CLINICAL RESEARCH
Initial Experience With a Magnetic Navigation System for Percutaneous Coronary Intervention in Complex Coronary Artery Lesions
Satya Reddy Atmakuri, MD*,
Eli I. Lev, MD ,
Carlos Alviar, MD*,
Edward Ibarra, RRT,
Albert E. Raizner, MD,
Stuart L. Solomon, MD and
Neal S. Kleiman, MD,*
* Baylor College of Medicine
Methodist DeBakey Heart Center, Houston, Texas
Manuscript received August 28, 2005;
revised manuscript received October 12, 2005,
accepted October 18, 2005.
* Reprint requests and correspondence: Dr. Neal S. Kleiman, 6565 Fannin, MS F-1090, Houston, Texas 77030 (Email: nkleiman{at}tmh.tmc.edu).
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Abstract
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OBJECTIVES: The aim of this study was to evaluate the feasibility of a magnetic-assisted navigation system during percutaneous coronary intervention (PCI) of tortuous and severely angulated coronary arteries.
BACKGROUND: The magnetic navigation system consists of two 0.8-T permanent magnets which generate a magnetic field over the heart. Altering the magnetic vector deflects a coronary guidewire with a magnetic tip.
METHODS: Patients were selected for magnetic-assisted intervention (MAI) for potentially difficult to cross lesions. The time required for placement of the guidewire, total procedure time, fluoroscopy time, and amount of contrast for the procedure were recorded. There were a total of 59 patients undergoing PCI of 68 lesions.
RESULTS: Patients were grouped based on whether MAI was attempted as a first option ("primary attempt"; n = 46) or following failure to pass a conventional guidewire ("secondary attempt"; n = 13). The target lesion was successfully crossed in 49 of 55 lesions (89%) and 9 of 13 lesions (69%) in patients undergoing primary and secondary attempts, respectively. The procedural success rates were 84% and 62%, respectively. Most lesions were located in the circumflex artery territory (39% and 62% of lesions, respectively). The median (25th and 75th percentiles) time for crossing the lesion was longer in the secondary attempt group (14.8 [5, 15.5] vs. 28.9 [8, 38] min). Median fluoroscopy time and median contrast used were also higher among the secondary attempt group.
CONCLUSIONS: This first report of MAI suggests that it may become a useful adjunct for wire placement in difficult coronary interventions.
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Abbreviations and Acronyms
| | MAI = magnetic-assisted intervention | | PCI = percutaneous coronary intervention | | SVG = saphenous venous graft |
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Percutaneous coronary intervention (PCI) has revolutionized treatment of atherosclerotic coronary artery heart disease. As the technical facets of imaging and interventional equipment have improved, and as adverse clinical events such as myocardial infarction and restenosis have declined (1–4), it is likely that the spectrum of coronary lesions approached by interventional cardiologists will include increasingly complex stenoses. However, interventions are still limited in a number of anatomic lesion subsets in which placement of guidewires is highly difficult. Such lesions include chronic total occlusions, tortuous arteries proximal to the target stenoses, severely angulated vessels and narrowing in native coronaries that require retrograde approach through saphenous venous grafts (SVG).
Recently, a magnetic navigation system was introduced for use during PCI and electrophysiologic procedures (5–7). This system was recently approved by the U.S. Food and Drug Administration for use during both types of procedures. The system consists of three basic components: two permanent magnets which generate a 0.08-T field over a spherical region of approximately 15 cm superimposed on the patient's mid-thorax, a magnet-tipped 0.014-inch-diameter guidewire, and a computer system to direct movement of the permanent magnets. By rotating the permanent magnets in three planes, the magnetic field can be directed to coincide with the long axis of a coronary artery. Consequently, the magnetic tip of the guidewire can be deflected along a line (or a series of lines) parallel with this axis. The system has been shown to be effective and safe for cardiac electrophysiology procedures, such as mapping and ablation (5,6). In addition, a preliminary case report describing use of the system for coronary intervention has been published on a commercial website (7). The magnetic navigation system can facilitate navigation through coronary arteries or coronary artery bypass grafts in which guidewire placement would otherwise be difficult or impossible.
