Epidural regional hypothermia for prevention of paraplegia after aortic occlusion: experimental evaluation in a rabbit model

Near infrared fluorescence imaging of rabbit thyroid and parathyroid glands

BACKGROUND

Near infrared fluorescence imaging using intravenous methylene blue (MB) is a novel technique that has potential to aid the parathyroid gland (PG) localization during thyroid and parathyroid surgery. The aim of this study was to examine MB fluorescence in the rabbit neck and determine the influence of MB dose and time following administration on fluorescence from thyroid and PGs.

METHODS

Thyroid and external PGs were exposed in six New Zealand white rabbits under anesthesia. Varying doses of MB (0.025-3 mg/kg) were injected through the marginal ear vein. Near infrared fluorescence from exposed tissues was recorded at different time intervals (10-74 min) using Fluobeam 700. Specimens of identified glands were then resected for histologic assessment.

RESULTS

Histology confirmed accurate identification of all excised thyroid and PGs; these were the only neck structures to demonstrate significant fluorescence. The parathyroid demonstrated lower fluorescence intensities and reduced washout times at all MB doses compared with the thyroid gland. A dose of 0.1 mg/kg MB was adequate to identify fluorescence; this also delineated the blood supply of the external PGs.

CONCLUSIONS

The study demonstrates that near infrared fluorescence with intravenous MB helps differentiate between thyroid and PGs in the rabbit. This has potential to improve outcomes in thyroid and parathyroid surgery by increasing the accuracy of parathyroid identification; however, the findings require replication in human surgery. The use of low doses of MB may also avoid the side effects associated with currently used doses in humans (3-7 mg/kg).

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.

Effects of hyperbaric oxygen therapy after spinal cord injury: systematic review

OBJECTIVE: to conduct a systematic review of experimental and clinical studies evaluating the effect of hyperbaric oxygen therapy on the spinal cord injury. METHODS: ninety-three studies were identified in the database Pubmed. Among these, through a set of inclusion/exclusion criteria, 11 articles published between 1963 and 2009 were selected. In the nine experimental studies, different ways to apply the treatment were observed. The measured outcomes were: functional, histological, biochemical and electrophysiological. RESULTS: in most of the studies, the results show recovery of locomotor function, histology and/or biochemical features. Regarding the two studies in clinical samples, the results are controversial. The samples are heterogeneous and the application of hyperbaric oxygen therapy is not the same for all patients in each study. CONCLUSION: considering the results of this review, further studies are necessary to define the role of hyperbaric oxygen therapy in acute spinal cord injury.

Effect of delayed induction of postischemic hypothermia on spinal cord damage induced by transient ischemic insult in rabbits

OBJECTIVE

This study was performed to determine the effect of delayed induction of mild hypothermia after transient spinal cord ischemia in rabbits.

METHODS

Abdominal aortic occlusion was performed for 15 minutes to induce spinal cord ischemia at a rectal temperature of 37.3 +/- 0.3 degrees C. Four groups of rabbits were investigated: Group 1 (n = 8) was subjected to ischemia and reperfused at the same temperature for 7 hours; Group 2 (n = 8) was subjected to ischemia and reperfused at the same temperature for 1 hour, followed by 6 hours of systemic hypothermia (32.5 +/- 0.5 degrees C); Group 3 (n = 8) was subjected to ischemia, reperfusion at the same temperature for 3 hours and then 6 hours of systemic hypothermia (32.5 +/- 0.5 degrees C); and Group 4 (n = 8) comprised non-ischemic controls. Neurological status of all rabbits in Groups 1-3 was recorded and animals were sacrificed 1 week after ischemic injury. Spinal cord sections were examined microscopically to determine the extent of ischemic neuronal damage.

RESULTS

Mean modified Tarlov's score at 1 week after ischemic insult was 0.5 +/- 0.8 in Group 1, compared to 43 +/- 1.5 in Group 2 and 2.9 +/- 1.8 in Group 3. Mean total number of surviving neurons within examined sections of spinal cord was significantly greater for Groups 2 and 3 compared with Group 1 (Group 1, 81 +/- 66.1; Group 2, 293.4 +/- 110.9; Group 3, 227.1 +/- 105.5; p < 0.001).

