Superior Vena Caval Flow:
Since left ventricular output includes blood about to pass through the PDA and right ventricular output includes blood which has already passed through the patent foramen ovale neither measure reflects true systemic blood flow. To circumvent this problem Nick Evans and Martin Kluckow realised the potential of measuring the volume of superior vena caval flow as a marker of upper body blood flow.
To measure the diameter of the vessel use an adapted parasternal long axis view - slide the probe superiorly and rotate slightly clockwise - essentially the SVC diameter view is a hybrid between the parasternal long axis view and the ductal view. Identify the ascending aorta then tilt the probe to look slightly to the subject’s right and the SVC should appear.
However most would agree that this view is hard to obtain. The appearance of spontaneous contrast can make the SVC distinctive. Be aware that the right atrial appendage wraps around the posterior aspect of the SVC, and can be mistaken for the SVC, especially on the M mode view.
Some of the difficulty in obtaining a reliable SVC diameter measurement may be due to the unusual shape of the SVC. In cross-section it appears oval, or even crescent-shaped as it adheres to the posterior wall of the ascending aorta:
For flow velocity classically a low subcostal view between the xiphisternum and the umbilicus is used. Tilt to look anterior, and slide as low as possible until you can see a clear length of the SVC flowing into the right atrium.
Flow can be measured by pulse wave Doppler. Most experts recommend taking a mean of 10 cycles due to the significant variation with respiration:
Flow velocity may also be measured from a suprasternal view, as originally described by Harabor et al. It may be that this method gives more robust measures, since it may be less afected by respiratory motion, and imaging further away from the right atrium may be a site with less turbulence, however again further study is required to fully assess the methodology.