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Ann Thorac Surg 2010;89:446-452. doi:10.1016/j.athoracsur.2009.10.068
© 2010 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Utility of Remote Wireless Pressure Sensing for Endovascular Leak Detection After Endovascular Thoracic Aneurysm Repair

Cyrus J. Parsa, MDa, Mani A. Daneshmand, MDa, Brian Lima, MDa, Keki Balsara, MDa, Richard L. McCann, MDb, G. Chad Hughes, MDa,*

a Division of Thoracic and Cardiovascular Surgery, Duke University Medical Center, Durham, North Carolina
b Section of Vascular Surgery, Department of Surgery, Duke University Medical Center, Durham, North Carolina

Accepted for publication October 26, 2009.

* Address correspondence to Dr Hughes, Thoracic Aortic Surgery Program, Division of Thoracic and Cardiovascular Surgery, Department of Surgery, Box 3051, Duke University Medical Center, Durham, NC 27710 (Email: gchad.hughes{at}duke.edu).

Presented at the Poster Session of the Forty-fifth Annual Meeting of The Society of Thoracic Surgeons, San Francisco, CA, Jan 26–28, 2009.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: The goal of thoracic endovascular aneurysm repair (TEVAR) is to exclude and depressurize the aneurysm sac. Type I and III endovascular leaks (EL) transmit systemic pressure and represent treatment failures. The significance of type II EL is more controversial. Remote pressure sensing is a novel nonradiographic technology for EL detection and monitoring. However, little experience exists with regard to use in the thoracic aorta. We present our experience with the EndoSure wireless pressure measurement system (CardioMEMS, Atlanta, GA) for monitoring aneurysm sac pulse pressure (ASP) after TEVAR.

Methods: Beginning May 2006, the EndoSure system was routinely implanted in TEVAR patients with suitable anatomy (36 aneurysm patients; 7 chronic dissection patients). The ASP measurements were taken predischarge and at scheduled follow-up visits. Computed tomography angiograms were performed at scheduled follow-up appointments. Data were prospectively maintained in an institutional aortic database.

Results: Through June 2008, 43 patients (34% of TEVARs performed during this interval) underwent implantation. In 10 patients (23%), the device was suboptimally positioned between the endovascular graft and the aortic wall, rather than in an area of thrombus-free lumen, with subsequent transmission of systemic pressure despite no radiographic evidence of EL. In patients with well-positioned sensors, predischarge ASP averaged 43% ± 22% of systemic. In 2 patients, systemic ASP measurements before discharge prompted imaging, confirming type I EL; both patients were treated successfully with cuff extension. One patient exhibited reduced ASP before discharge but exhibited increased ASP (70% systemic) at 1 month; computed tomography scan confirmed a type I EL. Additional TEVAR sealed the EL and reduced ASP to 39% systemic. For all patients at midterm follow-up, ASP decreased further, averaging 19% ± 12% systemic (p = 0.019); this correlates with computed tomography imaging demonstrating a 5 mm or greater reduction in aortic diameter in 76% of patients (25 of 33) with follow-up of 6 months or longer. No patients manifested a recurrent type I or type III EL at latest follow-up. The device has also been used to follow 8 patients with type II EL with low ASP.

Conclusions: Implantation of a wireless ASP sensor provides useful information regarding type I and type III EL after TEVAR and permits serial observation of type II EL. This information may guide clinical therapy and improve outcomes. Longer term follow-up will define sensor reliability in postoperative surveillance.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Thoracic endovascular aortic repair (TEVAR) has become an integral facet of the surgical armamentarium to contend with various pathologies of the descending thoracic aorta [1, 2]. Endovascular graft deployment excludes the aneurysmal aortic segment from the circulation and systemic pressure, thus mitigating the risk of expansion and rupture. Numerous published reports emphasize the need for long-term postoperative surveillance imaging [1, 2] owing to the late risk of endovascular graft migration or disruption, endovascular leak (EL) development (Table 1), aneurysm expansion or rupture, and disease progression in the remnant of native aorta [1]. Diagnostic surveillance modalities include serial computed tomography (CT) or magnetic resonance angiograms and some form of intravenous contrast agent; these procedures foster additional risk including radiation exposure for CT and potential renal and systemic toxicity secondary to contrast agents.


