Cat Cardiac Output: 140-220 bpm and 3x Reserve

A resting cat moves blood with a small, fast pump: 140 to 220 beats per minute, 1.2 to 2 mL per kg with each beat, which works out to roughly 0.7 to 1.8 L per minute in a 4 kg cat. During a sprint the same heart can reach 3 times that output, and it does so in seconds rather than minutes. The number that matters clinically is not the heart rate on its own and not the murmur grade, but the product of the two, and that product is invisible without imaging.

Three numbers, one equation

Cardiac output equals heart rate times stroke volume. At 180 bpm with a 6 mL stroke, a 4 kg cat delivers 1.08 L per minute, about 270 mL per kg per minute. Halve the stroke volume and the heart must run at 360 bpm to hold the same output, which a cat cannot do for long: maximum heart rate lands near 240 to 260 bpm, so a cat that loses stroke volume loses output, and it hides that loss by lying still.

Stroke volume depends on 3 things: how much blood returns to the heart (preload), how hard the muscle contracts (contractility), and how much resistance the aorta pushes back with (afterload). A cat's blood volume is roughly 60 to 70 mL/kg, so a 4 kg cat holds 240 to 280 mL in total. Losing 40 mL of that, about 15 percent, is enough to drop preload and output before the gums change color.

What 1.2 to 2 mL/kg looks like at different weights

The per-kg figure is the useful one because it scales with the animal. A 2.5 kg cat moves 3.0 to 5.0 mL per beat, a 6 kg cat moves 7.2 to 12 mL. Multiply by the rate and the output range opens up.

Weight	Stroke low	Stroke high	Output at 140	Output at 220
2.5 kg	3.0 mL	5.0 mL	0.42 L/min	1.10 L/min
4.0 kg	4.8 mL	8.0 mL	0.67 L/min	1.76 L/min
5.0 kg	6.0 mL	10.0 mL	0.84 L/min	2.20 L/min
6.0 kg	7.2 mL	12.0 mL	1.01 L/min	2.64 L/min

The table makes one clinical point: a big cat at a low normal rate can deliver more blood than a small cat at a high rate. Judging perfusion by heart rate alone gets the answer wrong in both directions, which is why perfusion is read from gum color, capillary refill of 1 to 2 seconds, and the temperature of the extremities.

How output triples in a sprint

Three mechanisms fire together. Heart rate climbs from 140 to 220 or higher, stroke volume rises 20 to 40 percent as the muscle contracts harder and empties more completely, and veins tighten to push blood back to the chest. The result is roughly 3 times resting output, and a 4 kg cat at full sprint can move close to 3 L per minute, which is 750 mL per kg per minute — a figure that would be unremarkable in a horse and is extreme for a 28 g heart.

Recovery is fast because the cat's lifestyle demands it. An ambush hunter sprints for 10 to 20 seconds, then stops. Output falls back toward 1 L per minute within 30 to 60 seconds, and a cat that stays at 200 bpm five minutes after play, or breathes with its mouth open at rest, is not recovering normally.

Why a thick heart loses output before it loses a beat

Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats: the left ventricular wall thickens, the cavity shrinks, and stroke volume falls even while the heart rate stays normal. The body compensates by raising rate, so the first sign at home is often not a slow heart but a cat that naps more and plays for 5 minutes instead of 15. In the later stage, fluid backs up into the lungs and the resting respiratory rate rises above 30 breaths per minute.

Counting breaths at home is the cheapest measurement available. Watch the chest while the cat sleeps, count 1 rise and fall as 1 breath, and time 30 seconds. Under 30 per minute is the target; 30 to 40 sustained is a call to the clinic; over 40, or any open-mouth breathing, is an emergency. Do it 3 times a week in a cat over 8 years, or in any cat with a known murmur.

Murmurs and output: why the grade lies

A murmur is sound, not flow. The 1 to 6 scale describes loudness, and loudness depends on turbulence, which depends on velocity and on the pressure difference across a narrow orifice, not on how much blood is moving forward. Three consequences follow. A cat with severe HCM and dynamic outflow obstruction can have a loud grade 4 to 5 murmur and a normal output. A cat with dilated, thin-walled failure can have a soft grade 1 murmur and a badly reduced output. And a cat with severe disease can have no murmur at all, which is why many cats with HCM are first found by screening rather than by stethoscope.

The same logic applies to arrhythmia. An irregular rhythm with a normal average rate can cut output by 20 to 30 percent, because beats that arrive too early eject against an empty ventricle. A gallop sound, a resting rate over 200 at home, or an episode of collapse lasting 5 to 15 seconds all justify an echocardiogram rather than a wait-and-see.

What the numbers cannot tell you

Output is a flow, and flow is not the same as delivery. Anemia drops oxygen carrying capacity while output stays normal or rises. A cat with gas distension from a disturbed gut microbiome pushes on the vena cava and drops preload. Pain raises rate and afterload together. This is why a single heart rate reading, taken in a clinic where the cat is frightened, is close to useless: a healthy cat in a carrier can hit 220 bpm, a sick cat in heart failure can sit at 160, and a cat whose hearing picks up the clinic door from 3 rooms away starts climbing before the exam begins.

Five measurements worth doing at home

• Count sleeping breaths for 30 seconds, 3 times a week, and log the number against the date.
• Take a resting heart rate only when the cat is asleep: 15 seconds times 4, and repeat once.
• Check gum color and capillary refill, 1 to 2 seconds, and note any change from pale pink.
• Time play sessions and record how long the cat lasts before it lies down: 15 minutes down to 5 is a trend.
• Ask for a heart screen with auscultation plus echocardiogram at 8 years on the cat age chart, or sooner if a murmur is heard once.

Terms mentioned in this article

HCM

Frequently asked