NAVLE Cardiovascular

Feline Cardiomyopathy Study Guide

📅 March 28, 2026 ⏱ 25 min read

Feline cardiomyopathies represent the most common form of acquired heart disease in cats, affecting approximately 1 in 7 cats (approximately 15%) in the general population.

Overview and Clinical Importance

Feline cardiomyopathies represent the most common form of acquired heart disease in cats, affecting approximately 1 in 7 cats (approximately 15%) in the general population. These primary myocardial disorders cause structural and functional abnormalities of the heart muscle, leading to congestive heart failure (CHF), arterial thromboembolism (ATE), and sudden death. Understanding the classification, diagnosis, and management of feline cardiomyopathies is essential for NAVLE success and clinical practice.

Cardiomyopathies are classified by phenotype (structural and functional characteristics) rather than etiology, as the underlying cause remains unknown in most cases. The ACVIM Consensus Statement (2020) provides the current classification framework used in clinical practice and board examinations.

Type	Primary Abnormality	Key Echocardiographic Features
HCM	Left ventricular hypertrophy (thick, stiff myocardium)	LV wall thickness greater than 6 mm in diastole; may have SAM, LA enlargement
RCM	Diastolic dysfunction due to endomyocardial fibrosis (stiff myocardium)	Normal to mildly increased LV walls; severe LA/biatrial enlargement; restrictive filling pattern
DCM	Decreased myocardial contractility (weak myocardium)	Dilated LV chamber; thin walls; reduced fractional shortening (less than 25%); LA enlargement
ARVC	Fibro-fatty replacement of RV myocardium	Severe RV enlargement; RV dysfunction; ventricular arrhythmias; right heart failure signs
End-stage HCM	HCM with systolic dysfunction	Persistent LV hypertrophy with reduced FS; regional wall motion abnormalities; severe LA enlargement

Classification of Feline Cardiomyopathies

Feline cardiomyopathies are classified based on echocardiographic phenotype. The primary phenotypic categories include hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), dilated cardiomyopathy (DCM), and arrhythmogenic right ventricular cardiomyopathy (ARVC). HCM accounts for approximately 60% of cases, while RCM and nonspecific cardiomyopathy (NCM) comprise 20-30%. DCM and ARVC are rare.

High-YieldFor NAVLE: HCM = thick walls with diastolic dysfunction; DCM = thin walls with systolic dysfunction; RCM = normal-appearing LV with massive LA enlargement; ARVC = right heart involvement with arrhythmias.

Breed	Mutation	Clinical Significance
Maine Coon	MYBPC3-A31P	Homozygotes develop severe HCM; heterozygotes may have subtle dysfunction; 34-41% prevalence in breed
Ragdoll	MYBPC3-R820W	Homozygotes at high risk of severe HCM by 1-2 years; earlier onset than Maine Coons
Other Predisposed Breeds	Unknown mutations	British Shorthair, Sphynx, Bengal, Persian, Norwegian Forest, Siberian, Chartreux

Hypertrophic Cardiomyopathy (HCM)

Hypertrophic cardiomyopathy is the most common form of feline heart disease, characterized by primary left ventricular concentric hypertrophy in the absence of other causes of LV thickening (such as systemic hypertension, hyperthyroidism, or aortic stenosis). HCM is primarily a diastolic dysfunction disorder - the thickened, stiff ventricle cannot relax and fill properly during diastole.

Etiology and Genetics

While the cause of HCM is unknown in most cats, genetic mutations have been identified in certain breeds. The condition follows an autosomal dominant pattern with incomplete penetrance.

Board Tip - Mnemonic: 'MYBPC3 = My Big Plump Cat' - Remember MYBPC3 (Myosin Binding Protein C3) is the gene mutated in Maine Coons and Ragdolls!

Pathophysiology

The thickened LV walls in HCM lead to several pathophysiological consequences. Diastolic dysfunction is the primary abnormality - the stiff ventricle cannot relax properly, leading to elevated filling pressures that are transmitted to the left atrium and pulmonary veins. This results in LA enlargement and predisposes to pulmonary edema and pleural effusion.

