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Ann Thorac Surg 2007;83:2130-2134
© 2007 The Society of Thoracic Surgeons
a Department of Cardiothoracic Surgery, The Mount Sinai Medical Center/New York University, New York, New York
b Department of Cardiothoracic Surgery, University Hospital of Cologne, Cologne, Germany
Accepted for publication January 29, 2007.
* Address correspondence to Dr Strauch, University Hospital of Cologne, Department of Cardiothoracic Surgery, Kerpener Strasse 62, 50924 Cologne, Germany (Email: justus.strauch{at}uk-koeln.de).
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Abstract |
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Methods: To visualize vessels contributing to the blood supply of the spinal cord, x-ray digital subtraction angiography and vascular casting by injection of a polymerizing solution of methylmethacrylate were used.
Results: The pig has larger internal thoracic arteries and subscapular arteries than the human, providing extensive collateral flow to the lower body, which offers blood supply to the spinal cord through the collaterals. The pig has a fine-caliber vessel plexus providing blood to the neck area, from which flow will reach both the spinal cord and the base of the brain. The segmental thoracic and lumbar arteries are relatively small in pigs, and they almost all originate as a single branch from the aorta and divide after 3 to 4 mm. The segmental vessels show a clear diminution after 2 to 3 cm at the level of the vertebral bodies. Pigs show major differences in the anatomy of the aortic bifurcation compared with humans. The median sacral artery in pigs is a large-caliber vessel, of a size almost comparable to the common iliac artery, with an isolated single dorsal branch leading to the spinal cord.
Conclusions: Documenting the anatomic differences in spinal cord blood supply between pigs and humans will aid in the planning of future experimental studies and in determining the clinical relevance of such studies.
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Introduction |
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Adjunctive procedures to preserve spinal cord integrity during surgery—including monitoring of motor evoked potentials, cerebrospinal fluid drainage, distal aortic perfusion, and neuroprotective hypothermia—have reduced the incidence of postoperative dysfunction of the cord. However, a small number of patients still suffer from paraplegia or paraparesis postoperatively. At a minimum, such patients have an impaired quality of life and a prolonged hospital stay, and incur significant costs for treatment. More ominously, there is a significant association of postoperative paraplegia with mortality.
Factors influencing long-term spinal cord function are more complex and less well understood. In a number of patients, evidence of impaired spinal cord function only becomes manifest days or weeks after the operation despite normal neurologic recovery early postoperatively. In many of these cases, the loss of spinal cord function can be linked to episodes of hypotension, suggesting that inadequate perfusion is the underlying mechanism. Occasionally, spinal cord function can be restored by countermeasures such as spinal cord drainage and hypertensive therapy, but these attempts fail in a significant number of cases.
For better understanding of the pathophysiology and the mechanisms responsible for the development of both immediate and delayed paraplegia, extensive research is still necessary to investigate the dynamics of blood supply to the spinal cord and the reaction of the spinal cord to ischemia. Little is known about the existence and possible duration of a period of increased vulnerability that may occur as a consequence of operative ischemia and sudden reduction in blood flow, or about time-related changes in spinal cord blood supply and the development of collaterals.
In humans, blood flow to the spinal cord relies not only on branches of the vertebral, deep cervical, intercostal, and lumbar arteries, all of which contribute to the feeding of the anterior spinal artery, but also on more distant arteries such as the hypogastric and subclavian arteries, which feed into the same network. However, in a species as popular and widespread in research as the pig, little information is available concerning the blood supply and vascular anatomy of the spinal cord and its comparability to the human. For the purpose of devising an animal model for investigating delayed paraplegia, we believe this knowledge is essential. The aim of this study was to point out the differences between the anatomic situation in humans and in pigs to be able to better interpret the results of future experiments involving spinal cord blood supply in the pig.
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Material and Methods |
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Anesthesia
Anesthesia was induced using ketamine hydrochloride (10 mg/kg intramuscularly) and sodium thiopental (20 mg/kg). After endotracheal intubation, the pigs were ventilated mechanically with an inspired oxygen fraction of 0.5 and isoflurane of 2% to maintain anesthesia. Pancuronium (0.1 mg/kg) was administered intravenously to achieve muscular paralysis and heparin (300 IU/kg) to avoid the formation of blood clots that could occlude peripheral arterial vessels.
