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Ann Thorac Surg 2002;74:S1864-S1866
© 2002 The Society of Thoracic Surgeons
a Department of Surgery, University Hospital of Maastricht, Maastricht, The Netherlands
* Address reprint requests to Dr Jacobs, University Hospital Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands.
e-mail: m.jacobs{at}surgery.azm.nl
Presented at the Aortic Surgery Symposium VIII, May 2–3, 2002, New York, NY.
Abstract
BACKGROUND: Monitoring motor-evoked potentials (MEPs) is an accurate technique to assess spinal cord integrity during thoracoabdominal aortic aneurysm (TAAA) repair, guiding surgical strategies to prevent paraplegia.
METHODS: In 210 consecutive patients with type I (n = 75), type II (n = 103), and type III (n = 32) TAAA surgical repair was performed using left heart bypass, cerebrospinal fluid drainage, and MEPs monitoring.
RESULTS: Reliable MEPs were registered in all patients. The median total number of patent intercostal and lumbar arteries was five. After proximal aortic crossclamping, MEP decreased below 25% of base line in 72 patients (34%) indicating critical spinal cord ischemia, which could be corrected by increasing distal aortic pressure. By using sequential clamping it appeared that in 43% of type I and II cases spinal cord circulation was supplied between T5 and L1, and 57% between L1 and L5. In type II and III cases cord perfusion was dependent upon lower lumbar arteries in 16% and pelvic circulation in 8%, necessitating reattachment of these segmental arteries. In 9% of patients critical ischemic MEP changes occurred without visible arteries, requiring aortic endarterectomy and selective grafting. One patient suffered early paraplegia and 2 delayed, and 2 patients had temporary neurologic deficit (5 of 210; 2.4%).
CONCLUSIONS: In patients with TAAA, blood supply to the spinal cord depends upon a highly variable collateral system. Monitoring MEPs is an accurate technique for detecting cord ischemia, guiding surgical tactics to reduce neurologic deficit (2.4%).
Survival and functional outcome after thoracoabdominal aortic aneurysm (TAAA) repair are significantly less successful than commonly reported [1, 2] especially in patients with postoperative complications. Renal failure, pulmonary insufficiency, extensive aneurysms, and neurologic deficit are major determinants of operative mortality [3] and length of hospital stay [4].
Spinal cord ischemic injury remains the most devastating complication. Despite protective measures such as distal aortic perfusion, cerebrospinal fluid (CSF) drainage, epidural cooling, and reattachment of intercostal arteries, paraplegia occurs with an incidence of 5 to 15% in experienced centers. One of the main limitations of protective strategies is the inability to assess the adequacy of spinal cord perfusion and cord function intraoperatively. Monitoring motor-evoked potentials (MEPs) is a technique to continuously assess motor tract function, including the ischemia-sensitive anterior horn motor neurons [5], allowing detection of cord ischemia and guiding operative strategies to restore spinal cord perfusion [6, 7]. In this study we prospectively evaluated the functional value of MEP monitoring during TAAA repair.
Patients and methods
TAAA repair was performed in 210 consecutive patients (75 type I, 103 type II, 32 type III) and included patients reported previously [8]. Patients with a type IV aneurysm were excluded because of the relatively low risk of paraplegia.
The surgical protocol included left-side heart bypass, CSF drainage (72 hours) and monitoring MEPs. After limited heparinization (0.5 mg/kg) distal aortic perfusion was established with cannulation of the femoral artery and left pulmonary vein. In patients with type II and III aneurysms the celiac axis, superior mesenteric, and both renal arteries were selectively perfused with assessment of volume flow and perfusion pressure in each artery [9]. Temperature was allowed to decrease spontaneously to 31°C to 34°C, and after the aortic repair left heart bypass was used to rewarm the patient.
Anesthetic technique
Adequate anesthetic techniques are essential because complete neuromuscular blockade is not compatible with myogenic MEP monitoring. The level of neuromuscular blockade is assessed with a relaxograph neuromuscular transmission monitor. This device applies a drain of four supramaximal stimuli to the ulnar nerve at the wrist every 20 seconds and records the resulting hypothenar compound muscle action potential. The level of neuromuscular blockade is expressed as the amplitude of the response to the first stimulus expressed as a percentage of control. Whenever the neuromuscular blockade decreases below the set point the relaxograph initiates the delivery of vecuronium at 2 mg · kg-1 · min-1. Anesthesia was maintained with sufentanil and ketamine. Mean arterial pressure (MAP) was maintained between 60 and 100 mg/Hg. During aortic cross clamping blood pressure was regulated by adjusting left heart bypass flow. Meticulous administration of intravenous nitroglycerine was used if preload management could not control high blood pressure.
