Left Ventricular Output:
This measurement is robust and well validated. However in the newborn infant it is obviously confounded by ductal shunting.
Measure the aortic diameter from the parasternal long axis view.
Zoom in on the valve, and try to get the outflow tract to look as broad as possible.
Then freeze the image, and scroll back to measure the diameter of the valve hinge points at end systole.
M mode measurement is not recommended as the aortic valve moves downwards during systole, and accurate placement of the M mode plane is essentially impossible.
For flow velocity use a modified apical 4 chamber view. As much as possible slide the transducer to the subject’s left, and caudally, before rotating clockwise and tilting to look anterior to the subjects right shoulder (this is the 'five chamber view').
Always use pulsed wave Doppler to prevent contamination from other sites of flow