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Ann Thorac Surg 1997;64:999-1003
© 1997 The Society of Thoracic Surgeons

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Original Article: Cardiovascular

Clinical Markers in CSF for Determining Neurologic Deficits After Thoracoabdominal Aortic Aneurysm Repairs

Division of Cardiothoracic Surgery and the Kennedy Krieger Institute, Johns Hopkins Medical Institutions, Baltimore, Maryland

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Background. Spinal cord ischemia is a major cause of morbidity and mortality after thoracoabdominal aortic aneurysm operations. The incidence of paraplegia is high even in experienced institutions.

Methods. We investigated whether neurotransmitter excitotoxicity is associated with neurologic deficits after thoracoabdominal aortic aneurysm operations. We hypothesized that patients with spinal cord injury would manifest elevated levels of excitatory amino acids in their cerebrospinal fluid. Sixteen patients undergoing thoracoabdominal aortic aneurysm operations had cerebrospinal fluid drawn through lumbar spinal drains preoperatively, intraoperatively, and postoperatively. Excitatory amino acid levels (glutamate, aspartate, glycine) were measured using high-performance liquid chromatography. Excitatory amino acid levels were compared in patients who exhibited no neurologic deficits postoperatively (group I; n = 12) with patients who had clinically evident lower extremity and cerebral neurologic deficits (group II; n = 4).

Results. Significant elevations in glutamate and aspartate levels from baseline (p < 0.05) were limited to group II. Excitatory amino acid levels in group II were significantly elevated (p < 0.05) compared with those observed in group I. Glutamate levels were especially increased during aortic cross-clamping and late reperfusion, whereas aspartate levels were increased only during late reperfusion.

Conclusions. These data suggest that neurotransmitter excitotoxicity plays a significant role in central nervous system injury.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Spinal cord ischemia is a major cause of morbidity and mortality after the surgical repair of descending thoracoabdominal aortic aneurysms (TAA). The incidence of paraplegia in experienced institutions performing elective repairs is 6% to 30% depending largely on the extent and the cause. The introduction of shunts and bypass grafts to maintain distal aortic perfusion and the development of various adjuncts to preserve spinal cord function such as cerebrospinal fluid (CSF) drainage have had mixed results in ameliorating the effects of spinal cord ischemia.

It is well established that excessive accumulation in the central nervous system (CNS) of amino acid neurotransmitters such as glutamate are responsible for substantial neurotoxicity. Glutamate contributes to neuronal injury by overactivating neuronal receptors precipitating a cascade of intracellular events that leads to neuronal death, a phenomenon termed glutamate excitotoxicity. First described by Olney and colleagues in 1969 [1], glutamate excitotoxity has been linked not only to acute neuronal diseases such as seizure disorders and cerebral ischemia, but also to chronic degenerative diseases such as Alzheimer's, Huntington's disease, and amyotrophic lateral sclerosis.

We describe a clinical demonstration detecting increased levels of neurotransmitter amino acids in neurologically impaired patients after TAA operation. We correlated the measured levels of excitatory amino acids (EAAs) in the CSF perioperatively to patient outcomes. The hypothesis tested was that patients exhibiting clinical signs of spinal cord injury would manifest elevated levels of EAAs in their CSF.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Patient Selection
Eighteen consecutive patients with TAA were treated surgically by the same surgeon during a 12-month period (1994). Two patients were excluded because of gross blood in their CSF intraoperatively. Gross blood in the CSF produces variability in measured amino acid levels. All patients were prospectively studied with motor (MEPS) and somatosensory evoked potential (SEPS) monitoring along with clinical evaluations for evidence of evolving spinal cord ischemia. The mean age of the patients was 68.5 ± 1.6 years. There were 10 women and 6 men. Three patients were diagnosed with Debakey type IIIB dissections, which are associated with a greater risk of neurologic morbidity than other aneurysmal types.

