Cardiac output (CO) is the total volume of blood moved by the heart per minute. It is a product of the left ventricular stroke volume (SV) and the heart rate (HR).
Cardiac output (mL/min) = Stroke Volume (mL/cycle) x Heart Rate (bpm)
By being able to calculate a patient’s cardiac output you can quantitively assess the patient’s cardiac function, optimize vasopressors and fluid resuscitation in critically ill patients, and even distinguish between undifferentiated shock types in critically ill patients.
But how can one estimate cardiac output? Early clinicians relied on using the Fick’s principle to measure cardiac output through thermodilution via a pulmonary artery catheterization. This required an invasive and potentially dangerous procedure, which wasn’t feasible for rapid assessment.
Sonographers in the 1980s began experimenting with alternative methods of estimating cardiac output such as Doppler-assisted calculations that would be more rapidly accessible to clinicians outside of the ICU.
The main barriers for learners attempting to obtain these measurements with ultrasound are:
- Pulse wave Doppler seems too advanced
- There seem to be too many measurements and math following
- Interpreting the waveforms seems difficult and intimidating
Well, the good news is that you only really need to learn how to do TWO measurements: LVOT Diameter and Velocity Time Integral (VTI) to calculate cardiac output with ultrasound. Yes, that’s it!
In this post you will learn:
- How to actually understand the concept of cardiac output and the basic physiology surrounding it
- How to obtain the left ventricular outflow tract (LVOT) diameter to determine the LVOT area
- How to obtain the LVOT velocity time integral (VTI)
- How to use these values to calculate stroke volume and cardiac output
To make this easier, we have put all of the details in a Cardiac Output PDF pocket card, so you can download and keep all of the most important information right at your fingertips. Click HERE to download.
Check out our stroke volume/cardiac output calculator below! You can also always access it HERE is well.
How to Think about Cardiac Output
The first thing we need to focus on when trying to calculate cardiac output is figuring out what the stroke volume is.
This just involves estimating the amount of blood (stroke volume) that comes out of the left ventricle at the left ventricular outflow tract (LVOT). If you think of the volume coming out of the LVOT as the shape of a cylinder you will just need two measurements to make the stroke volume calculation.
First is the LVOT Diameter and the second is the distance that one blood cell travels during one heartbeat (this is calculated by using the velocity-time integral or VTI). The stroke volume is then simply calculated using the formula for the volume of a cylinder.
Once you have the stroke volume just multiply by the heart rate to get the Cardiac Output!
Now that you understand these simple principles let’s show you exactly how to obtain these measurements to calculate SV and Cardiac Output.
Machine and Patient Preparation for Exam
You will need to be able to obtain the parasternal long axis and apical 5 chamber views to measure cardiac output. For a full post on how to obtain each cardiac view in detail, check out our post HERE.
- The patient should be supine or in the left lateral decubitus position
- Ideally, place the machine on the patient’s right side so you can scan with your right hand and manipulate ultrasound buttons with your left hand.
- Transducer: Phased Array Probe
- Exam Preset: Cardiac
- The Indicator should be on the RIGHT side of the screen (cardiac mode)
- You will need a cardiac package on your ultrasound machine in order to do these measurements. If you don’t, unfortunately, you will not be able to measure the velocity time integral (VTI) described below.
- You must also have Pulse Wave Doppler capability on your ultrasound system. Most cart-based systems have this, but unfortunately, most hand-held ultrasounds don’t have Pulse Wave Doppler. For a review on how pulse wave doppler works click HERE.
Step by Step Guide to Cardiac Output Measurement
Below we describe the exact steps for you to measure cardiac output in your patients using ultrasound.
Editor’s Note: Our team published one of the first studies using POCUS to measure cardiac output (Dinh 2012). We found it takes about 20 cardiac output measurements until you start getting reliable results. So if you are first starting out don’t be discouraged and keep practicing!
STEP 1: Get the Parasternal Long Axis View
Obtain a Parasternal Long Axis (PSLA) View. You want to clearly visualize your left ventricular outflow tract (LVOT) and the aortic valve. Look to ensure you can see your aortic valve opening and closing.
STEP 2: Measure LVOT Diameter
Once you have obtained the PSLA view, freeze your screen when you have the best view of your aortic valve at mid-systole (when the valves are wide open).
