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15 Levinsky NG, Levy M: Clearance techniques, Handbook of Physiology, Section 8, Renal physiology, Washington, D. C; 1973, American Physiology Society. 16 Martinez-Maldonado M: Methods in Pharmacology, Vol 4B, Renal Pharmacology, New York, 1978, Plenum Press 17 Roberts AJ, Hoover EL, Alonso DR, Combes JR, Dinen P, Gay WA Jr, Subramanian VA: Prolonged intraaortic balloon pumping. Klebsiella-induced hypodynamic shock: Cardiopulmonary, hematological, metabolic, and pathological observations. Ann Thorac Surg 28:73, 1979 18 Lamprecht W, Trautschold I: Adenosine-5 ' -triphosphate. Determination with hexokinase and glucose-6-phosphate dehydrogenase, Methods of Enzymatic Analysis, Vol 4, 2nd English edition, HU Bergmeyer, ed. New York, 1974, Academic Press, Inc. 19 Luft JH: Improvements in epoxy resin embedding methods. J Biophys Biochem Cytol 9:409, 1961 20 Buckberg GD, Fixler DE, Archie JP, Hoffman JIE: Experimental subendocardial ischemia in dogs with normal coronary arteries. Circ Res 30:67, 1972 21 Rudolph AM, Heymann MA: Circulation of the fetus in utero. Methods for studying distribution of blood flow, cardiac output and organ blood flow. Circ Res 21:163, 1967 22 Hughes RK, Reemtsma K: Correction of coarctation of the aorta. Manometric determinations of safety during test occlusion. J THORAC CARDIOVASC SURG 62:31, 1971 23 Moreno NN, de Campo T, Kaiser GA, Pallares VS: Technical and pharmacologic management of distal hypotension during repair of coarctation of the aorta. J THORAC CARDIOVASC SURG 80:182, 1980 24 Roberts AJ, Michaelis LL: The use of bypass techniques and other forms of organ protection during thoracic aortic cross-clamping, Surgery of the Aorta and its Body Branches, JJ Bergan, JST Yao, eds., New York, 1979, Grune & Stratton, Inc. 25 Cooley DA, DeBakey ME, Morris GC Jr: Controlled extracorporeal circulation in surgical treatment of aortic aneurysm. Ann Surg 146:473, 1957 26 Symbas PN: Trauma to the Heart and Great Vessels, New York, 1978, Grune & Stratton, Inc. 27 Wakabayashi A, Nakamura Y, Woolley T, Chen CC, Mullin PJ, Connolly JE: Successful prolonged heparinless venoarterial bypass in sheep. J THORAC CARDIOVASC SURG 71:648, 1976 18 Donahoo JS, Brawley RK, Gott VL: The heparin-coated vascular shunt for thoracic aortic and great vessel procedures. A ten-year experience. Ann Thorac Surg 23:507, 1977 29 Crawford ES, Walker HSJ III, Saleh SA, Normann NA: Graft replacement of aneurysm in descending thoracic aorta. Results without bypass or shunting. Surgery 89:73, 1981 30 Djindjian R, Hurth M, Houdard R, Laborit G, Julian H, Mamo H: Summary of the anatomy of the blood supply to

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the spinal cord, Angiography of the Spinal Cord, Baltimore, 1970, University Park Press, pp 2-13 Katz NM, Blackstone EH, Kirklin JW, Karp RB: Incremental risk factors for spinal cord injury following operation for acute traumatic aortic transection. J THORAC CARDIOVASC SURG 81:669, 1981 Blaisdell FW, Cooley DA: The mechanism of paraplegia after temporary thoracic aortic occlusion and its relationship to spinal fluid pressure. Surgery 51:351, 1962 Leshnower AC, Shumacker HB Jr: Experimental unassisted right brachial-to-femoral bypass. Arch Surg 109:542, 1974 Coles JG, Wilson GJ, Sima AF, Klement P, Tait GA: Intraoperative detection of spinal cord ischemia using somatosensory cortical evoked potentials during thoracic aortic occlusion. Ann Thorac Surg 34:299, 1982. Nylander WA, Plunkett RJ, Hammon JW Jr, Oldfield EH, Meacham WF: Thiopental modification of ischemic spinal cord injury in the dog. Ann Thorac Surg 33:64, 1982 DeBakey ME, Cooley DA, Crawford ES, Morris GC Jr: Aneurysms of the thoracic aorta. Analysis of 179 patients treated by resection. J THORAC SURG 36:393, 1958 Carlson DE, Karp RB, Kouchoukos NT: Surgical treatment of aneurysms of the descending thoracic aorta. An analysis of 85 patients. Ann Thorac Surg 34:512, 1982 Connolly JE, Kountz SL, Boyd RJ: Left heart bypass. Experimental and clinical observations of its regulation with particular reference to maintenance of maximal renal blood flow. J THORAC CARDIOVASC SURG 44:577, 1962 Kirsh MM, Behrendt DM, Orringer MB, Gago 0, Gray LA, Mills LJ, Walter JF, Sloan H: The treatment of acute traumatic rupture of the aorta. A IO-year experience. Ann Surg 184:308, 1976 Wolfe WG, Kleinman LH, Wechsler AS, Sabiston DC Jr: Heparin-coated shunts for lesions of the descending thoracic aorta. Arch Surg 112:1481, 1977 Symbas PN: Crossclamping the descending aorta. Hemodynamic and neurohumoral effects. (in press)

