Indications,

PERIOPERATIVE

MANAGEMENT, AND

LONG-TERM RESULTS

Within the last 50 years, the entity currently known as aortic dissection has evolved in importance from an incidental curiosity at autopsy into a frequently fatal, antemortem diagnosis which continues to challenge and frustrate the best therapeutic aspirations of physicians and surgeons. Although the natural history of untreated patients with aortic dissections continues to be debated [1], aortic dissection is a fatal disorder among the vast majority of patients unless the con­dition is recognized and treated promptly. In Hirst's carefully analyzed, large-scale, and representative autopsy series, 21 percent of patients succumbed within 24 h, 37 percent by 48 h, 49 percent by 4 days, and 74 percent by 2 weeks [2]. Moreover, aortic dissection is the most commonly occurring clinical catastrophe involving the aorta, far overshadowing the incidence of ruptured atherosclerotic abdominal aortic aneurysm [3]. Further, the incidence of the disease may be increasing in industrialized portions of the world (approaching 20 cases per mil­lion population per year in one study from Memphis) [4].

Effective surgical therapy has been available for almost 30 years [5]; however, rigorous scientific proof of the efficacy of surgical treatment is nonexistent. No controlled studies comparing medical with surgical therapy have been performed, and it is unlikely that any will be designed and implemented in the future because of logistic and philosophic constraints. Thus, the rationale behind current medical and surgical treatment regimens remains mostly empirical. Despite these

scientific limitations, a great deal of clinical progress has been accomplished since I960. Clinicians today have a much greater awareness of the protean manifesta­tions of aortic dissections [6] and understanding of the pathogenesis of the disease [7-9]. General consensus exists regarding the most optimum plan of treatment (medical vs. combined surgical and medical) for certain subsets of patients, e.g., those with acute type A dissections, but controversy still surrounds the choice of treatment for other patient subsets, e.g., those with acute type B dissections [9-13]. The purpose of this chapter is to outline the details of the Stanford surgical strategy for patients with acute and chronic aortic dissections, including nomen­clature, indications, operative features and technical points of debate, periopera-tive management, and long-term results. The goals and limitations of surgical treatment of patients with aortic dissections will be emphasized throughout this essay for perspective.

EVOLUTION OF SURGICAL THERAPY

Gurin is credited with the first surgical attempt to palliate a patient with an aortic dissection in 1935 [14]. The patient's acute ischemia of a lower extremity was relieved by surgically creating a "fenestration" between the true and false lumen of the dissected abdominal aorta. This decompressed the false lumen which had been extrinsically occluding the iliac artery. Although a femoral pulse was restored, the patient died of other complications shortly thereafter. Early investi­gators also attempted fenestration procedures [15], and others wrapped the dis­section with cellophane in an attempt to prevent rupture [16], but it was not until after the hallmark 1955 paper by DeBakey, Cooley, and Creech [5] that many surgeons began exploring direct techniques to salvage these patients. By 1965, the Houston group had operated on 179 patients with aortic dissection and reported a 21 percent operative mortality rate [17]. This encouraging report spawned wide­spread enthusiasm for surgical treatment of these patients, but few groups could duplicate this relatively low mortality rate [18-21]. Retrospectively, it was recog­nized that this discrepancy in outcome was probably related to the fact that DeBakey's series was markedly skewed; 68 percent of cases were chronic and 79 percent were type III (or type B), which did not involve the ascending aorta and the transverse aortic arch [18]. The relatively inferior surgical results attained at other institutions, however, prompted the development and introduction of mod­ern medical therapy by Wheat et al. based on reducing peak systolic and mean arterial pressure, diastolic recoil pressure, and aortic dp/dt [18]. Wheat's initial experience was favorable in 6 selected patients (no deaths compared to a 100 percent mortality rate in 6 previous surgically treated patients), but a subsequent report in 1969 revealed a 50 percent mortality rate over an average follow-up interval of 3.8 years [21]. As the limitations of medical therapy (including drug-related morbidity and side effects, late attrition rates, contraindications to drug therapy, lack of compliance on the part of the patient, and the need for late surgical intervention) became appreciated, other institutions embarked upon selective

