The heart's primary function is to pump blood throughout the body. Various measurements are used to determine the effectiveness of the heart's pumping action. Certain cardiovascular disorders can affect the ability of the heart to effectively pump blood, which can reduce blood delivery to vital organs such as the brain, kidneys and liver. An inadequate supply of oxygen-rich blood to these organs can result in a range of symptoms and complications. The cardiac index is one method of measuring how much blood is being pumped by your heart. Various methods can be used to calculate the cardiac index, from relatively simple to complex. Specialized medical equipment, or even hospitalization, is required to accurately determine cardiac index.
Traditional measure of cardiac function
Cardiac output is a traditional measure of the heart's ability to pump blood. The cardiac output represents the amount of blood ejected from the heart in one minute and is measured in liters. Calculating cardiac output is done by multiplying stroke volume, or the amount of blood ejected by the heart in one beat, by the heart rate, the number of heart beats in one minute. The normal range for cardiac output in a healthy adult at rest is 4 to 6 liters of blood per minute.
Measurement of cardiac index
A large person has a higher cardiac output than a small person. The cardiac index represents cardiac output that has been adjusted to a person's size. Dividing cardiac output by the person's body surface area, or BSA, will provide the cardiac index. In their book "Cardiac Physiology", Drs. David Mohrman and Lois Jane Heller report that cardiac output correlates better with body surface area than weight. Cardiac output that is expressed per square meter of body surface area is termed cardiac index.
Formulas for determining cardiac output and cardiac index
Calculating cardiac index requires you to first determine cardiac output. Cardiac output can be calculated by using the following formula:
Heart rate (beats/min) X stroke volume (mL/beat) = cardiac output (mL/min). The stroke volume is the amount of blood pumped by the heart with each beat, according to the Cardiovascular Physioloty Concepts website.
Proceed to calculate cardiac index using the following formula:
Cardiac output (mL/min) / body surface area (m2) = cardiac index (mL/min/m2).
Cardiac index normally ranges from 2.6 to 4.2 L/min/m2 in adults at rest.
Causes of low cardiac index
Certain conditions which reduce the pumping ability of the heart will result in a lowered cardiac index. Myocardial infarction, or heart attack, heart failure, cardiogenic shock and cardiomyopathy are all conditions in which heart muscle is damaged and weakened, thereby reducing the heart's ability to pump blood effectively. The less blood that is pumped by the heart, the lower your cardiac output and cardiac index.