Spinal Cord Protection for Thoracic Aortic Surgery: Bench to Bedside

A Surgical Protocol for Establishing Spinal Cord Ischemia with Extended Lifespan and Low Complication Rates in Rats

BACKGROUND

Experimental animal models of ischemic spinal cord injury (iSCI) are essential for studying its pathogenesis and for developing new therapeutic strategies to improve functional recovery in humans. Many existing models, however, exhibit high variability or early lethality. A reliable experimental iSCI model would significantly advance novel treatment approaches for these severe neurological disorders. To this end, we have established a rat model of persistent iSCI with an extended lifespan.

METHODS

We have developed a novel iSCI model that induces localized ischemic lesions in the spinal cord of male Sprague-Dawley rats. This is achieved by cross clamping the descending aorta just rostral the azygos vein using an atraumatic bulldog clamp.

RESULTS

The experimental iSCI model consistently demonstrated symptoms specific to spinal cord ischemia at the lumbar level. The procedure takes approximately 50 min and does not require specialized surgical equipment. It has a survival rate of 84%, a recovery rate of 40%, and a complication rate of 16%.

CONCLUSIONS

We have successfully developed a rat model of persistent iSCI. This protocol proves to be highly reliable and holds promise for evaluating new therapeutic strategies aimed at promoting functional recovery in patients suffering from spinal cord ischemia.

Computational fluid dynamics simulate optimal design of segmental arteries reattachment: Influence of blood flow stagnation

Objectives

This study aimed to simulate blood flow stagnation using computational fluid dynamics and to clarify the optimal design of segmental artery reattachment for thoracoabdominal aortic repair.

Methods

Blood flow stagnation, defined by low-velocity volume or area of the segmental artery, was simulated by a 3-dimensional model emulating the systolic phase. Four groups were evaluated: direct anastomosis, graft interposition, loop-graft, and end graft. Based on contemporary clinical studies, direct anastomosis can provide a superior patency rate than other reattachment methods. We hypothesized that stagnation of the blood flow is negatively associated with patency rates. Over time, velocity changes were evaluated.

Results

The direct anastomosis method led to the least blood flow stagnation, whilst the end-graft reattachment method resulted in worse blood flow stagnation. The loop-graft method was comparatively during late systole, which was also influenced by configuration of the side branch. Graft interposition using 20 mm showed a low-velocity area in the distal part of the side graft. When comparing length and diameter of an interposed graft, shorter and smaller branches resulted in less blood flow stagnation.

Conclusions

In our simulation, direct anastomosis of the segmental artery resulted in the most efficient design in terms of blood flow stagnation. A shorter (<20 mm) and smaller (<10 mm) branch should be used for graft interposition. Loop-graft is an attractive alternative to direct anastomosis; however, its blood flow pattern can be influenced.

A long-term survival rat model of spinal cord ischemia injury: Thoracic aortic occlusion combined with aortic bypass circulation

OBJECTIVE

This study aims to investigate the methods for rat spinal cord ischemia injury models with a high long-term survival rate.

METHODS

The rats were divided into three groups: the treatment group, the control group, and the sham operation group. The treatment group had a blocked thoracic aorta (landing zone 3 by Ishimaru - T11) + aortic bypass circulation for 20 min. In the control group, the thoracic aorta at the landing zone 3 was blocked for 20 min. In the sham operation group, only thoracotomy without thoracic aortic occlusion was performed. The mean arterial blood pressure (MABP) of the thoracic aorta and caudal artery before and after thoracic aortic occlusion was monitored intraoperatively. Spinal cord function was monitored by a transcranial motor evoked potential (Tc-MEP) during the operation. Spinal cord function was evaluated by the BBB scale (Basso, Beattie, & Bresnahan locomotor rating scale) scores at multiple postoperative time points. The spinal cord sections of the rats were observed for 7 days after surgery, and the survival curves were analyzed for 28 days after surgery.

