Advancing the Field of Coronary Intervention: Magnetic Guidewire Navigation

Why Do We Need Such Technology? While the currently available guide catheters, highly specialized guidewires, and transit catheters have allowed operators to gain access to distal vasculature not reachable five years ago, we still run into the case where the operator has exhausted all of the available wires and tricks without success. In case you are having difficulty remembering the last time this happened, just check the lab procedure log for the last case with 62 minutes of fluoroscopy time and more than 300 cc of contrast delivered! The American College of Cardiology/American Heart Association Classification for Coronary Lesions defines characteristics that have been shown to predict success in percutaneous intervention. Type A lesions have been shown to have a high degree of procedure success with the least associated complications. It is the Type C lesions which have always brought the lowest success rates and the highest complication rates. Table 1 shows the morphologic characteristics that make up Type C lesions. Extreme angulation and tortuosity represent features that cause difficulty in passing the guidewire distal to the lesion needing treatment. These features may be overcome by magnetic navigation. What is Magnetic Navigation? Essentially, it is a catheterization lab suite that has been married to two permanent magnets and a sophisticated software operating system (Figure 1). Currently, the only system available is the Niobe system, developed by Stereotaxis Inc., Saint Louis, Missouri. Unfortunately, this is not a technology that can be easily integrated into an existing lab like intravascular ultrasound or rotoblation. Obtaining this system basically requires a complete renovation of the existing lab. The magnets are the key feature of this system and are mounted on either side of the patient table to produce a 0.08-T field, directed in a spherical region encompassing about 15 cm of the patient's mid-chest. The software operating system allows the interventionalist to manipulate the magnetic field in order to direct the dedicated guidewire safely and smoothly through a coronary artery by deflecting the tip in any direction while manually advancing the wire. There currently are two separate families of guidewires that are available with differing handling characteristics: Cronus and Titan. Both wires have a magnet located within the distal tip of the wire (Figure 2), allowing it to be deflected in any direction by moving the magnetic field. Otherwise, the wires have the same general construction of currently available non-magnetic wires. They are mostly available in rapid-exchange lengths and vary in their supportive characteristics by differences in material and design of the shaft of the wire. By bringing together a magnetic wire and a moveable magnetic field, the operator has enhanced tip-control of the guidewire, allowing movements across heavily angulated segments with multiple bends previously not achievable even in the best hands of the most experienced operator. The magnets do not provide a forward driving force to advance the wire in the current system, but simply deflect the tip of the wire. The operator will advance the wire, change the direction of the field so that the tip of the wire is pointed in a desirable direction, and then advance the wire until the tip direction needs further changing. A Typical Case At this point in the development of the technology, there are relatively few labs in the United States that have the ability to perform a magnetically guided coronary intervention. So, the typical patient will likely be one that has undergone an attempted difficult percutaneous intervention at another lab without success. As described above, the patient likely to benefit from this technology is one in which the previous attempt failed because the anatomy did not allow standard guidewires to access the vessel distal to the lesion. This patient will then be brought to a magnetic cath lab suite for another percutaneous revascularization attempt. The operator will take at least two orthogonal angiograms of the vessel in question and then utilize the software to create a three-dimensional reconstruction (Figure 3). This image is then saved as a roadmap for the operator and a magnetic wire is placed in the vessel. The operator can manually steer this wire or can utilize the software to deflect the tip in an appropriate direction while advancing the wire down the lumen. The operator also has the ability to over-steer the wire by selecting extreme magnetic vectors to access sidebranches or other tortuous portions of the vessel. The vector is changed by a touch screen attached to the table and demarcated by an arrow on the roadmap showing the direction of tip deflection created by the magnetic field. Essentially, the wire-torqueing device is removed and significantly upgraded to the magnetics, which maintain wire tip position at an operator-prescribed location. Once