Monitoring motor-evoked potentials and surgical strategies
Transcranial stimulation and the technique of motor evoked potential recording has been described in detail before [5]. With this technique, single transcranial MEPs are evoked by applying paired stimuli to the scalp through four electroencephalographic disc electrodes. Potentials are recorded from the skin over the left and right anterior tibialis muscles. During crossclamping MEP levels are measured every minute. A reduction of MEP amplitude to less than 25% of base line is considered an indication of ischemic spinal cord dysfunction.
The first phase of the aortic reconstruction consisted of proximal double aortic clamping and complete transection of the aorta. If ischemic MEP changes occurred distal aortic pressure (DAP) was increased as well as MAP. Thereafter the distal clamp was moved down, excluding a thoracic segment when sequential clamping was feasible or, if not, excluding the entire thoracic aorta. If MEPs remained normal, intercostal arteries were only reattached if the aortic wall allowed a safe anastomosis. If MEPs decreased to critical levels, patent intercostal or lumbar arteries were revascularized. If the aorta did not allow a safe anastomosis, local endarterectomy was performed and a selective bypass anastomosed in an end-to-end fashion. In case the MEPs disappeared and no patent segmental vessels could be found, rapid aortic endarterectomy was executed with subsequent selective grafting of segmental arteries. Finally these selective grafts were anastomosed to the aortic tube graft in an end-to-side fashion. In any case attempts to revascularize the spinal cord were carried out until the MEPs restored.
Results
The in-hospital mortality was 10.4% (22 of 210 patients). Major complications comprised pulmonary insufficiency (48%), cardiac events (23%), renal failure (2%) and stroke (2%). There were no intraoperative deaths.
At the end of the procedure all but 1 patient had adequate MEPs. One patient (type III) showed absence of MEPs after crossclamping the thoracic aorta. Despite revascularization of segmental arteries MEPs did not return and the patient awoke paraplegic. Two patients suffered from delayed neurologic deficit on the sixth and ninth postoperative day, respectively. Another 2 patients complained of temporary paraparesis but recovered after several weeks. Strikingly enough, MEP amplitudes in the affected limbs were significantly lower at the end of the procedure compared with the unaffected limb. In total, paraplegia (3 of 210) and temporary paraparesis (2 of 210) accounted for a total neurologic deficit of 2.4%, 1.4% of which was permanent.
After proximal double cross clamping, MEPs remained normal in 162 of 210 patients (77%) with a mean DAP of 58 mm/Hg. In 48 patients (23%) MEP changes below 25% of base line amplitudes indicated critical spinal cord ischemia. Increasing DAP with a mean of pressure of 69 mm/Hg immediately corrected cord ischemia. The DAP required to maintain adequate MEPs was considered to be the minimal pressure to be maintained in the postoperative phase. Exclusion of the entire thoracic aorta resulted in normal MEPs in 53%, 59%, and 56% of type I, II, and III aneurysms, respectively. However, critical MEPs developed in 47%, 41%, and 44%, respectively. In these cases increased DAP (30%) or reattachment (52%) or selective grafting (18%) of vessels were necessary to regain acceptable MEPs. In type II (n = 103) and III (n = 32) patients additional MEP changes occurred after excluding the aortic segment between L1 and L5 in 15 and 7 patients, respectively. In these 22 patients (16% of type II and III) MEPs returned after revascularization of lumbar arteries between L3 and L5.
In order to perform the distal aortic or iliac anastomosis in type II and III patients left-side heart bypass was stopped. During that phase complete disappearance of MEPs occurred in 11 patients (8%) indicating the significance of the pelvic circulation with regard to spinal cord perfusion.
Critical ischemic MEP changes without visible segmental arteries occurred in 18 patients (9%) and required aortic endarterectomy and selective grafting. The mean number of patent intercostal and lumbar arteries in type I, II, and III aneurysms was 3.1, 5.4, and 5.1, respectively.
Comment
During the last decade several techniques and intraoperative measures have been developed and proven to reduce the incidence of neurologic deficit after TAAA repair. Recently Coselli and coworkers [10] performed a randomized clinical trial and nicely demonstrated the significant reduction of paraplegia (2.6 versus 13%) using CSF drainage. Spinal cord ischemia has a multifactorial etiology, therefore requiring a multimodality approach. In general the surgical protocol of most centers includes left heart bypass, CSF drainage, and reattachment of segmental arteries. The critical question remains, however, whether additional information is necessary to ameliorate the neurologic outcome. Retrograde aortic perfusion provides supplemental blood flow during the period of crossclamping but obviously does not perfuse the excluded aortic segment. Reattachment of the excluded intercostal or lumbar arteries is the logical surgical strategy to restore the interrupted blood supply, avoiding irreversible spinal cord ischemic damage.
