Aneurysm Sac Pressure Monitoring: Effect of Technique on Interpretation of Measurements

Assessment of three-dimensional stent-graft dynamics by using fluoroscopic roentgenographic stereophotogrammetric analysis

OBJECTIVE

To validate the use of fluoroscopic roentgenographic stereophotogrammetric analysis (FRSA) for its feasibility and accuracy for measuring the three-dimensional dynamic motion of stent grafts.

METHODS

A digital biplane fluoroscopy setup was calibrated (Siemens Axiom Artis dBc). Stereo images were acquired of a static aortic model with a stent graft in different axial positions, imposed by a micromanipulator. The three-dimensional measurement error of FRSA was determined by comparing FRSA measurements with the micromanipulator. An aortic model with a stent graft was constructed and connected to an artificial circulation with a physiological flow and pressure profile. Markers were added to the spine (tantalum spherical markers; diameter 1 mm) and stent (welding tin; diameter 1 mm). The three-dimensional measurement precision was determined by measuring the position of a single (stable) spine marker during two pulsatile cycles. Finally, three-dimensional stent marker motion was analyzed with a frame rate of 30 images per second, including three-dimensional marker position (change), diameter change, and center of circle position change.

RESULTS

The mean error of FRSA measurement of displacement was 0.003 mm (SD, 0.019 mm; maximum error, 0.058 mm). A very high precision of position measurement was found (SD, 0.009-0.015 mm). During pulsatile motion, the position (changes) of the markers could be assessed in the x, y, and z directions, as well as the stent diameter change and center of circle position change.

CONCLUSIONS

FRSA has proven to be a method with very high accuracy and temporal resolution to measure three-dimensional stent-graft motion in a pulsatile environment. This technique has the potential to contribute significantly to the knowledge of stent-graft behavior after endovascular aneurysm repair and improvements in stent-graft design. The technique is ready for clinical testing.

Accurate Detection of Stent-Graft Migration in a Pulsatile Aortic Model Using Roentgen Stereophotogrammetric Analysis

PURPOSE

To evaluate the feasibility and accuracy of Roentgen stereophotogrammetric analysis (RSA) versus computed tomography (CT) for detecting stent-graft migration in an in vitro pulsatile circulation model and to study the feasibility of a nitinol endovascular clip (NEC) as an aortic wall reference marker for RSA.

METHODS

An aortic model with stent-graft was constructed and connected to an artificial circulation with a physiological flow and pressure profile. Tantalum markers and NECs were used as aortic reference markers for RSA analysis. Stent-graft migrations were measured during pulsatile circulation with RSA and CT. CT images acquired with 64 x 0.5-mm beam collimation were analyzed with Vitrea postprocessing software using a standard clinical protocol and central lumen line reconstruction. RSA in the model with the circulation switched off was used as the reference standard to determine stent-graft migration. The measurement errors of RSA and CT were determined during pulsatile circulation.

RESULTS

The mean measurement error +/- standard deviation (maximum) of RSA during pulsatile circulation using the tantalum markers was -0.5+/-0.16 (0.7) mm. Using the NEC, the mean (maximum) measurement error was -0.4+/-0.25 (1.1) mm. The mean (maximum) measurement error of CT was -1.1+/-1.17 (2.8) mm.

CONCLUSION

RSA is an accurate and feasible tool to measure stent-graft migration in a pulsatile environment. Migration measurement with RSA was more accurate than CT in this experimental setup. The nitinol clip tested in this study is potentially feasible as an aortic reference marker in patients after endovascular repair.

The Intraluminal Thrombus as a Source of Proteolytic Activity

Most abdominal aortic aneurysms (AAAs) with a diameter indicating need for surgical repair contain intraluminal thrombus (ILT). The development of AAA is linked to degradation of elastin and collagen. These changes are more pronounced in the aneurysm wall covered by the ILT, which also shows more signs of inflammation and is thinner compared to the aneurysm wall exposed to flowing blood. The rate of increase in diameter of AAA correlates with increased thrombus growth and rupture. CT examinations of patients with rupture have demonstrated contrast appearing in the thrombus suggesting bleeding into it. Studies using gene array of human aneurysm specimens have shown that most matrix metalloproteinases (MMP) were upregulated in the thrombus-free wall. Analyses by zymography, however, demonstrate gelatinase activity in the interface between the thrombus and the underlying wall and in the media of the wall not covered by a thrombus. The thrombus contains large amounts of neutrophils. Neutrophil gelatinase associated lipocalin (NGAL) is involved in the regulation of MMP-9 activity and prevents its inactivation, thus augmenting the proteolytic effect. It has been identified in all layers of the ILT. The presence of NGAL/MMP-9 complexes throughout the thrombus and in the thrombus-covered wall may contribute to the increased proteolytic degradation seen in this wall segment. In conclusion, the presence, growth, and thickness of the ILT have been shown to be associated with growth and risk of rupture. The wall underlying the thrombus is thinner and shows more signs of proteolytic degradation. Increased proteolytic activity by MMP-9 may be mediated by binding to NGAL.

Aneurysm Sac Pressure Monitoring: Effect of Pulsatile Motion of the Pressure Sensor on the Interpretation of Measurements

PURPOSE

To determine the effect of pulsatile motion of graft-attached pressure sensors on the accuracy of aneurysm sac pressure measurement.

