Asian Cardiovasc Thorac Ann 2007;15:24-29
© 2007 Asia Publishing EXchange Ltd
MRI-Based Safety Evaluation of the Ventrica Magnetic Coronary Anastomotic System
Uwe Klima, MD,
Janina Beilner, MD,
Eric Bagaev, MD,
Stefan Fischer, MD,
Theo Kofidis, MD,
Joachim Lotz, MD1
Thoracic and Cardiovascular Surgery
1 Department of Radiology, Hannover Medical School, Hannover, Germany
For reprint information contact: Uwe Klima, MD Tel: 65 6772 2859 Fax: 65 6776 6475 Email: suruk{at}nus.edu.sg, Department of Cardiac, Thoracic and Vascular Surgery, Yong Loo Lin School of Medicine, National University of Singapore, 5 Lower Kent Ridge Road, 119074 Singapore.
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ABSTRACT
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The Ventrica Magnetic Vascular Positioner system is a novel automatic anastomotic coupling device for distal coronary artery anastomosis. There is concern that enormous magnetic fields may negatively affect graft anastomosis or coronary artery blood flow, or that they may lead to disconnection of the magnetic ports. Forty-five domestic swine (26.6 ± 5.9 kg) underwent magnetic resonance imaging after a single Ventrica anastomosis of the left internal mammary artery to the left anterior descending artery. Group A (n = 15) underwent magnetic resonance imaging immediately after surgery, group B (n = 15) was studied after 1 week, and group C (n = 15) after 2 weeks. The animals were sacrificed and the anastomotic sites were examined. All animals survived the imaging procedure. Mean imaging time was 25 ± 6 min. Although imaging artifacts occurred in the area surrounding the Ventrica port, there were no disconnections or electrocardiographic signs of ischemia during the study period. Upon sacrifice, all anastomoses were patent without alterations in the alignment of the magnetic components. Clinically relevant tests such as cranial magnetic resonance imaging may be safe after use of the Ventrica system for coronary artery revascularization.
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INTRODUCTION
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Techniques of coronary artery bypass surgery have evolved over the past 35 years. Constant refinements of technique are required to minimize morbidity and mortality risks for the patient. Currently, the hand-sewn anastomosis is considered the gold standard in coronary artery bypass grafting. To reduce surgical trauma and avoid the side effects of extracorporeal circulation, less invasive surgical approaches, such as minimally invasive coronary artery bypass (MIDCAB) and off-pump coronary artery bypass, have been introduced and tested in a variety of clinical studies.1,2 Thoracoscopic video-assisted coronary artery revascularization has been considered, but hand-sewn anastomosis under these conditions has proved to be exceedingly difficult. The feasibility of these conceptually beneficial, less invasive surgical approaches is limited. Numerous attempts have been made to develop micromechanical bonding techniques for coronary artery bypass grafting. Scheltes and colleagues3 reviewed 51 different devices for distal anastomoses in 4 major categories; including staples, clips, mounting systems, and intraluminal stents.
We previously reported successful use of the Ventrica Magnetic Vascular Positioner (MVP) system (Ventrica Inc., Fremont, CA, USA) in a multicenter trial involving 32 patients from 3 major cardiac surgical centers in Germany.4 The use of the device for distal coronary artery anastomoses was shown to be safe and yielded acceptable graft patency rates. We have further reported on the successful use of MVP anastomosis in a patient who had total arterial revascularization using bilateral internal thoracic arteries and the left radial artery as a T-graft from the left internal thoracic artery (LITA).5 In that patient, all anastomoses, including the T-anastomosis, were performed with the device. In a subsequent study, we described a series of patients undergoing MIDCAB with a single LITA-to-left anterior descending artery (LAD) anastomosis utilizing the MVP system.6 Since our initial experience, we have evaluated the safety of the device when used with an anti-platelet regimen, before recommending its routine clinical use.4 The effect of intense magnetic fields, as applied during postoperative magnetic resonance imaging (MRI), on these magnetic anastomoses has not been evaluated. There is concern that an enormous magnetic field might negatively affect the graft, the anastomosis, or the coronary artery blood flow, or even cause disconnection of the magnetic rings. These potential effects might put the patient at risk of coronary ischemia or bleeding from the anastomosis. This experimental study examined whether there were any deleterious effects from several clinically relevant MRI protocols on graft function, anastomotic integrity, coronary artery blood flow, and myocardial function at selected times after MIDCAB surgery using the MVP.
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MATERIALS AND METHODS
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All animals received humane care in compliance with the German animal protection legislation, the "Principles of Laboratory Animal Care", and the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Resources and published by the National Institutes of Health (NIH publication No. 85–23, revised 1985). Forty-five domestic swine (mean weight, 26.6 ± 5.9 kg) underwent postoperative MRI after a MIDCAB procedure that employed an automated MVP anastomotic device. The animals were randomized into 3 study groups: group A (n = 15) underwent MRI immediately after surgery, group B animals (n = 15) were studied after one week, and group C (n = 15) after 2 weeks.