Our results indicate, however, that the blood supply to the cord is anatomically completely disturbed in the presence of aneurysms: most segmental arteries are occluded and spinal cord integrity is maintained by an extensive collateral network, in which the lumbar arteries (L3-L5) and the pelvic circulation are responsible for the main blood supply in almost a quarter of the patients. These patients obviously profit from retrograde aortic perfusion. The fact that the anatomic structure is disturbed indicates that the functional contribution of the collateral network is unpredictable: it is difficult to predict which intercostal or lumbar artery is important and should be revascularized or can be oversewn. This question would be irrelevant if all segmental arteries could be reattached but we know that in practice many arteries arise from a friable aorta, not allowing a safe and adequate button-reattachment. Furthermore, if an aortic segment does not contain patent arteries the surgeon will not necessarily search for them.
Monitoring motor evoked potentials appears to be a sensitive technique to assess spinal cord integrity intraoperatively, identifying insufficient blood flow to the spinal cord, either through left heart bypass (pelvic circulation, lumbar arteries) during cross clamping or through excluded segmental arteries that make a critical contribution to cord perfusion. Based on MEP monitoring the latter arteries should be reimplanted even if these vessels are located in a severely diseased aortic segment, requiring endarterectomy and selective grafting. If this is not undertaken and MEPs are not restored the patient will awake paraplegic.
The situation that best illustrates the usefulness of MEPs is when the MEPs disappear and no arteries are present in the excluded aortic segment. Without the MEP information no surgeon would consider performing an aortic endarterectomy and searching for segmental arteries. Certainly the question can be raised as to whether occluded segmental vessels can contribute to spinal cord perfusion. First, the clinical finding that endarterectomy visualizes intercostal arteries with back bleeding indicates that these vessels must still be patent and interconnected. Second, reperfusion through these arteries restores MEPs, suggesting that these arteries are not only interconnected but also that they participate in cord perfusion.
We believe that this functional assessment of spinal cord integrity allowing immediate surgical strategies to correct ischemic circumstances is a significant adjunct in the battle against paraplegia. However, we still encountered some patients with neurologic deficits, perfectly elucidated by MEP-changes, which could not be corrected by any surgical intervention. That indicated that complete prevention of paraplegia after TAAA repair (especially of type II anuerysms) is still an elusive goal.
References
This article has been cited by other articles:
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  J. S. Coselli and S. A. LeMaire Descending and Thoracoabdominal Aortic Aneurysms Card. Surg. Adult, January 1, 2008; 3(2008): 1277 - 1298. [Full Text]
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 R. A. Kahn, M. E. Stone, and D. M. Moskowitz Anesthetic Consideration for Descending Thoracic Aortic Aneurysm Repair Seminars in Cardiothoracic and Vascular Anesthesia, September 1, 2007; 11(3): 205 - 223. [Abstract] [PDF]
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 D. R. Wong, J. S. Coselli, K. Amerman, J. Bozinovski, S. A. Carter, W. K. Vaughn, and S. A. LeMaire Delayed Spinal Cord Deficits After Thoracoabdominal Aortic Aneurysm Repair Ann. Thorac. Surg., April 1, 2007; 83(4): 1345 - 1355. [Abstract] [Full Text] [PDF]
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 S. A. LeMaire, L. N. Ochoa, L. D. Conklin, R. A. Widman, F. J. Clubb Jr, A. Undar, Z. C. Schmittling, X. L. Wang, C. D. Fraser Jr, and J. S. Coselli Transcutaneous near-infrared spectroscopy for detection of regional spinal ischemia during intercostal artery ligation: preliminary experimental results. J. Thorac. Cardiovasc. Surg., November 1, 2006; 132(5): 1150 - 1155. [Abstract] [Full Text] [PDF]
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T. B. Sloan Electrophysiologic Monitoring during Surgery to Repair the Thoracoabdominal Aorta Seminars in Cardiothoracic and Vascular Anesthesia, June 1, 2004; 8(2): 113 - 125. [Abstract] [PDF]
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 R E Bell and J F Reidy Endovascular treatment of thoracic aortic disease Heart, August 1, 2003; 89(8): 823 - 824. [Full Text] [PDF]
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