The principal of access recirculation measurement is to see how much of substance injected in to the venous line is returned (or recirculated) back to the arterial line for measurement - there should be none. So with ultrasound dilution, the velocity of sound in a column of saline injected in the venous line is compared to measurement of sound velocity at the arterial line which should only be filled with blood. Any 'dilution' of the velocity at the arterial line will be due to recirculation. And this relationship is established by a known formula. Please see the attached PDF and diagram.