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Ann Thorac Surg 1996;62:1152-1157
© 1996 The Society of Thoracic Surgeons
Division of Cardiothoracic Surgery, Department of Surgery, New York University Medical Center, New York, New York
Accepted for publication May 17, 1996.
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Abstract |
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 TopFootnotes Abstract IntroductionMaterial and MethodsResultsCommentReferences |
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Methods. Seventy-eight consecutive patients underwent repair of descending thoracic aortic aneurysm between 1983 and 1993. Two basic repair strategies were used: (1) distal perfusion with somatosensory evoked potential monitoring (n = 54) and (2) cross-clamping (n = 24), alone (n = 6) or with controlled distal exsanguination (n = 18).
Results. The operative mortality rate was 6.5% for elective repair (n = 62), 25.0% for emergent repair (n = 16), and 10.3% overall. Univariate predictors of increased operative risk were emergent operation, rupture, and shock. Neither death nor paraplegia was related to the operative technique used. The incidence of paraplegia was 3.7% in perfused patients and 4.2% in cross-clamping patients (p > 0.05). Paraplegia did not occur after any elective operation (zero of 62) but occurred in 18.6% of emergent cases (p < 0.01). In perfused patients, paraplegia did not occur when the distal pressure was maintained above 55 mm Hg and somatosensory evoked potentials remained intact. When somatosensory evoked potentials were lost (n = 7) in perfused patients, the operative technique was altered successfully in 5 patients, whereas in 2 patients (28.6%), paraplegia developed.
Conclusions. The risks associated with elective descending thoracic aortic aneurysm repair were extremely low using an operative strategy that was flexible but skewed toward perfusion with somatosensory evoked potential monitoring. In perfused patients, paraplegia did not occur when distal pressure was greater than 55 mm Hg and somatosensory evoked potentials remained intact. However, the risks of death and paraplegia were primarily related to emergent presentation, not to technique, and the technique of cross clamping with controlled distal exsanguination was found to be valuable in unstable or in anatomically complicated subsets of patients.
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Introduction |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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At New York University Medical Center, the preferred technique for most elective procedures for more than a decade has been perfusion of the distal aorta while the aorta is cross-clamped, with either left atrial–femoral artery bypass or femoral vein–femoral artery bypass. At New York University Medical Center, Cunningham and associates [5] developed the technique of SEP monitoring to detect progressive spinal cord ischemia and reported their initial experience in 1982. In 1983, Culliford and colleagues [3] reported their experience at New York University Medical Center with 43 patients.
Unfortunately, complicating factors in some patients make heparin treatment and distal perfusion difficult or hazardous. With these patients, the simpler technique of proximal aortic cross-clamping with controlled distal exsanguination, without heparin, has been used, as proposed by Crawford (personal observation, F.C.S., 1982) and Cooley and Baldwin [4].
This report analyzes our experience with 78 patients undergoing repair at New York University Medical Center between 1983 and 1993, comparing the distal perfusion SEP monitoring technique with cross-clamp and distal exsanguination.
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Material and Methods |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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The operation was considered elective if it was performed in a planned fashion after appropriate studies and preoperative preparation. The operation was considered emergent when it was performed for bleeding or rupture or with associated hypotension, shock, airway compromise, or major vascular occlusion.
All data are expressed as mean ± standard deviation unless indicated otherwise and were analyzed by the statistics program SPSS (SPSS Inc, Chicago, IL). Analysis of the significance of differences in preoperative and postoperative characteristics was performed by 
2 analysis or the Student's t test as appropriate. The frequency of late death was obtained by the actuarial life-table method. Multivariate analysis to determine significant predictors of hospital death was performed by means of stepwise logistic regression. Values of p less than or equal to 0.05 were considered statistically significant.
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Results |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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). Of the nine Stanford type B aortic dissections, six were chronic and three were acute. With acute type B dissection, the indication for operation was rupture with hypotension or rapid expansion with pain and contained rupture.
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Emergent operation was performed in 16 patients (20.5%), using femoral–femoral bypass in 7 and cross clamp with controlled distal exsanguination in 8. One patient with a traumatic aortic transection received an emergent double cross-clamp technique for repair. Eight of the patients having emergency repairs (50%) had hemodynamic instability with preoperative rupture.