During angiography, the ventilator rate and tidal volume were adjusted to maintain arterial carbon dioxide tension at approximately 35 to 40 mm Hg. End-expiratory carbon dioxide was continuously monitored (model 2010–200 R; PPG Biomedical Systems, Lenexa, KS).
In pigs undergoing vascular casting, animals were sacrificed shortly after aortic cannulation.
Illustration of the Arterial Vascular System
To visualize vessels contributing to the blood supply of the spinal cord, x-ray digital subtraction angiography and vascular casting by injection of a polymerizing solution of methylmethacrylate were used. Thus, two-dimensional angiographic images and a three-dimensional cast of each animal’s arterial vascular system were obtained, illustrating vessels and collaterals by visualizing even arteries of very small caliber.
Two-Dimensional Visualization of the Arterial System: X-Ray Digital Subtraction Angiographic Imaging
With the animal in a right-sided position, the chest was entered through the fourth intercostal space and the pericardium opened. To avoid accidental dislocation of the catheter, the anticipated site for catheterization on the ascending aorta was prepared using a 4-0 pursestring suture. Catheterization was carried out using the Seldinger technique with a regular 5F sheath inserted into the aorta through a conventional 18G needle.
Flush arteriograms (ascending, descending, and abdominal aorta, iliac and median sacral arteries) and selective angiograms (truncus bicaroticus and left subclavian arteries) were obtained during injection of 60 mL of undiluted Conray contrasting agent (Iothalamate Meglumine Injection; Mallinckrodt Inc, St. Louis, MO) into the ascending aorta.
For image recording, a C-arched angiography system (BV 29; Phillips Medical Systems North America, Denver, CO) was used.
Three-Dimensional Visualization of the Descending Aorta and Branches: Vascular Casting
A left anterior thoracotomy was carried out in the fourth intercostal space to allow exposure of the heart and the great vessels. The ascending aorta was identified, and a 4-0 pursestring put in place for cannulation. Vessel loops were placed around the ascending and descending aorta to allow occlusion by placement of vessel clamps.
Using a retroperitoneal approach, the renal arteries, the celiac trunk, the superior and inferior mesenteric arteries, and the femoral arteries were identified and ligated to avoid loss and diversion of the polymerizing solution of methylmethacrylate to the kidneys and to organs of the peritoneal cavity. The ascending aorta was cannulated with a 16F aortic cannula normally used for extracorporeal circulation. By incising the superior and inferior venae cavae, the animal was exsanguinated. The arterial system was flushed through the aortic cannula using 4 L of 4°C cold saline solution until the return from the venous system was clear and free of blood.
Vessel clamps were positioned on the descending aorta and on the ascending aorta proximal to the cannulation site. The injection of 1.5 L of Batson’s No. 17 solution (Polysciences, Warrington, PA) was started into the ascending aorta at a constant rate of 150 mL/min. On appearance of the solution in the superior vena cava, the superior vena cava was occluded to allow complete filling of the vessels of the upper body. After the first 500 mL was injected, the distal clamp was opened and the Batson’s No. 17 solution was allowed to perfuse the lower body. By clamping the inferior vena cava on appearance of the solution, the entire vascular system was allowed to fill. After complete injection and decannulation, the animal was transferred into a water bath of 4°C for 6 hours to allow the methylmethacrylate to cure.
Baths of sodium hydroxide were used to dissolve the tissue surrounding the cast. With proper safety measures, a 5N solution of the base was prepared, and the solution in the bath was changed regularly.
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Results |
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Angiography
Angiography reveals large subscapular arteries and internal thoracic arteries feeding collateral pathways in pigs (Fig 1; lateral view). Also present in angiographic views are segmental arteries in the lumbar area, here L3, L4, and L5 (Fig 2; lateral view), and the median sacral artery and its major side branches (Fig 3; lateral and anteroposterior view).
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Comment |
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Studies in animals are essential to understand physiologic principles and to provide a suitable environment to safely verify and refine protective measures subsequently to be used clinically. The pig has become a popular and widely accepted model for investigating different strategies for preventing neurologic dysfunction during operations on the thoracoabdominal aorta [7–9].