treatment protocols, including surgery, for patients with complicated dissections and those with dissections involving the ascending aorta (type A) [10,11,20,22,23). Parallel refinements in cardiovascular surgical techniques, improvements in car-diopulmonary bypass technology, and increased cardiologic, anesthetic, intensive care, and nursing expertise gradually led to lower operative mortality and mor­bidity rates. Subsequently, the pendulum has again swung back toward aggressive surgical treatment of most patients with acute aortic dissections and selected patients with chronic dissections [9-11,13,22-24].

Despite the substantial number of clinical reports in the literature, data sup­porting medical or surgical therapy based on large, representative patient popu­lations analyzed comprehensively in meaningful (time-related) statistical terms were previously scarce. As noted in Chapter 1, T.B. Peacock clearly described 140 years ago a dramatic difference in prognosis pivoting on whether the dissection involved the ascending or the descending aorta [25]; moreover, Hirst's [2] and Shennan's [26] pathological investigations vividly demonstrated the rapidly lethal nature of acute dissections compared to chronic cases. Unfortunately, these salient points were not utilized in the critical analysis of results of modern therapy until Daily and his colleagues from Stanford [22] in 1970 and the Peter Bent Brigham Hospital group [23] reminded us of the importance of strict definition of the patient population under study. Similar lack of critical insight led to misinter­pretation of the implications of other early reports, as alluded to above [17-20]. Furthermore, valid interinstitutional comparative analysis has been handi­capped by the fact that more meaningful actuarial (or life table) statistics have been employed in only two articles [10,11].

TERMINOLOGY AND CLASSIFICATION

For the purposes of clarity and to obviate misrepresentation, the Stanford classification scheme will be briefly described. This is a functional classification

system predicated upon involvement or lack of involvement of the ascending aorta, irrespective of the site of the primary intimal tear and regardless of the distal extent of the dissection (Figure 8-1). As well-known for over a century [2,25,26], it is ascending aortic involvement that determines the biological behavior and gov­erns the potential lethality of dissections; moreover, this functional classification system simplifies clinical decision making and facilitates comparative analysis. As illustrated in Figure 8-1, dissections resulting from tears in the aortic arch or proximal descending aorta may propagate retrograde (to involve the ascending aorta) (Figures 8-l£, c, and /) or antegrade (to involve the descending aorta) (Figure 8-lrf) or both (Figure 8-la) (27J.

Daily first described this classification system in 1970 [20], and it has slowly gained broad conceptual acceptance [10,12,13,24,28]. Recently, DeBakey has stated that his type I classification encompasses all patients with dissections in­volving the ascending aorta, whether the tear is in the ascending aorta, the aortic arch, or the descending aorta [29]. Reul and Cooley subdivided DeBakey's

three-type (I, II, HI) nomenclature into six types (I, II, Hl-a, IH-b, III-c, Hl-d) to account for antegrade and retrograde progression [30], and other groups continue to prefer to use two primary subdivisions, "proximal" and "distal" or "ascending" and "descending" [12,31]. The simple Stanford classification scheme applies equally well to these possibilities [32,33]. In our opinion, whether or not the lissection involves the ascending aorta is of critical, prime importance; the actual site of the primary tear and the longitudinal extent of the dissection represent complimentary pathoanatomic information that is of secondary importance.