The current report describes our initial experience using this unique system in the first 59 patients undergoing intervention for 68 lesions.
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Methods
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Patient selection.
Patients were judged to be candidates for PCI based on typical clinical criteria. Two additional criteria were used to select patients for magnetic navigation: first, the operators' perception that access to the target vessel with the guidewire would be difficult or impossible using conventional means (manual manipulation of an angioplasty guidewire); and second, if patients had previously undergone attempted but unsuccessful PCI using conventional guidewires but the operator felt that a second attempt at approaching the lesion might be possible using the magnetic guidance system. Patients were excluded if they had contraindications to undergoing magnetic resonance imaging, such as a pacemaker, defibrillator, or metallic implants.
Standard 6- or 7-F guiding catheters were used, and angiographic guiding views were selected to delineate the proximal segments of the target vessels as well as to display the target lesions without significant overlap. The magnetic navigation system was then used to position the guidewire across the lesion. Once the operator confirmed that placement of the wire was satisfactory, balloons and stents were advanced to the target lesion and deployed. The remainder of the intervention and post-procedural care was completed according to institutional standards.
The magnetic navigation system.
The magnetic navigation system (Niobe; Stereotaxis, St. Louis, Missouri) is commercially available and has been installed in a dedicated catheterization suite (Siemens, Forcheim, Germany). The system consists of two permanent magnets spaced on either side of the X-ray gantry (Fig. 1). When not in use, the magnets are rotated away from the gantry, and are rotated into position electronically when needed. In working position, they reside approximately 10 cm on either side of the table. The magnetic field over the patient's heart is approximately 0.08-T in strength and has a radius of 15 cm. A dedicated software program to control the position of the magnets can be run from either the control room or the tableside. Rotation of the magnets in three planes allows the vector of the resulting magnetic field to be rotated about the center of a virtual sphere located in the center of the patient's thorax. The direction of the vector is selected manually by the operator and is directed to correspond to the long axis of the coronary arterial segment being traversed with the guidewire. Software applications allow the wire tip to be directed longitudinally, to interrogate angles up to 120° from the long axis, or to proceed around the long axis of the vessel. The resulting vector is displayed on a monitor and is superimposed upon a rendition of the coronary anatomy as seen in that view. To guide the operator in selecting the appropriate vectors, two features are available. The target vessel as seen in the appropriate view can be selected from a library based on compilation of approximately 200 normal angiograms, or angiographic views of the patient's own vessels can be imported directly from the angiographic imaging chain (Fig. 2). Once these views are imported, it is no longer necessary to maintain the X-ray gantry in the position that was used to acquire them.
Use of the system requires a dedicated 0.014-inch-diameter coronary guidewire (Stereotaxis). The wire contains a magnet embedded in its tip that allows the tip to be deflected by the magnetic field applied to the patient. The guidewire is not hydrophilic and does not have "memory." The stiffness of the wire is less than most commonly used guidewires. The operator advances the wire manually. The software is used to predict and generate the vector necessary for the deflection of the guidewire tip.
Data collection.
Procedural data were analyzed retrospectively to evaluate the feasibility and benefits of using the magnetic-assisted intervention (MAI) system during PCI. The guidewire placement time was defined as the time required to place the magnet-tipped guidewire past the target lesion and was recorded by the control room operator. The procedure time was defined as the time from the start of guidewire placement to the completion of stent placement and removal of interventional equipment from the coronary artery for a single target lesion. The fluoroscopic time was recorded by the cardiac catheterization laboratory staff and included the time spent for acquiring vascular access and completion of PCI of other lesions by the same operator in the same setting. Patient data were gathered from retrospective chart reviews.
Statistical analysis.
