Three cardiomyopathies cover nearly every cat with primary heart muscle disease: hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC). They are told apart by 3 echo numbers — wall thickness at end-diastole, the left atrium to aorta ratio, and right ventricular size — plus a 24-hour rhythm recording for ARVC. The type sets the recheck interval, and the interval is what keeps a cat out of the emergency room.
Three phenotypes, one echo
HCM thickens the left ventricular wall without another cause such as hyperthyroidism or high blood pressure. RCM leaves the wall at normal thickness but stiffens it, so the atria dilate while the ventricle stays small. ARVC replaces muscle in the right ventricular free wall with fat and scar, giving a dilated right ventricle and ventricular beats on a strip. The measurements are taken at end-diastole, in 2D, from a short-axis view at the papillary muscle level, and one number outside the reference range should be repeated before it is treated as a diagnosis.
HCM: the 6 mm line and what sits behind it
A left ventricular wall of 6.0 mm or more at end-diastole, in a cat with no thyroid or pressure cause, is the working definition; 5.0 to 5.9 mm is equivocal and needs a repeat in 3 months. Measure 3 segments and use the thickest, because HCM is often focal at the papillary muscle or the apex. Left atrium to aorta ratio above 1.5 means atrial enlargement and a higher clot risk; above 1.8 with a smoke sign in the atrium means the cat needs a plan now. Check total thyroxine and blood pressure first: a hyperthyroid cat at 4.5 kg with a 6.2 mm wall can normalize on thyroid treatment alone.
RCM: normal wall, huge atria
RCM shows a wall of 4.0 to 5.5 mm, a left atrium to aorta ratio above 1.8, and restrictive filling on Doppler, with a small ventricular chamber. It is the type most often missed on a quick scan because the wall looks fine and only the atria are wrong. Recheck every 3 to 6 months, since atrial enlargement can progress between visits, and image the atrium for spontaneous echo contrast at every visit. A cat with RCM and a ratio of 2.2 at diagnosis is a cat to see every 3 months, not every year.
ARVC: the right ventricle and the rhythm
ARVC is a right-sided disease: a right ventricular outflow tract above 2.0 cm, a thin free wall, and ventricular premature complexes on a 24-hour Holter. The usual trigger for diagnosis is a cat with syncope and a normal-looking left heart. A Holter showing more than 100 ventricular beats per 24 hours, or any run of 4 or more, supports the diagnosis and changes the recheck to 3 months. Some cats present first with a clot or with sudden collapse, which is why a right-sided murmur deserves a strip even in a young cat.
| Type | Wall at end-diastole | LA to Ao ratio | Holter beats per 24 h | Recheck |
|---|---|---|---|---|
| HCM, equivocal | 5.0 to 5.9 mm | under 1.5 | under 50 | 3 months |
| HCM, confirmed | 6.0 mm or more | 1.5 to 1.8 | 50 to 100 | 6 months |
| RCM | 4.0 to 5.5 mm | over 1.8 | variable | 3 months |
| ARVC | 4.0 to 5.5 mm | under 1.5 | over 100 | 3 months |
Tests that go with the echo
Four add-ons change the plan. A 6-lead ECG and a 24-hour Holter if there is any syncope. Blood pressure, because 160 mmHg or higher causes a thickening that is not HCM. Total thyroxine, because thyroid disease mimics HCM in cats over 8. A renal panel and potassium before any cardiac prescription, then again at 7 to 14 days, since kidney values shift once diuretics start. Cardiac troponin and NT-proBNP are useful when the echo is borderline, but they do not replace the measurement.
Two numbers the owner owns
Resting respiratory rate: count 30 seconds during sleep, multiply by 2, twice a day, target under 30 per minute. A rate of 40 or more that holds for 2 days means fluid is building, and the cat needs to be seen within 24 hours. Weight: weekly, same scale; a 5 percent drop in a month is a reason to call, not to wait for the next scheduled visit. Both numbers catch decompensation 2 to 5 days before the cat looks sick.
What counts as an emergency
Cold, painful hind legs with no femoral pulse is a clot and gives a 6-hour window before the muscle damage is permanent; drag the cat in immediately rather than calling. Open-mouth breathing, blue gums, or a respiratory rate above 60 at rest means minutes, not hours. A respiratory rate of 40 to 50 that appears overnight is a same-day visit. Syncope lasting over 10 seconds, or 2 episodes in 24 hours, needs a Holter within a week. A cat that has stopped eating for 24 hours with a heart condition also needs to be seen the same day, because dehydration and kidney values move fast on diuretics.
The order of work
- Get one complete echo with 3 measured segments and a documented LA to Ao ratio, not a one-line report.
- Exclude thyroid disease and hypertension before accepting an HCM label, especially after age 8.
- Start the respiratory rate log the day of diagnosis; 14 days of numbers beats any single clinic reading.
- Book the recheck by type: 3 months for equivocal HCM, RCM and ARVC, 6 months for confirmed HCM.
- Repeat bloodwork 7 to 14 days after any prescription change, then every 3 to 6 months.
Prices for echo, Holter and panels are listed in the cost tool. A cat losing weight with a heart murmur should also have the stool panel in fecal test types ruled out, and a glucose curve in insulin types is the next step if bloodwork shows high glucose alongside the cardiac findings.
Terms mentioned in this article
Frequently asked
Can a murmur alone diagnose HCM?
No. A murmur only says blood is moving abnormally; the diagnosis needs a measured wall thickness of 6.0 mm or more at end-diastole, with thyroid disease and hypertension excluded. Cats with a murmur and a normal wall can be rechecked in 12 months, while a cat with a 6.2 mm wall needs a plan now.
How often should a cat with cardiomyopathy be rechecked?
Every 3 months for equivocal HCM, RCM and ARVC, and every 6 months for confirmed HCM with a left atrium to aorta ratio under 1.8. Any new syncope, a rising respiratory rate, or a change in prescription moves the next visit sooner, usually within 7 to 14 days.
What is the emergency window for a clot?
About 6 hours from the moment the hind legs go cold and painful. The clot blocks blood flow to the legs, and muscle damage becomes permanent after that window. Open-mouth breathing or a resting respiratory rate above 60 is an equally immediate emergency and should never wait for a scheduled visit.