Manuscript reviewer's comment The management of descending thoracic aortic lesions that require cross-damping of the aorta remains controversial. The controversy is centered around the issue of whether some form of bypass or other adjunct should be used during the aortic occlusion period. Cross-clamping of the aorta alone has been shown to result in a decrease of the arterial pressure distal to the aortic occlusion, decrease of cardiac index, decrease of renal and distal spinal cord blood flow, and an increase of LVEDP and left atrial pressure.!" These effects have been shown to result in various degrees of transient or permanent impairment of renal, circulatory, and particularly distal spinal cord function. Partial cardiopulmonary bypass or external shunt during cross-clamping of the descending aorta in experimental animals has been shown to decrease or ameliorate these hemody-

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namic and spinal cord abnormalities." 3, 7 It also has been shown that the spinal cord abnormalities in patients undergoing repair of descending aortic lesions with or without any form of bypass were not significantly different and that the overall results were better in those patients in whom no bypass or shunt was utilized.v" Dr. Roberts and associates are reporting that no overwhelming advantages were found as to the hemodynamic and renal effects of cross-clamping of the descending aorta with their use of femoral vein-femoral artery bypass or external shunt. Unfortunately, they did not study the effects of the investigated techniques upon the spinal cord function. Since similar changes in hemodynamic, renal, and particularly spinal cord function do not occur to such a degree during cardiopulmonary bypass without descending aortic crossclamping, perhaps these differences in the observed effects of cross-clamping of the aorta may be explained by the degree of effectiveness of the bypass techniques utilized. Various factors may influence the effectiveness of the bypass techniques and the different changes that occur during aortic cross-clamping. The hemodynamic effects may be influenced by the size and malfunction of the external shunt, if such is utilized; i.e., the size of the cannula may be inadequate or the shunt might be kinked, which will affect the blood flow. Dr. Roberts and associates used a 7 mm in diameter Gott shunt. Similarly, the hemodynamic effects in femoral vein-femoral artery bypass, when such is utilized, can be influenced by the flow rates of the pump. The pump output may be influenced, in addition to other reasons, by the amount of venous return to the pump. Venous return may be inadequate if the venous cannula is not advanced into the inferior vena cava. The effectiveness of bypass is not confirmed and its potential not realized unless, in addition to other parameters, the arterial pressure in the right arm and leg and the left atrial pressure are measured and the bypass flow is monitored and adjusted to maintain balanced and effective perfusion of the vascular compartments above and below the aortic occlusion. The flow rates in the femoral vein-femoral artery bypass group studied by Dr. Roberts and associates were 33 to 40 ml/kg/rnin and in the Gott shunt group they were 35 ml/kg/min. Previously it has been shown, as cited by Dr. Roberts in his text, that these flow rates are inadequate and that a flow rate of more than 60 ml/kgymin is required to effectively unload the left ventricle.' Spinal cord function has been shown in experimental animals to be influenced by the duration of aortic crossclamping. Aortic occlusion of more than 30 minutes proportionally increases the incidence of spinal cord malfunction.' Spinal cord malfunction in patients is also influenced by the length of the aortic occlusion, by the presence or absence of collateral blood flow between the vascular compartments above and below the aortic occlusion, by the number of intercostal arteries interrupted and their significance to the perfusion of the spinal cord, and by the patient's preoperative, intraoperative, and postoperative hemodynamic status. Since so many variables influence the effects of crossclamping of the descending aorta, the authors' conclusion that

the supportive methods to be used during repair of descending aortic lesions should be individualized is valid. Its validity, however, according to other available data.v' cannot be adequately supported by the fact that the hemodynamic and renal effects of the various supportive methods that they studied were not greatly dissimilar. These effects, and particularly the effects on spinal cord function, in fact can be different if the employed methods are appropriately used. Rather, the individualization should be dependent upon the type of the aortic lesion to be corrected, the time required.to repair the lesion, and the patient's condition. Until more data become available, it appears that when the repair of an aortic lesion can be accomplished in less than 30 minutes the repair may be safely done without an adjunct in the majority of patients. Similarly, when good collateral blood flow between the vascular compartment above and below the aortic crossclamp is sufficient, repair of the aortic lesion can be done without an adjunct. However, in patients with an otherwise normal aorta in whom the aortic occlusion time will or may be more than 30 minutes or in patients with impaired cardiac function, effective bypass appears to be desirable. If no central nervous system or extensive pelvic injuries are present, femoral vein-femoral artery bypass appears to provide good support; otherwise, an external temporary shunt should be employed. Perhaps in the future the monitoring of the somatosensory evoked potentials or other modalities will discriminate which is the appropriate method for the repair of a given descending aortic lesion. Panagiotis N. Symbas, M.D. Atlanta, Ga.

