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Ann Thorac Surg 1997;64:388-389
© 1997 The Society of Thoracic Surgeons
Department of Cardiothoracic Surgery, School of Medicine, University of Cape Town, 7925 Cape Town, South Africa e-mail: uvonopp{at}thoracic.cts.uct.ac.za
Sweeney and coworkers are to be congratulated on their excellent results: an incidence of new paraplegia of 1.5% and mortality of 12.6% after repair of acute descending aortic transections using only simple aortic cross-clamping. However, the conclusion drawn by Sweeney and co-workers that "the simple "clamp-sew" technique is a safe and effective method for the treatment of traumatic aortic transections" can be misleading, as 94.4% of their patients had cross-clamp times less than the reported critical 30 minutes. Their average aortic cross-clamp time of 24 minutes (range, 14 to 36 minutes) is significantly shorter than the international average time of 41 minutes (range, 13 to 79 minutes) required to repair this lesion [1]. Hence, the following caveat should be added to their conclusion, "provided the surgical repair is completed within 30 minutes of aortic cross-clamping." It is presumed that their shorter average cross-clamp time was as a result of both their institution treating a large number of aortic transections annually (on average 9.4 per year, compared with the norm of 2.6 per year [1]) and concentrating experience in the hands of 4 attending surgeons as surgical expertise is a known variable [2], as well as routine insertion of an interposition graft.
Sweeney and co-workers provide no results to substantiate the statement that "the development of paraplegia relates to many variables, the least of which may be clamp time or prevention of ischemia to the spinal cord with a shunt." The published metaanalysis of 1,492 patients [1] has reported a stronger statistical association between postoperative new paraplegia and the variable "spinal cord ischemia," albeit univariate, than any other published variable [1]. The results of this metaanalysis predict that each individual patient of Sweeney and co-workers was exposed to a variable paraplegia risk ranging from 1.1% to 23.1% depending on actual individual cross-clamp time, within their 14- to 36-minute range of cross-clamp times (Fig 1
). This increasing risk associated with longer cross-clamp times during simple aortic clamping contrasts to the low 1.3% predicted risk of paraplegia associated with cross-clamp times of up to 40 minutes if active distal perfusion is used (see Fig 1) [3].
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) [1], as ischemia-related paraplegia can occur within this reported "safe" time. The maximum safe period of total occlusion of the major radicular artery in the absence of any collateral flow is 15 to 18 minutes [4], which can occur if this artery originates in the aortic segment isolated by the aortic cross-clamps (the major radicular artery originates immediately distal to the isthmic rupture, between T-5 and T-8 in 12% to 15% of patients [5, 6]). Hypotensive perfusion (<40 mm Hg), which occurs in the distal aorta during simple aortic cross-clamping, only extends this "safe" period to between 24 minutes (earliest reported time of paraplegia with simple aortic cross-clamping) and 30 minutes (statistical significance) [1, 3, 7, 8]. The efficacy of providing distal perfusion is also technique dependent: "active" or "passive" distal perfusion (see Fig 1) [1, 3]. "Active" distal perfusion (the method reported to have the lowest average risk of paraplegia regardless of duration of aortic cross-clamping) can also be provided without the use of systemic heparin by means of a heparinless centrifugal pump or heparinless femoral venoarterial bypass without an oxygenator [1, 3]. Sweeney and co-workers correctly state that "it is wisest for surgeons to use the techniques with which they are most efficient," but it is also equally important for surgeons to fully understand both the risks and benefits as well as the physiologic limitations of techniques they may elect to use.
References
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  E. A. Hessel Bypass Techniques for Descending Thoracic Aortic Surgery Seminars in Cardiothoracic and Vascular Anesthesia, November 1, 2001; 5(4): 293 - 320. [Abstract] [PDF]
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