During the past ten years, considerable progress has been made in improving surgery of the thoracic aorta. In most instances, it is now technically possible to reconstruct any or all of the thoracic aorta thanks to the availability of impermeable grafts and the routine use of antifibrinolytic agents. The morbidity and mortality of elective aortic surgery continue to fall, and are now comparable to results following valve surgery. Since each area of the aorta warrants special consideration because of unique factors with regard to operative indications and issues related to organ preservation, the discussion which follows will be divided to allow separate consideration of the ascending aorta, the aortic arch, and the descending thoracic and thoracoabdominal aorta. Complete comprehensiveness is not possible in a brief review, so the special circumstances surrounding specific entities requiring aortic surgery, such as acute dissection, will not be addressed, and only general issues will be considered.

ascending aorta:

There is now a reasonable consensus that an ascending aortic diameter >6 cm poses a sufficient risk of dissection or rupture that elective resection should be recommended; in asymptomatic patients with Marfan's syndrome, most surgeons would operate when the ascending aortic diameter exceeds 5 cm. Notwithstanding this consensus, however, it should be noted that there is little objective data on which this almost universal recommendation is based.

There are more data to support the general acceptance of the idea that when dilatation of the ascending aorta involves the aortic sinuses and the aortic valve, the best operation is a modified Bentall procedure, in which the coronary arteries are attached to a valved conduit by means of buttons of aortic tissue, and the distal anastomosis is carried out open, during a brief interval of hypothermic circulatory arrest (HCA). In older patients, a bioprosthetic valve within the conduit is often an attractive option. Alternatives in which the valve is spared while the surrounding aorta and the aortic sinuses are replaced are promising but unproved. Similarly, the appeal of the Ross procedure, in which the aortic valve and ascending aorta are replaced by the patient's own pulmonary valve and main pulmonary artery (with a homograft then necessary in the pulmonary position) should perhaps be resisted, at least in patients with aneurysms, until more centers are able to replicate the excellent long-term results achieved by the pioneers of this approach.

Only rarely is a separate ascending aortic graft and aortic valve replacement justified. However, in older patients, in whom an ascending aortic aneurysm may begin above the sinotubular junction, isolated replacement of the ascending aorta, with or without a simple valvuloplasty, is an excellent option.

aortic arch

Indications for operation of the aortic arch include symptoms (pain or hoarseness), and dilatation of the aorta to a diameter exceeding 6 cm. Most commonly, aortic arch surgery is required because of extension of an aneurysm antegrade from the ascending aorta or retrograde from the descending aorta. The major concerns of aortic arch surgery revolve around how to avoid diffuse cerebral injury, manifest immediately after surgery as transient prolonged postoperative obtundation, lethargy or confusion, and later as obvious cognitive deficits or impaired performance on formal psychometric testing. HCAF is the standard method for cerebral protection during aortic arch surgery, but a consensus is gradually emerging that absence of diffuse injury can only be guaranteed for 30 minutes, particularly in the older patient, with HCA carried out even under optimal circumstances: when cerebral metabolic rate has been reduced to a minimum with cooling to 12 - 15C (esophageal temperature), reflected by jugular venous saturations exceeding 95% before arrest of the circulation. Although many patients appear to recover fully following longer intervals of HCA, sometimes exceeding 60 minutes, complete safety cannot be assured for durations of HCA exceeding 30 minutes. When surgery requires that the aortic arch remain open longer, selective antegrade cerebral perfusion seems to offer the best hope of avoiding cerebral injury. Although retrograde cerebral perfusion (RCP) has been embraced enthusiastically by many aortic surgeons and holds out some promise of enhancing cerebral protection, unequivocal laboratory or clinical evidence that it safely and significantly prolongs the permissible interval of HCA does not exist. Protecting the brain from focal embolic injury during aortic arch surgery is also a major concern. Focal injury occurs more frequently in older patients and in those with loose atherosclerotic debris in the aortic arch. The surgeon can have an impact on the incidence of focal cerebral injury by avoiding cannulation and clamping techniques which will dislodge such debris and by taking care to clear the arch and cerebral vessels of potentially embolic material before reinstituting antegrade perfusion. In this context, use of RCP may help to remove loose atheromatous debris from the arch vessels during surgical manipulation, but it probably does not dislodge emboli which have already gained access to small intracranial vessels. Further improvements in cerebral protection may come from use of additional strategies, such as evoked potential monitoring, to assure that cerebral metabolism is at a minimum before the circulation is arrested; use of pharmacological inhibitors of cerebral metabolism during circulatory arrest, and minimization of the consequences of reperfusion injury by mitigating the impact of harmful substances such as excitotoxic amino acids and free radicals.

descending thoracic and thoracoabdominal aorta

The indications for operations in the thoracic and thoracoabdominal aorta include pain, irregular dilatation of the aorta suggestive of localized wall weakness, and rapid rate of expansion. Because there is now good evidence that age, the presence of chronic obstructive pulmonary disease, and significant dilatation of the abdominal as well as the thoracic aorta influence the likelihood of rupture, a decision to resect an aneurysm based solely on the presence of a single diameter in the descending thoracic aorta >6 cm is no longer acceptable.

The central issue in operations on the descending thoracic and thoracoabdominal aorta is the preservation of spinal cord integrity. Although multiple factors and techniques have been advocated to avoid paraplegia or paraparesis, there is widespread consensus regarding the value of only three: use of active distal perfusion during aortic crossclamping; cooling to moderately hypothermic temperatures (31-33C) to reduce metabolic requirements, and administration of steroids perioperatively. Other techniques which have been recommended to reduce the incidence of spinal cord injury include: gradual serial intercostal sacrifice with monitoring of somatosensory evoked potentials; preoperative identification of the artery of Adamkiewicz and its selective reimplantation; blind implantation of peridiaphragmatic intercostal arteries; use of HCA, and cerebrospinal fluid drainage intraoperatively and postoperatively. Monitoring of spinal cord function by means of somatosensory, motor, and direct spinal cord evoked potentials is under active investigation and offers the advantage of assessing the behavior of the spinal cord in each individual patient during and following operation, as well as contributing to a deeper understanding of the mechanisms and timing of spinal cord injury which should lead to development of better strategies to avoid its occurrence in the future.

As more extensive procedure involving the abdominal aorta become more frequent, methods for providing intraoperative protection of visceral function are receiving increasing attention: there is a consensus emerging in favor of selective visceral perfusion using balloon-tipped catheters in the open aorta.

Endovascular treatment of thoracic aneurysms is an emerging possibility, building on techniques developed for use in infrarenal aneurysms: because the insult of conventional surgery in the frail elderly population generally afflicted with aortic aneurysms takes a considerable toll, nonsurgical options are extremely appealing. Most of the technical problems of deployment and fixation of the graft are probably amenable to solution, although current efforts are being challenged by difficulties in assuring stable fixation both proximally and distally, and in obtaining a secure seal in an aorta which may be continuing to expand. Enough experience with endovascular stents has accumulated to confirm that the simultaneous occlusion of a large number of intercostal arteries using this technique can also, unfortunately, result in paraplegia.

Conclusions

The many improvements which have occurred in aortic surgery during the past twenty years now allow elective replacement of most portions of the aorta with an acceptable risk. Future challenges include development of more refined operative indications which will allow surgery to be carried out only in those patients in whom rupture or dissection is truly imminent, and evolution of a better understanding of intraoperative ischemic injury so that central nervous system and other complications following surgery can be further reduced.

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