In humans, blood flow to the spinal cord depends on branches of the vertebral, deep cervical, intercostal, and lumbar arteries that contribute to the feeding of the anterior spinal artery. Since the advent of routine replacement of the thoracoabdominal aorta for extended aneurysms, a number of imaging methods, such as roentgenography, contrast angiography, computerized tomography, and magnetic resonance imaging, have succeeded to a greater or lesser extent in visualizing the blood supply to the spinal cord in humans [10]. This knowledge has been supplemented by observations at the time of surgery.
Both techniques used in this study to visualize the anatomy of the vascular supply to the spinal cord in pigs revealed interesting differences from the human situation. These differences should be noted by any researcher considering establishing a spinal cord ischemia model.
In correlation with its weight and body surface area, the pig has much larger internal thoracic arteries and subscapular arteries than the human, providing extensive collateral flow to the lower body, which offers additional blood supply to the spinal cord through the chest and abdominal walls. The pig also has a large fine-caliber vessel plexus providing blood to the neck area, from which we believe flow will reach both the spinal cord and the base of the brain. Furthermore, large bilateral vertebral arteries feed the circle of Willis in pigs, with abundant small branches. The first two branches on each side are major tributaries, which arise in a right angle fashion from the vertebral artery, and head toward the spinal cord in the cervical area: it seems evident that they are of major importance for the spinal cord above the segmental arteries.
We also found large arteriovenous shunts between the subclavian artery branches in the deep neck muscles and the muscles of mastication (Figs 1 and 4): we are not sure whether they have a beneficial effect on spinal cord blood supply or whether they provide an opportunity for steal. It is of course true that the anatomy of the neck and muscles of mastication are very different in the pig and the human, and the vessels reflect these differences. The arteriovenous shunt connections may be important to take into consideration not only for studies of spinal cord blood supply but also for studies dealing with cerebral blood flow and cerebral protection models. In the pig, these vessels are supplying a much larger volume of tissue in the head and neck than in humans. The blood distribution in the pig’s head is different from that in humans: a high percentage of flow goes into a large and densely vascularized nose plexus.
Against the background of body weight, the segmental thoracic and lumbar arteries are relatively small in pigs, and they almost all originate as a single branch from the aorta and divide after 3 to 4 mm. The segmental vessels show a clear diminution after 2 to 3 cm at the level of the vertebral bodies. In the experimental model, one might want to dissect the thoracic and abdominal aorta circumferentially in each case, so as not to miss the occasional segmental artery that arises bilaterally, most frequently in the high lumbar area. With the methods we used, we were not able to visualize and describe direct connections between the intersegmental arteries and the anterior spinal artery (Figs 2, 5 and 8) [11, 12]. There are a total of 16 to 17 segmental arteries in pigs: usually 9 to 11 thoracic, and 6 lumbar arteries.
Pigs show major differences in the anatomy of the aortic bifurcation compared with humans. The median sacral artery in pigs (roughly equivalent to the hypogastric arteries in humans) is a large-caliber vessel, of a size almost comparable to the common iliac artery, with an isolated single dorsal branch leading to the spinal cord area. One centimeter after its origin from the aortic bifurcation, the median sacral artery also splits, again with big branches going dorsally and dorsocaudally (Figs 3 and 7) to supply blood to the spinal cord and to the muscles of the buttocks [13–15].
We believe that there is a large quantity of flow going from these pelvic vessels to the lower spinal cord, and that they have to be taken into account in spinal cord studies. These major collateral vessels need to be identified and isolated in each experimental setup to properly interpret results involving intersegmental artery sacrifice and reconstitution.
Documenting the anatomic differences in spinal cord blood supply between the pig and human will aid in the planning of future experimental studies and in determining the clinical relevance of such studies. [4]
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  C. D. Etz, M. Luehr, F. A. Kari, C. A. Bodian, D. Smego, K. A. Plestis, and R. B. Griepp Paraplegia after extensive thoracic and thoracoabdominal aortic aneurysm repair: does critical spinal cord ischemia occur postoperatively? J. Thorac. Cardiovasc. Surg., February 1, 2008; 135(2): 324 - 330. [Abstract] [Full Text] [PDF]
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