Systolic anterior motion (SAM) of the mitral valve occurs in approximately 50-67% of cats with HCM. The anterior mitral leaflet is pulled into the left ventricular outflow tract (LVOT) during systole, causing dynamic obstruction (obstructive HCM or HOCM) and mitral regurgitation. SAM is associated with louder murmurs and may have a better prognosis than non-obstructive HCM.

Clinical Signs

Many cats with HCM are asymptomatic (subclinical) - more than half of affected cats show no clinical signs. When present, clinical signs relate to CHF, ATE, or arrhythmias.

Respiratory distress: Dyspnea, tachypnea, open-mouth breathing (pulmonary edema or pleural effusion)
Non-specific signs: Lethargy, decreased appetite, hiding, weight loss
Syncope: Due to arrhythmias or dynamic LVOT obstruction
Sudden death: Due to fatal arrhythmias or ATE
ATE signs: Acute hindlimb paralysis/paresis, painful limbs, absent femoral pulses, cold extremities

Physical Examination Findings

Heart murmur: Systolic murmur (parasternal), often grade II-IV/VI; louder with SAM/LVOT obstruction
Gallop rhythm: S3 or S4 sound - highly suggestive of myocardial disease
Arrhythmia: Irregular rhythm, tachycardia, or premature beats
Muffled heart/lung sounds: If pleural effusion present
Prominent apical impulse: Due to LV hypertrophy

High-YieldMany cats with HCM have NORMAL auscultation! A gallop sound is highly suggestive of cardiomyopathy and should prompt echocardiography. Heart murmurs in cats are not specific - many cats with murmurs have normal hearts.

Stage	Description	Management
Stage A	At-risk cats (predisposed breeds) with no evidence of disease	Genetic testing for Maine Coons/Ragdolls; periodic screening echocardiography starting age 2-3 years
Stage B1	Subclinical cardiomyopathy with mild LA enlargement (LA:Ao less than 1.6-1.8); low risk of CHF/ATE	Monitor; no specific therapy proven beneficial; recheck echo in 6-12 months
Stage B2	Subclinical with moderate-severe LA enlargement (LA:Ao greater than 1.8-2.0 or LA greater than 18mm); HIGH RISK of CHF/ATE	Clopidogrel 18.75mg PO q24h for thromboprophylaxis; consider atenolol if significant SAM; monitor sleeping respiratory rate
Stage C	Current or prior CHF (pulmonary edema or pleural effusion) or ATE	Furosemide, ACE inhibitor, clopidogrel; consider pimobendan (non-obstructive); thoracocentesis if needed
Stage D	Refractory CHF despite standard therapy	Escalate diuretics (torasemide); add hydrochlorothiazide; pimobendan; frequent monitoring; palliative care discussions

ACVIM Staging System for Feline Cardiomyopathy

The 2020 ACVIM Consensus Statement introduced a staging system for feline cardiomyopathy that guides clinical management and prognosis. Understanding this staging is essential for board examinations.

Board Tip - LA:Ao Memory: '1.6 is FIX, 2.0 is TROUBLE' - Normal LA:Ao is less than 1.6; values greater than 1.8-2.0 indicate increased risk of CHF and ATE requiring thromboprophylaxis!

Parameter	Normal Value	Clinical Significance
LV Wall Thickness (diastole)	Less than 6 mm	Greater than 6 mm = HCM (greater than 5 mm if cat less than 3 kg)
LA:Ao Ratio (short-axis)	Less than 1.6	Greater than 1.8-2.0 = increased risk CHF/ATE; greater than 1.9 suggests CHF likely
LA Diameter	Less than 15-16 mm	Greater than 18-19 mm = high risk for CHF/ATE
LV Fractional Shortening	35-65%	Less than 25% = systolic dysfunction (DCM or end-stage HCM)
Spontaneous Echo Contrast	Absent	'Smoke' in LA = blood stasis; high risk for thrombus formation

Diagnostic Approach

Echocardiography - Gold Standard

Echocardiography is the definitive diagnostic test for all feline cardiomyopathies. It allows classification by phenotype, assessment of severity, and identification of risk factors for complications.

Cardiac Biomarkers

NT-proBNP (N-terminal pro-B-type Natriuretic Peptide)

NT-proBNP is released by cardiac myocytes in response to stretching and wall stress. It is useful for distinguishing cardiac from respiratory causes of dyspnea and screening for occult cardiomyopathy.

