Cardiac Index

The heart is one of the most well-researched and vital organs. The state of its function and health can be described in many ways. The cardiac index is one way to evaluate heart function. It is based on cardiac output. The index converts the output into a standardised number that accounts for the patient’s body size. For example, the cardiac index of a person weighing 150 kg may differ from that of a person weighing 80 kg.

Cardiac Index

Cardiac index (CI) is a dynamic parameter that relates the cardiac output (CO) from the left ventricle of the heart in one minute to the body surface area (BSA) and thus, relating the performance of the heart to the size of the individual.

It calculates the heart’s output value depending on the size of the patient’s heart. To calculate the cardiac index, you need to divide an individual’s body surface area by the cardiac output. The cardiac index is calculated by dividing the volume of blood the heart pumps (litres per minute) by the body surface area. This can give you a simple difference in the cardiac indices of large and small individuals.

The function of Cardiac Index

The cardiac index generates a normalised number for heart function that efficiently corrects for the patient’s body size. The heart’s purpose is to maintain blood flowing at a suitable volume to fulfil the body’s present metabolic needs. The cardiac output varies according to the body size and the amount of exercise an individual does.

A normal range for resting cardiac output lies between 4.0 to 8.0 litres per minute. The normal value for the cardiac index should lie between 2.5 and 4.0 l/min/m2.

A cardiac index below 2.0 can be considered a cardiogenic shock.

Unit of Cardiac Index

Cardiac index is measured in litres per minute(l/min).

Factors Affecting Cardiac Index

Two factors determine cardiac index:

• Heart rate: The number of heartbeats per minute.

• Stroke volume index: The volume of blood pumped through the heart with each beat.

Formula to Measure Cardiac Index

Cardiac Index = Heart Rate x Stroke Volume Index/1000

Testing for Cardiac Index

There are a few techniques for measuring the cardiac index. The doctor selects the method that best fits the patient’s requirement based on the patient’s medical condition, need, and intensity.

These methods include non-invasive imaging techniques and intrusive pressure measurements. It is important to note that, while non-invasive methods can offer an appropriate figure of the readings, there is minimal evidence that the advantages of value exceed the dangers and issues associated with invasive procedures.

Since there is no ideal standard for assessing cardiac index, patients should be careful when selecting proper tests after considering the reasons for testing, the aims of testing, and their state.

Organ Systems Involved in Cardiac Index

The main organ related to cardiac index is the heart. Secondary organs are the autonomic nervous system, vascular system and endocrine system.

Uses of Cardiac Index

• Cardiac output helps quantify cardiac shunts and calculate pulmonary resistance.

• Cardiac index helps assess patients with heart disease, critically sick patients, and patients undergoing general anaesthesia.

Calculation of Cardiac Index

CO [L/min] = HR x SV

A simple way is to calculate cardiac output by multiplying the heart rate (HR) by the stroke volume (SV)

CI [L/min/m2] = CO/BSA

MRI in Cardiac Index

Multiplying flow volume by heart rate using velocity-encoded phase-contrast imaging gives cardiac output. The difference in antegrade and retrograde volumes, determined from mean velocity and the vessel’s cross-sectional area, can be used to compute net volume or stroke volume.

CO [L/min] = HR [bpm] x (antegrade – retrograde volume [mL] / 1000)

Conclusion

Cardiac index (CI) is a dynamic parameter that relates the cardiac output (CO) from the left ventricle of the heart in one minute to the body surface area (BSA) and thus, relating the performance of the heart to the size of the individual. Cardiac index generates a normalised number for heart function that efficiently corrects for the patient’s body size. The normal value for the cardiac index should lie between 2.5 and 4.0 l/min/m2. A cardiac index below 2.0 can be considered a cardiogenic shock.