Implantable Ultrasound Doppler Device that Measures Blood Flow: The Future of Bypass Graft Surveillance

Objective: Literature describing implantable ultrasound devices that measure blood flow is sparse. The majority are limited by their comparison of flow to electromagnetic flowmeters, which are outdated and no longer used clinically to monitor blood flow. We introduce a new implantable stamp-sized flow sensor device and evaluate its ability to accurately measure blood flow in an animal model using conventional duplex ultrasound as the clinical standard.
Methods: Acute non-survival in vivo experiments were performed to assess accuracy and performance of the device. A flow sensor made of 12-micron thick piezopolymer diffraction-grating transducer with silicone housing (12 x 10 mm), was placed on the infra-renal aorta of two 4.7 kg New Zealand White Rabbits under general anesthesia. The device was secured in close apposition with the infra-renal aorta. Sample aortic velocity measurements were recorded using both conventional duplex ultrasound, with probe placed directly on the infra-renal aorta and compared with implanted flow sensor measurements. With the abdomen closed, flow-volume measurements taken by the sensor were recorded every five minutes for 5 hours. Flow-volume error and reproducibility were calculated. Rabbits were euthanized at the end and the device retrieved.
Results: Aortic waveforms and peak systolic velocities between duplex ultrasound and sensor had excellent correlation. Measured blood flow by the implanted device was both consistent and accurate over 5 hours. Flow-volume error was +/- 6%, (cited variation for conventional duplex ultrasound <+/- 11%). The sensor’s reproducibility of velocity had a coefficient of variation of 2.9%.
Conclusions: Preliminary results show that a small, light, and flexible ultrasonic flow sensor can accurately measure blood flow in a living animal with minimal variability over 5 hours. This device has potential to be implanted around bypass grafts or other vascular reconstructions (i.e. organ transplants, tissue flaps) for continuous peri-operative and long-term surveillance. Further long-term animal studies assuring biocompatibility and functionality are required before going forward with human trials.