A 14-year-old 2.7-kg (5.94-lb) spayed female domestic shorthair cat was examined at The Ohio State University Veterinary Medical Center because of weight loss of 6 months’ duration and decreased appetite of 3 days’ duration. The cat was not receiving any medication at the time of the evaluation. The cat's medical history included a cystotomy for removal of uroliths performed 4 years prior to the evaluation. Pertinent physical examination abnormalities were a palpable thyroid gland nodule and abnormal cardiac auscultation characterized by an irregular rhythm and a bradycardia of 130 beats/min. The remainder of the physical examination findings were unremarkable.
A CBC, serum biochemical analysis, and urinalysis were performed and revealed no clinically notable abnormalities. Serum thyroxine (total T4) concentration was markedly high (10.5 μg/dL; reference interval, 1.0 to 3.0 μg/dL), consistent with hyperthyroidism. Indirect systolic arterial blood pressure (measured by means of a Doppler ultrasonographic method) was 130 mm Hg. Echocardiography was performed, and the heart appeared unremarkable with the exception of mild, 4-chamber dilatation. A standard 6-lead ECG was acquired to characterize the cat's irregular rhythm and bradycardia (Figure 1).
1. Schamroth L. Escape-capture bigeminy. In: Schamroth L, ed. The disorders of cardiac rhythm. 2nd ed. Oxford, England: Blackwell, 1980;329–331.
2. Schamroth L, Dubb A. Escape-capture bigeminy. Mechanisms in S-A block, A-V block, and reversed reciprocal rhythm. Br Heart J 1965; 27: 667–669.
3. Bradley SM, Marriott HJ. Escape-capture bigeminy; report of a case of A-V dissociation initiated by 2:1 S-A block with resulting bigeminal rhythm. Am J Cardiol 1958; 1: 640–643.
4. Henry SA, Makaryus AN, Loewinger L, et al. The great escape: junctional escape-capture bigeminy. Am J Geriatr Cardiol 2006; 15: 377–378.
5. Olgin J, Zipes DP. Specific arrhythmias: diagnosis and treatment. In: Bonow RO, Mann DL, Zipes DP, et al, eds. Braunwald's heart disease: a textbook of cardiovascular medicine. 9th ed. Philadelphia: Elsevier Saunders, 2012;771–824.
6. Fife WD, Cote E. ECG of the month. Second-degree atrioventricular (AV) block, type II advanced (3:1), with ventricular escape complexes. J Am Vet Med Assoc 2002; 220: 172–173.
7. Tilley LP. Uncommon complex arrhythmias. In: Tilley LP, ed. Essentials of canine and feline electrocardiography: interpretation and treatment. 3rd ed. Philadelphia: Lea & Febiger, 1992;385–416.
10. Peterson ME, Keene B, Ferguson DC, et al. Electrocardiographic findings in 45 cats with hyperthyroidism. J Am Vet Med Assoc 1982; 180: 934–937.
11. Sriussadaporn S, Vannasaeng S, Trisukosol D, et al. Complete heart block complicating hyperthyroidism: a case report. J Med Assoc Thai 1990; 73: 53–57.
12. Miller RH, Corcoran FH, Baker WP. Second and third degree atrioventricular block with Graves' disease: a case report and review of the literature. Pacing Clin Electrophysiol 1980; 3: 702–711.
13. Topaloglu S, Topaloglu OY, Ozdemir O, et al. Hyperthyroidism and complete atrioventricular block—a report of 2 cases with electrophysiologic assessment. Angiology 2005; 56: 217–220.
14. Atri SK, Chugh SN, Goya S, et al. Reversible atrioventricular blocks in thyroid storm. J Assoc Physicians India 2011; 59: 178–179.
15. Silva JE, Bianco SD. Thyroid-adrenergic interactions: physiological and clinical implications. Thyroid 2008; 18: 157–165.
16. Tilley LP. Analysis of common feline cardiac arrhythmias. In: Tilley LP, ed. Essentials of canine and feline electrocardiography: interpretation and treatment. 3rd ed. Philadelphia: Lea & Febiger, 1992;208–252.
17. Kaneshige T, Machida N, Itoh H, et al. The anatomical basis of complete atrioventricular block in cats with hypertrophic cardiomyopathy. J Comp Pathol 2006; 135: 25–31.
18. Liu SK, Tilley LP, Tashjian RJ. Lesions of the conduction system in the cat with cardiomyopathy. Recent Adv Stud Cardiac Struct Metab 1975; 10: 681–693.