Continuous data are presented as median values (25th, 75th percentiles) because of wide scatter within each data set. Categorical variables are presented as frequency (percentage). Categorical variables were compared using Fisher exact tests, and p < 0.05 was considered significant.
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Results
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A total of 14 operators performed PCI in 59 patients using magnetic-assisted navigation. Tables 1 and 2 present the patient and procedural characteristics among patients who underwent MAI as a primary attempt (n = 46) or a secondary attempt following failed conventional angioplasty (n = 13). The mean age of patients was 63 ± 12 years (Table 1). In all of the procedures, the patient's own angiogram was used as reference to navigate the coronary arteries. Most of the lesions that were approached using MAI were in the circumflex territory (43%). The rest were distributed in the left anterior descending artery (25%) and right coronary artery (32%) territories. The lesions that were attempted were mostly American College of Cardiology/American Heart Association (ACC/AHA) type B2 lesions (53%) followed by ACC/AHA type C lesions (43%). Two patients had chronic total occlusions, which could not be crossed using magnetic-assisted navigation. In three instances the target lesion was in a native coronary artery and was approached retrograde through an SVG. Nine lesions that could not be accessed previously using a manually directed approach were crossed using the MAI system.
The target lesion could be crossed with the magnet-tipped guidewire in 85% of cases (Table 2). In four of these cases a balloon could not be passed across the lesion following guidewire placement, leading to a procedural success rate of intervention of 79%. In most of the cases, a stiffer guidewire was exchanged to deliver the stents to the lesion. Therefore, a stent could be placed in 98% of all the lesions in which a balloon could be passed. Figures 3 and 4 illustrate successful interventions in typical lesions attempted. Among the three patients who underwent attempted PCI using a retrograde approach through an SVG, a balloon could be advanced across the lesion in two patients. The angiogram of one of these patients is shown in Figure 5.
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Figure 3 An example of successful magnetic-assisted intervention (MAI) in a tortuous saphenous venous graft.
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Figure 4 Angiogram depicting successful guidewire placement using magnetic-assisted intervention (MAI) in the circumflex coronary artery with severely angulated takeoff at the origin.
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Figure 5 Example of successful placement of guidewire in a native coronary artery using a retrograde approach through a saphenous vein graft in a patient with coronary artery bypass surgery. In this case, the wire could be placed but the operator was unable to position a balloon in the target segment beyond the acute bend at the vein graft anastomosis. MAI = magnetic-assisted intervention.
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The median time required to cross a lesion using the MAI was 10.5 (6, 18) min with a wide range for all the procedures (2 to 103 min). The median procedure time was 64 (41, 76) min, and median fluoroscopy time was 37 (15, 60) min. The median contrast used for the procedure was 190 (140, 270) ml. Because 19 patients underwent PCI of more than one vessel and 3 patients had brachytherapy, the estimate for contrast as well as the fluoroscopic time likely overestimates the requirements for crossing the target lesion using MAI.
When comparing the primary attempt and secondary attempt groups, most of the patients who underwent MAI as a secondary attempt had lesions within the circumflex artery territory (Table 1). Magnetic-assisted intervention was more likely to result in a successful procedure in patients who had undergone the PCI as a primary attempt (procedure success: 84% vs. 62%, respectively; p = 0.07; success in crossing the lesion with the wire: 89% vs. 69% respectively; p = 0.07). The median time (25th, 75th percentiles) for crossing the lesion was longer in the secondary attempt group (14.8 [5, 15.5] min vs. 28.9 [8, 38] min) (Table 2). In accordance, the median procedure time and median fluoroscopy time were longer in the secondary group (procedural time: 62 [38, 74] min vs. 70 [59, 89] min; fluoroscopy time: 23 [14, 42] min vs. 59 [41, 75] min, respectively).
In the secondary attempt group, patients who had failed manual guidewire placement were brought back to the cardiac catheterization laboratory on a different day or MAI was performed in the same setting. The median number of guidewires used during the initial failed attempt was 2 (2, 3), and median time devoted to guidewire placement attempts was shorter using the MAI system compared with the previous manual attempt (14 [8, 38] min vs. 30 [17, 35] min, respectively).
