Cat Heart: 4 Chambers, 140-220 bpm and the 6 mm Line

A cat's heart is 4 chambers in a shell about the size of a walnut, roughly 0.5 percent of body weight, and it beats 140 to 220 times a minute while the cat is awake. Those 3 numbers set the whole clinical picture: a heart this small has very little reserve, and a disease that adds 1 to 2 mm of muscle to a 4 mm wall has already changed how the organ fills.

Two measurements decide most cases. A murmur graded 3 or louder, or a wall thicker than 6 mm on ultrasound, moves a cat from watch-and-see to cardiology.

4 chambers, 20 grams

The heart of a 4 kg cat weighs 15 to 25 grams, about 0.5 percent of body weight, and sits between the 3rd and 6th ribs. Two thin-walled atria receive, 2 thick-walled ventricles pump: the right ventricle drives blood through the lungs at low pressure, the left drives it to the body at 120 to 170 mmHg systolic. The left wall is normally 3 to 5 mm thick at end-diastole; the right is 2 mm or less.

Output follows from rate. At 180 beats per minute with 2 to 3 mL per beat, a resting cat moves roughly 0.4 to 0.8 L of blood a minute, and that flow can triple in a sprint. Cats are built for 20 second bursts, not for 20 minute runs, and the heart reflects it.

140 to 220 at rest

The normal awake rate is 140 to 220 beats per minute. A sleeping cat drops to 120 to 140; a frightened one in a clinic hits 220 to 240, which is why a single high reading in the exam room means very little. Count for a full 30 seconds with a stethoscope on the left chest, 1 hand on the chest wall, and record the rate and the rhythm separately: a rate of 200 that is regular is usually stress, and a rate of 160 that is irregular is not.

Kittens run faster, 200 to 260 in the first 8 weeks, settling toward adult rates by 6 months. A resting rate above 220 at home, measured twice on 2 different days while the cat sleeps, deserves a look.

Murmurs, graded 1 to 6

A murmur is turbulence, and it is graded on a 6 point scale. Grade 1 is the softest sound a vet can hear in a quiet room after 30 seconds of listening; grade 2 is soft but audible at once; grade 3 is moderate and easy to hear; grade 4 is loud with a thrill you can feel through the chest wall; grade 5 has a thrill and is audible with the stethoscope edge lifted off the skin; grade 6 is heard with the stethoscope 1 cm away from the chest.

Grade alone does not measure severity. A grade 2 murmur in a 2-year-old Maine Coon can be early HCM, while a grade 4 in a 14-year-old with a normal wall is often a leaking valve that needs monitoring rather than treatment. What grade does decide is speed: 1 to 2 gets an echo within 3 months, 3 and above within 2 to 4 weeks.

Grade	What the vet hears	Next step	Echo within
1	faint, only in a silent room	recheck at next visit	3 months
2	soft, audible at once	book an echo	3 months
3	moderate, easy to hear	echo plus blood pressure	2 to 4 weeks
4	loud, thrill on the chest	echo, chest x-ray	1 to 2 weeks
5	thrill, heard off the skin	cardiology referral	7 days
6	heard 1 cm from the chest	same-day review	24 hours

The 6 mm line

Hypertrophic cardiomyopathy is diagnosed on ultrasound by wall thickness. The usual cutoff is 6 mm at end-diastole in a cat; 5.5 to 5.9 mm is borderline and gets repeated in 6 to 12 months, and 4 to 5 mm is normal for most cats. The measurement is taken on a relaxed cat, because stress and dehydration both thicken the reading by 0.5 to 1 mm.

Wall thickness alone is not the whole report. A left atrium to aorta ratio above 1.5 means enlargement, and above 1.7 raises the risk of heart failure and of a clot leaving the atrium. That clot is the reason a cat with HCM can lose the use of both hind legs in under 10 minutes, and why the antiplatelet drug clopidogrel, usually 18.75 mg once a day, a quarter of a 75 mg tablet, enters the plan long before any cough does. Costs of the workup, from 400 to 800 RMB for an echo in most Chinese cities, are broken down in what a vet bill is made of.

Screening, by age and breed

Not every cat needs an echo. Cats with a murmur, an arrhythmia, a gallop sound, a breed predisposition (Maine Coon, Ragdoll, Persian, Sphynx, British Shorthair) or a relative with HCM should be scanned at 12 months or later, then every 1 to 2 years until 8 years old. Breeding cats are usually scanned before each litter. A normal scan at 2 years does not clear a cat for life, because HCM can appear at 5 or 7.

Many cats with HCM have no murmur at all, especially when calm, so a quiet chest is not a negative test. If the vet hears nothing but the cat is a predisposed breed, one echo at 12 months is still the cheaper option; the screening pathway is set out in murmur and HCM screening. A cat age calculator is a rough guide to when to start: a 1-year-old cat is roughly a 15-year-old human, and HCM risk climbs across the equivalent of the 30s and 40s.

The number to count at home

Resting respiratory rate is the most useful home measure in feline cardiology, and it costs nothing. Count chest movements for 30 seconds while the cat sleeps, multiply by 2, and write it down 3 times a week. Under 30 breaths a minute is normal; 30 to 40 means call within 2 days; over 40 at rest, or any open-mouth breathing, is an emergency. A rise of 10 breaths a minute over 2 to 3 days often comes before a cat stops eating.

When it is an emergency

Both hind legs suddenly weak or cold, with the pads pale and the cat crying when touched, is an arterial thromboembolism, and the first 2 to 4 hours decide whether the leg can be saved. Open-mouth breathing, blue gums, or a cat that will not lie down and cannot settle are heart failure until proven otherwise. In either case the cat should not be carried up stairs or allowed to walk; a carrier, a cool room and a call ahead to the clinic are the whole first aid.

4 steps after a murmur

Record the grade, the date and the heart rate at the visit, and photograph the page. Ask for the actual wall measurement in millimetres, not the word normal. Start the sleeping breathing count the same week, 3 readings a week. And repeat the echo on the schedule the cardiologist writes, usually 6 to 12 months, rather than when a symptom appears.

Frequently asked