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*r2) * 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.

Parasternal long axis view – measuring LVOT diameter

In this case, the diameter is 1.75 cm. Then the cylinder’s cross sectional area = π*r2 = π*(1.75/2)2 ~ 2.405 cm2

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.

Apical 5 chamber – tracing LVOT VTI

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*r2) * h = (2.4 cm2) * (11.8 cm) = 28.3 cm3

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 ~ 3.0 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.

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6 Comments
  1. Giada says

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

    1. Rishi says

      Thanks Giada! 🙂

  2. Emelie says

    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 😀

    1. Rishi says

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

  3. Jake says

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

    1. Rishi says

      Hey Jake, you can find the post here.

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