CONCLUSIONS

Delayed postischemic hypothermia induced within 3 hours after reperfusion significantly reduces ischemia-induced spinal cord neuronal damage in rabbits.

Effect of post-ischemic hypothermia on spinal cord damage induced by transient ischemic insult in rabbits

OBJECTIVE

The effect of post-ischemic mild hypothermia applied immediately after induced transient ischemia on the extent of neuronal damage to the spinal cord was investigated in rabbit.

SUBJECTS AND METHODS

A 15-minute period of transient abdominal aortic occlusion for spinal cord ischemia at a rectal temperature of 37.3 +/- 0.3 degrees C was performed just below the left renal vein via median laparotomy. Three groups of rabbits were investigated; Group 1 (n = 8) subjected to ischemia and reperfused at the same temperature for 7 hours, Group 2 (n = 8) also subjected to ischemia and then to 6 hours of systemic hypothermia (32.5 +/- 0.5 degrees C), and Group 3 (n = 8) non-ischemic controls. All the rabbits in Group 1 and Group 2 were sacrificed at 1 week after ischemic injury. Spinal cord sections were examined microscopically to determine the extent of ischemic neuronal damage.

RESULTS

The mean modified Tarlov's score at 1 week after ischemic injury was 0.5 +/- 0.8 in Group 1, whereas it was 4.4 +/- 1.4 (p < .001) in Group 2. The mean total number of surviving neurons within examined sections of the spinal cord was significantly greater in Group 2 than in Group 1 (Group 1: 81 +/- 66.1 vs Group 2: 300.9 +/- 154.1, p < .001).

CONCLUSION

Post-ischemic hypothermia induced immediately after reperfusion significantly reduced ischemia-induced neuronal damage in rabbit.

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

PURPOSE

This experiment was designed for the determination of the optimal epidural cooling temperature for the allowance of spinal cord protection with minimal side effects during an aortic occlusion-induced spinal cord ischemia model in rabbits.

METHODS

Spinal cord ischemia was induced in rabbits with infrarenal aortic occlusion for 40 minutes. Spinal cord cooling was effected with epidural infusion of normal saline solution at the following different temperatures: group 1, 17 degrees C (n = 6); group 2, 24 degrees C (n = 6); group 3, 32 degrees C (n = 6); and group 4, 39 degrees C (n = 3). Sham-operated rabbits without aortic occlusion were used as controls with epidural infusion at healthy body temperature (39 degrees C; n = 3). Motor function was assessed at 48 hours with Tarlov's criteria, and the animals were killed. The spinal cord was sectioned into multiple segments, and semiquantitative histologic scoring (0 to 5) was used to grade ischemic injury.

RESULTS

Cooling solution and spinal cord temperatures showed linear correlation (r = 0.95). All the rabbits in groups 1 (except one with mild weakness), 2, and 3 were neurologically intact, and all in group 4 had paraplegia develop (P < .001). One rabbit in group 1 died from increased intracranial pressure (ICP). Mean blood pressure, ICP, and body temperature were similar among the groups. Histology correlated with the clinical findings. In groups 1 and 2, minimal histologic changes were noted. Low-grade ischemic changes were present in group 3 in the low-lumbar and mid-lumbar segments. Severe ischemic injury occurred at the same segments in group 4 (P < .05).

CONCLUSION

These study results suggest that in rabbits satisfactory spinal cord protection during aortic occlusion can be achieved at moderate regional hypothermia (24 degrees C). Large volume infusion for the achievement of profound hypothermia may cause deleterious effects of increased ICP and is not warranted.

Single-clamp technique for aneurysms of the descending thoracic aorta: report of 132 consecutive cases

OBJECTIVE

To determine the efficacy of a single-clamp technique in preventing spinal cord ischemia during repair of aneurysms of the descending thoracic aorta.