RESULTS

After aortic occlusion, the MABP of thoracic aorta decreased to 6% of that before occlusion, and the MABP of caudal artery decreased to 63% of that before occlusion in the treatment group. In the control group, the MABP of both thoracic aorta and caudal artery decreased to 19% of that before occlusion. The Tc-MEP waveform of the treatment group disappeared after 6 min, and that of the control group disappeared after 8 min until the end of surgery. There was no change in the Tc-MEP waveform in the sham operation group. The BBB score of the treatment group decreased more obviously than the control group, and there was a significant difference. There was no decrease in the sham group. Spinal cord sections showed a large number of degeneration and necrosis of neurons, infiltration of inflammatory cells, and proliferation of surrounding glial cells in the treatment group. In the control group, multiple neurons were necrotic. The histology of the sham operation group was normal. The 28-day survival rate of the treatment group was 73.3%, which was higher than the control group (40.0%), and there was a significant difference (p < 0.05).

CONCLUSION

Thoracic aortic occlusion combined with aortic bypass is an effective modeling method for rats with accurate modeling effects and high long-term survival rates.

Direct and indirect activation of the adenosine triphosphate-sensitive potassium channel to induce spinal cord ischemic metabolic tolerance

OBJECTIVES

The mitochondrial adenosine triphosphate-sensitive potassium channel is central to pharmacologically induced tolerance to spinal cord injury. We hypothesized that both direct and nitric oxide-dependent indirect activation of the adenosine triphosphate-sensitive potassium channel contribute to the induction of ischemic metabolic tolerance.

METHODS

Spinal cord injury was induced in adult male C57BL/6 mice through 7 minutes of thoracic aortic crossclamping. Pretreatment consisted of intraperitoneal injection 3 consecutive days before injury. Experimental groups were sham (no pretreatment or ischemia, n = 10), spinal cord injury control (pretreatment with normal saline, n = 27), Nicorandil 1.0 mg/kg (direct and indirect adenosine triphosphate-sensitive potassium channel opener, n = 20), Nicorandil 1 mg/kg + carboxy-PTIO 1 mg/kg (nitric oxide scavenger, n = 21), carboxy-PTIO (n = 12), diazoxide 5 mg/kg (selective direct adenosine triphosphate-sensitive potassium channel opener, n = 25), and DZ 5 mg/kg+ carboxy-PTIO 1 mg/kg, carboxy-PTIO (n = 23). Limb motor function was assessed using the Basso Mouse Score (0-9) at 12-hour intervals for 48 hours after ischemia.

RESULTS

Motor function was significantly preserved at all time points after ischemia in the Nicorandil pretreatment group compared with ischemic control. The addition of carboxy-PTIO partially attenuated Nicorandil's motor-preserving effect. Motor function in the Nicorandil + carboxy-PTIO group was significantly preserved compared with the spinal cord injury control group (P < .001), but worse than in the Nicorandil group (P = .078). Motor preservation in the diazoxide group was similar to the Nicorandil + carboxy-PTIO group. There was no significant difference between the diazoxide and diazoxide + carboxy-PTIO groups.

CONCLUSIONS

Both direct and nitric oxide-dependent indirect activation of the mitochondrial adenosine triphosphate-sensitive potassium channel play an important role in pharmacologically induced motor function preservation.

Distinct Polarization Dynamics of Microglia and Infiltrating Macrophages: A Novel Mechanism of Spinal Cord Ischemia/Reperfusion Injury

Purpose

Recent studies indicate that microglia and monocyte-derived macrophages (MDMs) have different roles in diseases such as stroke and spinal cord injury, yet their respective polarized phenotypes and roles remain unclear in spinal cord ischemia/reperfusion injury (SCIRI).

Methods

We established a mouse model of SCIRI by transient aortic occlusion followed by reperfusion. Basso mouse scale (BMS) scores were used to test the locomotor functions. The histopathological changes in spinal cord were assessed by hematoxylin-eosin staining and NF-200 immunohistochemistry. Real-time PCR, immunofluorescence and flow cytometry were employed to analyze the polarized phenotypes of the microglia and infiltrating MDMs, and the resulting inflammatory responses. Furthermore, the role of infiltrating MDMs were investigated by MDMs depletion using systemic administration of clodronate-liposomes.

