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Cardiac Dysrhythmias in Cats- Part 2



 

Arrhythmias associated with cardiac disease Figure 12

Atrial fibrillation (AF) occurs because of atrial stretch (Figure 12). Whether atrial fibrillation is able to develop and become sustained depends on atrial mass, so cats, with small body size, are much less likely to develop AF than larger animals and therefore the presence of AF normally indicates advanced heart disease and severe atrial dilation (14). It is therefore considered to be a poor prognostic indicator in this species. In humans, AF is associated with risk of thrombus formation and thromboembolic stroke. In cats, atrial dilatation and spontaneous echo contrast (“smoke”) has been associated with arterial thromboembolism and it is likely that concurrent atrial fibrillation exacerbates this risk.

 

Supraventricular (atrial) premature complexes may occur with less severe atrial stretch (Figure 13), but their presence may herald the future development of AF.

 

Ventricular premature complexes (VPCs) or ventricular tachycardia can occur in cardiac disease for various reasons, and most cats with ventricular tachyarrhythmias have cardiac disease (15). Increased left ventricular wall stress can occur in both acquired (Figure 14) and congenital heart disease (Figure 15). With hypertrophy, myocardial hypoxia may occur giving the potential for arrhythmias. Arteriosclerosis is known to occur in HCM leading to myocardial infarction or ischemia (12).

 Figure 13 and 14

The presence of arrhythmias or a history of syncope are negative prognostic factors in feline HCM (16). Finally, the progression of heart disease can result in histological changes of the myocardium, such as myocardial fibrosis. In arrhythmogenic right ventricular cardiomyopathy (ARVC), there is a marked fat or fibrofatty infiltrate into the myocardium; a change of substrate resulting in malignant ventricular  arrhythmias (17).

 

Some cats with cardiac disease, particularly cardiomyopathies, may show bradyarrhythmias. Although not clinically relevant on its own, an ECG diagnosis of 1st degree atrioventricular (AV) block in myocardial disease is not uncommon. Second and third degree AV blocks are often not clinically significant as cats have a high escape rate (e.g. 100-120 bpm) and compensate for any bradyarrhythmia (18). Furthermore, they may be transient arrhythmias, and recognition may depend on device monitoring (7). AV blocks may be associated with HCM if there is gross fibrosis around the AV node and conduction system (19) (Figure 16). Note that many cats with symptomatic third degree AV blocks may present with seizures rather than a classic flaccid syncope (20). AV blocks may also be identified in the absence of any other recognized structural or functional heart disease, i.e. idiopathic cases.


Figure 15 and 16


Possible causes of arrhythmias

It is important to remember that arrhythmias may result from other systemic conditions. Potential causes of tachyarrhythmias, especially ventricular arrhythmias include myocardial contusions associated with trauma, hyperthyroidism, causes of systemic hypertension, and myocarditis (e.g. Toxoplasma, Bartonella). Many cats with systemic conditions may show bradyarrhythmias (Figure 1).

 Figure 1

An important cause of a bradyarrhythmia, which is vital to recognize promptly, is hyperkalemia. On ECG, there may be no evidence of atrial activity (atrial standstill; no visible P waves) and tall spiky T waves. QRS complexes may become wide. Hyperkalemia can be a consequence of urinary tract obstruction or anuric acute renal failure. Sepsis, hypoglycemia and intoxications may also result in bradycardia or bradyarrhythmias.

 

It is unusual for a cat to show sinus arrhythmia in a consulting room situation or during an ECG recording (Figure 17). If sinus arrhythmia is detected, it is important to exclude causes of high vagal tone, such as feline asthma and other thoracic, abdominal or ocular disease.

 

If arrhythmias are suspected to be due to systemic diseases, other additional investigations warranted include total T4 (in cats > 7 years of age), blood pressure measurement, routine hematology and biochemistry to check for other dysfunction, and abdominal ultrasound.

 

Figure 17 and 18

 

Treatment of arrhythmias

In cats, arrhythmias are most commonly secondary to either cardiac or systemic disease. The most important treatment of the arrhythmia is therefore to recognize and treat the underlying condition; optimize CHF treatment in a case with known cardiac disease; and correct the systemic or metabolic disturbance.

 

In atrial fibrillation, assess the ventricular rate once treatment of CHF has been optimized. Once atrial fibrillation has been established, it is very unlikely that conversion to sinus rhythm will be achievable, so controlling the ventricular response and overall heart rate is the treatment goal. In cats, unlike dogs, once the underlying disease and heart failure have been treated, additional drugs to slow the ventricular rate may not be required, but options are detailed in Table 1.

 

If the arrhythmia is not hemodynamically significant (e.g. a cat with occasional single, uniform VPCs with HCM but no exercise intolerance or collapse), no further action may be required other than treating the CHF. It should be appreciated that antiarrhythmic drugs are not benign, and they have the potential, difficult to predict in an individual animal, to be pro-arrhythmic. Some anti-arrhythmic drugs which the author has used for feline arrhythmias are listed in Table 1.