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  		Table 1 Classification of Endovascular Leaks
 
Remote wireless pressure sensor monitoring is a novel technology with potential benefit for postoperative surveillance of thoracic endovascular grafts. While radiologic surveillance is essential to the care of this patient population, measurement of aneurysm sac pressure (ASP) is linked to effective therapeutic exclusion of the aneurysm sac [3]. Importantly, ASP sensors monitor the ideal metric of endovascular graft efficacy, namely sac pressure, while changes in aneurysm size as well as contrast enhancement of the aneurysm sac serve as indirect surrogates for ongoing pressurization of the sac.

Clinical feasibility trials confirm device durability and efficacy and conclude that a reduction in the ASP ratio of less than 30% (ie, sac pulse pressure remains greater than 70% of systemic pulse pressure) after endovascular graft deployment is consistent with type I or type III EL (sensitivity 94%, specificity 80%) [4]. The US Food and Drug Administration (FDA) has approved device utilization for intraoperative detection of ELs in the thoracic aorta. However, FDA approval excludes longer-term surveillance, although this is an important potential utilization of the technology.

While short-term (1-month postoperative) data series exist for endovascular abdominal aortic repair [4, 5], little information is available regarding application of this technology in TEVAR [6]. This technology may mitigate the frequency of scheduled imaging, and perhaps more importantly, prompt urgent imaging if an abnormality is detected. We hypothesized that ASP would be a useful adjunct in TEVAR cases with suitable anatomy. Therefore, ASP data were gathered prospectively in an institutional thoracic aortic database and retrospectively evaluated for intraoperative and midterm results, specifically with regard to EL detection both early and late after TEVAR for degenerative aneurysms due to atherosclerotic disease and chronic dissection.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Acknowledgments
 References
 
Between May 2006 and June 2008, 125 patients underwent TEVAR at our institution for the treatment of descending aortic pathology, of whom 43 (34%) underwent concomitant placement of the EndoSure device (CardioMEMS, Atlanta, GA). The device was placed in patients meeting the following criteria: (1) at least 10 mm of anticipated thrombus-free lumen remaining after stent graft deployment to prevent "sandwiching" the device between the aortic wall and the endograft with subsequent erroneous trans-graft transmission of systemic pressure (Fig 1); and (2) suitable access vessel anatomy as described in further detail below. Aortic pathology in patients undergoing EndoSure placement included descending thoracic aneurysm (n = 36, 33 fusiform and 3 saccular) and chronic type B dissection with secondary aneurysmal degeneration (n = 7; Fig 2). The study was approved by the Duke Institutional Review Board (IRB) who waived the need for individual patient consent.


Figure 1
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  		Fig 1. Axial computed tomography angiography images of (A) a "sandwiched" aneurysm sac pressure sensor between stent graft and aortic wall, and (B) a well-positioned sensor with adequate space between the endovascular graft and aortic wall. The sensor in (A) measured systemic transendovascular graft pressure despite successful aneurysm sac exclusion. Arrows depict sensor location.

 

Figure 2
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  		Fig 2. Patient allocation: distribution of the 43 patients who received an aneurysm sac pressure sensor in the study interval. The principal indications included atherosclerotic descending thoracic aneurysm and chronic type B dissection with aneurysm (aneurysm sac pressure sensor placed in the aneurysmal false lumen adjacent to covered primary tear). (EL = endovascular leak; TEVAR = thoracic endovascular aortic repair.)

 
The mechanism of the EndoSure sensor has been described [7]. Briefly, alterations in surrounding ASP change the capacitance, and therefore the resonant frequency, of the sensor. An external antenna activates the sensor through a radiofrequency impulse and receives the pressure-dependent change through the resonant frequency of the sensor that is converted into a real-time pressure measurement (Fig 3). During the course of this study, the design of the device changed to a basket and dual-loop design, which we implanted as of April 2007. Twenty-three patients received the first-generation EndoSure sensor and 20 patients received the second-generation EndoSure s2 device (Fig 4). The mechanical advantage of the s2 device is that the hoops are tethered to the delivery catheter, which secures the sensor for delivery. Additionally, the hoops and the basket bodyguard the sensor to mitigate "crowding" against the wall of the stent graft or the aneurysm and purportedly aide stabilization of the sensor in the sac.


