what is cardiac index
Cardiac index is a way of measuring how well the heart is pumping, adjusted for a person’s body size.
It is calculated as cardiac output (the amount of blood the heart pumps in one minute) divided by body surface area, and is expressed in liters per minute per square meter L/min/m2\text{L/min/m}^2L/min/m2.
What is cardiac index?
- Definition: Cardiac index (CI) is cardiac output divided by body surface area, so it shows how much blood the heart pumps per minute for each square meter of body size.
- Why adjust for size? A large person naturally has a higher raw cardiac output than a small person, so indexing to body surface area helps compare heart performance fairly between different people.
In simple terms, cardiac output is “how much blood per minute,” while cardiac index is “how much blood per minute for a body of this size.”
The formula (made simple)
The standard formula is:
Cardiac index=Cardiac outputBody surface area\text{Cardiac index}=\frac{\text{Cardiac output}}{\text{Body surface area}}Cardiac index=Body surface areaCardiac output
Since cardiac output itself equals heart rate times stroke volume, you can also write:
CI=Heart rate×Stroke volumeBody surface area\text{CI}=\frac{\text{Heart rate}\times \text{Stroke volume}}{\text{Body surface area}}CI=Body surface areaHeart rate×Stroke volume
- Cardiac output is usually 4–8 L/min at rest in adults.
- Body surface area for adults is often around 1.6–2.0 m², depending on height and weight.
Normal values and what they mean
- Typical normal range in healthy adults: about 2.5–4.0 L/min/m² (some sources quote 2.6–4.2 L/min/m²).
- In older adults, slightly lower normal ranges (around 2.1–3.2 L/min/m²) are often accepted.
When cardiac index is low, it suggests the heart is not pumping enough blood for the body’s needs relative to the person’s size.
Why doctors care about cardiac index
Cardiac index is especially important in serious heart and circulation problems.
- It helps assess:
- Heart failure severity.
* Different types of shock (like cardiogenic shock), where CI may fall below about 2.0 L/min/m².
* Whether tissues are likely getting enough blood and oxygen overall.
Because it accounts for body size, CI is often more useful than raw cardiac output when doctors compare patients or track the same patient over time.
Quick illustration
Imagine two people have the same cardiac output of 5 L/min:
- Person A has a body surface area of 1.5 m² → CI ≈ 3.3 L/min/m².
- Person B has a body surface area of 2.2 m² → CI ≈ 2.3 L/min/m².
Even though the heart pumps the same total amount in both, Person B has a lower cardiac index, meaning that relative to their size, their heart is delivering less blood per unit body area.
TL;DR: Cardiac index is cardiac output divided by body surface area, showing how much blood the heart pumps per minute for a body of a given size; normal is roughly 2.5–4.0 L/min/m², and low values can signal serious heart or circulatory problems.
Information gathered from public forums or data available on the internet and portrayed here.