 

For cats which are symptomatic due to bradyarrhythmias such as 3rd degree AV block, the treatment of choice is a pacemaker. Transvenous leads (via the jugular) are usually avoided in cats, because of the risk of cranial vena cava obstruction and chylothorax. Leads are therefore placed epicardially, with the pulse generator in the abdominal wall musculature and the lead passed transdiaphragmatically and attached to the epicardium through a pericardial incision (Figure 18). The beta-2 agonist terbutaline can be tried which may have a positive chronotropic and dromotropic effect for symptomatic bradyarrythmias.


Table 1


Conclusion

Dysrhythmias in cats normally indicate the presence of cardiac disease although systemic conditions may also result in tachyarrhythmias or bradyarrhythmias (most importantly, hyperkalemia). Cats with significant myocardial disease may have no arrhythmia or other auscultatory abnormality on clinical examination so other investigations are warranted to identify cardiac disease and any arrhythmia. Myocardial infarction is likely an under-recognized consequence of feline myocardial disease. Hemodynamically significant arrhythmias which are transient may result in episodic seizure-like episodes rather than syncope.

 

CLICK HERE for Part 1 covering examination and diagnosis!

 

This article was kindly provided by Royal Canin, makers of a range of veterinary diets for dogs and cats. For the full range please visit www.RoyalCanin.co.uk or speak to your Veterinary Business Manager:

 

 


REFERENCES

1. Hanas S, Tidholm A, Egenvall A, et al. 24 hour Holter monitoring of unsedated healthy cats in the home environment. J Vet Cardiol 2009;11:17-22.

2. Paige CF, Abbott JA, Elvinger F, et al. Prevalence of cardiomyopathy in apparently healthy cats. J Am Vet Med Assoc 2009;234:1398-1403.

3. Ruthrauff CM, Smith J, Glerum L. Primary bacterial septic peritonitis in cats: 13 cases. J Am Anim Hosp Assoc 2009;45:268-276.

4. Little CJ. Hypoglycaemic bradycardia and circulatory collapse in a dog and a cat. J Small Anim Pract 2005;46:445-448.

5. Cote E. Feline arrhythmias: an update. Vet Clin North Am Small Anim Pract;40:643-650.

6. Harvey AM, Faena M, Darke PG, et al. Effect of body position on feline electrocardiographic recordings. J Vet Intern Med 2005;19:533-536.

7. Ferasin L, van de Stad M, Rudorf H, et al. Syncope associated with paroxysmal atrioventricular block and ventricular standstill in a cat. J Small Anim Pract 2002;43:124-128.

8. Bright JM, Cali JV. Clinical usefulness of cardiac event recording in dogs and cats examined because of syncope, episodic collapse, or intermittent weakness: 60 cases (1997-1999). J Am Vet Med Assoc 2000;216:1110-1114.

9. Ferasin L. Recurrent syncope associated with paroxysmal supraventricular tachycardia in a Devon Rex cat diagnosed by implantable loop recorder. J Fel Med Surg 2009;11:149-152.

10. Willis R, McLeod K, Cusack J, et al. Use of an implantable loop recorder to investigate syncope in a cat. J Small Anim Pract 2003;44:181-183.

11. Schober KE, Fuentes VL, Bonagura JD. Comparison between invasive hemodynamic measurements and non-invasive assessment of left ventricular diastolic function by use of Doppler echocardiography in healthy anesthetized cats. Am J Vet Res 2003;64:93-103.

12. Fox PR, Liu SK, Maron BJ. Echocardiographic assessment of spontaneously occurring feline hypertrophic cardiomyopathy; an animal model of human disease. Circulation 1995;92:2645-2651.

13. Fox PR, Rush JE, Reynolds CA, et al. Multicenter evaluation of plasma N-terminal pro-brain natriuretic peptide (NT-Pro BNP) as a biochemical screening test for asymptomatic (occult) cardiomyopathy in cats. J Vet Int Med 2012;25:1010-1016.

14. Cote E, Harpster NK, Laste NJ, et al. Atrial fibrillation in cats: 50 cases (1979-2002). J Am Vet Med Assoc 2004;225:256-260.

15. Cote E, Jaeger R. Ventricular tachyarrhythmias in 106 cats: associated structural cardiac disorders. J Vet Int Med 2008;22:1444-1446.

16. Rush JE, Freeman LM, Fenollosa NK, et al. Population and survival characteristics of cats with hypertrophic cardiomyopathy: 260 cases (1990-1999). J Am Vet Med Assoc 2002;220:202-207.

17. Fox PR, Maron BJ, Basso C, et al. Spontaneously occurring arrhythmogenic right ventricular cardiomyopathy in the domestic cat: a new animal model similar to the human disease. Circulation 2000;102:1863-1870.

18. Kellum HB, Stepien RL. Third-degree atrioventricular block in 21 cats (1997-2004). J Vet Int Med 2006;20:97-103.

19. 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.

20. Penning VA, Connolly DJ, Gajanayake I, et al. Seizure-like episodes in 3 cats with intermittent high-grade atrioventricular dysfunction. J Vet Int Med 2009;23:200-205.

This article was previously published on VetGrad.co.uk in 2012.


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