Epidural cooling for the prevention of ischemic injury to the spinal cord during aortic occlusion in a rabbit model: determination of the optimal temperature

Role of induced hypothermia in thoracoabdominal aortic aneurysm surgery

For more than 50 years, hypothermia has been used in aortic surgery as a tool for neuroprotection. Hypothermia has been introduced into thoracoabdominal aortic aneurysm (TAAA) surgery by many cardiovascular centers to protect the body's organs, including the spinal cord. Numerous publications have shown that hypothermia can prevent immediate and delayed motor dysfunction after aortic cross-clamping. Here, we reviewed the historical application of hypothermia in aortic surgery, role of hypothermia in preclinical studies, cellular and molecular mechanisms by which hypothermia confers neuroprotection, and the role of systemic and regional hypothermia in clinical protocols to reduce and/or eliminate the devastating consequences of ischemic spinal cord injury after TAAA repair.

Spinal cord hypothermia without systemic hypothermia

BACKGROUND AND PURPOSE

Hypothermia has been shown to be beneficial in the setting of acute SCI. However, widespread use has been hindered by the need for systemic hypothermia as the vehicle for achieving spinal cord hypothermia. This study demonstrates that localized spinal cord hypothermia can be achieved via a percutaneous approach while maintaining systemic normothermia.

MATERIALS AND METHODS

Five Yucatan swine underwent catheterization of the subarachnoid space and infusion of room temperature, chilled, and iced PL solutions into the cervical spinal canal, with drainage from the lumbar canal. Thermocouples were placed within the spinal cord and in the subarachnoid space and recorded during infusions and recovery from hypothermia.

RESULTS

Results demonstrated that hypothermia as low as 16.8°C is feasible in the spinal cord with retention of systemic normothermia, with strong (r = 0.95) correlation between the spinal cord temperature and the CSF temperature. Degrees of cooling varied with flow rates and with infusate temperature.

CONCLUSIONS

While the data are preliminary in a small group of animals, the ability to rapidly create a wide range of controlled spinal cord hypothermia while preserving normal body temperature warrants wider exploration. The study also indicates that further investigation of the hypothesis that CSF temperature monitoring may be an acceptable surrogate for direct spinal cord temperature monitoring should be pursued.

Moderate hypothermia prevents neural cell apoptosis following spinal cord ischemia in rabbits

Paraplegia is a disastrous complication after operations of descending and thoracoabdominal aortic aneurysm. Regional hypothermia protects against spinal cord ischemia although the protective mechanism is not well know. The objective of this study is to examine whether hypothermia protects the spinal cord by preventing apoptosis of nerve cell and also investigate a possible mechanism involved in hypothermia neuroprotection. Cell apoptosis with necrosis was evident in the spinal cord 24 h after 30 min of ischemia. Moderate hypothermia decreased the incidence of apoptotic nerve cells. Both cell apoptosis and necrosis were attenuated by hypothermia. p53 expression increased and bcl-2 expression declined after ischemia, while hypothermia mitigated these changes. This study suggests that apoptosis contributes to cell death after spinal cord ischemia, and that moderate hypothermia can prevent nerve cell apoptosis by a mechanism associated with bcl-2 and p53 genes.

Spinal cord ischemia after endovascular repair of the descending thoracic aorta in a sheep model

OBJECTIVES

Spinal cord ischemia remains a devastating complication after thoracic aortic surgery. The aim of this study was to investigate the pathophysiology of spinal cord ischemia after thoracic aortic endografting and the role of intercostal artery blood supply for the spinal cord in a standardized animal model.

METHODS

Female merino sheep were randomized to either I, open thoracotomy with cross-clamping of the descending aorta for 50 min (n=7), II, endograft implantation (TAG, WL Gore & Ass.), (n=6) or III open thoracotomy with clipping of all intercostal arteries (n=5) . CT-angiography was used to assess completion of surgical protocol and assess the fate of intercostal arteries. Tarloy score was used for daily neurological examination for up to 7 days post-operatively. Histological cross sections of the lumbar, thoracic and cervical spinal cords were scored for ischemic damage after stained with Hematoxylin-Eosin, Klüver-Barrrera and antibodies. Exact Kruskall-Wallis-Test was used for statistical assessment (p<0.05).

RESULTS

Incidence of paraplegia was 100% in group I and 0% in group II (p=0.0004). When compared to the endovascular group, there was a higher rate of histological changes associated with spinal cord ischemia in the animals of the control group (p=0.0096). Group III animals showed no permanent neurological deficit and only 20% infarction rate (p=0.0318 compared to group I).

CONCLUSIONS

In sheep, incidence of histological and clinical ischemic injury of the spinal cord following endografting was very low. Complete thoracic aortic stent-grafting was feasible without permanent neurologic deficit. Following endovascular coverage or clipping of their origins, there is retrograde filling of the intercostal arteries which remain patent.

Cold spinoplegia and transvertebral cooling pad reduce spinal cord injury during thoracoabdominal aortic surgery

OBJECTIVE

We examined the protective effects of the new selective spinal cord cooling by using cold saline infusion into the cross-clamped aorta and a transvertebral cooling pad placed over the lumbar vertebral column from paraplegia caused by ischemic spinal cord injury on thoracoabdominal aortic surgery.

