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Department of Cardiovascular Surgery, Hokkaido University Hospital, Sapporo, Japan
Accepted for publication May 8, 2009.
* Address correspondence to Dr Shiiya, Department of Cardiovascular Surgery, Hokkaido University Hospital, N14W5, Kita-ku, Sapporo, 060-8648, Japan (Email: shiyanor{at}med.hokudai.ac.jp).
Presented at the Forty-fifth Annual Meeting of The Society of Thoracic Surgeons, San Francisco, CA, Jan 26–28, 2009.
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Abstract |
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Methods: One hundred sixteen patients underwent multidetector row computed tomography before descending or thoracoabdominal replacement. Segmental arteries feeding the spinal cord were detected in 92 patients (79%), and were classified into "critical" (isolated hairpin shaped) or "supplemental" (confluence-shaped or multiple). Spinal cord ischemia was monitored together with distal aortic perfusion in 53 of them by motor-evoked potentials, evoked spinal cord potentials, or both. The relationship between monitoring results and operative management to the detected feeding arteries was analyzed.
Results: When no feeding segmental artery was involved in the extent of replacement (n = 18), spinal cord ischemia was detected in 1 (6%), which was due to cross-clamping the subclavian artery. When a supplemental feeding artery was involved (n = 15), ischemia was detected in 7 patients (47%), and was reversed by stopping back-bleeding. When a critical feeding artery was involved (n = 20), ischemia was detected in 6 (30%). In 3 of them, ischemia was reversed by stopping back-bleeding, whereas it was reversed only after reconstruction of the critical feeder in the remaining 3. Paraparesis occurred in 1 of the latter 3, and the incidence of spinal cord injury was 2% (1 of 53).
Conclusions: When the involved feeding artery is a supplemental one, the steal phenomenon is the predominant mechanism of ischemia. Conversely, blood flow interruption to the critical feeding artery may cause spinal cord ischemia without steal phenomenon.
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Introduction |
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With the recent advance in the imaging technologies, noninvasive visualization of the spinal cord feeding arteries has become reliable, either by magnetic resonance imaging (MRI) [4–6] or multidetector row computed tomography (MD-CT) scan [7, 8]. We have been using MD-CT scan for this purpose [8]. Based on this experience, we hypothesized that anatomical pattern of the spinal cord feeding arteries, visualized by MD-CT scan, reflects hemodynamic continuity of the anterior spinal artery and is related to the prevalence and mechanism of intraoperative spinal cord ischemia. The study aim was to test this hypothesis in our clinical experiences.
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Material and Methods |
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Segmental arteries that fed the spinal cord were detected in 92 patients (79%), and were multiple in 34 patients (29%). Their anatomical pattern was classified into the "hairpin" shaped and the "confluence" shaped (Fig 1). The latter were frequently accompanied by a more proximally located hairpin-shaped feeding artery. Distribution of these feeding arteries is shown in Figure 2, with respect to each anatomical pattern. The hairpin-shaped ones were arising from T7 to L1, predominantly in the left. This finding is consistent with our current knowledge of the great radicular artery. On the other hand, confluence shaped ones were distributed more distally, in accordance with a previous anatomical study [9].
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Study on Prevalence and Mechanism of Intraoperative Spinal Cord Ischemia
Among the 92 patients in whom spinal cord feeding arteries were detected, 28 without electrophysiologic monitoring, 10 undergoing deep hypothermic operation, and 1 without distal aortic perfusion were excluded from the study. Therefore, the study subjects consisted of the remaining 53 patients who underwent surgery with monitoring and distal aortic perfusion. There were 17 descending and 36 thoracoabdominal lesions, and 18 of the latter had Crawford I or II extent. Sixteen had aortic dissection, and none had acute presentation.
Our surgical technique, which we call the multisegmental sequential repair, has been reported previously [10]. Briefly, the distal one third of the descending thoracic aorta, where spinal cord feeding arteries were usually present, was opened in two or more sequences, and segmental arteries, usually one pair from each segment, were reattached sequentially by separate tube grafts. Presence of mural thrombus did not preclude the use of this technique as long as the aorta could be clamped and blood flow through distal aortic perfusion could effectively be blocked. The rationale of this technique was to expect collateral blood flow through the neighboring segmental arteries during reattachment of a segmental artery, and to minimize the steal phenomenon in opening the aorta. In this technique, we always noticed active back-bleeding from the patent segmental arteries, reflecting the presence of rich collateral blood flow. We usually used a specially designed occlusion balloon catheter (A shield; Asahi Intecc, Nagoya, Japan) to stop back-bleeding during reconstruction. In addition, mild hypothermia and cerebrospinal fluid drainage, to maintain its pressure at 13 cmH2O, were also employed.