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REFERENCES Gelman S, Reves JG, Fowler K, Samuelson PN, Lell WA, Smith LR: Regional blood flow during cross-clamping of the thoracic aorta and infusion of sodium nitroprusside. J THORAC CARDIOVASC SURG 85:287-291, 1983 Symbas PN, Pfaender LM, Drucker MH, Lester JL, Gravanis MB, Zacharopoulos L: Cross-clamping of the descending aorta. J THORAC CARDIOVASC SURG 85:300-305, 1983 Hug HR, Taber RE: Bypass flow requirements during thoracic aneurysmectomy with particular attention to the prevention of left heart failure. J THORAC CARDIOVASC SURG 57:203-213, 1969 Valiathan MS, Weldon CS, Bender HW Jr, Topaz SR, Gott VL: Resection of aneurysms of the descending thoracic aorta using a GBH-coated shunt bypass. J Surg Res 8:197-205,1968 Kouchoukos NT, Lell WA, Karp RB, Samuelson PN: Hemodynamic effects of aortic clamping and decompression with a temporary shunt for resection of the descending thoracic aorta. Surgery 85:25-30, 1979 Blaisdell FW, Cooley DA: The mechanism of paraplegia after temporary thoracic aortic occlusion and its relationship to spinal fluid pressure. Surgery 51:351-355, 1962 Katz NM, Blackstone EH, Kirklin JW, Karp RB: Incremental risk factors for spinal cord injury following operation

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for acute traumatic aortic transection. J THoRAc CARDIOVASC SURG 81:669-674, 1981 8 Crawford ES, Rubio PA: Reappraisal of adjuncts to avoid ischemia in the treatment of aneurysms of descending thoracic aorta. J THoRAc CARDIOVASC SURG 66:693-704, 1973 9 Najafi H, Javid H, Hunter J, Serry C, Monson D: Descending aortic aneursymectomy without adjuncts to avoid ischemia. Ann Thorac Surg 30:326-335, 1980

Response We recognize the important contributions that Dr. Symbas has made in the field of descending thoracic aortic surgery. In fact, early in the course of our research in this area, we were fortunate to have his assistance in the planning of certain aspects of our study. We did not include studies of either spinal cord function or anatomy in our experimental protocol. We acknowledge that such information is very important and have learned from the recent literature on this topic.':' We agree that the effectiveness of the bypass technique is critical in the analysis of experimental or clinical results. The factors which Dr. Symbas cites as influential in the performance of effective bypass are valid. The 7 mm diameter Gott shunt may be too small for adequate left ventricular decompression in some adult patients, but we believe it is more than reasonable in 25 kg dogs. The flow rate that we selected arbitrarily for femoral vein-femoral artery bypass was approximately 40 mljkgjmin. In contrast to the study in 1969 by Hug and Taber, which tested the efficacy of left atrial-femoral artery bypass, we did not observe marked elevation in LVEDP during aortic crossclamping in either the control group (no form of bypass used) or the partial femoral-femoral cardiopulmonary bypass group at the stated flow rates. In fact, the mean LVEDP during aortic cross-clamping for the partial cardiopulmonary bypass group was approximately 4 mm Hg, with no individual

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experimental values sustained at a level greater than 10 mm Hg. Whether higher bypass flow rates would yield different results cannot be determined from our study. We also have the clinical impression that descending thoracic aortic cross-clamping which lasts longer than 30 minutes is associated with greater neurologic sequelae related to spinal cord ischemia; however, it is not clear that currently available techniques used during cross-clamping can limit the magnitude of this damage. We agree with Dr. Symbas that the particular method used surgically to repair descending thoracic aortic lesions should be individualized. We acknowledge that recent experimental articles!" suggest that sodium nitroprusside may normalize hypertension proximal to the descending thoracic aortic cross-clamp, yet still be associated with paraplegia and evidence of unacceptable spinal cord and renal blood flow distal to the aortic occlusion. The examples of some of the clinical variables which might influence the choice of techniques used in operations on the descending thoracic aorta are well stated by Dr. Symbas, and we agree with their general importance.

Arthur J. Roberts, M.D. Gainesville, Fla. REFERENCES Gelman S, Reves JG, Fowler K, Samuelson PN, Lell WA, Smith LR: Regional blood flow during cross-clamping of the thoracic aorta and infusion of sodium nitroprusside. J THORAC CARDIOVASC SURG 85:287-291, 1983 2 Symbas PN, Pfaender LM, Drucker MH, Lester JL, Gravanis MB, Zacharopoulos L: Cross-clamping of the descending aorta. J THORAC CARDIOVASC SURG 85:300-305, 1983 3 Hantler CB, Knight PR: Intracranial hypertension following cross-clamping of the thoracic aorta. Anesthesiology 56:146-147,1982