The mean cross-clamp time was 48.5 ± 16.6 minutes when a bypass technique was used and 27.2 ± 12.4 minutes when a nonperfused cross-clamp technique was used.
Mortality and Morbidity
The hospital mortality rate was 6.4% (4 of 62) for elective repair, 25% (4 of 16) for emergent repair, and 10.3% (8 of 78) overall. The operative techniques used in the elective-repair patients who died were cross-clamp/open distal anastomosis (n = 1), left atrial–femoral bypass (n = 1), and femoral–femoral bypass (n = 2). The operative techniques used in emergent-repair patients who died were cross-clamp/open distal anastomosis (n = 2) and femorofemoral bypass (n = 2). Operative risk was not related to the technique used. The univariate significant predictors of death were emergent operation, acute rupture, and shock (p < 0.05). None of these predictors was significant by multivariate analysis, possibly because of the small sample size.
Before discharge, reoperation was required in 9 patients (11.5%), and renal failure requiring dialysis occurred in 6 (7.7%). One patient (1.3%) suffered a stroke and 1 (1.3%) had a myocardial infarction. Eleven patients (14.1%) remained on a respirator for more than 7 days.
Paraplegia
Paraplegia did not occur in any of the 62 patients undergoing elective excision of a thoracic aortic aneurysm. A bypass technique was used in 47 patients, whereas 15 patients had repair with nonperfused cross clamping (5 patients with standard proximal and distal cross clamping; 10 patients with proximal cross clamping and controlled distal exsanguination). The cross-clamp time was much shorter in the nonperfused patients (27 ± 12 minutes) than in perfused patients (48 ± 17 minutes). The longest cross-clamp time in the nonbypass group was only 39 minutes.
Paraplegia developed after three of 16 emergency operations (Table 2). The cross-clamp open distal exsanguination technique was used in 1 of the 3 patients, and a femoral–femoral bypass technique was used in the other 2. One of the 2 perfused patients was in shock from rupture of the aneurysm. The shunt flow rate was inadequate at less than 2 L/min, with a low distal perfusion pressure, which could not be increased because of the combination of hypovolemia and shock. The second perfused patient in whom paraplegia developed had a technical complication resulting in graft compression, with acute loss of distal pressure and SEPs. The distal anastomosis was revised and an island of intercostal arteries was reimplanted, but paraplegia occurred nonetheless.
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). Thirty-six of these maintained intact SEPs with a pressure greater than 55 mm Hg, whereas 2 patients maintained intact SEPs with lower pressures. None of the 38 patients with intact SEPs experienced paraplegia. In contrast, 7 patients lost SEPs during the procedure, with paraplegia developing ultimately in 2 patients (28.6%). In each patient whose SEPs were lost intraoperatively, either the perfusion rate was increased (n = 5) or the operative technique was changed (n = 2). The 2 perfused patients in whom paraplegia developed were both emergent cases, as described earlier. Factors increasing the risk of paraplegia by univariate analysis were emergency operation, aneurysm rupture, shock, a distal perfusion pressure less than 55 mm Hg in patients receiving a perfusion strategy, and a loss of SEPs.
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). For elective procedures, 97.2% ± 2.8% of hospital survivors were alive at 1 year and 86.4% ± 10.5% were alive at 3 years; for emergency procedures, only 81.3% ± 11.9% were alive at 1 year and 69.7% ± 14.8% were alive at 3 years (p < 0.01). The causes of late death were cardiac in 2 patients, rupture of an abdominal aneurysm in 1 patient, and unrelated disease in 4 patients.
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Comment |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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During emergency procedures, anatomic concerns and coagulopathy frequently make heparin treatment and the establishment of distal bypass difficult or hazardous. When this scenario is encountered, we recommend use of the proximal cross-clamp and controlled distal exsanguination technique, without heparin. Based on our experience, this technique may have been life-saving in several patients, lowering the risk of bleeding without increasing the risk of paraplegia.
Spinal Cord Ischemia
GENERAL CONSIDERATIONS.