The only complication noted was one episode of coronary artery perforation. In this case, the patient underwent PCI using the MAI system as a primary attempt and the lesion was crossed successfully with the MAI wire. The patient subsequently had a guidewire exchange with a conventional wire guidewire over which a stent was placed, but during multiple balloon dilations the perforation occurred.
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Discussion
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This is the first peer-reviewed report on the use of a magnetic navigation system to facilitate the performance of technically complex PCI in a consecutive series of patients. In our initial experience, magnetic navigation enabled the range of PCI to be extended to certain patients with challenging highly tortuous vessels and acute bends. Although the current report represents a consecutive series of individual cases without a control group, it demonstrates that this novel near-robotic technique is feasible in human PCI and has the potential to expand the utility of the procedure. Previously, the system had been shown to be useful for cardiac electrophysiology procedures (5,6).
As percutaneous revascularization of the coronary arteries further evolves in the drug-eluting stent era, it is likely that the quarry of the interventional cardiologist will include increasingly complex anatomic substrates. Currently, angiographic success rates following PCI are reported to be in the range of 88% to 95% (1,2). Reasons to refer cases for medical management or coronary artery bypass surgery rather than PCI include the risk of restenosis, ischemia, abrupt vessel closure, or perceived difficulty accessing lesions with guidewires and balloons. Intracoronary stenting has nearly eliminated the risk of abrupt vessel closure. With modern anticoagulant and antiplatelet regimens, the risk of periprocedural infarction has been reduced. The advent of drug-eluting stents has reduced the likelihood of restenosis to <10% even in long lesions (8,9). The principal difficulty limiting PCI in anatomically complex cases is now difficulty crossing the target lesion with guidewires or stent delivery devices.
In the current report, we observed that use of a novel magnetic navigation system appears to extend the ability of PCI operators to instrument lesions that were otherwise either difficult or impossible to access. Traditionally, excessively tortuous lesions, lesions located in vessels with sharp angles of origin from the left main trunk, have been regarded as difficult to instrument. In the modified ACC/AHA lesion classification, both severe angulation (>90°) and extreme proximal tortuosity result in a lesion being given a type B or C classification, indicating a lower than average likelihood of success (10). Lesions located in proximal coronary arteries that must be reached through bypass conduits have been exceptionally difficult or impossible to reach using conventional guidewire techniques. Although the advent of newer-generation hydrophilic wires with continuous cores has facilitated wire placement in complex coronary anatomy, cases remain in which conventional techniques are not able to result in successful wire placement.
Compared with the overall population of patients undergoing PCI, the patient group in the current study represents an older population with more extensive coronary artery disease. Thirty-seven percent had diabetes mellitus, in contrast with the generally quoted figure of about 25% to 30% (1,2,11) in the overall PCI population. Twenty-seven percent of patients had prior coronary artery bypass surgery; 19% had previous failed attempts at PCI using conventional guidewire techniques. The fact that four lesions could be crossed with the guidewire but not with a balloon illustrates the complexity of the lesions approached. Although fluoroscopic times and total procedural times were considerably longer than are generally reported, it is likely that there is a learning curve for use of this device.
It is not yet clear how wide the applicability of magnetic-assisted navigation is likely to be. Very few clinical trials or registries report the severity of angulation or tortuosity and rarely report the frequency of failure to cross a lesion with a guidewire in vessels that are not totally occluded. Although the presence of a severely tortuous vessel was a reason for exclusion from the pivotal trials of drug-eluting stents, Wilensky et al. (11) report that 24% of the broad range of patients represented in the National Heart, Lung, and Blood Institute-sponsored Dynamic registry had a moderately or severely tortuous vessel. It is also likely that as a result of selection criteria many patients with severely angulated or tortuous lesions are not referred at all for PCI.
