Cardiac Index: Normal Range, Formula & Clinical Meaning

Cardiac index takes cardiac output and adjusts it for body size, so a petite patient and a tall one can be compared on the same scale. Here's the formula, the normal range, and what abnormal values reveal.

What is cardiac index?

Cardiac index (CI) is cardiac output normalised to body surface area — the amount of blood the heart pumps per minute for each square metre of body size. Because a 50 kg and a 100 kg person need very different absolute outputs, indexing to size makes the number comparable between patients.

Cardiac index formula

Cardiac index = cardiac output ÷ body surface area

Cardiac output is in L/min and body surface area (BSA) in m², so cardiac index is expressed in L/min/m². Compute BSA with the Mosteller BSA calculator, get cardiac output from the Fick calculator, or do it all at once in the cardiac index calculator.

Normal cardiac index range

Cardiac index	Interpretation
2.5–4.0 L/min/m²	Normal
2.2–2.5 L/min/m²	Borderline low
< 2.2 L/min/m²	Cardiogenic shock range
< 1.8 L/min/m²	Severe, without support
> 4.0 L/min/m²	High output (e.g. sepsis)

What does a low cardiac index mean?

A low cardiac index means the heart isn't delivering enough blood for the body's size. It is a defining feature of cardiogenic shock (classically CI < 2.2 L/min/m² with a high filling pressure) and severe heart failure. The lower it falls, the more the tissues are starved of oxygen — reflected in a falling mixed-venous oxygen saturation.

What does a high cardiac index mean?

A cardiac index above the normal range is a high-output state. Causes include sepsis (early, "warm" shock), severe anaemia, hyperthyroidism, pregnancy, and large arteriovenous shunts. Here the problem usually isn't the pump — it's excessive demand or low vascular resistance.

Cardiac index vs cardiac output

They measure the same thing at different scales:

	Cardiac output	Cardiac index
Definition	Total blood pumped per minute	Cardiac output per m² of BSA
Units	L/min	L/min/m²
Normal	4–8	2.5–4.0
Adjusts for body size?	No	Yes

Because it accounts for size, cardiac index is often the better number for grading shock severity.

How is cardiac index measured?

Cardiac output is measured first — by thermodilution through a pulmonary-artery catheter, by the Fick principle, or non-invasively by echocardiography — and then divided by body surface area. Learn the underlying methods in our hemodynamics study guide.

Key takeaways

• Cardiac index = cardiac output ÷ body surface area (L/min/m²).
• Normal is 2.5–4.0 L/min/m².
• Below 2.2 signals cardiogenic shock; above 4.0 signals a high-output state.
• It adjusts for body size, so it grades shock better than raw cardiac output.

Frequently asked questions

What is a normal cardiac index?

A normal cardiac index is 2.5–4.0 L/min/m². Below 2.2 suggests cardiogenic shock, and above 4.0 indicates a high-output state such as sepsis.

What is the cardiac index formula?

Cardiac index = cardiac output (L/min) ÷ body surface area (m²), expressed in L/min/m².

What is the difference between cardiac output and cardiac index?

Cardiac output is the total blood pumped per minute; cardiac index divides that by body surface area so values can be compared between people of different sizes.

What does a low cardiac index mean?

It means the heart isn't pumping enough blood for the body's size — a hallmark of cardiogenic shock and severe heart failure. A value under 2.2 L/min/m² is the classic cut-off.

What does a high cardiac index mean?

A high-output state such as sepsis, anaemia, hyperthyroidism, pregnancy, or a large arteriovenous shunt.

How do you calculate cardiac index from cardiac output?

Divide the cardiac output in litres per minute by the body surface area in square metres.

Why is cardiac index used instead of cardiac output?

Because it accounts for body size, letting clinicians compare and grade heart function fairly across small and large patients.

How is cardiac index measured?

Cardiac output is measured by thermodilution, the Fick method, or echocardiography, then divided by body surface area.