Less than 100 pmol/L: Normal - significant cardiomyopathy unlikely
Greater than 100 pmol/L: Suggests cardiac disease present - echocardiography recommended
Greater than 270 pmol/L: In dyspneic cat, CHF is likely cause of respiratory distress (90% sensitivity, 88% specificity)
SNAP NT-proBNP: Point-of-care test; positive result correlates to greater than 270 pmol/L; useful in emergency settings

High-YieldNT-proBNP can be falsely elevated in hyperthyroidism, hypertension, and renal disease. Always rule out these conditions! A normal NT-proBNP does NOT exclude mild/early HCM.

Radiography

Thoracic radiography cannot diagnose specific cardiomyopathy types but is valuable for detecting CHF complications:

Valentine-shaped heart: Classic DV/VD appearance due to LA enlargement (left auricular bulge)
Pulmonary edema: Patchy, diffuse interstitial to alveolar pattern - more ventral distribution in cats than dogs
Pleural effusion: Common in feline CHF; may obscure cardiac silhouette

Drug Class	Drug/Dose	Indication	Notes
Loop Diuretic	Furosemide 1-2 mg/kg PO q12-24h (chronic); 2-4 mg/kg IV/IM q2-6h (acute)	CHF (pulmonary edema, pleural effusion)	Mainstay of CHF treatment; monitor renal function
ACE Inhibitor	Enalapril/Benazepril 0.25-0.5 mg/kg PO q12-24h	CHF management; LA enlargement	Limited evidence of benefit in feline CHF; use with caution in renal disease
Beta-Blocker	Atenolol 6.25-12.5 mg/cat PO q12-24h	Obstructive HCM (SAM); ventricular arrhythmias	Avoid in acute CHF; may worsen heart failure; target HR 140-160 bpm
Calcium Channel Blocker	Diltiazem 7.5 mg/cat PO q8h or 30 mg SR q12h	Non-obstructive HCM; rate control	Avoid with beta-blockers; not in acute CHF; may cause anorexia
Inodilator	Pimobendan 0.625-1.25 mg/cat PO q12h	Non-obstructive CM with CHF; DCM; end-stage HCM	AVOID in obstructive HCM; may worsen SAM; controversial in cats
Antiplatelet	Clopidogrel 18.75 mg/cat PO q24h	Thromboprophylaxis (Stage B2 and higher); post-ATE	Superior to aspirin for ATE prevention; FAT CAT study

Treatment of Feline Cardiomyopathy

Pharmacological Management

High-YieldPIMOBENDAN + OBSTRUCTIVE HCM = BAD! Pimobendan increases contractility and can worsen dynamic LVOT obstruction in cats with SAM. It is only indicated for non-obstructive cardiomyopathies with systolic dysfunction.

Feline Arterial Thromboembolism (FATE)

Arterial thromboembolism is a devastating complication of feline cardiomyopathy, affecting 12-28% of cats with HCM. Thrombi form in the dilated left atrium or left auricle due to blood stasis, endothelial dysfunction, and hypercoagulability. The embolus most commonly lodges at the aortic trifurcation ('saddle thrombus') in approximately 90% of cases, causing acute ischemia of the hindlimbs.

Clinical Presentation - The 5 P's

Classic presentation involves acute onset of hindlimb dysfunction with the characteristic '5 P's':

Pain: Severe - cats often vocalize; gastrocnemius muscles hard and painful
Paralysis/Paresis: Complete paralysis or weakness of affected limb(s)
Pulselessness: Absent femoral pulses (unilateral or bilateral)
Pallor: Pale or cyanotic nail beds and foot pads
Poikilothermia: Cold extremities compared to unaffected limbs

Board Tip - Memory Aid for ATE: 'PAINFUL Cat with PALE, PULSELESS, PARALYZED, POIKILOTHERMIC limbs' = Think FATE! Also remember: hypothermia at presentation (rectal temp less than 37.2°C/99°F) is a NEGATIVE prognostic indicator.

Prognosis and Survival

Initial survival: 27-35% survive to discharge with treatment
Median survival with clopidogrel: 443 days (vs 192 days with aspirin)
Recurrence rate: ~25% with aspirin; lower with clopidogrel
Poor prognostic factors: Hypothermia, bilateral involvement, CHF, severe LA enlargement

Due to the severity of the condition and guarded prognosis, euthanasia rates are 50-75% at initial presentation.