Study limitations.
The primary limitation of the current report is that it is a descriptive report of consecutive cases. Selection of a concurrent control group was not possible, because the criteria used for selection of cases were largely intuitive and operator dependent rather than based on well-defined criteria. In addition, the operators may have been biased by the availability of the MAI system, thus proceeding to a magnetic-assisted PCI following a relatively short period of time allocated to attempts with a conventional approach. Obviously, further comparison with conventional techniques will require a randomized trial.
Conclusions.
Although this report describes an acceptably high success rate in a particularly complex lesion subset, the available interventional equipment (for example, MAI guidewires and catheters) represents early and perhaps "primitive" technology. As advancements in equipment specifically engineered to maximize the potential of MAI evolves, higher success rates in even more complex lesions would be anticipated.
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Footnotes
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Drs. Kleiman and Raizner have received modest speakers' honoraria from Stereotaxis, St. Louis, Missouri. Since the initial submission of the manuscript, Mr. Ibarra has become an employee of Stereotaxis.
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References
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1. Ellis SG, Guetta V, Miller D, Whitlow PL, Topol EJ. Relation between lesion characteristics and risk with percutaneous intervention in the stent and glycoprotein IIb/IIIa era: an analysis of results from 10,907 lesions and proposal for new classification scheme Circulation 1999;100:1971-1976.[Abstract/Free Full Text]2. Krone RJ, Shaw RE, Klein LW, et al. Evaluation of the American College of Cardiology/American Heart Association and the Society for Coronary Angiography and Interventions lesion classification system in the current "stent era" of coronary interventions (from the ACC-National Cardiovascular Data Registry) Am J Cardiol 2003;93:389-394. 3. Halkin A, Stone GW. Polymer-based paclitaxel-eluting stents in percutaneous coronary intervention: a review of the TAXUS trials J Interv Cardiol 2004;17:271-282.[CrossRef][Medline] 4. Moses JW, Leon MB, Popma JJ, et al. Sirolimus-eluting stents versus standard stents in patients with stenosis in a native coronary artery N Engl J Med 2003;349:1315-1323.[Abstract/Free Full Text] 5. Faddis MN, Chen J, Osborn J, Talcott M, Cain ME, Lindsay BD. Magnetic guidance system for cardiac electrophysiology: a prospective trial of safety and efficacy in humans J Am Coll Cardiol 2003;42:1952-1958.[Abstract/Free Full Text] 6. Ernst S, Ouyang F, Linder C, et al. Initial experience with remote catheter ablation using a novel magnetic navigation system: magnetic remote catheter ablation Circulation 2004;109:1472-1475.[Abstract/Free Full Text] 7. Stereotaxis. Digital Solutions for Interventional Medicine. Available at: http://www.stereotaxis.com/content/4_3_caseStudies.php. Accessed October 7, 2005.. 8. Moses JW, Leon MB, Popma JJ, et al. SIRIUS Investigators Sirolimus-eluting stents versus standard stents in patients with stenosis in a native coronary artery N Engl J Med 2003;349:1315-1323.[Abstract/Free Full Text] 9. Stone GW, Ellis SG, Cox DA, et al. TAXUS-IV Investigators A polymer-based, paclitaxel-eluting stent in patients with coronary artery disease N Engl J Med 2004;350:221-231.[Abstract/Free Full Text] 10. Ellis SG, Vandormael MG, Cowley MJ, et al. Coronary morphologic and clinical determinants of procedural outcome with angioplasty for multivessel coronary diseaseImplications for patient selection. Multivessel Angioplasty Prognosis Study Group. Circulation 1990;82:1193-1202.[Abstract/Free Full Text] 11. Wilensky RL, Selzer F, Johnston J, et al. Relation of percutaneous coronary intervention of complex lesions to clinical outcomes (from the NHLBI DYNAMIC registry) Am J Cardiol 2002;90:216-221.[CrossRef][Web of Science][Medline]
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Abstract
Methods
