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.
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.
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 cm3 ~ 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.