Intraoperative spinal cord ischemia was monitored by transcranial motor evoked potentials, evoked spinal cord potentials, or both, using a Neuropack MEB-2204 system (Nihon Kohden, Tokyo, Japan). For motor evoked potentials, myogenic responses of tibialis anterior and abductor hallucis muscles were recorded, together with the abductor pollicis muscle as a control. In order not to interfere with motor evoked potentials monitoring, total intravenous anesthesia was used and the anesthetic depth was controlled by bispectral index level. Muscle relaxants were not used except for anesthetic induction. Evoked spinal cord potentials were monitored using two epidural electrodes, one for stimulation and one for recording. The detail of monitoring technique was reported elsewhere [11, 12].
The relationship between monitoring results and operative management to the detected feeding arteries was analyzed.
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Results |
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When extent of repair was taken into account, critical feeding arteries were involved in 11 patients with Crawford I or II extent and in 9 patients with less extensive repair. Prevalence of intraoperative ischemia was higher with Crawford I or II extent, and all the 3 patients in whom ischemia was not reversed by stopping back-bleeding had Crawford I or II extent (Table 1).
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Comment |
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These results not only are consistent with the collateral network concept proposed by Griepp and Griepp [13], but also may improve our understanding as to when collateral blood flow fails to maintain spinal cord perfusion within a viable range. Namely, visualization of a hairpin-shaped feeding artery means that there is a great discrepancy between the diameter of the anterior spinal artery above and below the junction, and spinal cord perfusion proximal to this point is vulnerable to ischemia even with the use of distal aortic perfusion, as pointed out by Svensson and colleagues [3] in 1986.
The prevalence of intraoperative ischemia in the absence of steal phenomenon was 6% (3 of 53) in this series, which seemed higher than that reported by the Griepp group [1, 14]. However, it is not different from the incidence of postoperative spinal cord injury reported by the groups who do not reattach segmental arteries [15, 16] or that after thoracic endovascular aneurysm repair [17, 18]. The difference from the Griepp series may be explained by their technique of sequentially dividing segmental arteries before opening the aorta.
The clinical implication of the present results is clear. When no spinal cord feeding artery is involved in the extent of repair, there is no need for segmental artery reattachment, which has also been shown by previous studies [19]. When an isolated hairpin-shaped feeding artery is involved in the extensive Crawford I or II lesion, it may be better to consider the segmental artery reattachment, using a more aggressive method of spinal cord protection than that depending upon collateral flow, such as active segmental artery perfusion or deep hypothermia. For the remaining cases, segmental artery reattachment, using our optimized surgical technique that maximally takes advantage of collateral blood flow, seems at least not detrimental. Therefore, we will continue to use this strategy, expecting that it will reduce the risk of delayed-onset injury resulting from the postoperative hemodynamic deterioration [20].
Study Limitations
One may argue against the accuracy of MD-CT scan to detect the spinal cord feeding arteries, especially those with confluence shape arising from distal aortic segment. Nijenhuis and colleagues [6], using MRI with two consecutive dynamic phases, have reported that such a vessel is the great anterior radiculomedullary vein, and reported its validation in the postmortem examination of 1 patient [21]. However, Hyodoh and colleagues [22], using MRI, have reported that spinal cord drainage vein merged at T9 to L2 levels, which suggests that distally located vessels detected in the present study were not necessarily veins. In contrast to the four-row machine and fixed scan delay used in the Nijenhuis series [6], we used a 64-row machine and an automated trigger system to acquire images in the arterial phase. In our experience, change from the four-row machine to the 64-row one dramatically improved the imaging quality. In addition, most recent series have reported the use of 64-row machines, suggesting that image resolution in our series is accurate enough, although machines with 128 or 256 rows may further improve the imaging quality. Because the anatomy of the spinal cord drainage vein has not been extensively studied, further works will thus be required on this issue.
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Discussion |
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DR SHIIYA: We use cerebrospinal fluid drainage. We try to maintain the proximal blood pressure at more than 120 to 130 mm Hg and distal perfusion pressure should be around 60 to 80 mm Hg. Postoperatively, we continue the drainage until we found that the patient is intact. We don't continue it up to 72 hours, but we would remove it the next day.