Monitoring Procedures
After the induction of general anesthesia, radial, femoral, or distal aortic catheters were used to monitor arterial pressures above and below the cross-clamp. These were continuously and simultaneously displayed. We have previously detailed the methods of our evoked cortical potential and CSF drainage procedures in patients undergoing thoracoabdominal operations [2]. A 3F subarachnoid drain was inserted in the third or fourth lumbar interspace with a 14-gauge Tuohy needle. Cerebrospinal fluid (2 ml) used in the measurement of amino acids was drawn directly from the patient at baseline, during cooling, after aortic cross-clamping, and then postoperatively. In between measurements, the CSF was allowed to drain at a pressure of 5 cm H₂O into a reservoir and then was discarded. The CSF was drained by a lumbar spinal catheter for at least 24 hours postoperatively at a pressure of 10 cm H₂O above the ear. Somatosensory evoked potentials (Nicolet CA 2000, Madison, WI) were elicited with stimulating electrodes placed bilaterally in the tibial and ulnar nerves peripherally, and the responses were recorded peripherally, cervically, and cortically. Motor evoked potentials were generated by direct electrical cord stimulation and the responses recorded peripherally. In addition to monitoring evoked potentials, four-channel electroencephalograms were recorded on each patient using a bihemispheric montage. Motor evoked potential and SEPS recordings were obtained preoperatively and continuously during the operation. If SEPS or MEPS were lost intraoperatively, little corrective action was taken, as a plan for intercostal attachment was developed preoperatively and this was adhered to intraoperatively. Postoperative SEPS testing was obtained on 2 patients who clinically demonstrated lower extremity deficits after the operation. Standard elicitation and recording parameters were used.

Perfusion and Surgical Technique
Our surgical technique using moderate hypothermia with partial bypass for the segmental sequential repair of TAA has been previously described [3]. Surgical exposure was through a left posterolateral thoracoabdominal incision. The left atrium or aorta was cannulated with a 22F to 24F catheter proximally, and the left femoral artery was cannulated distally with a 22F catheter. A centrifugal blood pump (Biomedicus, Eden Prairie, MN) with an Omnitherm heart exchanger (Avecor, Plymouth, MN) was used for partial bypass after heparin 100 IU/kg was given. By adjusting bypass flows, intravascular volume, and depth of general anesthesia, the mean arterial blood pressure was maintained between 70 and 90 mm Hg both above and below the cross-clamp. Bypass flows initially were about one half cardiac output and were reduced accordingly as different segments of the aorta were repaired and the cross clamps were moved more distally. Moderate hypothermia on bypass was considered to be 30°C as measured by nasopharyngeal probes, and this level was maintained until all the anastomoses were completed.

Woven polyethylene terephthalate (Dacron; Meadox Medicals, Inc, Oakland, NJ) grafts were used in all patients. All aneurysms were repaired segmentally from proximal to distal. This allowed adequate distal perfusion of the spinal cord except for the time required for direct anastomosis. Proximal anastomoses were all completed with distal perfusion. Rewarming to a nasopharyngeal temperature of 36° to 37°C was usually done while the distal anastomosis was being performed, and then bypass was discontinued.

Excitatory Amino Acid Determination
Cerebrospinal fluid samples were isocratically separated and assayed for the amino acids (glutamate, aspartate, and glycine) using high-performance liquid chromatography with electrochemical detection. This method combines the rapidity of precolumn derivatization and reverse-phase separation with the sensitivity and reproducibility of electrochemical detection (ESA Inc model 5200 Coulochem II Multielectrode Detector, Bedford, MA). All assays were carried out following the methods of Donzanti and Yamamoto [4]. The precolumn amine derivatization reagents of o-phthalic aldehyde and 2-mercaptoethanol were freshly prepared the day of the experiment. The mobile phase, consisting of 0.1 mol/L Na₂HPO₄, 0.13 mmol/L Na₂EDTA, and 28% methanol, was continuously recycled every 2 weeks and then changed with a fresh solution. Amino acid content was quantified by comparing the peaks of sample chromatograms with the peaks of external amino acid standards. Only samples that were not grossly contaminated with blood were used.

Statistical Analysis
The data were analyzed for continuous measures. A nonpaired Student's t test was used to test for differences between groups. A p value less than 0.05 was considered significant.

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
After surgical repair, 4 of the 16 patients had clinical signs of postoperative lower extremity motor deficits indicative of spinal cord ischemia. For comparison, the patients were divided into two groups based on the clinical development of postoperative lower extremity neurologic deficits. The 4 patients who clinically demonstrated lower extremity motor deficits perioperatively were identified and placed in group II, whereas 12 patients who had no deficits were assigned to group I.

The results of the CSF analysis for EAAs are graphically displayed in Figures 1, 2, and 3. There was a significant difference seen in the elevation of both glutamate and aspartate in those patients who experienced neurologic deficits caused by spinal cord ischemia. Elevations in glutamate were measured at the time of aortic cross-clamp, as well as during the early and late perfusion periods. Significantly elevated aspartate levels were only detected during the late reperfusion period. Although never reaching statistical significance, there is a trend of increased glycine levels during cross-clamping and reperfusion in patients with spinal cord injuries.