PATIENTS AND METHODS

From January 1989 to May 1999, 132 consecutive patients (91 men and 41 women, aged 31-86 years), with aneurysms of the descending thoracic aorta underwent repair using a single-clamp technique and temporary partial distal exsanguination. The diseased aortic segment was replaced with a Dacron graft. Blood was re-infused from an auto-transfusion device, and the segmental vessels were over-sewn but not implanted into the graft.

RESULTS

The average aortic cross-clamp time was 26.4 min (range, 11-67 min) for the overall group and 37.4 min for patients who had spinal cord complications. An average of 2066 ml of blood was auto-transfused (range, 450-6100 ml). During the first 30 postoperative days, 17 patients (12.9 %) died. Eleven patients (8.3%) had spinal cord dysfunction, six patients (4.5%) had lower-extremity paraparesis, and five patients (3.8%) had paraplegia. Nine patients (6.8%) had renal failure necessitating hemodialysis. Other complications included bleeding in 15 cases (11.4%), respiratory failure in 12 cases (9.1%), wound-related sequelae in five cases (3.8%), distal embolism in five cases (3.8%), and bowel ischemia in two cases (1.5%).

CONCLUSION

The single-clamp technique yielded an acceptable complication rate, and the mortality was comparable to that seen after the use of more complex methods. For satisfactory results, the cross-clamp time should not exceed 30 min.

Epidural versus subdural spinal cord cooling: cerebrospinal fluid temperature and pressure changes

BACKGROUND

Regional spinal cord cooling can increase the tolerable duration for spinal cord ischemia resulting from aortic clamping. We compared the efficacy of epidural and subdural cooling and the effect of the resulting cerebrospinal fluid-pressure (CSF) increases on spinal cord motor neuron function.

METHODS

In 8 pigs, CSF temperature and pressure were assessed in the subdural space at L4, T15, and T7. Saline was infused at 333, 666, and 999 ml/h at four consecutive locations: L4 subdural, L4 epidural, T15 subdural, and T15 epidural. First, the influence of CSF-pressure increases during normothermic infusion on transcranial motor evoked potentials (tc-MEPs) was assessed. Then, hypothermic infusion (4 degrees C) was performed to assess CSF-temperature changes.

RESULTS

During normothermic infusion, baseline CSF pressures increased uniformly from 6 +/- 4 mm Hg to 34 +/- 18, 42 +/- 17, and 50 +/- 18 mm Hg with increasing infusion rates (p < 0.001), and did not differ between epidural or subdural infusion. Tc-MEPs indicated spinal cord ischemia in 6 animals when CSF pressures reached 65 +/- 11 mm Hg. During hypothermic infusion, CSF temperatures decreased from 37 degrees to 35 +/- 1.2 degrees, 31 +/- 2.2 degrees, and 28 +/- 2.8 degrees C, but increasing CSF-temperature gradients were observed between the infusion location and distant segments. Subdural cooling resulted in lower CSF temperatures (p < 0.001), but caused larger CSF-pressure increases (p < 0.001).

CONCLUSIONS

Subdural and epidural infusion cooling produce localized spinal cord hypothermia in pigs. The concurrent pressure increases, however, are uniformly distributed and can result in tc-MEP evidence of ischemia.

Clinical experience with epidural cooling for spinal cord protection during thoracic and thoracoabdominal aneurysm repair

PURPOSE

This report summarizes our experience with epidural cooling (EC) to achieve regional spinal cord hypothermia and thereby decrease the risk of spinal cord ischemic injury during the course of descending thoracic aneurysm (TA) and thoracoabdominal aneurysm (TAA) repair.