Results

SCIRI significantly impaired locomotor function of mice, accompanied with progressed necrosis, infiltration of inflammatory cells and neuron loss in the spinal cord. M1-related pro-inflammatory markers (iNOS, CD16, CD86 and TNF-α) increased dramatically in the early phase following SCIRI. In contrast, M2-related anti-inflammatory markers (CD204 and CD206) elevated at later stage. Besides, the invading MDMs were principally pro-inflammatory M1 type, transiently restricted to the first week after SCIRI. In contrast, microglia were the main source of anti-inflammatory M2 type. Furthermore, depletion of MDMs by clodronate-liposomes significantly preserved neurological functions and relieved neuronal damage caused by SCIRI.

Conclusion

These findings suggested distinct polarized status of resident microglia and MDMs following SCIRI. Inhibition of the invading MDMs may represent a novel approach for SCIRI treatment.

Postoperative spinal cord ischaemia: magnetic resonance imaging and clinical features

OBJECTIVES

Ischaemic spinal cord injury (SCI) is one of the most serious complications of aortic surgery. Ischaemic SCIs occur due to various aetiologies, and prediction of the risk is difficult. Magnetic resonance imaging (MRI) is useful to detect the details of spinal cord infarction. There are few studies about MRI for evaluating ischaemic SCI after cardiovascular surgery and aortic events. We report 9 cases of postoperative ischaemic SCI and analyse their MRI features.

METHODS

T2-weighted MRI scans of 9 patients who developed ischaemic SCI due to cardiovascular surgery and aortic events between 2012 and 2017 were evaluated.

RESULTS

In all patients, high-intensity areas were observed on T2-weighted magnetic resonance images. The site of infarction was the thoracic spinal cord level (9 cases) and additionally at the lumbar spinal cord level (5 cases). The area of infarction area was categorized based on the arterial territory: anterior spinal artery territory (3 cases), posterior spinal artery territory (2 cases), spinal sulcal artery territory (1 case) and artery of Adamkiewicz territory (3 cases).

CONCLUSIONS

MRI revealed the infarction sites in all cases and the differences in the infarction patterns in each case. MRI could thus be useful for investigating the aetiology of ischaemic SCI following aortic surgeries and events.

Optimizing Nicorandil for Spinal Cord Protection in a Murine Model of Complex Aortic Intervention

There are currently no clinically utilized pharmacological agents for the induction of metabolic tolerance to spinal cord ischemia-reperfusion injury in the setting of complex aortic intervention. Nicorandil, a nitric oxide donor and ATP-sensitive potassium (KATP) channel opener, has shown promise in neuroprotection. However, the optimized clinical application of the drug and its mechanism of neuroprotection remains unclear. We hypothesized that 3-days pretreatment would confer the most effective neuroprotection, mediated by mitochondrial KATP channel activation. Spinal cord injury was induced by 7 minutes of thoracic aortic cross-clamping in adult male C57BL/6 mice. Time course: mice received 0.1 mg/kg nicorandil for 10 min, 4 hours, and 3 consecutive days prior to ischemia compared with control. Dose challenge: mice received 3-days nicorandil pretreatment comparing 0.1 mg/kg, 1.0 mg/kg, 5.0 mg/kg, and saline administration. Mitochondrial KATP channel blocker 5-hydroxy-decanoate (5HD) was co-administered to elucidate mechanism. Limb motor function was evaluated, and viable anterior horn neurons quantified. Nicorandil pretreatment at 4 hours and 3 days before ischemia demonstrated significant motor function preservation; administration 10 minutes before ischemia showed no neuroprotection. All nicorandil doses showed significant motor function preservation. Three days administration of Nicorandil 1.0 mg/kg was most potent. Neuroprotection was completely abolished by 5HD co-administration. Histological analysis showed significant neuron preservation with nicorandil pretreatment, which was attenuated by 5HD co-administration. Three days administration of Nicorandil 1.0 mg/kg showed near-total motor function preservation in a murine spinal cord ischemia-reperfusion model, mediated by the mitochondrial KATP channel.

Reactive Oxygen Species Mediate Nicorandil-induced Metabolic Tolerance to Spinal Cord Injury

BACKGROUND

Spinal cord injury remains a devastating complication of thoracoabdominal aortic surgery. We previously demonstrated that pretreatment with nicorandil preserved motor function in a murine spinal cord injury model through mitochondrial adenosine triphosphate-sensitive potassium channel activation. We hypothesized that the neuroprotective effect of nicorandil is mediated by downstream generation of reactive oxygen species.

