged to propagate the impulse; consequently, conduction through the ventricle may be altered. If phase 4 aberration occurred solely as a result of bradycardia, one would expect it to be more common in small animal patients. Phase 4 aberration is uncommon, and most reported cases involve underlying cardiac disease.6 The cat of the present report had apparently normal echocardiographic findings, but disease within the conduction system cannot be ruled out and may have had a role in conjunction with abnormal vagal tone associated with gastrointestinal tract disease. Lymphoma of the intestines and spleen in this patient was suspected, and early infiltrative cardiac disease could have been present despite the lack of echocardiographic evidence. Bradycardia and many different conduction abnormalities in both humans and other animals with cardiac lymphoma have been reported.7,8

Another consideration for the variable QRS complex morphology in the cat of the present report was the Brody effect. Prolonged diastole results in an extended period of ventricular filling and a greater-thannormal volume of blood within the heart.9,10 Given that blood has even greater electrical conductivity than the ventricular myocardium, a QRS complex of greaterthan-normal amplitude is evident in a surface ECG lead

II tracing. Although the Brody effect could explain the QRS complex amplitude changes in the lead II tracing associated with increasing cycle duration in this cat and may have had some role, it cannot explain the other morphological changes of the QRS complexes in leads III and aVL.

A final consideration for the variable QRS complex morphology identified in the cat of the present report was sinus pauses with fusion escape complexes. Such complexes were considered unlikely because the P-R interval was consistent, and the R-R interval of the pauses was not exactly consistent throughout; escape foci typically occur with a very regular interval.1

For the cat of the present report, the owner declined further diagnostic testing including microscopic examination of splenic aspirate and intestinal biopsy specimens. The cat was discharged from the hospital with supportive therapy including maropitant, mirtazapine, metronidazole, and omeprazole.

References

  • 1. Tilly LP, Miller MS, Smith FWK. Canine and feline cardiac arrhythmias. 3rd ed. Philadelphia: Lea & Febiger, 1993; 133137.

  • 2. Hanås S, Tidholm A, Egenvall A, et al. Twenty-four hour Holter monitoring of unsedated healthy cats in the home environment. J Vet Cardiol 2009; 11: 1722.

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  • 3. Lewis KA, Scansen BA, Aarnes TK. ECG of the month. Respiratory sinus arrhythmia in an anesthetized cat. J Am Vet Med Assoc 2013; 242: 623625.

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  • 4. Kittleson MD, Kienle RD. Small animal cardiovascular medicine. St Louis: Mosby, 1998;449524.

  • 5. Moïse S. Diagnosis and management of canine arrhythmias. In: Fox P, Sisson D, Moïse S, eds. Textbook of canine and feline cardiology. 2nd ed. Philadelphia: Saunders, 1999; 9293, 371385.

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  • 6. Marriott HJL, Conover MB. Advanced concepts in arrhythmias. 3rd ed. St Louis: Mosby, 1998; 215220.

  • 7. Stern JA, Tobias JR, Keene BW. Complete atrioventricular block secondary to cardiac lymphoma in a dog. J Vet Cardiol 2012; 14: 537539.

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  • 8. Miguel CE, Bestetti RB. Primary cardiac lymphoma. Int J Cardiol 2011; 149: 358363.

  • 9. Brody DA. A theoretical analysis of intracavitary blood mass influence on the heart-lead relationship. Circ Res 1956; 4: 731738.

  • 10. Amoore JN. The Brody effect and change of volume of the heart. J Electrocardiol 1985; 18: 7175.

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