DR FRIEDRICH W. MOHR (Leipzig, Germany): How is your strategy on reimplantation of the feeding arteries or blockade? If you go, let's say, with the type 2, you see a has-been feeder, is that your first step of reimplantation and reperfusion to do the proximal anastomosis first, or how do you sequence your anastomosis?
DR SHIIYA: In our technique, we either do the proximal or the distal anastomosis first and then we implant the segmental arteries sequentially. We tried to limit the segment of clamping as short as possible, so that one or two pairs of segmental arteries should be included there. Once we reattach it, then we go next segment and reattach the second.
If the critical artery is exactly in the middle of the extent of the repair, we usually reattach the neighboring segment artery just proximal to the critical one first and then we go to this critical segment, so that the collateral blood flow from the neighboring artery could benefit during the reconstruction of the critical artery.
DR MOHR: Do you base your decision, let's say, if you go through the CT scans, usually one can make a decision a patient has multiple feeding arteries and usually one does not reimplant all of them, do you base your decision on the preoperative diagnosis and CT scan and say, you know, I mark it, I'm going to go there?
DR SHIIYA: When the feeding artery is outside the extent of repair, we ignore it. We do nothing. But when it is within the extent of repair, we usually reattach it, even though we don't have change during operation, because the reconstruction of the feeding artery may be useful for preventing postoperative delayed onset injury. That's our concept.
DR CHRIS K. ROKKAS (Athens, Greece): Congratulations on a much needed study. I just wanted to make a comment and ask a question. Occasionally, in thoracoabdominal aneurysms, because of the presence of extensive atheromas within the lumen of the aorta, we don't find any intercostal or lumbar arteries to reattach. Despite that, the patients don't have any evidence of spinal cord ischemia preoperatively, but they do develop spinal cord ischemia postoperatively. On the basis of your anatomic findings, how do we explain this phenomenon?
And second, how do you protect such a patient from spinal cord ischemic injury when there is really nothing to reattach? Thank you.
DR SHIIYA: First question, when there is no intercostal artery patent within the extent of the repair, we usually find nothing on the preoperative MD-CT scan. At this time we are not sure where is the feeding artery, so we don't rely on the preoperative diagnosis. We do exactly what we are planning to do, namely sequential reconstruction of the segmental arteries.
The second question, how do we protect the spinal cord in such a situation is now answered by my comment. We do the sequential reconstruction, that's all.
DR MOHR: In which cases do you use profound hypothermia? Because I noticed you had 20 cases out of 20. What are your criteria for selecting those patients?
DR SHIIYA: In the beginning of our series, we used it for the extensive repair, but now we limit the indication for those involving the distal aortic arch.
DR JOSEPH S. COSELLI (Houston, TX): Congratulations on your excellent results and a provocative study. Did I understand correctly that in every case in which you were able to identify preoperatively the important blood supply to the anterior spinal artery that you were able to specifically reattach it during the operation? And if that was the case, how did you confirm postoperatively that you had reattached the correct vessels and that there was patency?
The second question, although not specifically related to this presentation is this: what was your strategy for managing the 19% of patients in whom you could not identify the circulation preoperatively?
DR SHIIYA: To answer your second question first, in about one fifth of the patients we certainly did not identify the feeding arteries, and in such cases, I would like to repeat my strategy, we sequentially reconstruct the segmental arteries that's located within the distal third of the descending thoracic aorta, in two or three sequences, not in the en bloc island technique, but we use a separate graft technique and do one by one. One reconstruction, and then we go second. So to some segmental arteries, there is always flow and that may serve as a source of collaterals. That's our strategy.
And for the first question—I'm sorry, what was the first question? I don't remember.
DR COSELLI: The first question was, how do you identify what you assume to be the feeding vessel on the CT scan, and then how do you ensure patency?
DR SHIIYA: We usually repeat the MD-CT scan postoperatively, but we do not try to visualize the feeding artery. What we simply see is the patency of the graft attached, and its continuation until the intercostal arteries, that's all. Even though they are not patent, we don't see any ischemic spinal cord complications, which maybe due to collateral flow.
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References |
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This article has been cited by other articles:
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  T. Kuratani, M. Kato, Y. Shirakawa, K. Shimamura, and Y. Sawa Long-term results of hybrid endovascular repair for thoraco-abdominal aortic aneurysms Eur J Cardiothorac Surg, September 1, 2010; 38(3): 299 - 304. [Abstract] [Full Text] [PDF]
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