A crucial component of ECMO circuits are the flow sensors. At minimum, they provide measurement of blood flow rate and bubble detection. Newer sensors can also estimate hematocrit and oxygen saturation. The flow sensor is incorporated into the downstream side of the centrifugal pump. An additional clip-on sensor is sometimes included downstream of the oxygenator. In more advanced compact systems such as the Maquet CARDIOHELP, the sensors are built into the pump/oxygenator disposable component.

ECMO Ultrasonic Flow Bubble Sensor

Blood Flow Sensor

With positive displacement pumps, the blood flow flux through the pump is determined solely by the pump speed. However, with centrifugal flow pumps that rotate at set speeds, the flow rate through the pump also depends on the upstream and downstream pressures. Therefore, an external sensor is required to provide the blood flow rate information.

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The sensors work by by measuring the transit-time of ultrasonic pulses through the blood flow. The flow sensor surrounds the ECMO circuit tubing. Ultrasonic impulses are sent from one piezo crystal to another (either directly in an X-pattern or by a reflector with the crystals acting alternately as transmitters and receivers depending on the manufacturer). The transit time taken by each impulse is determined, and the differences times taken by the impulses flowing upstream and the ones flowing downstream are proportional to the flow rate.

ECMO Ultrasonic Flow Sensor Doppler Transit Schematic

Even though the physical path lengths of the ultrasound beams within the the sensor are the same, the transit time is longer for the beam going upstream is longer because of the Doppler effect. An object moving towards the ultrasonic beam compresses the wave, thereby increasing the signal’s frequency, whereas an object moving away from the beam reduces the signal’s frequency. The change in frequency, which is also termed the Doppler shift, provides information about the object’s speed and direction of motion.

Bubble Detection Sensor

Bubble detection is critical to maintain the steady-flowing circuit and prevent complications from air embolism especially with veno-arterial ECMo circuits. Non-invasive ultrasonic bubble sensor uses an active piezoelectric element as a piezoelectric transmitter to generate a high frequency acoustic wave. This acoustic wave travels through the sensor wall and is coupled (using coupling paste) to the tubing that is in contact with that wall. The wave then travels through the fluid-filled tubing to the opposite sensor wall and is received by a passive piezoelectric element on the other side (figure below).

ECMO Ultrasonic Bubble Detector Acoustic Window

The ability to sense when air is present is due to the large acoustic impedance difference that exists between the tubing wall or fluid and air (igure below). This large impedance mismatch creates an acoustic mirror which reflects the ultrasonic wave back in the direction of the transmitter. Since energy does not reach the receiver side, the sensor will indicate the presence of air.

ECMO Ultrasonic Bubble Detector Acoustic Mirror

Blind spots are areas within the tubing that do not lie in the acoustic path. Since they are not in the acoustic path, ai