Ultrasound dilution technology is a synthesis of two widely accepted technologies: differential transit-time ultrasound and classic indicator dilution technology.

With transit-time ultrasound technology, a clip-on sensor transmits a beam of ultrasound through the blood line. Four transducers pass ultrasonic signals back and forth, alternately intersecting the flowing blood in upstream and downstream directions. The ELSA monitor derives an accurate measure of the changes in the time it takes for the wave of ultrasound to travel from one pair of transducers to the other (“transit time”) resulting from the flow of blood in the vessel. The difference between the upstream and downstream transit times provides a measure of volume flow.

Ultrasound dilution technology is used to measure ultrasound velocity. The velocity of blood (1560-1590 m/sec) is determined primarily by its blood protein concentration. When a bolus of isotonic saline (ultrasound velocity: 1533 m/sec) is introduced into the bloodstream the blood becomes diluted and its ultrasound velocity is reduced. The ELSA monitor and sensors measure these changes in ultrasound velocity and from the indicator dilution curves that the monitor generates is able to calculate actual flows.