Investigation of the pressure gradient of embolic protection devices

1 Introduction

Carotid artery stenting is a standard method to treat calcified vessels. One risk during this operation is that plaque particles come to the brain and cause stroke. To minimize the risk of the cerebral infarction embolic protection devices (EPD) are used. They are applied during the minimal invasive operation distal to the constricted region of the vessel where the stent is set. They consist of a guiding catheter and a filter which is a barrier for the particles. The flow of blood should be reduced by the membrane of the device as few as possible to minimize the risk of complications with the blood pressure and the cerebral blood supply [1]. So the pressure gradient should be a minimum. Figure 1 shows the Angioguard RX as an example of an EPD.

Figure 1

Angioguard RX as an example for an embolic protection device for capturing plaque particles in the carotid artery.

The objective of this study is to measure the pressure gradient of five different types of EPD.

2 Material and methods

To evaluate the pressure gradient of five different EPD, a test setup containing a silicone tube vessel model with an inner diameter of 5 mm was used to simulate the internal carotid artery [2]. The setup is built after the one used in previous studies [3], [4] and is shown in Figure 2.

Figure 2

Test setup for the evaluation of the pressure gradient of different embolic protection devices.

The test was performed with clean water as test medium. It circulated with a flow rate of 942 ml/min (calculated after the maximal systolic velocity in the carotid artery [2]). A pre-filter was used to avoid contamination of the circuit and the devices were implanted through a sheath with haemostatic valve. At the beginning of the test spherical standard particles (polystyrene, COOH-covered, D = 150 μm) were brought into the model by connecting the particle reservoir to simulate plaque particles. To assure that all particles come from the reservoir to the EPD, the suspension was homogenized with a magnetic stirrer. The particles were mainly caught by the EPD and those that were missed by the protection system were collected by an inline filter. The systems were tested for 5 min each and the flow rate and the pressure were measured by a flow sensor (Leviflow LFS-008, Levitronix) and a pressure sensor (PCE-917). Table 1 shows the tested embolic systems and their pore size. The FiberNet does not consist of a porous membrane but of a collection of fibres so the pore size is defined as an equivalent by the manufacturer.

Additionally, microscopic images of the systems were taken to measure the pore size of the membranes and to get an impression of the pore arrangement. The pictures were taken with a light microscope and a lens magnification of 11.5 and the diameter of the pore sizes were measured exemplary.

3 Results

The pressure gradient was calculated for each system by subtracting the differential pressure value of the test setup without system P₀ of the measured pressure value during the test with the EPD P_s. Both values are mean values of 300 measurements during the test duration of 5 min. The pressure was held nearly constant during the test procedure because the particles were introduced in the circuit in a relatively short time period of a few seconds in the beginning of the test. Otherwise one would have expected an increasing pressure over time.

The results are shown in Figure 3.

Figure 3

Pressure gradient of the Embolic protection devices, *the test procedure was repeated with a lower flow rate.

The device AngioguardRX shows the highest pressure gradient of P = 180 mbar during the test in the vessel model with a flow rate of 942 ml/min. The lowest pressure gradient occurs during the test with the FiberNet. It is around zero and the pressure values with and without system are within the uncertainty of the pressure sensor equal. During the test of the device EmboshieldNAV⁶ the measured pressure raised over 1.8 bar. The pressure resistance of connectors of the test circuit and the inline filter is lower so that the test with this system was completed with a lower flow rate of 450 ml/min and showed a pressure gradient of 130 mbar.

Figure 4 shows the morphology of membrane pores. The FiberNet does not contain a porous membrane so it was taken an overview image of the device. In accordance with the statements of the producers the EPD EmboshieldNAV has the highest pore diameter in the microscopic images. The exemplary measured pores of the RX Accunet and FilterWireEZ are smaller than the pore size as claimed by the manufacturers.

Figure 4

Microscopic images of the embolic protection device: (A) Angioguard RX, (B) RXAccunet, (C) FiberNet, (D) FilterWireEZ, (E) EmboshieldNAV [10].

4 Discussion

The study shows that different EPD cause different pressure gradients in the test setup. This is in accordance with previous studies [11], [12]. It is obvious that the pressure gradients are influenced by the morphology of the systems. The pore size and density might have the highest effect on the pressure gradient beside the general design of the capture device.

The soft fibres of the FiberNet have the lowest influence on the flow of the liquid, in this case water. The pressure gradient is lower than for the systems which contain porous membranes. The EmboshieldNAV causes the highest pressure gradient and has consequently the highest effect on the antegrade blood flow. The pore size is the largest but the microscopic image support the impression that the density of pores is lower than at the other three systems also containing a porous membrane. Higher number of pores in the membrane causes a lower flow resistance which has a positive influence on the pressure gradient of the entire system. The same applies for the effective surface area of the porous membranes.

Hydrodynamic differences resulting from the different design of the devices also have an influence on the pressure gradient.

The study was made under the condition of a round cross profile of the vessel model. In a real vessel one can find certain asymmetries so that the wall apposition of the device might not be optimal. That could have a decreasing effect on the pressure gradient because of gaps between vessel and device allowing a less restricted blood flow. Under these circumstances, however, a lower particle capture efficiency of the systems is expected [4].

The values for the pressure gradients of several systems were taken at a constant flow rate. The results and the relation of the pressure gradients to each other might differ under other conditions. The higher the flow rate the higher the resulting pressure difference will be. Assuming laminar flow conditions, flow rate and pressure are proportional (Darcy’s law) but the complex pore structures of the membranes might lead to turbulences and to a non-linear relation between pressure and flow rate [13].

5 Conclusion

The pressure gradients of five EPD could be measured with the vessel model of the internal carotid artery. Under the condition that the particle capture efficiency is given it is an advantage to choose a filter design that causes a low pressure gradient.