Remote Pressure Sensing and Aneurysm Repair Monitoring

Can this technology change the way we monitor patients after endovascular aneurysm repair?
BY ROSS MILNER, MD, AND JAN D. BLANKENSTEIJN, MD

To view the figure that corresponds with this article, please refer to the print version of our July/August 2003 issue, page 49.

Because as many as 10% of endovascular aneurysm repair (EVAR) patients per year will require some reintervention, be it endoluminal or open therapy,¹ aggressive follow-up to prevent aneurysm rupture is necessary. Computed tomographic angiography is the most widely used imaging modality for identifying problems such as aneurysm expansion, endoleak, or device migration. Some authors prefer magnetic resonance angiography (MRA) or ultrasound as their primary imaging modality. The information obtained from any of these studies is only valid in the setting of a previous study for comparison. It does not provide instantaneous information about the patient, and a comparison must be made to the previous study.

The endpoint of these studies is to identify problems that will lead to repressurization of the excluded aneurysm sac. Some authors have attempted to investigate pressure changes within an excluded aneurysm sac using invasive means.^2,3 These techniques are highly invasive and carry a small risk of infection. Routine puncture of a sac combined with the resultant infection risk could be a lethal problem.

A technological advancement that would allow for the identification of pressure changes within an excluded aneurysm sac would be welcomed with great enthusiasm.

REMOTE PRESSURE SENSING

Wireless, remote pressure sensors would provide an ad hoc appraisal of the status of an aneurysm sac. In other words, the systolic and diastolic pressures within the sac could be obtained at any moment in time. In fact, one of the postulated benefits of remote pressure sensing is that frequent checks of the excluded aneurysm sac could be performed without posing any risk to the patient. If an elevated pressure were detected, the appropriate imaging study could be obtained. It would be unnecessary for patients to undergo routine imaging studies unless a pressure change is noted. Therapy could then be directed toward an abnormality seen on a cross-sectional imaging study.

Work on pressure sensors remains experimental. Ohki et al⁴ demonstrated the efficacy of remote pressure sensing in a chronic canine model of abdominal aortic aneurysm using the CardioMEMS device (CardioMEMS, Inc., Atlanta, GA). The CardioMEMS sensor functions through radiofrequency technology. It is completely wireless and remote. The work by Ohki et al demonstrated the potential of the technology to function in this situation.

At the Society for Vascular Surgery meeting in May, we presented our initial results in an animal model using the ImPressure pressure sensor (Remon Medical Technologies, Caesarea, Israel). The sensor is a miniaturized device that measures 3 mm x 9 mm x 1.5 mm (Figure 1). It is an ultrasound-based technology that remains quiescent until acoustically activated. Once activated, acoustic energy is converted into electrical energy and a pressure measurement is made. The measurement is transferred to the monitor through acoustic energy. A printout of the pressure is then made.

Our work was in a porcine model with the surgical creation of an abdominal aortic aneurysm. The model allows for type II and type III endoleaks. The type II endoleaks are created acutely. The type III endoleaks were delayed and created in a remote fashion that would not disturb the thrombus within the sac or the sac itself.

This was the first demonstration of the utility of this sensor to detect type II and type III endoleaks in an animal model that mimics the human situation of an excluded AAA. We also demonstrated that the sensor can function through 2-week-old thrombus.

The translation of this experimental work to clinical application is in progress. In our model, we sutured the sensor to an interposition graft within the surgically created aneurysm. The placement of the sensor for clinical use is still being determined. It may be possible to place this sensor in the body or limb of an endograft without altering the size of the delivery system. This would allow continuous pressure monitoring of a patient after EVAR even if no complications are encountered. An alternative is a self-standing sensor that can be implanted after an endograft has been delivered. For instance, if an interventionalist is concerned about the status of a patient after an endograft has been inserted, it may be possible to deploy a sensor within the aneurysm sac. Pressure determinations can then be made and difficult decisions regarding management of the patient may become clearer.

FUTURE DIRECTIONS

We believe that remote pressure sensing has the potential of replacing routine imaging studies as the standard of care for follow-up of patients after EVAR. Several limitations are important to discuss here. Our initial work is based on short-term data, and long-term data are needed. Also of concern is the issue of compartmentalization of the aneurysm sac. It is possible that a single sensor within a sac might not be adequate to detect a problem at a distant site from the sensor. The answer to this dilemma is unknown. Finally, does the recognition of a pressurized aneurysm sac change patient outcome? As mentioned earlier, more frequent examinations are possible given the noninvasive nature of this technology. More frequent examinations might allow for the earlier detection of problems that can lead to aneurysm rupture, whereas an annual computed tomographic angiography allows for a long interval during which dramatic changes can occur. Remote pressure sensing may be able to eliminate this issue. 

Ross Milner, MD, is Assistant Professor of Surgery in the Division of Vascular Surgery at Emory University School of Medicine, Atlanta, Georgia. He holds no financial interest in any of the companies mentioned herein. Dr. Milner may be reached at (404) 778-5451; ross_milner@emoryhealthcare.org.

Jan D. Blankensteijn, MD, is Professor of Surgery and Chief, Division of Vascular Surgery, University Medical Center, St. Radboud, Nijmegen, The Netherlands. He holds no financial interest in any of the companies or products mentioned herein. Dr. Blankensteijn may be reached at (+31) 24-3615333; j.blankensteijn@chir.umcn.nl.

1. Sampram EK, Karafa MT, Mascha EJ, et al. Nature, frequency, and predictors of secondary procedures after endovascular repair of abdominal aortic aneurysm. J Vasc Surg. 2003;37:930-937.
2. Baum RA, Carpenter JP, Cope C, et al. Aneurysm sac pressure measurements after endovascular repair of abdominal aortic aneurysms. J Vasc Surg. 2001;33:32-41.
3. Sonesson B, Dias N, Malina M, et al. Intra-aneurysm pressure measurements in successfully excluded abdominal aortic aneurysm after endovascular repair. J Vasc Surg. 2003;33:733-738.
4. Ohki T, Yadav J, Gargiulo N, et al. Preliminary results of an implantable wireless aneurysm pressure sensor in a canine model: will surveillance CT scan following endovascular AAA repair become obsolete? J Endovasc Ther. 2003;10(Supplement I):32.