Despite substantial improvements in operative techniques, ischemia of the spinal cord remains a serious unsolved problem after descending thoracic aortic aneurysm repair. The reported risk of paraplegia ranges from 0% to 20%, averaging 2% to 5%, and is known to be significantly higher after repair of traumatic aortic transection [12], after emergent repair of ruptured aneurysm [13, 14], and after repair of acute aortic dissection. Elective procedures and repairs of atherosclerotic aneurysms [15, 16] have a lower risk.
The cause of postoperative spinal cord injury may be related to the interruption of critical intercostal arteries in the replaced aortic segment, to insufficient perfusion of the distal aorta in perfused patients, or to long unprotected aortic cross-clamp time [4]. A variety of other factors, such as preoperative or postoperative hypotension, intraoperative hypertension in the proximal aorta, and elevation of spinal fluid pressure, also have been implicated. Each of these variables may lead to spinal cord ischemia with subsequent transient neurologic deficit or permanent paraplegia.
The severity of the neurologic deficit increases with the duration of ischemia. From laboratory and clinical studies, the critical time period for spinal cord ischemia appears to be approximately 30 minutes [10, 12, 17, 18], with the risk of paraplegia rising precipitously after 30 minutes and approaching 100% after 60 minutes. Spinal cord ischemic intervals of less than 20 to 30 minutes appear to be tolerated quite well.
Similarly, placement of the aortic cross-clamp results in proximal hypertension, left ventricular strain, increased cerebrospinal fluid pressure, and distal aortic hypotension [1, 4, 18]. This combination of factors decreases blood flow to the spinal cord.
DANGERS OF NITROPRUSSIDE.
The traditional approach to controlling proximal hypertension has been to use pharmacologic vasodilators, such as sodium nitroprusside. However, sodium nitroprusside has been shown to increase cerebrospinal fluid pressure through a loss of autoregulation of the spinal vasculature, shunting blood away from the high-resistance spinal collaterals to the peripheral dilated vessels [12].
A more favorable method to decrease proximal hypertension may be either the use of mechanical shunts (such as left atrial–femoral bypass) to decrease proximal hypertension while simultaneously increasing the distal perfusion pressure, or the use of partial distal exsanguination, which lowers proximal and distal blood pressure while diminishing cerebrospinal fluid pressure and minimizing ischemia to the spinal cord [1, 2, 4]. Both methods were used in this series, depending upon the anatomy and the clinical conditions present.
What Flow Rate and Pressure Are Adequate to Prevent Spinal Cord Ischemia?
The most important conclusion from these data from 78 patients is that paraplegia did not occur after elective operations if perfusion of the distal aorta maintained a pressure of greater than 55 mm Hg with intact SEPs. The flow rate required to maintain distal pressure above 55 mm Hg varied widely with different patients and was related to both vasodilatation and the degree of collateral circulation. The average flow rate in the perfused patients was 3 L/min, but ranged between 1,600 mL and 5 L. Eleven of the group required flow rates of 4 to 5 L/min to maintain distal perfusion pressure and SEPs. For unknown reasons, the important fact that a high flow rate is required in a substantial number of patients has not been emphasized in previous reports. The ability to increase shunt flow as much as needed is an important advantage of left atrial–femoral artery bypass as opposed to standard femoral vein–femoral artery bypass. With newer venous cannulas that can be introduced from the femoral vein into the right atrium, high flow rates should be feasible routinely with the femoral vein cannulation technique. The simple fact that flow rates greater than 4 L/min often were needed to maintain a distal aortic pressure greater than 55 mm Hg and to keep SEPs intact may be the major reason that previous studies comparing a bypass technique with a simple cross-clamp technique have found no differences in mortality or morbidity; ie, the flow rates used in the shunts (often not described) were inadequate. Likewise, this may be a major reason that distal perfusion techniques have not been completely effective in protecting patients from paraplegia after elective aneurysm repair in previous studies.
The Value of Somatosensory Evoked Potential Monitoring
The technique of SEP monitoring requires considerable experience and skill to use effectively. Technical errors such as faulty electrode placement, hypothermia, and others can lead to a loss of SEPs, with uncertainty about its importance. A major question is whether simple monitoring of distal aortic pressure is sufficient. This question cannot be answered from these data because both distal aortic pressure and SEPs were monitored in the majority of perfused patients. The important fact remains, however, that the presence of intact SEPs throughout the period of aortic clamping virtually guaranteed that marked spinal ischemic injury was not occurring. Paraplegia did not occur in any patient whose SEPs remained intact.
