Measuring the cardiac output is straightforward, and relies on a few simple concepts which we review here:
- The Left Ventricular Outflow Tract (LVOT) is assumed to be roughly circular. Measure a diameter and you can calculate the area of the circle.
- Pulsed Wave Doppler (PW) through the same point, in the centre of the LVOT tells us how fast that blood is travelling at any time.
- The area under the curve then tells us how far the column of blood has been pushed. (Y axis is in m/sec, X axis in seconds, so the area under the curve will tell us how far the blood has moved travelling at these velocities for this amount of time.)
- Work out the volume of the cylinder – Multiply the area of the LVOT (a circle) by the length the blood travels and you get the stroke volume (ie volume ejected per beat)
- The stroke volume multiplied by the heart rate gives us the cardiac output (expressed as L/Min).
- Divide the cardiac output by the body surface area and we get the Cardiac Index.
To summarise the required echo steps:
Measure the LVOT diameter (Parasternal long axis, 2D). Zoom in to be accurate. Measure up to 0.5cm back from the aortic valve leaflet insertion points (on the ventricular side).
Using pulse wave Doppler (PW) line up the LVOT in the apical views, using either the apical 5 chamber or the apical 3 chamber. The flow of blood is laminar through the PW gate, which is why the all the velocities follow a narrow band and the PW waveform is not “filled in”. The PW gate should be 2-4mm.
Obtain the PW waveform. To get the most accurate reading, move sample volume toward aortic valve until flow accelerates
b) Then move sample volumes slightly away from the aortic valve, toward apex until laminar flow returns.
In a surface echo, the blood flows through the LVOT away from the probe so the curve is below the line. It should look hollow if the blood has laminar flow. Trace along the edge of the modal velocity (the outside of the chin, not the beard of the waveform) to measure the area under the curve (the Velocity Time Integral – VTI expressed in cm).
References:Back to basics cardiac output echocardiography featured TTE VTI