Acute Management of ATE

Analgesia: CRITICAL - opioids (buprenorphine, hydromorphone, methadone); fentanyl CRI
Anticoagulation: Unfractionated heparin 200-300 IU/kg IV then 150-200 IU/kg SC q6-8h; OR enoxaparin (LMWH) 0.75-1 mg/kg SC q6h
Antiplatelet: Clopidogrel 18.75 mg/cat PO q24h (start immediately)
Supportive care: IV fluids (cautiously if CHF), oxygen, thermoregulation
Monitor for reperfusion injury: Hyperkalemia, metabolic acidosis, rhabdomyolysis as perfusion returns

Other Feline Cardiomyopathies

Dilated Cardiomyopathy (DCM)

DCM is now rare in cats (less than 5%) since the discovery of taurine deficiency as a cause in the 1980s. When taurine supplementation became routine in commercial cat foods, DCM incidence decreased dramatically. Current cases are mostly idiopathic or occasionally due to taurine-deficient diets (boutique, vegetarian, or dog food diets).

Key feature: Systolic dysfunction - dilated, thin-walled LV with poor contractility
Echo findings: Increased LV end-systolic and diastolic diameters; FS less than 25% (often less than 15%); LA enlargement
Diagnosis: Measure whole blood and plasma taurine levels; obtain diet history
Treatment: Taurine supplementation (250-500 mg PO q12h); standard CHF therapy; pimobendan appropriate

High-YieldTaurine-deficient DCM is REVERSIBLE with supplementation! Clinical improvement occurs within weeks, but echocardiographic improvement takes 2-3 months. Always measure taurine levels and supplement any cat with DCM.

Restrictive Cardiomyopathy (RCM)

RCM is characterized by diastolic dysfunction due to endomyocardial fibrosis. The stiff ventricle cannot fill properly, leading to elevated atrial pressures and severe atrial enlargement out of proportion to ventricular changes.

Two forms: Myocardial (diffuse fibrosis) and Endomyocardial (visible fibrous bands bridging LV cavity)
Echo findings: Normal or mildly thickened LV walls; marked LA/biatrial enlargement; restrictive transmitral flow pattern on Doppler
Prognosis: Generally worse than HCM; high risk of CHF, ATE, and sudden death

Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC)

ARVC is rare in cats and involves fibro-fatty replacement of the right ventricular myocardium. This leads to RV dysfunction, ventricular arrhythmias, and right heart failure.

Clinical presentation: Right heart failure signs - ascites, pleural effusion, jugular distension; syncope; arrhythmias
Echo findings: Severe RV enlargement with poor RV wall motion; RA enlargement; often normal LV
Treatment: Antiarrhythmics (sotalol, atenolol); diuretics for R-CHF; thromboprophylaxis

Ruling Out Secondary Causes of LV Hypertrophy

Before diagnosing HCM, secondary causes of LV thickening must be excluded:

Hyperthyroidism: Measure T4 in all cats greater than or equal to 6 years with LV hypertrophy; LVH usually resolves with treatment
Systemic hypertension: Measure blood pressure; systolic BP greater than 160-180 mmHg can cause concentric LVH
Acromegaly: GH excess causes cardiac hypertrophy; measure IGF-1 if suspected
Congenital aortic/subaortic stenosis: Look for fixed LVOT obstruction and post-stenotic aortic dilation
Dehydration: Pseudohypertrophy due to decreased preload; reassess after rehydration

High-YieldIf LV wall thickness is greater than or equal to 7 mm AND hyperthyroidism or hypertension is present, the cat likely has PRIMARY HCM being exacerbated by the secondary condition - these conditions don't typically cause severe hypertrophy alone.

Prognosis

Subclinical HCM: Variable; many cats live normal lifespans; ~20% develop CHF within 5 years; ~9% experience ATE
CHF (Stage C): Median survival ~12 months with treatment; variable based on response
ATE: Guarded to poor; median survival ~6-12 months in survivors; high euthanasia rate
Poor prognostic indicators: Large LA (greater than 18-19 mm or LA:Ao greater than 2.0), CHF, ATE, hypothermia, arrhythmias, LV systolic dysfunction

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