METHODS

During the interval July 1993 to Dec. 1995, 70 patients underwent TA (n = 9, 13%) or TAA (n = 61) (type I, 24 [34%], type II, 11 [15%], type III, 26 [37%]) repair using the EC technique. The latter was accomplished by continuous infusion of normal saline (4 degrees C) into a T11-12 epidural catheter; an intrathecal catheter was placed at the L3-4 level for monitoring of cerebrospinal fluid temperature (CSFT) and pressure (CSFP). All operations (one exception, atriofemoral bypass) were performed with the clamp-and-sew technique, and 50% of patients had preservation of intercostal vessels at proximal or distal anastomoses (30%) or by separate inclusion button (20%). Neurologic outcome was compared with a published predictive model for the incidence of neurologic deficits after TAA repair and with a matched (Type IV excluded) consecutive, control group (n = 55) who underwent TAA repair in the period 1990 to 1993 before use of EC.

RESULTS

EC was successful in all patients, with a 1442 +/- 718 ml mean (range, 200 to 3500 ml) volume of infusate; CSFT was reduced to a mean of 24 degrees +/- 3 degrees C during aortic cross-clamping with maintenance of core temperature of 34 degrees +/- 0.8 +/- C. Mean CSFP increased from baseline values of 13 +/- 8 mm Hg to 31 +/- 6 mm Hg during cross-clamp. Seven patients (10%) died within 60 days of surgery, but all survived long enough for evaluation of neurologic deficits. The EC group and control group were well-matched with respect to mean age, incidence of acute presentations/aortic dissection/aneurysm rupture, TAA type distribution, and aortic cross-clamp times. Two lower extremity neurologic deficits (2.9%) were observed in the EC patients and 13 (23%) in the control group (p < 0.0001). Observed and predicted deficits in the EC patients were 2.9% and 20.0% (p = 0.001), and for the control group 23% and 17.8% (p = 0.48). In considering EC and control patients (n = 115), variables associated with postoperative neurologic deficit were prolonged (> 60 min) visceral aortic cross-clamp time (relative risk, 4.4; 95% CI, 1.2 to 16.5; p = 0.02) and lack of epidural cooling (relative risk, 9.8; 95% CI, 2 to 48; p = 0.005).

CONCLUSION

EC is a safe and effective technique to increase the ischemic tolerance of the spinal cord during TA or TAA repair. When used in conjunction with a clamp-and-sew technique and a strategy of selective intercostal reanastomosis, EC has significantly reduced the incidence of neurologic deficits after TAA repair.

The role of excitatory amino acids in hypothermic injury to mammalian spinal cord neurons

Hypothermia has been reported to be beneficial in CNS physical injury and ischemia. We previously reported that posttraumatic cooling to 17 degrees C for 2 h increased survival of mouse spinal cord (SC) neurons subjected to physical injury (dendrite transection) but that cooling below 17 degrees C caused a lethal NMDA receptor-linked stress to both lesioned and uninjured neurons. The present study tested whether cooling below 17 degrees C increases extracellular levels of excitatory amino acids (EAA). SC cultures were placed at 10 degrees C or 37 degrees C. Glutamate (Glu) and aspartate (Asp) levels were higher in the medium of the cooled cultures after 0.5 h (23 +/- 4 nM/microgram vs. 4 +/- 1 nM/microgram and 4 +/- 1 nM/microgram vs. 1 +/- 0 nM/microgram, respectively). The concentration of each EAA then declined and reached a plateau at 2-4 h that was still significantly higher than control levels (p < 0.0001, two-factor ANOVA, three cultures per group). Other amino acids (glycine, asparagine, glutamine, serine) showed an opposite pattern, with higher levels in the 37 degrees C group. Both NMDA and non-NMDA antagonists prevented the lethal cold injury. Survival of SC neurons cooled at 10 degrees C for 2 h and rewarmed for 22 h was 58% +/- 25% in the control group, 94% +/- 5% in the CNQX-treated group, 97% +/- 5% in the DAPV-treated group, and 99% +/- 2% in the group treated with both antagonists [p < 0.0006, one factor ANOVA, five cultures (> 120 neurons) per group]. These results show that death of neurons cooled to 10 degrees C is caused by elevated extracellular Glu and Asp and requires activation of both the NMDA and non-NMDA receptor subtypes.