Asian Cardiovasc Thorac Ann 2006;14:43-46
© 2006 Asia Publishing EXchange Ltd
Magnetic Resonance Angiography in Coronary Artery Bypass Grafting
Shingo Ohuchi, MD,
Kohei Kawazoe, MD,
Hiroshi Izumoto, MD,
Kunihiro Yoshioka, MD1
Department of Cardiovascular Surgery
1 Department of Radiology, Iwate Medical University Memorial Heart Center Morioka, Japan
For reprint information contact: Shingo Ohuchi, MD Tel: 81 19 651 5111 Fax: 81 19 624 8384 Email: gekai_ohuchi{at}hotmail.com, Department of Cardiovascular Surgery, Iwate Medical University Memorial Heart Center, 1-2-1 Chuohdori, Morioka 020-8505, Japan
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ABSTRACT
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An attempt was made to reduce the incidence of perioperative stroke by detecting cerebrovascular disease with preoperative head and neck magnetic resonance angiography and by selecting the coronary artery bypass grafting technique. This strategy was used in 268 patients with ischemic heart disease who had undergone both head and neck magnetic resonance angiography before elective coronary artery bypass in our hospital between May 1997 and April 2001. In patients with significant stenosis or obstruction detected by head and neck magnetic resonance angiography, the findings were evaluated and cerebral blood flow was examined using brain single-photon emission computed tomography. In those with a high risk of cerebrovascular ischemia, off-pump coronary artery bypass was performed to maintain cerebral blood flow. No stroke occurred during surgery, and hemodynamic cerebrovascular ischemia was prevented in all 268 patients. Postoperative stroke occurred in 3 patients (1.1%), but the incidence of perioperative stroke was reduced.
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INTRODUCTION
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Stroke is an important cause of morbidity and mortality in patients undergoing coronary artery bypass grafting (CABG). Therefore, a strategy to prevent perioperative cerebral complications is needed in patients with atherosclerotic lesions in the cerebrovascular system. For this purpose, accurate preoperative evaluation is important in determining the surgical technique to obtain the best outcome. We performed magnetic resonance angiography (MRA) of the arteries in the head and neck for screening before CABG, and attempted to reduce the incidence of perioperative cerebral complications by determining the best surgical technique on the basis of the MRA findings.
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PATIENTS AND METHODS
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There were 268 patients with ischemic heart disease who underwent both head and neck MRA before their first elective CABG in our hospital between May 1997 and April 2001. In patients with significant stenosis or obstruction detected by head or neck MRA, a high risk of cerebrovascular ischemia was indicated, and surgical techniques to maintain the cerebral blood flow (CBF) were selected. The incidence of perioperative stroke was investigated retrospectively. The percentage of stenosis was calculated by dividing the luminal diameter of the narrowest part of the artery by that of the segment distal to the lesion. The following findings on MRA were defined as significant: 
60% stenosis or total occlusion of the common carotid artery, internal carotid artery, basilar artery, dominant vertebral artery, or total occlusion of both vertebral arteries, and stenosis accompanied by poor peripheral blood perfusion or total occlusion of the anterior, middle, or posterior cerebral artery. Unruptured cerebral aneurysm, cerebral arteriovenous anomalies, and brain tumors were not regarded as positive findings.
The MRA was carried out using a SIGNA Horizon Hi-Speed LX-1.5 Tesla (GE Medical Systems, Milwaukee, WI, USA). The neck MRA images were constructed using the 2-dimensional time-of-flight procedure, and those of the head using the 3-dimensional time-of-flight procedure. In this study, no contrast medium was used for screening. MRA images were evaluated by a radiologist and a cardiovascular surgeon. MRA findings were individually assessed in the MRA-positive group. The following findings ruled out the possibility of cerebrovascular ischemia occurring during surgery: blood flow maintained through collateral circulatory pathways (e.g., circle of Willis); obstructed regions included in old cerebral infarction areas; or overestimation due to artifacts (no detection by cerebrovascular angiography, contrast MRA, or carotid ultrasonography). The CBF at rest and at the time of acetazolamide administration were quantitatively measured using 133Xe single-photon emission computed tomography (SPECT), and the CBF reserve capacity was evaluated using the following equation:
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CBF reserve capacity of +19% or greater was regarded as normal, and that of less than 19% as reduced. Patients with a normal reserve CBF capacity were regarded as being unlikely to experience cerebrovascular ischemia during surgery.
Surgical techniques were selected on the basis of MRA and brain SPECT findings. Off-pump coronary artery bypass (OPCAB) was the first choice in patients with reduced CBF. During the OPCAB procedure, every effort was made to stabilize the hemodynamics and prevent hypoperfusion. Systolic blood pressure was maintained at 80 – 90 mm Hg. An intra-aortic balloon pump or internal shunt was used in selected patients with unstable hemodynamics. However, if OPCAB could not be performed, carotid endarterectomy and conventional CABG were performed concomitantly in patients with carotid stenotic lesions.
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RESULTS
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The ages of the 268 patients ranged from 38 to 83 years and the majority were male; demographic data are listed in Table 1
. Sixty patients (22%) were classified as MRA-positive due to stenosis or obstruction of the common carotid artery in 3 patients (5%), the internal carotid artery in 32 (53%), vertebral artery in 3 (5%), middle cerebral artery in 14 (23%), posterior cerebral artery in 1 (2%), basilar artery in 4 (7%), internal carotid and middle cerebral artery in 1 (2%), and middle and posterior cerebral artery in 2 (3%). No patient had anterior cerebral artery disease.