Figure 3
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  		Fig 3. Pre- and post-exclusion pressure tracings as measured using implanted aneurysm sac pressure (ASP) monitor and external antenna/interrogator. (A) Pre-exclusion ASP reading is taken in the operating room once the device has been placed in the desired intraluminal position within the aneurysm sac, but before stent graft deployment. The pressure tracing indicates an operational EndoSure device with systemic sac pressure readings, including systolic and diastolic pressure, mean arterial pressure, and pulse pressure. (B) Post-exclusion ASP reading indicates diminution in all pressures after stent graft deployment. The ratio of aneurysm sac pulse pressure to a concomitantly obtained systemic pulse pressure measured via arterial line will determine ASP ratio. Tracings provided courtesy of and with reproduction permission of CardioMEMS, Inc. (C) Depiction of a second-generation interrogator with antenna visible (arrow). The antenna is placed externally over the patient's chest to obtain ASP readings. Picture provided courtesy of and with reproduction permission of CardioMEMS, Inc.

 

Figure 4
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  		Fig 4. EndoSure devices. (A) First-generation device. (B) Second-generation EndoSure s2 with two end-loop and basket design abdominal aortic aneurysm pressure sensor. (C) Comparison of the first-generation EndoSure (closest to dime on left) and second-generation device. The difference in length and width is evident. The radiopacity of the second-generation device is also increased. (D) Fluoroscopic image of an in situ device (arrow). (A), (B), and (C) reproduced with the permission of CardioMEMS, Inc.

 
Deployment was achieved through a 14F introducer sheath (Cook Medical, Bloomington, IN) in the surgically exposed femoral artery/iliac conduit. The EndoSure device was then positioned over a stiff guidewire in the aneurysm sac; for patients with aneurysms secondary to chronic dissection (n = 7), the sensor was intentionally positioned within the false lumen (adjacent to the primary tear) of the aneurysm sac through an existing distal reentry tear, which enabled measurement of potential false lumen pressurization. The sheath was then exchanged for a 20F to 24F introducer required for the Gore TAG (W. L. Gore & Associates, Flagstaff, AZ) endovascular graft with the EndoSure device exteriorized to the endovascular graft introducer sheath. This occasionally requires snaring the femoral artery/Dacron (C. R. Bard, Haverhill, PA) conduit with a vessel loop to minimize bleeding around the sheath. This variation in our approach differs from the instructions for use and other reported abdominal aortic series. The advantage of this approach is that it obviates contralateral femoral cutdown or percutaneous arterial closure devices necessary to accommodate the 14F introducer sheath. The disadvantage is that the access vessel diameter needs to be 1 to 2 mm greater than the introducer sheath so as to accommodate the EndoSure system, a requirement that may be problematic with small or borderline diameter arteries.

The endovascular graft utilized in all patients was the Gore TAG device. Details regarding the device [8], as well as procedural details of TEVAR and follow-up protocol performed at our institution have been described [9]. All procedural outcomes and complications were prospectively recorded. The ASP tracings were acquired intraoperatively, before and after exclusion of the aneurysm, at the time of discharge, and at each follow-up visit. All follow-up was done at the Duke University Center for Aortic Surgery. This report includes data collected through the patients' most recent follow-up visit.

Data were entered into a spreadsheet (Excel 2007; Microsoft, Redmond, WA). Descriptive and summary statistics with mean and standard deviation were calculated. Continuous variables were compared using the paired Student's t test (http://www.graphpad.com/quickcalcs/ttest1.cfm). Statistical significance was designated as p less than 0.05. All data are presented in accordance with the "Reporting Standards for Endovascular Aortic Aneurysm Repair" [10].


