Doppler estimation of cardiac output
Continuous or pulse wave Doppler is aimed at an area of the left ventricular outflow tract (LVOT) or at the descending aorta.
Calculation of cardiac output:
Its not rocket science.
The cross-sectional area of the aorta, multiplied by the distance traveled by the column of blood, gives you the volume ejected per beat. Once you have stroke volume and heart rate, you have cardiac output.