Distal aortic perfusion—still of relevance for spinal cord protection during complex aortic surgery?

In their manuscript, Haunschild et al. [1] established a complex animal model to assess spinal cord perfusion during aortic cross-clamping using microspheres. This animal study, which used German landrace pigs as a model for spinal cord perfusion, enabling reliable findings for the physiological and pathophysiological changes in humans, included 8 juvenile animals. During surgery, a cerebrospinal fluid drain was placed and cannulation for cardiopulmonary bypass realizing distal aortic perfusion was performed, followed by aortic cross-clamping of the descending thoracic and abdominal aorta. After injection of microspheres of different colours according to a standardized protocol, the authors were able to assess the regional distribution of spinal cord perfusion influenced by distal aortic perfusion. While distal aortic perfusion showed a massive increased perfusion of the distal spinal cord, which is directly influenced by left-heart bypass perfusion, the aortic segment, which is affected by cross-clamping, experienced only a minor increase in spinal cord perfusion. According to the performed experiment, a compensatory blood flow was only observable at the levels of C1 to Th7.

In the second part of their study, the authors increased the cerebrospinal fluid pressure three-fold by plasma injection through a lumbar catheter. As a consequence, the spinal cord perfusion, again assessed by the local distribution of microspheres, decreased even more, indicating an inappropriate counteracting force of the distal aortic perfusion. Yet, significant differences were solely observable in the C1-T7 segment when microspheres were injected proximally.

The observations of this experiment are relevant and the methodology of this animal study is strong, particularly as the swine is an established animal model for spinal cord perfusion, leading to transferable findings for human beings [2]. Yet, even if the collateral network concept of the spinal cord perfusion suggests a compensatory flow in all segments of the spinal cord as long as patent collateral vessels are available, the experiment may fail to display all aspects of collateralization, despite the methodological appropriateness. The transferability of these findings, gained in an animal study assessing juvenile animals with healthy vessels, to a patient’s diseased aorta may be not realistic. In case of a degenerative diseased aorta, the collateral network changes due to occlusion of multiple segmental arteries, stimulating a network of spinal cord collaterals to develop. These collaterals are often perfused proximately via subclavian artery side branches and distally via hypogastric arteries. Especially, the latter ones are continuously perfused during distal aortic perfusion, providing blood flow to all segments of the spinal cord. This collateral flow support is not available in healthy animals and therefore the finding of limited perfusion in the upper and middle parts of the spinal cord in the animal experiment might not be transferable to diseased aortas in men [3].

Finally, this study underlines the relevance of a multimodular spinal cord protection concept during open and endovascular complex aortic repair. Distal aortic perfusion is important to reduce the inflammation-reperfusion reaction following aortic cross-clamping in fields of open thoracoabdominal aortic surgery. Again, the relevance of distal aortic perfusion regarding an improved spinal cord supply may be underestimated if assessed in an animal model. The altered spinal cord collateral network in patients suffering from aortic pathologies benefits significantly more, as the relevant vessels enabling collateral flow are already established. As published recently, the usage of cerebrospinal fluid drainage should be considered as a useful and established tool to observe and treat increased spinal cord pressure [4]. Motor-evoked potentials are, in particular against the background of this study, of relevance to assess intraoperative spinal cord perfusion [5]. In case of open thoracoabominal aortic repair, an increased distal aortic perfusion or the implantation of selective intercostal artery bypass are well-described options to re-establish perfusion of the spinal cord if motor-evoked potentials are decreasing and the spinal cord is in utmost risk of ischaemic injury [6].

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