Cardiac Output Calculator

Cardiac output is the amount of blood that the heart pumps per minute and is one of the clearest single indicators of whether circulation meets the body's needs. In a healthy adult at rest, cardiac output is about five liters per minute, which is nearly the total volume of blood circulating through the body each minute.

This calculator starts with the basic formula that clinicians learn: Cardiac output is heart rate times stroke volume. One of three values can be hidden to calculate it. In addition, cardiac index using body surface area, ejection fraction using ventricular volumes and oxygen consumption are alternative methods used to estimate cardiac output.

What is cardiac output?

Cardiac output is the amount of blood that the heart pumps per minute. It's determined by two factors: how fast the heart beats and how much blood it pumps with each beat. The product of these two values gives cardiac output.

In a resting adult, the heart rate is about 70 beats per minute, and the stroke volume is about 70 mL, giving a cardiac output of about 5 L/min.

When you start training both of these values increase. The heart rate can go from 70 to 150 or even higher and the stroke volume increases from 70 ml to over 100 ml, thus increasing the cardiac output to over 15 L/min in order to supply enough oxygen to your muscles during exercise.

Here's how to use this calculator.

If you enter the heart rate and stroke volume per beat, then cardiac output is shown on the right side. If the cardiac output and one of the other two values are already known, then you can enter the value that you want to calculate in the appropriate field, and the calculator will immediately determine the result. In the units menu, you can change the units such as milliliters or liters without changing the calculation method.

In the three optional sections you can add additional values that are relevant for the treatment team at the bedside. If you activate the option "body measurements" and enter weight and size, you can calculate the heart index. If you activate the option "heart chamber" and enter filling volume at end diastole and filling volume at end systole, you can calculate the ejection fraction. By activating the option "fick method", cardiac output can be estimated from oxygen data.

Formula for calculating cardiac output:

The basic equation is to multiply heart rate by stroke volume per beat.

Example: A resting adult has a heart rate of 72 beats per minute and a stroke volume of 70 mL.

Multiplying these together gives 5,040 mL per minute, which is about 5.04 L/min.

Since two factors are multiplied together, by rearranging the equation one of the two factors can be calculated from the cardiac output. This is why this calculator allows for bi-directional calculations.

Heart factor and body surface area

Cardiac output of 5 L/min can have different meanings for people with smaller or larger physiques. So doctors calculate the cardiac index by dividing the cardiac output by body surface area. Body surface area is calculated using the Dubois formula, where weight in kilograms and height in centimeters are entered.

A normal cardiac index is generally between 2.5 and 4.0 L/min/m2. In patients with medical problems a value below about 2.2 indicates that the heart is unable to meet demand. Conversely, sepsis, anemia, and hyperthyroidism can lead to unusually high demands resulting in higher cardiac index values.

Calculation of ejection fraction from ventricular volume

If the volume of blood in the left ventricle at the end of filling (end-diastolic volume) and the volume at the end of contraction (end-systolic volume) are known, then the difference between these values is the stroke volume, and the percentage of blood ejected corresponds to the ejection fraction.

A normal ejection fraction is about 50 percent to 70 percent. If the EDV is 120 mL and the ESV is 50 mL, then the stroke volume is 70 mL and the ejection fraction is approximately 58 percent. A decrease in ejection fraction is a common sign of heart failure, where the ability of the heart to pump blood is weakened.

The fick principle for calculating cardiac output:

During a cardiac catheterization procedure the cardiac output is usually measured by other methods. The Fick principle states that the oxygen uptake of the body is equal to the product of blood flow and the amount of oxygen taken up by the blood as it passes through the lungs. Solving this equation for cardiac output gives:

The oxygen uptake is given in mL/min, the oxygen saturation of arterial and venous blood is expressed as a decimal number, and hemoglobin is measured in g/dL. 13.4 gives the amount of oxygen transported by one gram of hemoglobin, converted to liters of blood. The actual oxygen uptake is usually estimated at 125 mL/min per square meter of body surface area. 110 refers to a population aged 70 and over. It's rare for values estimated using the Fick principle and those derived from heart rate to be exactly the same, but comparing both can be useful.

Normal values at a glance:

The following table summarizes typical resting values used in this calculator. Deviations from these values do not necessarily indicate an abnormality but should be further investigated.

Factors that affect cardiac output.

The four main factors that play a role are: Preload, which describes the degree of stretching of the heart muscle before it contracts; generally speaking, more stretch leads to greater contraction. Contractility is the force with which the muscle contracts after filling; adrenaline increases contractility.

Afterload is the pressure that the heart must overcome to eject blood. Therefore, long term high blood pressure leads to a decrease in stroke volume. The autonomic nervous system constantly regulates both heart rate and contractility. When you are surprised, sprinting or at a tense meeting, the sympathetic nervous system is activated, which causes an increase in both heart rate and stroke volume simultaneously.

Tips for meaningful results:

The heart rate is easily measured precisely whereas stroke volume generally must be measured by echocardiography or MRI of the heart. So typical values are estimates rather than actual measurements. Be consistent with units. If stroke volume is in milliliters and heart rate is per minute, then cardiac output will be in milliliters per minute. Double check your weight and height when using heart factors as small errors here can change the factors significantly.