Integrated circuits for an implantable bidirectional ultrasonic flowmeter

A bidirectional continuous wave Doppler ultrasonic velocity flowmeter was designed and fabricated with the required telemetry link for chronic implantation. This flowmeter uses a family of monolithic integrated circuits designed for low-power and low-voltage operation. These circuits operate at the optimum bias current during the life of the single cell mercury battery. The flowmeter was used to measure bidirectional pulsatile blood velocity in a canine descending aorta; the results compare well with simultaneous measurements using an electromagnetic flowmeter with percutaneous leads. Bidirectional detection was accomplished using a quadrature technique because it is simpler to implement than the frequency-offset or single-sideband phasing alternatives. Time division multiplexing was employed to transmit the two quadrature signals over one FM telemetry channel.