The primary goal of endovascular aneurysm repair (EVAR) is clearly to achieve and maintain de-pressurisation of the aneurysm sac. The development of a late occurring endoleak or the onset of endotension (sac enlargement without the presence of a detectable endoleak) will cause the excluded sac to remain pressurised and thus, increase the chance of aneurysm rupture. Yet, the standard post-surgical monitoring techniques, CT with IV contrast, angiography and duplex ultrasound, provide only surrogate measurements of intra-aneurysmal pressure. A method for chronic measurements of sac pressure would greatly enhance current protocols for endograft surveillance.
For the past several years, the endovascular community has been calling for the availability of technology that would provide this critical piece of information. Reports of post-EVAR rupture without visualised leaks are of particular concern and demonstrate that endoleak or endotension is not always detected in a timely manner (1). Harris stated in an editorial in the European Journal of Vascular and Endovascular Surgery “measurement of intrasac pressure is likely to be the best predictor of success or failure of treatment following endovascular repair of an abdominal aortic aneurysm. What is needed is a telemetric device that can be implanted safely at operation for remote long-term monitoring of intra-sac pressure” (2).
The ideal device to accomplish this task would be small and flexible so that it can be delivered endovascularly, and not require a battery or external wire connection. Flexibility is essential because effective communication with a sensor located deep within the body restricts miniaturisation, requiring that the sensor be capable of being folded into a catheter for delivery. Because these sensors do not contain a battery, an external energy source (radiofrequency) is necessary to energise the device.
In cooperation with the Georgia Institute of Technology, we have adapted a sensor originally designed for use within a turbine engine into a sensor that can be implanted endovascularly at the time of EVAR. Construction of the sensor required full utilisation of recent advances in MEMS (Micro-Electrical Mechanical Systems) technology, antenna design, and digital signal processing. MEMS and Micro-machining lends itself to the fabrication of small, flat flexible sensors that can be formed using biocompatible polymers as substrate materials and are accurate, precise and durable. Patients will be able to perform monitoring at home with a hand held transmitter receiver or at their doctor’s office.
Chronic evaluation of prototype sensors has been performed in animals at the Montefiore Medical Center (Bronx, NY) and ACRI (Norcross, GA). A simulated aneurysm containing a commercially available wired pressure sensor was surgically implanted in a canine model. The MEMS pressure sensor was then endovascularly placed into the sac using a custom designed delivery system and the aneurysm was treated with an AneuRx stent graft. The wireless measurement system was capable of measuring both the systolic and diastolic pressure along with producing a high-resolution pressure waveform. Chronic testing has demonstrated that the sensors are both accurate and highly stable. Human testing is planned.
References:
1. Schurink GW, Aarts NJ, Malina M, van Bockel JH. Pulsatile wall motion and blood pressure in aneurysms with open and thrombosed endoleaks—comparison of a wall track system and M-mode ultrasound scanning: An in vitro and animal study.
J Vasc Surg 32: 795-803, 2000.
2. Harris PL, Dimitri S. Predicting Failure of Endovascular Aneurysm Repair. Eur J Vasc Endovasc Surg 17, 1–2, 1999
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