Feline Cardiac Arrhythmias Study Guide

Overview and Clinical Importance

Cardiac arrhythmias in cats represent a critical diagnostic and therapeutic challenge in veterinary cardiology. Unlike dogs, cats have a predominantly sympathetically-driven heart rate, making sinus arrhythmia an uncommon and often pathological finding. Feline arrhythmias are frequently associated with underlying hypertrophic cardiomyopathy (HCM), the most common acquired heart disease in cats, affecting approximately 10-15% of the feline population.

Recognition of arrhythmias is essential for the NAVLE, as these conditions can lead to sudden cardiac death, arterial thromboembolism (ATE), and congestive heart failure (CHF). The ECG remains the gold standard for arrhythmia diagnosis, though interpretation in cats requires understanding of species-specific peculiarities, including small QRS amplitude and wide variability in mean electrical axis.

Normal Feline ECG Parameters

Understanding normal feline ECG values is fundamental for recognizing pathological rhythms. Cats have several unique ECG characteristics that differ from dogs, including smaller waveform amplitudes, shorter intervals, and a wider normal mean electrical axis range (0 to +160 degrees).

Classification of Feline Arrhythmias

Feline arrhythmias are classified by origin (supraventricular vs. ventricular) and rate (tachyarrhythmia vs. bradyarrhythmia). This classification guides diagnosis and treatment selection.

Tachyarrhythmias in Cats

Sinus Tachycardia

Sinus tachycardia is the expected cardiac rhythm in essentially all cats in clinical settings due to anxiety and stress. A heart rate greater than 240 bpm in the absence of identifiable systemic disease should raise suspicion for pathological tachycardia. Common causes include hyperthyroidism, fever, pain, anemia, CHF, hypotension, and sympathomimetic drugs.

ECG Characteristics: Normal P-QRS-T morphology with rate greater than 220-240 bpm. P wave present before each QRS complex. Regular R-R intervals.

Treatment: Address underlying cause. For profoundly hyperthyroid cats with rate greater than 260 bpm, atenolol (6.25 mg per cat PO every 12-24 hours) may be initiated until euthyroidism is achieved.

Supraventricular Tachycardia (SVT)

SVT encompasses tachyarrhythmias originating above the ventricles, including atrial tachycardia and junctional tachycardia. These arrhythmias produce narrow QRS complexes (unless aberrantly conducted) and often occur in cats with significant cardiac disease and atrial enlargement.

ECG Characteristics: Rapid rate (greater than 240 bpm), narrow QRS complexes, regular rhythm. P waves may be absent, buried in QRS, or have abnormal morphology. P wave configuration differs from normal sinus P waves.

Clinical Signs: Weakness, lethargy, acute CHF, syncope, end-organ dysfunction (acute kidney injury) due to reduced cardiac output from decreased diastolic filling time.

Treatment: Acute management: IV diltiazem (0.1-0.25 mg/kg IV bolus over 5 minutes) or IV esmolol. Chronic management: oral diltiazem (1.5-3 mg/kg or 7.5-15 mg per cat PO every 8 hours), atenolol (6.25 mg per cat PO every 12-24 hours), or sotalol.

Atrial Fibrillation (AFib)

Atrial fibrillation in cats is almost exclusively associated with significant structural heart disease and atrial enlargement, most commonly HCM. Unlike large-breed dogs that can develop lone atrial fibrillation, cats require a critical mass of atrial tissue (achieved through dilation) to support the multiple reentry circuits necessary for AFib propagation. Atrial depolarization rates can exceed 500 bpm, with the AV node determining ventricular response rate.

ECG Characteristics: Absence of P waves, irregularly irregular baseline (f waves or fibrillation waves), irregularly irregular QRS rhythm, ventricular response rate typically 180-280 bpm. QRS complexes are usually narrow unless aberrant conduction occurs.

Treatment: Cats with AFib are generally treated with either a calcium-channel blocker (diltiazem) or beta-blocker (atenolol, sotalol), with caution in the face of CHF or systolic dysfunction. The combination of digoxin and diltiazem is superior to either drug alone for ventricular rate control.

Ventricular Premature Complexes (VPCs)

VPCs (also known as premature ventricular complexes or PVCs) are the most common arrhythmia identified in cats with structural heart disease. They arise from ectopic foci within the ventricular myocardium. In cats, ventricular hypertrophy increases myocardial susceptibility to arrhythmias. While only approximately 7% of cats with subclinical HCM have VPCs on resting ECG, almost all cats diagnosed with ventricular tachyarrhythmias have some form of cardiomyopathy.