The Ventrica series 6000 distal anastomosis system consists of two sets of elliptical-shaped intravascular magnets and 2 extravascular magnetic clips; each set is preloaded on a delivery instrument.4–6 One set of magnetic clips is deployed to form the anastomotic port in the graft vessel; the other set is deployed to form an identical anastomotic port in the target coronary artery. Once both ports are created, coupling is accomplished by simply bringing the 2 ports into close proximity and magnetic attraction establishes a self-sealing anastomosis instantly. The MVP anastomosis has been tested by the manufacturer and when the anastomosis is created according to their instructions for use, it has an attachment strength exceeding that of 8/0 polyester suture. The MVP system is available in two sizes: 1.5 mm (series 6000 model 6150) for vessels between 1.5 and 2.0 mm internal diameter (ID), and a 2.0-mm device (series 6000 model 6200) for vessels of 2.0 to 4.0 mm ID. The device sizes refer to the approximate minor ID of the elliptical lumen of the device (actual, 1.6 mm and 2.1 mm, respectively). As all animals had target and graft arteries > 2 mm ID, only the MVP series 6000 model 6200 was used.
Anesthesia was induced with intramuscular azaperone (5 mg·kg–1) and atropine (0.5 mg total dose), and intravenous thiopental sodium (15 mg·kg–1). Animals were intubated and mechanically ventilated at a fraction of inspired oxygen of 0.5. All animals were further monitored by 12-channel electrocardiogram (EKG) during all surgical and radiographic interventions. Following induction of anesthesia, the animals were placed in a 30° right lateral decubitus position. The left hemithorax was entered via a 6- to 7-cm anterolateral minithoracotomy in the 4th or 5th intercostal space. The LITA was dissected free of the chest wall under direct vision, within a fascia-muscular pedicle. All side branches were clipped. The length of the pedicle was between 10 and 15 cm, permitting a tension-free LITA-to-LAD anastomosis. After completion of the LITA harvest and testing for adequate flow, papaverine was repeatedly applied to the LITA pedicle to prevent vascular spasm. Systemic heparinization (100 IU·kg–1) was established. Additional heparin was administered if necessary to maintain the activated clotting time above 350 sec at the time of MVP insertion. The LITA pedicle was transected distally and its distal end was occluded with 2 titanium clips. The pericardium was opened in a T-shaped manner, and the LAD target anastomotic site was identified. The LAD was loosely encircled with a 4/0 polypropylene suture proximal to the selected anastomotic site. The anastomotic site was free of atherosclerotic plaque and had an intraluminal diameter of at least 2 mm in all animals. A commercially available blower/mister system (Medtronic ClearView 22120; Medtronic, Inc., Minneapolis, MN, USA) maintained a bloodless operative field. A 5.0 mm incision was made in the distal LITA starting 2.0 mm proximal to the distal clips. The first set of magnets was deployed to create the graft port. After confirming adequate blood flow through the port, the LITA pedicle was temporarily occluded with a proximal plastic bulldog clamp and wrapped in a papaverine-soaked sponge. A plastic bulldog clamp was used to avoid attraction to the port magnets. By creating the graft port first, myocardial ischemic time during the performance of the rest of the anastomosis was minimized. Immediately prior to creating the target artery port, the LAD was permanently ligated with the previous loosely positioned polypropylene ligature, well proximal to the intended anastomotic site. A 5.0 mm incision was made in the LAD at the anastomotic site, and the second set of magnets was deployed to create the target artery port. The two ports were brought together, forming a reliable side-to-side anastomosis (functional end-to-side) held together by magnetic attraction. After completion of the anastomosis, the bulldog clamp was removed from the LAD pedicle and the pedicle was tacked to the epicardium with 2 stitches to prevent rotation or twisting, and to provide additional security to the anastomosis, as recommended by the manufacturer. Half of the administered heparin was reversed with protamine. Pericostal sutures were tied securely, and the thoracotomy was closed. After extubation, food and water were administered to the animal ad libitum. For anti-platelet treatment, all study animals were given 150 mg of clopidogrel and 100 mg of aspirin preoperatively. Postoperatively, they received 75 mg of clopidogrel and 100 mg of aspirin daily.
