How to Determine Cardiac Output

Three Methods:Determining Heart RateDetermining Stroke VolumeUnderstanding the Factors Affecting Cardiac Output

The term cardiac output refers to the quantity of the blood that your heart can pump in one minute, represented in litres per minute.[1] Cardiac output indicates how effectively your heart is supplying oxygen and nutrients throughout your body. It shows how well your heart is performing in relation to the rest of your cardiovascular system. In order to determine cardiac output, you need to determine both stroke volume and heart rate. This can only be done by a professional who will use an echocardiogram.

Method 1
Determining Heart Rate

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    Get a stopwatch or clock. The heart rate is simply the number of heartloads of blood that are ejected from the heart per unit of time.[2] Typically we measure heart rate by beats per minute. Measuring your heart rate is simple, but before attempting it, make sure you have an accurate device to count the seconds.
    • You can attempt to keep track of the the beats and seconds in your head, but this could be inaccurate.
    • It is better to have a timer set, so you can focus on counting the beats.
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    Find your pulse. Although there are many places on your body where you can find a pulse, the inner wrist is usually an easy place to find it. An alternative is on the side of your throat, on the jugular vein area. Once you have identified your pulse and have a clear beat, take the forefinger and middle finger of one hand and place them over the area where you can feel the pulse.[3]
    • You may need to move your fingers around a bit to find your heartbeat.
    • You may also need to apply a little pressure to feel it.
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    Begin counting the beats. Once you find your heartbeat, start your stopwatch or look at your clock with the second hand, wait until the second hand reaches the 12, and begin counting the beats. Count the beats for one minute (until the second hand returns to the 12). The total number of beats per minute is your heart rate.
    • If you find it difficult to count your heartbeats for an entire minute, you can count for 30 seconds (until the second hand reaches the 6) and then multiply that number by two.
    • Or you can count for 15 seconds and multiply it by four.

Method 2
Determining Stroke Volume

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    Have an echocardiogram. Whereas the heart rate is simply the number of times your heart beats in a minute, the stroke volume is the volume of blood pumped out by your heart's left ventricle with each beat. It is measured in millilitres and is much more complicated to determine that your heart rate.[4] A special test called an echocardiogram (aka echo) is used to determine your heart’s stroke volume.
    • An echocardiogram uses radio waves to create a picture of your heart so the volume of blood passing through it can be measured.[5]
    • An echocardiogram makes it possible to make the measurements of the heart that are required to calculate stroke volume.
    • Using the echocardiogram, you will be able to determine the numbers needed for the following calculations.
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    Calculate the area of the left ventricle’s outflow tract (aka LVOT). The left ventricle outflow tract is the portion of your heart through which blood passes to enter your arteries. In order to calculate the stroke volume you will need to determine the area the left ventricular outflow tract (LVOT), and the velocity time integral of the left ventricular outflow tract (LVOT VTI).
    • These calculations need to be done by a done professional reading a echocardiogram.[6] An expert may use the following equation to determine the area of the left ventricle’s outflow tract:
    • Area = 3.14 (LVOT diameter/2)^2[7]
    • This way of calculating the area is now beginning to be superseded by more advanced imaging technology.[8]
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    Determine the velocity time integral. The velocity time integral (VTI) is the integral of the velocities during the period of flow in a vessel or through a valve. In this instance it is used to determine the amount of the blood flowing through a ventricle.To determine the left ventricle’s VTI, your technician will measure the flow by doppler endocardiography. To do this the technician will use the tracing function on an endocardiography machine, which will then calculate the VTI.[9]
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    Assess stroke volume. To determine stroke volume, the amount of the blood in a ventricle (heart chamber) at the end of a beat (end-systolic volume, ESV) is subtracted from the amount of the blood in a ventricle just before a beat (end-diastolic volume, EDV). Stroke Volume = ESV – EDV.[10] While stroke volume usually refers to the left ventricle, it can also refer to the right ventricle. The stroke volume of both ventricles is usually equal.
    • To determine your stoke volume index, take the velocity time integral, which is the amount of blood being pumped with each heartbeat and divide it by the body surface area of the left ventricle (in square meters).
    • This formula allows direct analysis of the stroke volume for a patient of any size.[11]
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    Determining your cardiac output. Finally, to determine your cardiac output, multiply your heart rate by your stroke volume. This is a relatively simple calculation which identifies the amount of blood your heart pumps in one minute. The formula is Heart Rate x Stroke Volume = Cardiac Output.[12] For example, if your heart rate is 60 bpm and your stroke value is 70 ml, the equation looks like this:
    • 60 bpm x 70 ml = 4200 ml/min or 4.2 liters (1.1 US gal) a minute.[13]

Method 3
Understanding the Factors Affecting Cardiac Output

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    Understand how heart rate functions. You can get a fuller understanding of cardiac output by learning about what influences it. The most straight forward of these is heart rate, the number of beats the heart makes in a minute. The more it beats the more blood is pumped all over the body. A normal heart should beat at 60-100 in a minute. When the heart rate is too slow, it is also called bradycardia, a condition in which the heart ejects too little blood in circulation.[14]
    • If your heart beats really fast it may cause tachycardia (a heart rate that exceeds the normal range) or, in severe cases, arrhythmia (problem with the rate or rhythm of the heartbeat).
    • Although you may think that the faster the heart beats the more blood is circulated, in fact the heart ejects less blood with each stroke.
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    Learn about contractility. If you are interested in the influences of physical fitness on cardiac output learn about contractility. Contractility is the ability of the muscle to contract. The heart is made up of muscles that contract in a certain pattern to eject blood. As the heart contracts, for example during exercise, this equates to increased cardiac output.[15]
    • The stronger the heart contracts, the more blood it circulates.[16]
    • This is what is affected when a piece of the heart muscle dies, and the heart is able to eject less blood during circulation.
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    Investigate the importance of preload. Preload refers to the stretch of the heart before shortening and impacts on cardiac output. According to Starling’s law, the force of contraction depends on the length to which the heart muscle is stretched. Therefore, the greater the preload the greater the force of contraction, which results in a greater amount of blood being pumped by the heart.[17]
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    Analyse afterload. The final key factor that influences cardiac output and is connected to the condition of the heart is known as afterload. Afterload is simply the amount of force that the heart needs to overcome to pump blood, which is highly dependent on the tone of the blood vessels and arterial blood pressure. A decrease in afterload can increase cardiac output especially in cases when the heart’s contractility is impaired, as often seen in heart problems.[18]

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Categories: Cardiovascular Health and Blood Pressure