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Acknowledgments
 References
 
The distribution of patients (n = 43) is noted in Figure 2. The mean patient age was 71 ± 12 years (range, 31 to 83). A total of 23 patients received a first-generation device, with the remainder (n = 20) undergoing implantation of the current second-generation model. The majority of the sensor malposition cases occurred with the first-generation device. Specifically, 5 of 7 malpositioned ASP sensors (71%) in this series occurred with the first-generation device, with the remaining 2 (29%) occurring with the EndoSure s2 iteration. Overall, this equates to a 22% malposition rate with the first-generation device, which decreased to 10% with the s2. This difference likely relates to changes in device design, as well as a learning curve with regard to device positioning and patient/anatomy selection, given that the use of the first-generation device coincides with the earlier half of our TEVAR-ASP experience.

Intraoperative after-exclusion ASP measurements were obtainable in only 20 patients (47%). This limited intraoperative utility is attributable to radiofrequency interference from the transesophageal echocardiography probe and evoked potential monitors, which are routine adjuncts utilized at our institution [9]. This contrasts with the reported abdominal experience, likely because these adjuncts are not typical during infradiaphragmatic procedures. In patients in whom intraoperative pre- and post-exclusion ASP measurements were obtained, a reduction in sac pressure to 46% ± 25% systemic (p < 0.0001), consistent with successful aneurysm sac exclusion, was seen. A slight further reduction in aneurysm sac pulse pressure ratio to 43% ± 22% systemic (p = not significant, as compared with after-exclusion readings) was seen in patients (n = 36) with well-positioned sensors before discharge.

Mean duration of follow-up was 11 ± 8 months (range, 0 to 28). At the latest follow-up, this series exhibited further reduction in the ASP ratio to 19% ± 12% systemic (p = 0.019). This correlates with a statistically significant reduction in aortic diameter from a preoperative diameter of 6.5 ± 1.4 cm to a diameter of 5.6 ± 1.5 cm (p < 0.001) at latest follow-up in all patients (Fig 5). Overall, a 5 mm or greater reduction in aortic diameter as documented by CT angiography (CTA) was evident in 76% of patients (25 of 33) with follow-up of 6 months or longer.


Figure 5
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  		Fig 5. Relationship of aneurysm sac pressure and aortic aneurysm diameter. (A) Graphic representation of the ratio of aneurysm sac pressure to systemic pressure at the time of discharge as compared with last clinic follow-up. (B) Graphic representation of the preoperative maximum transverse aortic diameter as compared with last clinic follow-up. *Denotes p < 0.05 in both graphs. (F/U = follow-up.)

 
Eleven postoperative ELs were detected by CTA (Fig 2). In 2 patients, systemic ASP measurements before anticipated discharge prompted a CTA, confirming type I EL. Both were treated successfully with additional TEVAR (cuff extension) during the index hospitalization. One patient had reduced ASP at discharge, but exhibited an increased ASP (70% systemic) at 1-month follow-up, and CTA confirmed a type I EL, which was managed with additional TEVAR; that sealed the EL and subsequently reduced ASP to 39% systemic. One patient in the chronic dissection subset illustrated a reduction in ASP at 1 month, but demonstrated near-systemic pressures at 6-month follow-up. Subsequent CTA demonstrated no EL, but also revealed near-complete reverse remodeling of the thoracic aorta with obliteration of the false lumen, in essence "sandwiching" the ASP device (hence acquiring transendovascular graft pressures; Fig 1A).

Eight patients had type II ELs, which are being managed expectantly. Of these patients, the ASP sensor was malpositioned between the endovascular graft and aortic wall in 3, with subsequent systemic transendovascular graft pressure transmission; an additional patient with initially low ASP demonstrated significant reverse remodeling of the thoracic aorta so that the sensor began to transmit systemic transendovascular graft pressures at 4 months postoperatively. Of the remaining 4 patients with a type II EL and a well-positioned sensor, the mean ASP measured 27% systemic. None of the 8 patients with type II EL has manifested an increase in aortic diameter by CTA during a mean follow-up of 11 ± 6 months.

