Magnetically Supported PCI: Success after failed surgery and conventional PCI

The System

The Niobe® II magnetic navigation system (MNS: Stereotaxis, St Louis, Missouri) has two external magnets that produce a 15-cm uniform magnetic field. Through computer-controlled magnet movements, the magnetic field can be directed in all planes. When the field is changed, the deflection of the magnet at the wire tip also changes, resulting in reproducibly precise steering. The system has an adjustable touch screen at the lab table that is the interface between the operator and the MNS (Figure 1).

The current system has three main advantages with regards to tortuosity. First, it produces a 3-D, volume-rendered reconstruction from angiographic images, providing better morphological information about the artery. Second, the spatial coordinates, or location, of the reconstruction within the patient are known. Third, the model gives real-time, on-line vectors, which direct the external magnetic field to navigate the wire through the 3-D reconstruction. As a result, operators have the ability to utilize 3-D information in real-time patient therapy.

System costs. The current list price for the Stereotaxis Magnetic Navigation System is $1.9 million. The size of a typical modern Stereotaxis cath lab room is 770 square feet (72 square meters). The cost of magnetic shielding generally ranges from $50,000–$100,000 and consists of minimal common steel shielding in the walls, ceiling and floor.

There is a range of magnetically enabled Stereotaxis guidewires that are priced at a marginal premium over conventional wires. All other PCI equipment is standard and compatible with the Stereotaxis guidewires and MNS. No additional consumable items are required. All third-party billing occurs as part of a standard PCI intervention; therefore, all current reimbursement applies and there is no effect on current billing practices.

Case Report

This report describes a 60-year-old man with multiple medical pathologies and a burdened cardiac history. Past medical history included severe airways disease (FEV1 of 1 litre), ulcerative colitis with chronic iron deficiency anemia despite iron therapy, previous GI bleeding, gout, hypertension and hypecholesterolaemia, and a family history of ischemic heart disease and peripheral vessel disease.

The patient had undergone an aortic valve replacement 8 months earlier for significant calcific aortic stenosis. At that time, he received a jump saphenous vein graft (SVG) that anastomosed to the obtuse marginal branch (OM) and then the right posterior descending coronary artery (RDP). At the time of the operation, the cardiothoracic surgeon had noted that although the RCA had been grafted, it was heavily calcified, of poor quality and not suitable for further surgical treatment. The patient had a difficult recovery after the coronary artery bypass graft (CABG) operation with persistent hypotension, in part due to episodes of arrhythmia such as sinus bradycardia that was externally paced, and also atrial fibrillation (AF), for which he was later cardioverted. He had some generalized oedema and, once extubated, dyspnoea that resolved slowly.

Unfortunately, 2 months after surgery, the patient again began to suffer with chest pains (despite a satisfactory Hb) that could not be fully controlled with medication. Angiography showed the proximal end of the graft was occluded but a section of graft between the OM and the RDP was still patent. However, this remaining section of the SVG was compromised by severe lesions at both anastomoses with an adversely acute angle in the stenosis just before the OM anastomosis (Figure 2).

Further surgery was refused and conventional PCI using biplane angiography was attempted, with a view to reopening the RCA or if unsuccessful, intervening on the tortuous Cx-OM-SVG-RDP (Cx: circumflex artery). The RCA was heavily calcified. Despite the use of stiff occlusion wires and a variety of balloons for support, a wire could not be passed and the attempt was stopped due to the presence of dissection. An attempt to pass the wire into the SVG via the left coronary artery (LCA) was unsuccessful due to tortuosity and despite the use of multiple catheters and wires. For the attempts on both vessels, a total of 5 guiding catheters and 7 angioplasty guide wires, including Asahi Miracle 3, Asahi Miracle 6 (Abbott Vascular Devices, Redwood City, CA) and Crosswire NT (Terumo Medical Corporation, Somerset, NJ), were used.

The Magnetic Procedure Reconstruction. A decision was made to attempt a further procedure with the MNS. A 3-D reconstruction (Figures 3& 4) showed that the sum of all the angles from the LMS to the RCA was over (equivalent to over 3 complete turns of a corkscrew). Currently, an analysis of the reasons for prolonged procedures (with respect to wire passage across a lesion) is ongoing. Both the number of bends and the degree of angulation seem to be strongly related to a prolonged crossing time. Each bend adds friction to conventional wire rotation and therefore steering, and this friction becomes greater as the angulation increases (as the wire is more deformed and may also be pushed more forcefully against the vessel wall). Thus, the cumulative angulation may be extremely relevant to procedural duration and success.

Navigation. One of the prime benefits of the MNS 3-D reconstruction is that it provides an endoluminal view with a computed 3-D center line (Figure 5). This view offers a tailored direction for the magnetic field vector at every point in the vessel. There is also a white-line overlay that is a roadmap on the fluoroscopy screen. Tip advancement is shown on the white-line overlay in tandem with operator advancement of the wire. The 3-D vector changes simultaneously to keep the wire pointing down the chosen path.

