## **Core Concept**
The cardiac index is a measure that relates the cardiac output from left ventricle in one minute to body surface area, thus relating heart performance to body size. It is an important parameter in assessing the heart's pumping efficiency. The formula for cardiac index (CI) is: CI = Cardiac Output (CO) / Body Surface Area (BSA).

## **Why the Correct Answer is Right**
The cardiac index is calculated by dividing the cardiac output by the body surface area. Cardiac output is the volume of blood the heart pumps per minute, and body surface area is an estimate of the body size. This ratio helps in comparing cardiac outputs between individuals of different sizes. It is expressed in liters per minute per square meter (L/min/m^2).

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the components used to calculate the cardiac index.
- **Option B:** This option is incorrect as it inaccurately represents the formula for cardiac index.
- **Option D:** This option is incorrect because it does not correctly represent the relationship between cardiac output, body surface area, and cardiac index.

## **Clinical Pearl / High-Yield Fact**
A normal cardiac index ranges from 2.6 to 4.2 L/min/m^2. This value is crucial in assessing cardiac function, especially in critically ill patients or those with heart failure. Remember, the cardiac index helps in understanding if the heart is pumping adequately for the size of the patient.

## **Correct Answer:** . Cardiac Output / Body Surface Area