the lesion is traversed, the procedure will then be carried out as it would be in a non-magnetic catheterization lab with standard balloon catheters and stents. There is currently no magnetic assistance for any part of the procedure except for the manipulation of the coronary guidewire. Does it Work? Several case series have now been published that have outlined the initial experiences with magnetic navigation and coronary intervention. Tsuchida et al¹ reported on 21 consecutive coronary arteries in which magnetic navigation was used for wire manipulation and compared directly to traditional guidewire manipulation in the same vessel. There were no procedure-related adverse events and the magnetic guidance produced markedly shorter crossing times and less contrast utilization than traditional guidewire manipulation. Atmakuri et al² evaluated the utility of magnetic guidance in patients with previously failed percutaneous revascularization attempts or patients with potentially difficult-to-cross lesions. In these difficult 68 lesions, they showed an 85% success rate in delivering the guidewire distal to the lesion and a 79% overall procedural success rate. Current Limitations While reports have shown that magnetic navigation allows an operator to gain access to the distal vessel, overall procedural success is still subject to the limitations of the currently available balloon and stent delivery platforms. Thus, the calcification and tortuosity of the lesion and proximal vessel may be traversed by the magnetic guidewire, but the lesion may not be able to be angioplastied or, more likely, stented because of the inability to advance the catheter to the lesion. Operators have reported the need to exchange the presently available magnetic wires for conventional wires with more support, prompting the need for a wider array of magnetic wires with more delivery support. As the system has no direct forward power to push guidewires and the currently available guidewires do not have a great deal of stiffness for support, the use of magnetic navigation has not greatly improved the ability to cross chronic total occlusions. Operators frequently can get the magnetic wire to the occluded segment and direct the tip in the proper direction, but the wire cannot penetrate the fibrous cap and gain access distal to the occluded segment. From an imaging and technical standpoint, the presently available system has a limited ability to image at extreme angulations as the magnets limit the movement of the C-arm. Also, patients with potential inability to tolerate a magnetic field (e.g., those with pacemakers, defibrillators, or surgical clips) have been prohibited from undergoing a magnetic navigation procedure. As these devices become more common in our difficult-to-treat patients, we will need a full evaluation of any potentially negative effects that patients with these prostheses may incur if placed in this type of magnetic field. Future Directions One of the potential strengths of this type of system is the ability to integrate multiple imaging modalities such as CT angiography, MRI, and coronary angiography to produce virtual reconstructions of coronary vessels and a road map for the operator. Coupling the magnetic guidance with technology to automatically advance the guidewire may allow the operator to remotely wire lesions. This has the potential to decrease radiation exposure and the amount of time the operator is required to wear a leaded apron something that all interventionalists will eventually appreciate. Developments are also underway to couple a power-source to a magnetic guidewire to allow it to puncture fibrous caps of chronic total occlusions and potentially assist in the most difficult lesion subset currently faced in the cath lab. Other developments have focused on providing a wider array of magnetic wires with more delivery support and hydrophilic coatings. As these become available, magnetic navigation may become an option for all coronary angioplasty procedures, regardless of lesion complexity, as a way to decrease procedure time and contrast dosage for even the more straightforward angioplasties. One can imagine that as more cases are performed with magnetic navigation, further advances will be needed as we continue to find ways to enhance our ability to revascularize all of our patients successfully in the catheterization lab just as refinements in balloon angioplasty evolved from the initial case performed in 1977. In the field of interventional cardiology, change is a certainty! Dr. Lim can be contacted at limmj (at) slu. edu.

1. Tsuchida K, Garcia-Garcia HM, van der Giessen WJ, et al. Guidewire navigation in coronary artery stenoses using a novel magnetic navigation system: First clinical experience. Cathet Cardiovasc Intervent 2006; 67:356-363.

2. Atmakuri ST, Lev EI, Alviar C, et al. Initial experience with a magnetic navigation system for percutaneous coronary intervention in complex coronary artery lesions. J Am Coll Cardiol 2006; 47:515-521.