All animals were deeply sedated with propofol before and during examination. Electrocardiogram monitoring was carried out with an MRI-compatible EKG monitor (Medrad Multigas, MEDRAD Inc., Pittsburgh, PA, USA). The animals were closely monitored by a veterinary anesthesiologist during the entire MRI examination. All examinations were performed in a 1.5 Tesla whole-body MRI scanner (CV/I GE Medical, Milwaukee, WI, USA) using the body coil. To mimic the majority of clinically relevant MRI protocols, various sequences were employed during the MR-examination. All sequences, their indications, parameters, and potential impact on the mechanical anastomosis are described in Table 1
. All sequences were centered on the anastomosis as seen on the initial real-time localizer (Figure 1). Depending on the sequence type, different forms of stress were applied to the magnetic anastomosis: spin-echo-based sequences provoked heating induced by extensive radiofrequency impulses. Sequences based on gradient-echo and epitaxial semiconductor technology use extensive and rapid changes of the magnetic field, which induces mechanical stress on the anastomosis. The highest mechanical stress on the anastomosis was expected to occur on the way in and out of the center of the magnet, as this is the time when the magnetic anastomosis experiences an increase of external magnetic field strength from a few µT to 1.5 T. Vital signs were closely monitored during this time.
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Figure 1. Realtime localizer for planning MRI examination in a pig. The large black area in the middle of the thorax (*) resembles the artifact induced by the magnetic anastomosis.
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Study animals were sacrificed after MRI with a high dose of sodium pentobarbital. The heart was removed, and the anastomotic device was examined in situ. Special attention was paid to potential displacement of the magnets or peri-anastomotic blood accumulation. The heart was fixed and immersed in glutaraldehyde (1%) for histologic examination. The vessels were perfusion-fixed with 1% glutaraldehyde before en bloc explantation of the anastomosis. The explanted bloc consisted of the distal LITA and LAD from 1 cm proximal to 1 cm distal to the magnetic port. The magnetic implants were removed through incisions in the fibrous tissue pocket encapsulating them. The remaining tissue was mounted in paraffin. Cross-sections (5 to 10 µm thick) were taken every 0.5 mm across the longitudinal axis of the implants. The sections were stained with hematoxylin and eosin.
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RESULTS
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Fifty-three animals underwent a MIDCAB procedure with the Ventrica MVP anastomosis. Mean total procedure time was 92 ± 12 min. Mean anastomotic time (creation of graft and target artery port and coupling time) was 135 ± 45 sec. All animals survived the surgical procedure. There were 8 deaths during the postoperative period, prior to MRI examination. Four animals developed acute ventricular fibrillation within 2 hr of surgery. One animal died from postoperative hemorrhage, another died from severe myocardial infarction, and two died from porcine flu. In the autopsies of all animals that died from acute ventricular fibrillation, the anastomotic device was normally positioned and completely patent. There was no evidence of myocardial infarction. Acute ischemia secondary to spasm of the LITA graft was considered a possible cause of the arrhythmia because after ligating the proximal LAD, the LITA graft carried the entire blood supply to the anterior wall and septum. In the animal that died of postoperative hemorrhage, a titanium clip had slipped off a side branch of the LITA and this appeared to be the source of the bleeding. In the 2 pigs that died from influenza, the magnetic anastomosis was intact and there were no histological or macroscopical signs of myocardial infarction. Only in the animal that died from acute myocardial infarction was occlusion of the magnetic anastomosis found at autopsy. However, all magnetic elements were in their proper position. The reason for occlusion remains unclear. Perhaps the anticoagulation regimen in this single case was inadequate.
Mean MRI examination time was 25 ± 6 min. No deaths or arrhythmias occurred during the MRI examination. In all animals sacrificed after MRI, the anastomoses were found to be intact and there were no signs of myocardial infarction at autopsy. Large artifacts induced by the MVP magnets were seen in the area of all magnetic anastomoses examined. These artifacts impaired the assessment of cardiac function and thoracic morphology in these animals. (Figure 2).
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Figure 2. Susceptibility artifacts (*) induced by the magnetic material of the vascular positioner for anastomoses in a pig. Assessment of anatomy and cardiac function by MRI was impossible due to these artifacts. FIESTA = fast imaging employing steady-state acquisition.
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In all histologic specimens, the vessel wall including the intima at the anastomosis was intact, indicating no major intimal damage during surgery or mechanical trauma due to distortion during MRI. Histologic examination showed no evidence of heat damage, confirming our clinical impression that there was no significant local heat development in the magnetic implants during MRI.
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DISCUSSION
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A variety of novel automatic proximal and distal anastomotic devices for coronary artery revascularization are currently in clinical use, and their number is increasing. The clinical performance, special individual considerations, and the outcome following application, especially postoperative angiography, have been well described by us and others.3–6 With increasing popularity of such devices and intense worldwide demand for less invasive approaches in cardiac surgery, automatic anastomotic devices are most likely to be used in a routine clinical fashion in the near future. However, before their routine application, many important questions regarding the safety of these instruments should be answered. All possible clinical scenarios following coronary artery revascularization, especially those involving the most common complications, should be considered.