View larger version (16K): [in this window] [in a new window]   Fig 1. . Patients suffering spinal cord injury had significant elevations in the levels of the excitatory amino acid glutamate compared with those without injury. Glutamate levels were significantly increased during aortic cross-clamping, early reperfusion, and late reperfusion. (AXC = aortic cross-clamping; Early Rpf = early reperfusion [0 to 6 hours postoperatively]; Late Rpf = late reperfusion [>15 hours postoperatively]; Mid Rpf = mid-reperfusion [7 to 15 hours postoperatively].)

View larger version (16K): [in this window] [in a new window]   Fig 2. . Patients suffering spinal cord injury had significant elevations in the levels of the excitatory amino acid aspartate compared with those without injury. (AXC = aortic cross-clamping; Early Rpf = early reperfusion [0 to 6 hours postoperatively]; Late Rpf = late reperfusion [>15 hours postoperatively]; Mid Rpf = mid-reperfusion [7 to 15 hours postoperatively].)

View larger version (17K): [in this window] [in a new window]   Fig 3. . Patients suffering spinal cord injury showed a trend of having increased levels of the excitatory amino acid glycine compared with those without injury. Glycine levels in the patients with spinal cord injuries never reached statistical significance when compared with patients without injury at any point during the ischemic or reperfusion periods. (AXC = aortic cross-clamping; Early Rpf = early reperfusion [0 to 6 hours postoperatively]; Late Rpf = late reperfusion [>15 hours postoperatively]; Mid Rpf = mid-reperfusion [7 to 15 hours postoperatively].)

Group II patients differed significantly from those in group I in the length of aortic cross-clamp times (142 ± 16 minutes versus 88 ± 12 minutes, p 0.05). The duration of partial bypass between groups, however, was not significantly elevated (128 ± 10 minutes versus 190 ± 26 minutes) (Table 1). The intraoperative electrophysiologic data were not helpful as many patients had decreased amplitude and increased latency associated with cooling. One patient in group II with early paraparesis regained SEPS and MEPS from one leg at the conclusion of the operation, but not from the other. There was one false-positive electrophysiologic result during operation on a patient in group I who postoperatively had no neurologic deficits. Two patients, both in group I, had electrophysiologic monitoring that was prematurely terminated because of technical problems. Postoperative SEPS, performed on the 2 surviving patients in group II, showed degradation of neurologic potentials immediately postoperatively. During repeated postoperative testing, one of these patients showed the electrophysiologic data improving to baseline equivalents by time of discharge. Intraoperative electroencephalographic data were equivocal and did not serve as accurate predictors of subsequent neurologic dysfunction in this study.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

Elevations in extracellular glutamate concentration in the brain and spinal cord have been measured. In vivo microdialysis has been used to detect perturbations of EAAs both in animal models after the induction of spinal cord ischemia [6, 7], and more recently in patients after severe traumatic brain injury [8]. Fresh CSF sampled through ventricular drains from brain-injured patients has also been shown to have high levels of free amino acids such as glutamate and glycine [9, 10]. This study investigated whether fresh CSF sampled from the lumbar drains of patients undergoing TAA operations would reveal any perturbations in EAAs as the operation requires cross-clamping of the aorta and thus spinal cord ischemia. Using high-performance liquid chromatography with electrochemical detection, significant elevations of EAAs were detected in patients exhibiting neurologic deficits, and the degree of elevation was compatible with data from animal models of spinal cord ischemia as a result of aortic cross-clamping [6].

Although many patients in this study exhibited minimal, transient elevations of EAA during aortic cross-clamping, those patients experiencing neurologic events had larger increases in EAA concentration from baseline. Two patterns of EAA elevation were observed: an early, transient increase in glutamate and aspartate during periods of intraoperative spinal cord ischemia, and a late elevation seen hours after the early rise had returned to baseline levels. The initial, transient elevations in EAA concentration may be partially explained by the hypothesis that ischemia causes energy failure of the sodium-dependent transporters that clear glutamate. After aortic cross-clamping is removed and resolution of ischemia occurs, the transporters clear the EAA.

It is difficult to know whether late elevations of EAAs after reperfusion are from dying cells that have extruded their glutaminergic contents, represent ongoing ischemia from a second inhibition of the glutamate transporters, or are indicative of a vicious, positive feedback mechanism produced during the early ischemic period by a secondary chemical signal molecule. Although difficult to interpret, late elevations in EAA concentrations are consistent with observations in patients with severe CNS injury who have persistently elevated glutamate concentrations in their CSF [15]. Little is known to correlate EAA concentration in the CNS with neurologic function, although Panter and colleagues [11] suggest there is a linear relationship between the extracellular concentration of EAAs and the severity of blunt trauma to the spinal cord. Glutamate excitotoxic neuronal death is also a delayed event, often occurring 6 to 18 hours after glutamate exposure. Recently it was demonstrated that glutamate subtype receptor antagonists given even several hours after toxic glutamate exposure in animal models of hypothermic circulatory arrest are protective against neuronal death [12].