You can also use the zoom feature on your ultrasound machine to get a better view.
Click the calculation package on your ultrasound machine and choose LVOT Diameter. Using your measure tool, measure near the aortic annulus at the base of the leaflets. This is the diameter of your left ventricular outflow tract (LVOT), typically this measurement is around 2 cm.
STEP 3: Get the Apical 5-Chamber View
Using the same phased array transducer, obtain an Apical 5-Chamber View. Again, you will need to visualize the LVOT and aortic valve.
STEP 4: Place PW Doppler Gate at LVOT
Next, position the pulse wave Doppler gate at the LVOT in the Apical 5 Chamber view. This is at the aortic annulus, or base of the aortic valve leaflets. Make sure to align the pulse wave Doppler gate and LVOT as parallel as possible in order to get the best VTI tracing. The more your angle is off the more you will underestimate the cardiac output.
For more information on how to use the pulse wave Doppler, click HERE.
STEP 5: Trace LVOT VTI
Once your Doppler gate is in a good position, activate your pulse wave Doppler.
Then press the Freeze button and then select the cardiac calculation package. Look for LVOT VTI (every machine will have a different way of accessing this calculation).
You will then trace the outline of one of the systolic waveforms (yellow outline). The machine will then calculate the LVOT VTI for you. The LVOT VTI will output as a distance in centimeters (cm) and represents the distance that blood travels in one heartbeat.
POCUS 101 Tip: A rule of thumb we like to use is that a normal LVOT VTI is around 20cm. This is calculated that by assuming a normal stroke volume is 60ml and an average LVOT diameter is 2cm. If you use the cylinder formula to back-calculate this, a “normal” LVOT comes to just about 20cm!
Editor’s Note: Some ultrasound machines will have an “auto tracing VTI” package that does not make you trace the wave and you just need to place the cursor at the beginning and end of the waveform. The ultrasound then auto traces the VTI for you. I find these are hit or miss. Just make sure the tracing is accurate when the computer does it for you.
Here is a quick video we made summarizing exactly how to get the apical 5 chamber view and use the pulse wave Doppler to calculate the LVOT VTI:
STEP 6: Measure Heart Rate
The last step in calculating the cardiac output will be to just get the heart rate.
Measuring the patient’s heart rate can be done using the cardiac package on your ultrasound machine or you can use the heart rate from the patient monitor.
If calculating the heart rate using the ultrasound machine, press the heart rate function and then just place the cursor from one peak of a wave to another peak and the machine should automatically calculate the heart rate for you (the example below shows a heart rate of 75 bpm).
STEP 7: Calculate Cardiac Output
Now after you have completed the 3 measurements (LVOT diameter, LVOT VTI, and heart rate) the machine should automatically calculate the cardiac output for you and it should appear on the screen.
For example below, the cardiac output is 2.46 L/min
However, you can also calculate the Cardiac output yourself if you are having any issues:
|Step 1||Calculate the LVOT area||LVOT Area = π x (LVOT Diameter/2)2 LVOT Area = _______________ cm2|
|Step 2||Calculate Stroke Volume||SV (mL) = LVOT area (cm2) x VTI (cm) SV= _______________ mL|
|Step 3||Calculate Cardiac Output||CO (mL/min) = SV (mL/cycle) x HR (bpm) CO = __________________ mL/min|
Don’t forget to download the Cardiac Output Pocket Card summarizing these values and formulas.
Or you can use this calculator below for even more ease!
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- Schuster AH, Nanda NC. Doppler echocardiographic measurement of cardiac output: Comparison with a non-golden standard. The American Journal of Cardiology. 1984;53(1):257-259. doi:10.1016/0002-9149(84)90723-9
- Zhang Y, Wang Y, Shi J, Hua Z, Xu J. Cardiac output measurements via echocardiography versus thermodilution: A systematic review and meta-analysis. Plos One. 2019;14(10). doi:10.1371/journal.pone.0222105
- Costanzo LS. Cardiac Physiology. In: Physiology. Philadelphia, PA: Saunders/Elsevier; 2014:145-158.
- Dinh, V. A. et al. Measuring cardiac index with a focused cardiac ultrasound examination in the ED. Am J Emerg Medicine. 30, 1845–1851 (2012).