ECG Characteristics: Wide (greater than 0.04 sec), bizarre QRS complexes without preceding P waves. Occur earlier than expected in cardiac cycle. May be uniform (same morphology) or multiform (different morphologies, indicating multiple foci or increased electrical instability).

Causes: Primary cardiac disease (HCM, other cardiomyopathies), electrolyte abnormalities (hypokalemia), anemia, hypoxemia, systemic disease, drug effects (digoxin toxicity, anesthetics, sympathomimetics).

Treatment Indications: Address VPCs if patient is symptomatic (syncope), if there is evidence of R-on-T phenomenon, couplets, triplets, or runs of ventricular tachycardia, or if VPCs are multiform.

Ventricular Tachycardia (VT)

Ventricular tachycardia is defined as three or more VPCs in a row. It is a potentially life-threatening arrhythmia that can result in sudden death. VT may be intermittent (paroxysmal) or sustained and significantly reduces cardiac output due to decreased diastolic filling time.

ECG Characteristics: Wide, bizarre QRS complexes at rapid rate (greater than 220-240 bpm). May be monomorphic (uniform QRS morphology) or polymorphic. Usually regular R-R intervals, though some variation is not uncommon. Sinus P waves may be seen superimposed on or between ventricular complexes but are dissociated from QRS.

Clinical Signs: Lethargy, weakness, collapse, syncope, signs of acute CHF, or sudden death.

Treatment of Ventricular Tachycardia

Bradyarrhythmias in Cats

Sinus Bradycardia

Sinus bradycardia (heart rate less than 140 bpm) is abnormal in cats unless the animal is well-acclimated to the practice and calm. The presence of bradycardia should prompt investigation for serious underlying disorders.

Clinical Signs: Lethargy, depressed appetite, weakness, symptoms of underlying disease.

Treatment: Address underlying cause. Atropine (0.04 mg/kg IV, IM, or SC) may be used if clinical signs are present. If bradycardia is related to high vagal tone, it should be abolished by atropine administration.

Atrioventricular (AV) Block

AV block describes abnormalities affecting conduction from atria to ventricles at the level of the AV node or bundle of His. High-grade and third-degree AV blocks can cause syncope in cats due to cerebral hypoperfusion from bradycardia.

Treatment: Medical treatment for AV block is not well-established in cats. Atropine may be tried for vagally-mediated blocks. For symptomatic high-grade or third-degree AV block, pacemaker implantation is the standard recommended treatment. Cilostazol (phosphodiesterase inhibitor) has been reported to increase heart rate in some cats with AV block but requires further study.

Hypertrophic Cardiomyopathy and Arrhythmias

Hypertrophic cardiomyopathy (HCM) is the most common acquired heart disease in cats, affecting 10-15% of the feline population. Arrhythmias are frequently observed in cats with HCM and contribute significantly to morbidity and mortality. The thickened, hypertrophied myocardium creates electrical instability and predisposes to both ventricular and supraventricular arrhythmias.

Breed Predispositions for HCM

HCM is familial in many breeds, with genetic mutations in myosin-binding protein C (MYBPC3) identified in Maine Coon and Ragdoll cats. Other predisposed breeds include Persian, Sphynx, Norwegian Forest Cat, Bengal, Chartreux, Siberian, British Shorthair, and American Shorthair. Male cats tend to develop more severe disease at an earlier age, though both sexes are equally predisposed.

Diagnostic Approach to Feline Arrhythmias

A systematic approach to diagnosing arrhythmias involves clinical assessment, electrocardiography, and evaluation for underlying cardiac and systemic disease.

Step 1: Clinical Assessment

• Auscultation: Irregular rhythm, bradycardia (less than 140 bpm), tachycardia (greater than 240 bpm), gallop sound, murmur
• Clinical signs: Syncope, episodic weakness, exercise intolerance, dyspnea, lethargy
• History: Owner should video episodes; distinguish syncope from seizure (facial twitching and tonic-clonic motions are common in feline syncope)

Step 2: Electrocardiography

• Resting ECG: Standard 6-lead ECG; assess rate, rhythm, waveform morphology
• Holter monitoring: 24-hour ambulatory ECG for intermittent arrhythmias
• Event monitor: Owner-activated recording for infrequent episodes

Step 3: Additional Diagnostics

• Echocardiography: Gold standard for diagnosing HCM and other cardiomyopathies; assess LV wall thickness, LA size, systolic function
• Blood pressure: Rule out systemic hypertension as cause of secondary LV hypertrophy
• Thyroid testing: T4 to rule out hyperthyroidism-induced cardiomyopathy
• Serum chemistry: Evaluate electrolytes (potassium), renal function
• Cardiac biomarkers: NT-proBNP and troponin I may be elevated in severe disease

Antiarrhythmic Drug Summary for Cats