Magnetic resonance imaging has become a useful tool for sensitive and quick recognition of neurological disorders following surgical procedures, and has proved to be more sensitive then common computed tomography in diagnosing acute brain-stem hemorrhage. This complication may occur after cardiac surgery using extracorporeal circulation.7,8 Thus it is important that the safety of cranial MRI in patients with the MVP be known.
Recent developments in cardiac MRI have led to tremendous breakthroughs in the imaging of acute myocardial ischemia and infarction.9 Hardware and acquisition sequences have improved image quality while simplifying cardiac examinations. Cine-MRI allows for accurate time-resolved imaging of global and segmental left ventricular function with high spatial resolution.10 Dynamic multislice MRI of myocardial first-pass perfusion is now widely available, allowing the detection of induced ischemia as an early sign of significant coronary artery disease. Direct high-resolution MRI of myocardial infarction is well standardized, adding significant diagnostic value over the usual clinical and biological markers after non-ST-elevation coronary syndromes.11 However, due to high-power magnetic fields, it is known that MRI is not applicable in the presence of a variety of metallic implants and devices. There have been severe accidents and some deaths related to the use of MRI in patients with metallic implants and devices.12 The main purpose of this experimental study was to investigate the safety of MRI in the presence of the MVP and to determine the effect of MRI on the position and function of the magnetic anastomosis after coronary artery revascularization. We further examined whether the high-power magnetic fields during imaging influenced blood flow through the anastomosis, and whether myocardial ischemia might be caused by distraction of the magnetic ports. However, we found that none of the animals, regardless of the observation time after surgery, developed EKG signs of ischemia during MRI. In addition, none of the implanted devices were displaced by the magnetic field during MRI. No deaths or complications occurred during MRI. This study suggests that MRI following coronary artery revascularization using the Ventrica MVP device is safe at any postoperative time point, even in the very early period before wound healing has begun when only the magnetic force maintains the anastomosis.
Special attention was given to detect intimal damage in the LITA graft and in the LAD at the anastomosis. Damage might be anticipated to arise from surgical trauma, mechanical stress from the device, particularly during MRI, or from overheating by the magnetic field of the MRI. Histology showed an intact vessel wall in all specimens, including the intima at the anastomotic site, supporting the assumption that MRI does not cause excessive mechanical stress or heat at the anastomosis, even in cardiac MRI.
As a limitation of the study, it may be argued that no measurement of coronary flow was performed in the LAD distal to the anastomosis. Such a study might show potential ischemia during MRI secondary to distortion of the magnetic ports by the magnetic fields. External flow probes containing ferrous materials could not be used. An intracoronary flow probe would not be tolerated during the entire 2-week study period. This is especially true in pigs whose coronary arteries are particularly subject to spasm. As coronary perfusion of the anterior wall and the septum was solely dependent on the LITA graft after the LAD was ligated proximally, and because there was no EKG evidence of ischemia and no instance of ventricular fibrillation, we concluded that no major ischemia had occurred during MRI. Generally, as a consequence of severe ischemia, pigs’ hearts fibrillate within 2 min. The large imaging artifacts induced by the MVP system were due to the magnetic properties of the device. Even with careful choice of the applied scanning technique, a morphologic or functional assessment of the heart is not possible by MRI in the presence of an MVP. The artifacts are larger than the normal human heart.
Since the magnetic properties do not affect the quality of image in computed tomography, imaging of the heart and thorax is still possible in the presence of an MVP (Figure 3). Advances in multi-slice technology may make it possible to assess cardiac function adequately.
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Figure 3. Artifacts induced by the magnetic material of the vascular positioner in multislice computed tomography (MSCT) of a patient. While assessment of the anastomoses is not possible, diagnostic evaluation of the cardiac anatomy and imaging of the thorax is not impaired.
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As the artifacts of the MVP system are limited to the heart and thorax, MR imaging of the brain is not affected and can be safely performed. This is of great clinical importance as lesions of the brain stem after cardiac procedures can only be detected by MRI and not by computed tomography.
This study demonstrated that clinically relevant tests such as cranial MRI following coronary artery bypass grafting surgery may be safe after the use of the MVP system for fast and safe coronary artery revascularization.
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ACKNOWLEDGMENTS
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We cordially thank Horace MacVaugh III, MD, Consultant in Surgery, Philadelphia, Pennsylvania, USA, for providing know-how and for his co-ordinative work.
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REFERENCES
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- Scheltes JS, Heikens M, Pistecky PV, van Andel CJ, Borst C. Assessment of patented coronary end-to-side anastomotic devices using micromechanical bonding. Ann Thorac Surg 2000;70:218–21.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
