Implant, performance, and retrieval of an atrial leadless pacemaker in sheep.

BACKGROUND: Medtronic is developing an atrial Micra Transcatheter Pacing System (Medtronic, Minneapolis, Minnesota) and associated retrieval system. OBJECTIVE: The purpose of this study was to evaluate chronic atrial Micra retrieval, reimplantation, and chronic pacing performance. METHODS: Sheep were implanted in 2 groups: group 1 (n = 6) for 6 months, a second device implanted, and first retrieved and studied for an additional 6 months; group 2 (n = 6) for 6 months, devices were retrieved, and a second device implanted and observed acutely. Both groups underwent histopathological evaluation. Pacing capture thresholds (PCTs), p wave amplitude, and pacing impedances were measured chronically. Device retrieval times were recorded, and intracardiac echocardiography was used. RESULTS: At 24 weeks, PCTs for group 1 were low and stable for both the first device (0.55 ± 0.14 V) and the second device (0.57 ± 0.09 V), in which the average retrieval time was 17:35 minutes. For group 2, the average retrieval time was 6:12 minutes, chronic PCTs in the first device were 0.53 ± 0.11 V, and acute PCTs for the second device were 0.71 ± 0.19 V. Pathological findings were within an expected range of tissue responses for similar Micra acute and chronic implants and device retrievals. p waves and impedances were stable and within an expected range for implant site and electrode design. Complications included 1 early dislodgment and 1 death attributed to a prototype retrieval tool. CONCLUSION: In an animal model, an atrial Micra can be easily implanted with excellent chronic pacing performance and is easily retrievable at 6 months. A second device can successfully be implanted with low, chronic stable thresholds. A developed prototype retrieval tool was easy to use and, with modifications, complication free.

Acoustic Properties of Small Animal Soft Tissue in the Frequency Range 12-32 MHz.

Quality assurance phantoms are made of tissue-mimicking materials (TMMs) the acoustic properties of which mimic those of soft tissue. However, the acoustic properties of many soft tissue types have not been measured at ultrasonic frequencies >9 MHz. With the increasing use of high-frequency ultrasound for both clinical and pre-clinical applications, it is of increasing interest to ensure that TMMs accurately reflect the acoustic properties of soft tissue at these higher frequencies. In this study, the acoustic properties of ex vivo brain, liver and kidney samples from 50 mice were assessed in the frequency range 12-32 MHz. Measurements were performed within 6 min of euthanasia in a phosphate-buffered saline solution maintained at 37.2 ± 0.2 °C. The measured mean values for the speed of sound for all organs were found to be higher than the International Electrotechnical Commission guideline recommended value for TMMs. The attenuation coefficients measured for brain, liver and kidney samples were compared with the results of previous studies at lower frequencies. Only the measured kidney attenuation coefficient was found to be in good agreement with the International Electrotechnical Commission guideline. The information provided in this study can be used as a baseline on which to manufacture a TMM suitable for high-frequency applications.

Broadband Acoustic Measurement of an Agar-Based Tissue-Mimicking-Material: A Longitudinal Study.

Commercially available ultrasound quality assurance test phantoms rely on the long-term acoustic stability of the tissue-mimicking-material (TMM). Measurement of the acoustic properties of the TMM can be technically challenging, and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique was proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range 4.5-50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and attenuation were very similar and within the International Electrotechnical Commission-recommended value. However, the results obtained using the proposed modified technique exhibited greater stability over the 1-y period compared with the results acquired using the standard technique.