# Cardiac Output With LVOT Measurements Using Echocardiography

It’s possible to utilize **transthoracic echocardiography **(TTE), a noninvasive technique, to estimate a patient’s cardiac output. I’ll use some images from an echo I performed on a patient with a depressed left ventricular ejection fraction on low dose pressors. Yes, I know the echo images aren’t perfect. 😛

We know that cardiac output (CO), stroke volume (SV), and heart rate (HR) are related as follows:

**CO = HR * SV**

A cylinder’s **area = (π*r ^{2}) * h** where ‘r’ is the

**radius**of the cross sectional area and ‘h’ is the cylinder’s

**height**. If we model the

**left ventricular outflow tract**(LVOT) as a cylinder, this area would represent SV.

The radius of LVOT cylinder would be half the LVOT’s diameter measured at the aortic valve’s annulus during systole. This can be measured on the **parasternal long axis view**.

In this case, the diameter is 1.75 cm. Then the cylinder’s cross sectional area = π*r^{2} = π*(1.75/2)^{2} **~ 2.405 cm ^{2}**

Going back to calculus, we know that the integral of velocity over time (ie, the area under the curve) is equal to distance. In this case, the **velocity-time integral** (VTI) represents the distance that blood travels. This represents the “height” of our cylinder model and is obtained by applying pulse-wave doppler (PWD) across the LVOT in the apical 5 chamber view.

The blue tracing calculated the VTI as 11.8 cm. This represents the LVOT cylinder’s height. Now let’s put it all together:

**SV = (π*r ^{2}) * h = (2.4 cm^{2}) * (11.8 cm) = 28.3 cm^{3}**

Now that we know the SV, we multiply it by the HR (105 bpm at the time) to calculate the cardiac output:

**CO = HR * SV = 105 bpm * 28.3 cm ^{3 } ~ 3.0 liters/minute. **

We use the LVOT as a reference point because its diameter remains fairly fixed throughout systole and diastole; however, if the LVOT diameter is measured inaccurately, this error is squared. Additionally, the LVOT tends to be more **ellipsoid** rather than cylindrical, so the SV measurement may not be completely accurate. Keep these drawbacks in mind when interpreting CO from LVOT.

not a doc here but stumbled on this. i did not understand one word!! you are one smart cookie

Thanks Giada! 🙂

So well explained! This made it so much easier to understand, now I’ll know this forever! Thank you 🙂 I’m looking forward to another IG live! Even though she is like 3 am for me it s all worth it 😀

Thanks so much for the kind words! Feel free to always ask questions! 🙂

Nice! Have you posted the personal statement on the site? Would love to see it! Thank you!

Hey Jake, you can find the post here.