The Titan™ Soft Support 2 mm angled tip wire (Cordis Corp., Miami, FL) was navigated through the tortuous LMS, Cx and OM using the vectors as far as the stenosis before the first anastomosis (with the adverse angle). At this stage, finer navigation was required to reach the SVG. Careful interrogation, by changing the angulation using the endoluminal view (the vector can be changed in the endoluminal view by double-tapping the screen at any point), and increasing the magnetic field strength to 0.1 Tesla, managed to get the wire tip to angulate enough to pass into the body of the graft. At this point, a 1.5 mm Maverick over-the-wire balloon (Boston Scientific Corp., Natick, MA) was passed to the graft body and used for support. The wire was then advanced through the graft using vector control and then into the RDP. In order to maximize support, the balloon was advanced to the RDP and an Iron Man support wire (Abbott Vascular) exchanged.

Both anastomoses were predilated. However, on the first attempt to pass a stent, there was excess resistance in the Cx that nearly caused wire retraction. The Cx was predilated, and both the anastomoses and the Cx were then stented (Figure 6). There was a good result, with TIMI-3 flow at the conclusion of the procedure (Figure 7).

Discussion

The potential advantages of magnetic navigation are especially relevant to excessively tortuous vessels. It is in these cases where the advantages of magnetic steering are most obvious over conventional procedures:

1) A conventional procedure has to use the same tip shape throughout. While appropriate for some angles, it is not ideal for multiple bends of varying angulation.

2) Once inside the patient, the tip shape of a conventional wire cannot be changed without retrieving the wire and losing position or exchanging through a support catheter.

3) After a conventional wire has been passed distally, there may be resistance when trying to rotate the wire tip for manipulation.

In contrast, the tip of a magnetically-enabled wire can be directed at will in any direction. Therefore, the tip shape is variable at will, is adjustable in situ for a particular bend and is independent of rotation of the wire (and thus friction from the bends in the vessel). Such abilities may be of significant benefit in challenging cases.

While the system is relatively intuitive and usable after minimal experience, we are finding that further improvement continues steadily as the range of additional features is explored. The experience at our institutions is a little over 350 cases and rising. A variety of randomization studies and registries are currently ongoing or under analysis.

Future Directions

The MNS has other potentially valuable capabilities. For PCI, the integration of other 3-D, volume-rendered imaging such as multislice computed tomography (MSCT) (Figure 8), as well as the possibility of other useful functions such as magnetically-navigable ablation or remote control navigation, may prove advantageous. In addition, the integration of data such as the electromechanical maps from the Carto RMT™ system (Cordis, Figure 9) may enhance procedures such as stem cell therapy.

While 3-D imaging represents improved appreciation of coronary anatomy compared to the 2-D black-and-white x-ray images that have been the standard manner of visualizing coronary arteries, the current reconstructions are static. Dynamic road-mapping is an area under development.

Hardware. The internal guide wire magnets are currently rigid 3 mm wire tips, with a 2 mm option. While the rigidity can potentially restrict the movement of the magnet tip in a lesion of less than 3 mm diameter, to date, there has been only a mild clinical impact, mainly when attempting to enter side branches that may come off at right angles within a tight lesion. A flexible tip (with several magnets) is under development for enhanced navigation.

Strategies. The range of options within the system translate into a number of possible strategies that may develop in an evolutionary fashion. One example of a hybrid strategy is the manipulation of an angled wire without a magnetic field, bringing in the magnets as necessary. The use of presets and the 2-D clockface allow for relatively speedy use of the system, and may better suit procedures in simple lesions or where time plays a role, such as primary PCI. Performing a 3-D reconstruction may give a better idea of the anatomy and provide more individually tailored pre-calculated vectors for navigation. Bringing together reconstructions from diagnostic images and using navigation with the white-line overlay and minimal contrast may be helpful when it is necessary to reduce contrast volume. Additionally, 3-D reconstruction may be especially useful in extremely long and tortuous segments.

Conclusions

MNS has been shown to aid PCI of complex and tortuous vessels. System capabilities, together with other current and forthcoming options, indicate that MNS could have a major impact on the performance of PCI. In our opinion, the finer wire tip control, the extended anatomical information, the potential for integration in using other 3-D sources such as MSCT in real-time, together with forthcoming developments such as flexible wire-tips, ablative wire-tips and navigable injection catheters, may produce a formidable system.

Acknowledgement

The authors are grateful to Joep Maeijer for help with preparation of the images.

Dr. M. Patterson can be contacted at: Amsterdam Department of Interventional Cardiology, OLVG; 1e Oosterparkstraat 279; 1090 HM, Amsterdam, The Netherlands. Email: markspatterson (at) doctors. net. uk

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