METHODS

Eighteen rabbits were divided into three groups: groups I, II, and III (n = 6 for each group). In group I (37 degrees C; 5 mL) and group II (3 degrees C; 5 mL), saline was infused into the isolated aortic segment twice, at 0 and 5 minutes after aortic cross clamping. In group III, a 3 degrees C saline solution plus cooling pads placed just after cross clamping were combined. The infrarenal aorta was then isolated proximally and distally by vascular clamps for 12 minutes. In our preliminary study, only the abdominal aorta just distal to the left renal artery was clamped. At 48 hours after reperfusion, the groups clamped for 12 and 15 minutes were all paraplegic. The time of clamping the aorta was set at 12 minutes as the critical point when paraplegia occurred upon simple clamping of the infrarenal aorta only. The spinal cord temperature was monitored at the L4 level continuously during the procedures in all three groups. At 8, 24, and 48 hours after the operation, hind limb function was estimated by using the Tarlov score, which is often used for evaluating motor function in animals. A histopathologic study using hematoxylin and eosin stains was also performed.

RESULTS

At 48 hours after the operation, the Tarlov scores in groups I, II, and III were 0 +/- 0, 2.0 +/- 1.9, and 4.0 +/- 0 (mean +/- SD), respectively. The Tarlov score and histopathologic analysis in group III were significantly superior to those of groups I (P < .01) and II (P < .05). The spinal cord temperature in groups II and III decreased by -1.8 degrees C and -4.3 degrees C at its minimum. The rabbits in group III were also protected from paraplegia.

CONCLUSIONS

Selective spinal cord cooling with cold saline infusion into the isolated aortic segment and transvertebral regional cooling can reduce the neurologic damage of spinal cord ischemia.

Histochemical and Electron Microscopic Study on Motor Neurone Degeneration Following Transient Spinal Cord Ischaemia at Normothermic Conditions in Rabbits

This study was carried out to investigate the motor neurone degeneration in the ventral horn following transient spinal cord ischaemia at normothermic conditions in rabbits. Transient spinal cord ischaemia was induced by occlusion of the abdominal aorta underneath the left renal artery for 15 min at normothermia (38.7 degrees C). Sections at the level of L7 were examined using histochemical and electron microscopic methods. Cresyl violet-positive motor neurones began to reduce in number at 3 h after ischaemia reperfusion, and were not detectable at 48 h after ischaemia reperfusion. Acid fuchsin-positive motor neurones were detected at 1 h after ischaemia reperfusion, significantly increased up to 6 h after the ischaemia reperfusion, and eventually disappeared by 48 h after ischaemia reperfusion. In electron microscopic findings, the disintegration of cytoplasmic membranes, and the disruption of mitochondria and endoplasmic reticulum were observed in motor neurones at 30 min after ischaemia reperfusion. Motor neurones showed necrotic findings with pyknotic degeneration at 1 h after ischaemia reperfusion. The necrotic degeneration became severer time dependently after ischaemia reperfusion. At 48 h after ischaemia reperfusion, cellular components were not detectable in motor neurones. In conclusion, we suggest that the degeneration pattern of motor neurones of the ischaemic spinal cord was necrotic after ischaemia reperfusion under normothermic conditions.

NS-7, a novel Na+/Ca2+ channel blocker, prevents neurologic injury after spinal cord ischemia in rabbits

OBJECTIVE

We investigated the neuroprotective effect of NS-7 (4-[4-fluorophenyl]-2-methyl-6- [5-piperidinopntyloxy] pyrimidine hydrochloride), a novel Na(+)/Ca(2+) channel blocker, on transient spinal cord ischemia in rabbits.

METHODS

Spinal cord ischemia was induced in New Zealand white rabbits by means of infrarenal aortic occlusion for 20 minutes. Four experimental groups were enrolled. A sham group (n = 3) underwent the same operation without aortic occlusion. A control group (n = 7) received only saline before occlusion. Group A (n = 8) received NS-7 (1 mg/kg) 15 minutes before ischemia, and group B (n = 8) received NS-7 (1 mg/kg) at the onset of reperfusion. Neurologic function was assessed 24 and 48 hours after the operation with modified Tarlov criteria. Spinal cords were harvested for histopathologic examination and in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL staining). Spinal cord infarction was investigated with 2, 3, 5-triphenyltetrazonlium chloride staining.

RESULTS

Tarlov scoring demonstrated marked improvement in both group A and group B compared with the control group at 24 and 48 hours after the operation. Minimal histologic changes were found in lumbar spinal cords of the 2 NS-7-treated groups, whereas severe neuronal necrosis was shown in the control group. TUNEL-positive neurons and the infarct size of lumbar spinal cords were significantly reduced by NS-7 administered both before ischemia and at the onset of reperfusion. No significant difference was noted between group A and group B in terms of spinal cord protection.

CONCLUSION

These results indicate that NS-7 protects the spinal cord against ischemic injury by preventing both neuronal necrosis and apoptosis.