Both NMDA and opioid receptors have been implicated in the pathophysiology of spinal cord injury. The infusion of endogenous opioids, such as dynorphin, into the thoracic subarchnoid space of rats produces an acute hindlimb paresis with accompanying histologic changes that are ameliorated by NMDA antagonists [13]. Similarly, opiate receptor antagonists both in cell culture and in animal models have demonstrated neuroprotective effects by attenuating NMDA-mediated excitotoxicity. Acher and coworkers [14] used this neuroprotective quality of opioid antagonists in humans by administering naloxone with CSF drainage during TAA repairs and reported that patients had significantly less neurologic deficits than if they did not receive any adjuvant therapy. Our data showing increased concentrations of EAAs in the CSF suggest that the addition of a suitable NMDA antagonist to naloxone and spinal cord drainage may contribute additional neuroprotection during ischemia.

Less likely reasons for increased EAAs in the CSF may have been contamination by blood, especially in the patient in whom a subarachnoid hemorrhage later developed. Most of our samples, however, were grossly normal in color, and the 2 patients with blood contamination of their CSF were not included in the study. Although the transport of glutamate from the interstitial space across the blood–brain barrier to the CSF is possible, it is an unlikely explanation for the elevated glutamate concentrations measured in the CSF. There is some evidence to suggest that the equilibration time of glutamate between the spinal extracellular fluid and CSF compartments is rapid (<30 minutes), but its half-life in the CSF is long (6 to 8 hours). Any glutamate released into the extracellular space must move down a concentration gradient and be diluted by the CSF and other extracellular fluids. This would tend to lower its concentration in CSF rather than raise it.

Similarly, the effect of hypothermia on EAA concentration in the CSF would be to lower rather than raise the CSF concentration. A core temperature near 30°C is generally employed during the TAA repairs to minimize CNS morbidity after spinal cord ischemia. In addition to any effects hypothermia has on the metabolic rate of CNS tissue and a resultant decrease in oxygen consumption, there is now evidence that moderate hypothermia (30°C) also attenuates the release of glutamate and other amino acid neurotransmitters, in effect ameliorating EAA excitotoxicity [9].

We examined CSF levels of glycine in this study because this amino acid is an important neurotransmitter in both the brain and spinal cord. Glycine acts as both an excitatory cotransmitter at the NMDA-type glutamate receptor and an inhibitory transmitter at a separate receptor that is prominent in the brainstem and spinal cord. Glycine-containing inhibitory local circuit neurons in the spinal cord are especially sensitive to ischemia. Several animal and human studies observed increases in brain extracellular or CSF glycine levels after ischemic or traumatic insults [6, 9, 10]. Although the elevations in mean CSF glycine levels in our patients did not reach statistical significance, there was a trend for increased glycine levels during cross-clamping and in late reperfusion. Elevated glycine concentrations have been shown to enhance NMDA-evoked neuronal activity in the rat spinal cord, and several other studies demonstrated that glycine potentiates NMDA-mediated seizures and elevation of intraneuronal calcium levels [15, 16]. Therefore, in some cases, elevated glycine levels might enhance the excitotoxic effects of high glutamate levels on spinal cord neuronal tissue.

In summary, we observed a strong positive relationship between elevations of CSF EAAs during aortic cross-clamping and reperfusion, and subsequent development of clinical signs of spinal cord ischemia and infarction. Because the sample size was relatively small, replication in a larger patient group is needed. However, these results provide additional support for the hypothesis that spinal cord ischemia during aortic surgery may cause damage through an excitotoxic mechanism. If this is true, interventions that block EAA receptors or the release of excitatory neurotransmitters from ischemic nerve terminals might reduce the incidence of spinal cord injury. The results also suggest that an elevation in CSF EAA levels during the operation may be an early, sensitive indicator of cord ischemia. If this is confirmed in additional studies, development of technology that is much faster than the procedure we used might provide a better method for real-time intraoperative monitoring.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Presented at the Poster Session of the Thirty-second Annual Meeting of The Society of Thoracic Surgeons, Orlando, FL, Jan 29–31, 1996.

Address reprint requests to Dr Brock, Division of Cardiac Surgery, Johns Hopkins Hospital, Blalock 618, 600 North Wolfe St, Baltimore, MD 21287.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Malcolm V. Brock Michael V. Johnston William A. Baumgartner John C. Laschinger G. Melville Williams Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Brock, M. V. Articles by Williams, G. M. Search for Related Content PubMed PubMed Citation Articles by Brock, M. V. Articles by Williams, G. M.