In contrast, when SEPs become significantly blunted or disappear, the surgeon must assume that spinal cord ischemia is present. Despite immediate changes in operative technique to improve distal perfusion or to reimplant intercostal arteries, in this study paraplegia developed in more than one quarter (28.6%) of the patients who lost SEPs during the cross-clamp time.
If the distal perfusion pressure is less than 55 mm Hg and the SEPs are lost, we recommend increasing the bypass flow rate until the distal pressure is greater than 55 mm Hg and the SEP amplitude and latency improve. If adequate distal perfusion pressure cannot be maintained and the evoked potentials stay flat, the risk of paraplegia is high. A different problem develops in a few patients whose SEPs deteriorate despite a distal aortic pressure greater than 55 mm Hg. This probably indicates interruption of critical intercostal vessels, which should be reimplanted into the graft or incorporated into the circulation by beveling the distal suture line.
Perfusion Versus Cross-Clamp With Controlled Distal Exsanguination
The technique of proximal cross clamping with controlled distal exsanguination was developed and used by both Crawford (F.C. Spencer, personal observation, 1982) and Cooley and Baldwin [4]. In the current study, we used cross clamp and controlled distal exsanguination in 18 of the 78 patients, usually because different technical reasons made heparin use or exposure of the distal neck of a large aneurysm difficult or hazardous. In those patients, the distal anastomosis was done in an "open" fashion. Another 5 patients had repair with the standard proximal and distal cross-clamp technique without controlled distal exsanguination. Operative death and the frequency of paraplegia were similar in the cross-clamp and distal-perfusion patients. However, the aortic cross-clamp time was much shorter with the cross-clamp technique (27 ± 12 minutes) than with perfusion (48 ± 17 minutes). Hence, no nonperfused patient had a cross clamp applied for longer than 39 minutes.
It is well known from previous reports that aortic clamping can be done with almost total safety if occlusion lasts less than 20 minutes. As noted earlier, longer unprotected cross-clamp times are associated with a progressive increase in the frequency of paraplegia, especially after 45 to 60 minutes. In the patients in this series, simple cross clamping was as effective as bypass if clamping did not exceed 39 minutes, but no data are available here about longer periods. From previous experience of others using this technique, extension of the unprotected cross-clamp method for longer periods probably would have resulted in a higher frequency of paraplegia.
Reported experiences with both the cross-clamp and "open" distal exsanguination technique and with the bypass techniques [1, 2] have been favorable. The reported operative mortality rate for descending thoracic aorta aneurysm repair ranges from 6% to 20% [2–11], similar to the 6.5% elective risk and 10% overall mortality rate reported here. In the current study, the operative risk was primarily related to emergent operation, aneurysm rupture, and shock, and we did not demonstrate significant differences in either mortality or paraplegia risk related to the technique used. Admittedly, selection of the operative strategy was purposely biased in this study, based upon clinical conditions and anatomic findings. In view of the low risk of paraplegia when using a perfusion strategy with SEP monitoring, we continue to recommend this technique for most elective procedures. If descending thoracic aortic aneurysms are approached this way for elective procedures, the surgeon must remain prepared to abandon the perfusion strategy in some patients and to adopt the cross-clamp technique with controlled distal exsanguination, based upon intraoperative findings and clinical judgment.
Conclusions
These results demonstrate that elective repair of aneurysms of the descending thoracic aorta can be performed with a very low operative risk, whereas emergency operation for repair of ruptured aneurysms continues to produce major morbidity and mortality. Thus, we recommend prompt elective operation for most descending thoracic aortic aneurysms of greater than 5 to 6 cm in diameter. The current findings confirm that the risk of paraplegia is extremely low after elective descending thoracic aortic aneurysm repair, approaching zero when the distal perfusion pressure is adequate and SEPs remain intact. Accordingly, an operative strategy that uses distal aortic perfusion and SEP monitoring is recommended for most patients undergoing elective aneurysm repair. The proximal single cross-clamp with controlled distal exsanguination technique was found to be useful and safe in selected patients in whom rupture, bleeding, and distal anatomic control of the aneurysm were the paramount issues.
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Footnotes |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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References |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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