Effects of epidural hypothermic saline infusion on locomotor outcome and tissue preservation after moderate thoracic spinal cord contusion in rats

Object. Regionally delivered hypothermia has advantages over systemic hypothermia for clinical application following spinal cord injury (SCI). The effects of local hypothermia on tissue sparing, neuronal preservation, and locomotor outcome were studied in a moderate thoracic spinal cord contusion model.

Methods. Rats were randomized to four treatment groups and data were collected and analyzed in a blinded fashion. Chilled saline was perfused into the epidural space 30 minutes postcontusion to achieve the following epidural temperatures: 24 ± 2.3°C (16 rats), 30 ± 2.4°C (13 rats), and 35 ± 0.9°C (13 rats). Hypothermia was continued for 3 hours when a 45-minute period of rewarming was instituted. In a fourth group a moderate contusion only was induced in 14 animals. Rectal (core) and T9–10 (epidural) temperatures were measured continuously. Locomotor testing, using the Basso-Beattie-Bresnahan (Ba-Be-Br) scale, was performed for 6 weeks, and rats were videotaped for subsequent analysis. The lesion/preserved tissue ratio was calculated throughout the entire lesion cavity and the total lesion, spinal cord, and spared tissue volumes were determined. The rostral and caudal extent of gray matter loss was also measured. At 6 weeks locomotor recovery was similar in all groups (mean Ba-Be-Br Scale scores 14.88 ± 3.71, 14.83 ± 2.81, 14.50 ± 2.24, and 14.07 ± 2.39 [p = 0.77] for all four groups, respectively). No significant differences in spared tissue volumes were found when control and treatment groups were compared, but gray matter preservation was reduced in the infusion-treated groups.

Conclusions. Regional cooling applied 30 minutes after a moderate contusive SCI was not beneficial in terms of tissue sparing, neuronal preservation, or locomotor outcome. This method of cooling may reduce blood flow in the injured spinal cord and exacerbate secondary injury.

Ischémie médullaire

Traditional data and recent advances in the field of spinal cord ischemia are reviewed, with special attention to clinical and radiological features, as well as underlying etiology, outcome, and pathophysiology. Acute spinal cord ischemia includes arterial and venous infarction and global ischemia resulting from cardiac arrest or severe hypotension. MRI has become the technique of choice for the imaging diagnosis of spinal cord infarction. Correlation of clinical and MRI data has allowed diagnosis of clinical syndromes due to small infarcts in the central or peripheral arterial territory of the spinal cord. Diffusion-weighted MR imaging may increase the sensitivity and specificity for diagnosis of acute spinal cord infarction. Diagnosis of venous spinal cord infarction remains difficult. As for global ischemia, neuropathological studies demonstrated a great sensitivity of spinal cord to ischemia, with selective vulnerability of lumbosacral neurons. Chronic spinal cord ischemia results in a syndrome of progressive myelopathy. The cause is usually an arteriovenous malformation. Most often, diagnosis may be suspected on MRI, leading to diagnostic, and eventually therapeutic, spinal angiography.

Thoracoabdominal aneurysm repair: results with 337 operations performed over a 15-year interval

OBJECTIVE

To review perioperative results and late survival after thoracoabdominal aneurysm repair (TAA), in particular to assess the impact over time of epidural cooling (EC) on spinal cord ischemic complications (SCI).

SUMMARY BACKGROUND DATA

A variety of operative approaches and protective adjuncts have been used in TAA to minimize the major complications of perioperative death and SCI. There is no consensus with respect to the optimal approach.

METHODS

From January 1987 to November 2001, 337 consecutive TAA repairs were performed by a single surgeon. Clinical features included prior aortic grafts in 97 (28.8%) and emergent operation in 82 (24.6%), including rupture in 46 (13.6%) and dissection in 63 (19%). Operative management consisted of a clamp/sew technique with adjuncts in 93%. EC (since July 1993) to prevent SCI was used in 194 (57.6%) repairs. Variables associated with the end points of operative mortality and postoperative SCI were assessed with the Fisher exact test and logistic regression; late survival was estimated with the Kaplan-Meier method.

RESULTS

Operative mortality was 8.3% and was associated with nonelective operation, intraoperative hypotension, total transfusion requirement, and the postoperative complications of paraplegia, renal failure, and pulmonary insufficiency. Postoperative renal failure and transfusion requirement were independent correlates of mortality. SCI of any severity occurred in 38 of 334 (11.4%) operative survivors, with 22/38 (6.6% of cohort) sustaining total paraplegia. EC reduced the risk of SCI in patients with types I-III TAA (10.6% vs. 19.8%, =.04). Independent correlates of SCI over the entire study interval included types I/II TAA, rupture, cross-clamp duration, sacrifice of T9-L1 intercostal vessels, and intraoperative hypotension. Late survival rates at 2 and 5 years were 81.2 +/- 3% and 67.2 +/- 5%.

CONCLUSIONS

EC has decreased the risk of SCI after TAA repair. Decreasing the substantial proportion (nearly 25%) of patients requiring nonelective operation will improve results. Late survival is equal to that after routine AAA repair, indicating that the considerable resource expenditure required for TAA repair is worthwhile.