METHODS

Spinal cord injury was induced by 7 minutes of thoracic aortic cross-clamping in adult male C57BL/6 mice. Five groups were evaluated: ischemic control (n = 19); nicorandil 1.0 mg/kg (n = 17); nicorandil 1.0 mg/kg plus N acetyl L-cysteine (NAC [reactive oxygen species scavenger, n = 18)]) 150 mg/kg; NAC 150 mg/kg (n = 13); and sham (n = 10). Limb motor function and the number of viable neurons within the anterior horn of the spinal cord were evaluated.

RESULTS

Mice in the sham group showed no functional deficits after surgery. Compared with ischemic control, motor function was significantly preserved in the nicorandil pretreatment group at every timepoint after ischemia. In the nicorandil plus NAC group, the motor-preserving effect of nicorandil was completely abolished (P < .001). Viable neuron quantification showed significant neuron preservation in the nicorandil group (29.± 2.6) compared with the ischemic control group (18.5 ± 2.1, P = .024) and nicorandil plus NAC group (14 ± 8.3, P = .001); no significant difference was observed between the ischemic control group and nicorandil plus NAC group (P = 0.768).

CONCLUSIONS

Reactive oxygen species generation plays a key role in the nicorandil-induced metabolic tolerance to spinal cord injury. Manipulation of mitochondrial adenosine triphosphate-sensitive potassium channels may lead to improvement in preventing spinal cord injury after thoracoabdominal aortic interventions.

Low-density vulnerable thrombus/plaque volume on preoperative computed tomography predicts for spinal cord ischemia after endovascular repair for thoracic aortic aneurysm

BACKGROUND

Similar to open surgical repair, thoracic endovascular aortic repair (TEVAR) carries a risk of spinal cord ischemia (SCI). However, the generally lower incidence of SCI after TEVAR compared with that after open surgical repair, despite the inability to preserve the intercostal arteries, indicates different pathophysiologic mechanisms with the two procedures. We hypothesized that a microembolism from an aortic mural thrombus is the main cause of SCI. Thus, we evaluated the association between the density of a mural thrombus in the descending thoracic aorta and the development of SCI.

METHODS

A retrospective review of a prospectively assembled database was performed for all patients who had undergone surgery at a single institution from October 2008 to December 2018. Patient demographics and procedure-related variables were collected. The volume and Hounsfield unit (HU) value of mural thrombi in the whole descending thoracic aorta were estimated on preoperative computed tomography using a three-dimensional workstation. Logistic regression analysis was performed to identify the risk factors for SCI development.

RESULTS

Of the 367 patients who had undergone TEVAR during the study period, 155 were excluded because of previous arch surgery (n = 59), previous descending thoracic aortic surgery (n = 6), previous TEVAR (n = 6), unavailability of optimal preoperative computed tomography data (n = 17), double-barreled dissection (n = 40), and other reasons. The mean ± standard deviation age of the remaining 212 patients was 75.8 ± 6.4 years, and 42 (19.8%) were women. Of the 212 patients, 14 (6.6%) developed SCI after TEVAR. The low mean density of the mural thrombus, total thrombus volume, low-density (≥-100 HU but <30 HU) thrombus volume, intermediate-density (≥30 HU but <150 HU) thrombus volume, treatment length, urgent surgery, and baseline dialysis differed significantly between patients with and without SCI. Although subsequent multivariate analysis could not be performed owing to the small number of SCI events, vulnerable low-density thrombus/plaque was a stronger predictor among the aneurysm-related factors of SCI after TEVAR on univariate analysis. Well-known risk factors, such as distal coverage between T8 and L1, left subclavian artery coverage, previous abdominal aortic surgery, and prophylactic spinal drainage, did not show significant differences.

CONCLUSIONS

The results from the present study have demonstrated that among aneurysm-related factors, a lower density mural thrombus/plaque in the descending thoracic aorta is a predictor of SCI development after TEVAR. These results suggest that microembolism is one of the important mechanisms of SCI after TEVAR, which might change the prophylactic strategy.