The MRA findings of these 60 patients were evaluated and it was concluded that cerebrovascular ischemia was unlikely to occur in 14 (Figure 1); CABG was performed in 9, and OPCAB in 5. Brain SPECT was performed in 37 of the 46 patients (80%) in whom cerebrovascular ischemia was suspected on MRA. Reduction of the CBF reserve capacity was detected in 7 patients using brain SPECT; OPCAB was performed in 6, and CABG with carotid endarterectomy in 1. Brain SPECT showed normal CBF reserve capacity in 30 patients; CABG was performed in 25, and OPCAB in 5. In the remaining 9 patients, brain SPECT was not performed although cerebrovascular ischemia was suspected. Of these, 6 underwent OPCAB, 1 underwent CABG with carotid endarterectomy, and 2 underwent CABG with intra-aortic balloon pump support.
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Figure 1. Schematic representation of the selection of surgical methods. CABG = coronary artery bypass grafting, CBF = cerebral blood flow, CEA = carotid endarterectomy, IABP = intra-aortic balloon pumping, MRA = magnetic resonance angiography, OPCAB = off-pump coronary artery bypass, SPECT = single-photon emission computed tomography.
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No stroke occurred during surgery, and hemodynamic cerebrovascular ischemia was prevented. Postoperative stroke occurred in 3 patients (1.1%). One of them was in the MRA-positive group (60% stenosis of internal carotid artery) and an embolism occurred on the same side as the internal carotid artery stenosis on postoperative day 7. Both head and neck MRA was negative in the other 2 patients. Postoperative stroke occurred on days 8 and 21; both had 50% stenosis of the internal carotid artery and developed ipsilateral cerebral embolism and transient ischemic attacks. Two patients (0.7%) died in the hospital, both from low output syndrome. Cerebrovascular ischemia was not involved in any of the deaths.
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DISCUSSION
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The selection of surgical strategy at our hospital has been decided on the basis of neck vascular lesions determined by Doppler study and chest computed tomography findings in the ascending aortic wall. In patients with a severely diseased ascending aorta, crossclamping and/or placement of a proximal coronary anastomosis were avoided. However, the incidence of perioperative stroke in patients undergoing elective CABG was 2.5% (5/201) in our hospital between 1993 and 1996. Therefore, we examined vascular lesions using head and neck MRA to evaluate the causes and establish a strategy to prevent perioperative complications.
Our preoperative evaluation and surgical strategy consisted of the following steps. First, significant vascular stenosis of the head and neck were detected by MRA screening. A number of studies have found that hypertrophy of the carotid intima-media thickness is closely related to ischemic heart disease and stroke.1–3 To prevent cerebrovascular ischemia during surgery, it is necessary to detect not only lesions in the carotid artery but also in the vertebral-to-basilar artery and in the cerebral arteries. A Doppler study of the neck vessels is incapable of identifying intracranial arterial disease. Since MRA can show these arteries as well as the carotid arteries, it is useful for preoperative screening.4 Preoperative MRA revealed significant stenosis or obstruction in 22% of the patients in this study. Second, significant stenotic lesions were accurately evaluated. Blood flow is sometimes maintained through collateral circulatory pathways in the cerebrum.5 Since MRA occasionally leads to overestimation of the extent of a lesion due to the winding of blood vessels and turbulent flow distal to the constricted region, caution should be used in evaluation. If overestimation is suspected, further examination using other methods such as cerebrovascular angiography, contrast MRA, or carotid ultrasonography is desirable. Third, in selected patients, the CBF reserve capacity was examined using brain SPECT with the administration of acetazolamide.6,7 Whether cerebrovascular ischemia is likely to occur at the time of reduction of blood pressure during surgery is an important consideration, and cerebrovascular ischemia is more likely in patients with significant stenotic lesions. However, the CBF reserve capacity is maintained in some patients with stenotic lesions, in which case CABG may be performed without complications.
When the risk of cerebrovascular ischemia is considered high, surgery should be performed with great caution. No general consensus has been established on cerebral autoregulation under heavy narcotic anesthesia and cardiopulmonary bypass. However, we believe that low blood pressure during surgery may cause a decrease in CBF, and OPCAB is useful in maintaining blood pressure in the brain. Therefore, we perform OPCAB in patients at high risk of cerebrovascular ischemia.8 If the cerebrovascular ischemia is more severe than the ischemic heart disease, improvement of the cerebral circulation by carotid endarterectomy is considered safe. Guidelines for carotid endarterectomy were published by the American Heart Association.9,10
One of the major limitations of this study is its design. Randomized prospective selection of the surgical strategy, comparison of groups according to age and risk factors (including extra- or intra-cranial vascular lesion severity and number of grafts) could have been more convincing in proving the importance of preoperative head and neck MRA. Nevertheless, there was no stroke during surgery in this study, and hemodynamic cerebrovascular ischemia was prevented. We believe that our CABG operative strategy with head and neck MRA is effective in preventing intraoperative hemodynamic cerebrovascular ischemia in patients undergoing CABG.
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ACKNOWLEDGMENTS
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The authors thank Dr. Akira Ogawa and Dr. Kuniaki Ogasawara for their assistance in brain SPECT analysis.
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REFERENCES
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ABSTRACT
INTRODUCTION