In the subset of chronic type B dissection patients (n = 7) undergoing TEVAR for aneurysmal dilatation with EndoSure placement in the false lumen (Fig 6), all exhibited a decrease in ASP, although 1 patient, as described earlier, manifested subsequent near-systemic pressures at the 6-month reading owing to sensor "sandwiching" attributed to aortic remodeling. Of the other 6 patients with well-positioned sensors, the ASP ratio decreased from 52% ± 27% at the predischarge measurement to 14% ± 5% at the latest follow-up reading (p = 0.029). The difference of the means equals a 38% (95% confidence interval: 59 to 70.4) further reduction in false lumen/ASP during follow-up. Similarly, the reduction in aortic diameter was statistically significant, with maximal aortic diameter decreasing from 5.6 ± 0.8 cm preoperatively to 4.6 ± 0.9 cm at latest follow-up (p = 0.0024). The mean difference in aortic diameter is 1 cm (95% confidence interval: 0.536 to 1.431). All 7 chronic dissection patients with ASP sensor placement in the false lumen exhibited a 5 mm or greater reduction in aortic lumen caliber at midterm follow-up, with 1 EL (type II) observed in this group (mean follow-up 12 ± 5 months in dissection subset).


Figure 6
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  		Fig 6. Axial computed tomography angiography image of an aneurysm sac pressure sensor in a thrombosed false lumen after thoracic endovascular aortic repair of a complicated chronic type B dissection with aneurysm. Arrow depicts sensor location.

 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Acknowledgments
 References
 
Endovascular leak rates of 11% to 29% have been reported with respect to stent grafting of the thoracic aorta [11, 12]. The ELs result in pressurization of the aneurysm sac with a potential for rupture, despite exclusion of the aneurysm by an endovascular luminal graft. Both type I and type III ELs represent treatment failures and warrant intervention [11], whereas the clinical significance of type II ELs remains controversial [13]. Reintervention rates of approximately 12% are necessary when ELs are detected [12]. While ultrasound technology serves as a useful adjunct in the detection of abdominal ELs, its translation to thoracic endovascular grafting is hampered by poor visibility through air-filled lung parenchyma. As such, EL surveillance after TEVAR generally requires imaging with contrast-enhanced CTA or magnetic resonance angiography. Because of the potential toxicity related to contrast agents or repeated radiation exposure, alternative detection strategies are needed.

The EndoSure device utilizes wireless radiofrequency energy and can provide instantaneous confirmation of aneurysm sac (or false lumen) depressurization. Further, endovascular tension may be detected much earlier within a previously excluded aneurysm and may prompt evaluation with selective instead of routine imaging to confirm an EL. That occurred in 3 patients with type I EL in the present series, 2 of which were detected before discharge and prompted early additional intervention to seal the leak. The sensors have exhibited efficacy and accuracy in detecting type I and III ELs and may help clarify the physiologic relevance of type II ELs. In the current series, low ASP provided clinical reassurance with regard to observation of type II EL (all related to intercostals perfusion) in 4 patients with well-positioned sensors. The clinical significance of type V ELs, defined as aneurysm growth without detectable EL (endovascular tension), also remain uncertain [13], although we have yet to observe a type V EL in our series of nearly 300 thoracic aortic stent graft procedures performed to date (unpublished data).

Elucidation of the relationship between EL and pressure may help clinical decision making. This represents a major advantage of ASP monitoring over radiographic imaging, particularly with regard to type II ELs and the "persistent" false lumen of chronic dissection patients undergoing TEVAR therapy. In addition, the technique may abrogate the need for costly and time-consuming imaging procedures, as well as radiation and contrast dye exposure. To date, we have deferred CT and magnetic resonance imaging radiographic evaluation for 3 patients during longer term follow-up because of stable, low ASP readings and anticipate a general decrease in surveillance imaging in this cohort as we obtain further confirmation of sensor reliability over time.

There are notable limitations to ASP usage. For example, prior work has demonstrated that pressure measurements in the setting of documented EL may exhibit a lack of uniformity throughout the aneurysm sac, with consistently higher pressures measured in the EL channel as compared with the surrounding aneurysm thrombus [14]. Moreover, the orientation of the pressure sensor in relation to the source of pressure can also influence measurements [15, 16]. In some cases, a lack of uniformity in thrombus structure may influence the transmission of pressure [17]. Another potential limitation of the device is that the relatively simple construct of the sensor does not provide any error correction systems for interference from external radiofrequency fields, and as observed in the current series, the presence of multiple radiofrequency energy emitting devices in the operating room may limit utility in this setting. This interference is more pronounced in the TEVAR population owing to monitoring adjuncts such as transesophageal echocardiography and neurologic monitoring. Hence, 53% of patients in our series required postoperative calibration of the ASP. Another limitation, although anecdotal, is the difficulty in proper positioning in saccular aneurysms; positioning is easier and more effective in fusiform aneurysms. Yet another consideration is financial; although the interrogator is provided to the institution at no charge, each implant (sensor) is approximately $3,500, which approaches the cost of some endovascular graft components.