METHODS

Pressure inside a pressure box was measured with a sensor attached to a stent-graft (Sensorgraft) facing a sensor in fixed position (Sensorbox). Maximum inter-sensor variation of Sensorgraft and Sensorbox was determined in static experiments. Experiments with pulsatile circulation were performed with a compliant and a noncompliant stent-graft at 120/80 mmHg and 160/95 mmHg. Pressure measurements in the box were repeated after the box pressure was increased from 0 to 120 mmHg. Sensorgraft motion was measured by ultrasound. Measurements with Sensorgraft were compared to those with Sensorbox using Pearson correlation coefficients to determine the concordance between the sensors.

RESULTS

The maximum inter-sensor variation was 4 mmHg. Increased box pressure induced progressive pulsatile graft and sensor motion during all experiments. During the experiments with the compliant graft at systemic pressures of 120/80 and 160/95 mmHg, the maximum inter-sensor variation was exceeded at box pressures of 65 and 75 mmHg, respectively. The sensor motion at these box pressures was 214+/-2.70 microm and 210+/-0.93 microm, respectively. Measurements of Sensorgraft were higher than Sensorbox, up to 13 mmHg at a box pressure of 120 mmHg. The Pearson correlation coefficients during these experiments were 0.99 and 1.00 (p < 0.001), respectively. In the experiments with the noncompliant graft, the maximum inter-sensor variation was not exceeded, and sensor motion was only 7 +/- 0.46 microm and 26 +/- 1.48 microm, respectively. The Pearson correlation coefficient during these experiments was 1.00 (p < 0.001).

CONCLUSION

Pulsatile sensor motion can influence the accuracy of pressure measurement. More compliant grafts are more susceptible to this phenomenon. Despite false high pressure measurements, stent-graft-attached pressure sensors seem appropriate to follow pressure trends in the aneurysm sac.

Embolization-driven Occlusion of the Abdominal Aortic Aneurysmal Sac as the Basis of Prevention of Endoleaks in a New Swine Model

OBJECTIVES

To assess the effect of a new polymer in embolization of endoleaks using an animal model.

METHODS

A modified aortic stent-graft was placed in 20 pigs. Embolization was performed at the time of graft insertion with non-cytotoxic n-butyl-2-cyanoacrylate-metacryloxysulpholane and lipiodol (0.2:0.8ratio, 2 ml). Angiography, scanning electron microscopy and immuno-histochemistry were obtained at day 0, 1 week and 3 months.

RESULTS

In control animals both type I and II endoleaks were demonstrated. In treated animals, neither type-I nor type-II endoleaks were observed and a fibro-proliferative response was demonstrated within the aneurysm thrombus.

CONCLUSIONS

Host vascular responses govern the fate of the excluded aneurysm. Embolization of the sac and feeding arteries with non-cytotoxic glue sealed all occlusions by stimulating a massive restenosis-like process.

Effect of intraluminal thrombus on pressure transmission in the abdominal aortic aneurysm

OBJECTIVE

To clarify the effect of intraluminal thrombus on pressure transmission.

METHODS

A saccular aneurysm was inserted into an artificial circulation system. Subsequently, the saccular aneurysm was filled with eight different human aortic aneurysm thrombus samples. Starch solution in an empty aneurysm was used as a control. A pressure sensor measured the pressure in the circulation, and a second piezoelectric sensor measured the pressure in the saccular aneurysm at 3, 2, and 1 cm from the endoluminal surface (23 locations). The influence of the elastic characteristics of the aneurysm wall on the extent of pressure reduction was evaluated by experiments performed with aneurysms made of rubber and paraffin.

RESULTS

The pressures measured in the empty aneurysm were identical to those measured in circulation (P > .05). The pressure measured in the thrombus was significantly lower than the pressure measured in the circulation (P < .05). The mean pressure ratio between the systolic thrombus pressure and systolic circulation pressure at 1, 2, and 3 cm was 0.90 +/- 0.09, 0.86 +/- 0.10, and 0.81 +/- 0.09, respectively. However, there was a clear correlation between the pressure in the circulation and in the thrombus (Pearson correlation coefficient: mean, r = 0.997; range, 0.975-0.999; P < .01). The change in circulatory pressure was followed by an almost identical change in thrombus pressure (regression coefficient: mean, beta = .997; range, .983-1.000; P < .01). In stiff aneurysms, the pressure reduction is less than in more compliant ones (P < .05).

CONCLUSIONS

In an in vitro model, pressure in the aneurysmal sac is reduced by fibrinous thrombus.

CLINICAL RELEVANCE

Endovascular aneurysm repair (EVAR) aims at reducing the pressure in the aneurysmal sac. Therefore, it seems attractive to use pressure monitoring in the aneurysmal sac as a follow-up after EVAR. This study contributes to the development of the rationale of pressure monitoring in the aneurysmal sac as a follow-up method after EVAR. The aneurysmal sac is filled with thrombus. To interpret pressure measurements in the thrombus, we have to learn about the effect of the thrombus on pressure transmission and on the pressure measurements themselves. Our study demonstrates that reduction of pressure occurs as it is transmitted through a human aortic thrombus.