The acoustic filter: an ultrasonic blood filter for the heart-lung machine.

Cardiopulmonary bypass-associated encephalopathy is thought to be due in part to continuous microembolization of the brain with gas microbubbles more than 40 microns in diameter during bypass. Current barrier filter technology cannot effectively remove such small microbubbles in fragile fluids such as blood. The design concepts for a new nonbarrier ultrasound-based fluid filtration system (an "acoustic filter") capable of filtering small microbubbles from blood are presented. The acoustic filter uses a field of high-intensity ultrasound to push microbubbles down an acoustic gradient, where they can be collected and removed. To test the filtration efficiency of the system, a Doppler ultrasound bubble detector was built. By monitoring the prefilter and postfilter Doppler signal an assessment of filtration efficiency was made. A suspension of stable albumin-encapsulated microbubbles (4 to 32 microns) were used as a model of the microbubble contaminants that might be found in the arterial return line of the heart-lung machine. Inactivated, the acoustic filter neither removed nor added microbubbles to the fluid. Activated, the acoustic filter provided total or near-total clearing of microbubbles. We conclude that the acoustic filter can remove microbubbles from a cardiopulmonary bypass-like apparatus.