The rate of malpositioned sensors in our series is significant. These occurred earlier in the series, and all except 2 cases were with the first-generation EndoSure device. The second-generation device is shorter with a basket cage design, which requires a smaller area of thrombus-free lumen (10 x 21 mm) for optimal positioning (Fig 4). This technology may change current surveillance protocols after TEVAR if durability of the device can be validated during longer follow-up. While a device lifespan of at least 10 years is expected by the manufacturer (unpublished data), that must be weighed against data that suggest that the long-term survival of patients undergoing repair of degenerative thoracic and thoracoabdominal aneurysms is limited (eg, approximately 30% survival at 10 years after open thoracoabdominal aortic aneurysm repair); thus, device lifespan is unlikely to be an issue for the majority of these patients [18].

With regard to the chronic type B dissection patients in the present series, the ASP monitor was intentionally placed in the aneurysmal false lumen adjacent to the primary tear. Low ASP readings correlated with significant reverse aortic remodeling in all cases. For chronic dissection with aneurysm, TEVAR remains controversial [1] because of concerns over persistent retrograde false lumen flow and pressurization resulting from downstream fenestrations distal to the endovascular graft. The findings of this study would suggest a "proof of concept" that TEVAR is useful for the treatment of descending aneurysm resulting from chronic dissection. Low false lumen ASP readings were uniformly seen despite some element of retrograde false lumen flow distal to the endovascular graft in a majority of these patients. Regardless, while ASP data confirmed technical success in terms of primary tear and thoracic aneurysm stent graft exclusion, ongoing radiographic surveillance remains imperative for the distal native thoracic aorta. Isolated ASP readings can not identify metachronous aortic disease. In addition, reverse aortic remodeling can result in false positive elevated ASP readings, as seen in 1 of the 7 dissection patients in whom CTA 6 months postoperatively confirmed complete aortic remodeling with no EL or false lumen perfusion, implicating the elevated ASP as transmitted transendovascular graft pressure. This is a potential limitation of the device in midterm to long-term follow-up with continued aortic remodeling.

In summary, ASP represents the ideal metric for assessing endovascular tension. We report the first and largest series of ASP measurement in patients undergoing TEVAR. The study validates device durability as well as concordance with radiographic documentation of EL, or lack thereof, at midterm follow-up. The study also raises potential issues with the device, including limited intraoperative utility if extensive monitoring is used, problems with sensor malpositioning, as well as the potential for an initially well-positioned sensor to subsequently transmit systemic transendovascular graft pressure should significant reverse aortic remodeling occur. At least in current practice, ASP devices cannot supplant serial radiographic imaging for the reasons enumerated above. However, the device serves as a useful diagnostic adjunct that permits expectant management of type II EL and partially thrombosed false lumen given low sac pressure. The greatest benefit of this modality is that increasing ASP readings will prompt reimaging based upon a rapid bedside assessment. Longer term data are necessary to provide information regarding durability of the technology for postoperative surveillance, which may permit a possible reduction in follow-up radiographic requirements for these patients.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
References
 
Divisional funds covered study costs, and no corporate or grant assistance was utilized.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 

  1. Svensson LG, Kouchoukos NT, Miller DC, et al. Expert consensus document on the treatment of descending thoracic aortic disease using endovascular stent-grafts Ann Thorac Surg 2008;85(Suppl):1-41.[Free Full Text]
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  5. Hoppe H, Segall JA, Liem TK, et al. Aortic aneurysm sac pressure measurements after endovascular repair using an implantable remote sensor: initial experience and short-term follow-up Eur Radiol 2008;18:957-965.[Medline]
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