Open surgery for thoracic aortic disease.

Many recent technical advances have enhanced the safety of open surgery of the descending thoracic aorta. Previous to the refinement of these adjuncts and techniques, surgeons such as Stanley Crawford showed that simple aortic cross-clamping with expeditious surgery produced the best results. In Crawford’s era of “clamp and go”, time limitations pressured surgeons to perform anastomoses rapidly with perfect haemostasis. The duration of aortic crossclamping was directly related to survival and to serious complications such as paraplegia and visceral ischaemia. The role of bypass, intercostal reimplantation, and cerebrospinal fluid drainage was unclear, in that none of these techniques appeared to be beneficial. Cross-clamping the aorta below the left common carotid artery and above the coeliac axis increases proximal systemic pressure, which in turn increases the cerebrospinal fluid pressure. In addition, the mean arterial pressure distal to the clamp will fall, and therefore the distal spinal cord will be at risk of ischaemia from a combination of decreased arterial perfusion and increased cerebrospinal fluid pressure. Logically, techniques that help to reduce the cerebrospinal fluid pressure and increase the distal arterial pressure will help to treat these two adverse factors, and will consequently lower the incidence of paraplegia. Cerebrospinal fluid drainage prevents elevation of the cerebrospinal fluid pressure, and several techniques can increase the distal arterial perfusion pressure, such as a simple shunt, partial heart bypass (from the left atrium or the pulmonary veins to the left common femoral artery) or full cardiopulmonary bypass. Currently, there is overwhelming evidence that keeping the cerebrospinal fluid pressure low is beneficial. Practitioners recommend that cerebrospinal fluid drainage be implemented whenever the aorta is clamped above the level of the coeliac axis. Arterial bypass techniques to perfuse the distal aorta allow sequential clamping of the aorta so that each segment can be replaced at a relatively leisurely pace compared to the dispatch demanded by “clamp and go”. Problems with each anastomosis can be dealt with immediately, using devices such as Teflon buttressed box sutures, so that all anastomoses are secure before the next segment of aorta is clamped and opened. Distal bypass assures good perfusion of the legs, viscera, and kidneys while the proximal anastomoses are performed. However, when the aorta bearing the coeliac axis, superior mesenteric artery, and both renal arteries is clamped and opened, none of these vital organs will be perfused. In this instance selective perfusion of the visceral arteries using an octopus system can minimise organ ischaemia. 4 Current studies attach an additional benefit to cooling the kidneys although the advantage to cooling the remainder of the viscera is somewhat controversial. The routine reimplantation of intercostals has further decreased the incidence of paraplegia. Some authorities use somatosensory evoked potentials or motor evoked potentials to identify the critical vessels for reimplantation, 6 but the general consensus is that the large intercostals in the region of T8–T12 must be reperfused. Aneurysms that involve the descending and abdominal aorta used to be exposed by completely transecting the diaphragm. However, this is not necessary in the majority of cases, and if avoided, patients have better respiratory function and recover more quickly. Additional gains have been achieved with the use of moderate hypothermia, in which patient temperature is permitted to drift downward to between 32–33°C.