GOALS AND LIMITATIONS OF SURGICAL TREATMENT

It is implicit that operative intervention does not cure the disease called aortic dissection, but in certain circumstances (e.g., patients with acute type A dissec­tions) appropriate surgical treatment can salvage at least 85 to 92 percent or more of patients [24,34,35] who otherwise have less than a 20 percent chance of sur­viving [1,2,11,13]. Even without the potential of "cure," such results represent excellent palliation and prolongation of life expectancy. Surgery can offer such propitious results because the appropriate limited, conservative operation (see below) in properly diagnosed patients eliminates the leading cause of death, even though diseased aorta and various pathoanatomic derangements remain distally. In the case of acute type A dissections, 75 to 85 percent of deaths in untreated or medically treated patients are due to local complications, such as pericardial rupture with cardiac tamponade, fulminant left ventricular failure due to acute aortic regurgitation, and acute myocardial ischemia and/or infarction due to compromise of the coronary circulation [1-3,26]. All these complications can be prevented by successful surgery even though nothing is done directly to resect or repair the distal pathological injury, which frequently extends around the arch and into the abdominal aorta [11,13,29,33]. In the case of patients with acute type B dissections, the majority (greater than 60 percent) of deaths are due to local rupture into the pleural space (see Chapter 2) [1-3,26]. Hence, simple surgical replacement of a relatively short length of the descending aorta which is the most severely traumatized segment can obviate the most frequent causes of death [10,11,13,33]. The logic of these approaches pivots upon the premise that the most frequent causes of death are prevented and the likelihood that the distal false lumen will thrombose is higher after the primary intimal tear has been resected.

The fact that currently accepted surgical techniques do not entail extensive resection and replacement of all or most of the damaged aorta in cases of acute dissections explains in part some of the limitations of surgical treatment. The initial dissecting process usually creates multiple distal reentry points or fenestrations which form communications between the true and false lumens. The number and individual size of these fenestrations are largely unpredictable (see Chapters 2 and 6) [2,26]. They result from either spontaneous reentry into the true lumen at the terminal end of the dissection (Figure 8-2a) or from an aortic tributary being sheared off as the dissection propagates down the descending thoracic and abdom­inal aorta (Figure 8-26). When the primary intimal tear is resected and the false lumen obliterated surgically, it is hoped that rediversion of antegrade flow into the true lumen will compress the false lumen toward the adventitia from within and reduce the likelihood that the false lumen will remain patent; however, blood flow can enter the false lumen of the distal aorta via one or more of these distal fenestrations. This is probably the principal cause of a persistently patent false lumen postoperatively [28,36-39]. Such postoperative circumstances can give rise to saccular aneurysms located far distal to the site of the initial aortic repair [10,29,36,38,39]. These secondary saccular false aneurysms can enlarge and/or rupture [ 10,29]; this usually occurs years after operation, but also can occur in the

immediate postoperative period. Many of these false aneurysms require late reoperation.

Simple central redirection of flow into the aortic true lumen usually results in restitution of flow into most important aortic tributaries in cases of acute dissec­tions. As shown in Figure 8-2, the usual cause of compromise of a branch of the aorta is extrinsic compression of the true lumen by the false lumen, which can either be patent or thrombosed (Figure 8-2c and 24). If, on the other hand, the orifice of the compressed true lumen of an important aortic branch thromboses over time (Figure 2e), it is highly unlikely that flow from the aortic true lumen will reperfuse the true lumen of this branch vessel postoperatively. Furthermore, if the dissection reenters into the proximal segment of a tributary thereby perfusing the tributary only from the aortic false lumen (Figure 8-2/), the sheared offostial flap can heal in this configuration; thus, the organ(s) supplied by this branch has (have) become totally dependent on flow from the aortic false lumen. If surgery is under­taken in the chronic phase of the disease and all antegrade flow in the aortic false lumen is obliterated by the surgical repair (a relatively infrequent occurrence) [28,36-39], it is obvious that there will be no blood flow in this branch postopera­tively. If this tributary happens to supply major abdominal viscera, a kidney, or the spinal cord, then life-threatening organ ischemia can occur immediately post­operatively. This potential problem represents a recently appreciated incremental risk factor for patients undergoing operation in the chronic phase of the disease,

and explains how a "successful" thoracic aortic procedure can potentially result in the loss of peripheral pulses [28,32]. Concern over this potential (and, fortunately, rare problem) has prompted one major cardiovascular surgical group to redirect distal flow purposely into both the true and false lumens in certain patients with chronic dissections [28].

Since it is technically feasible to resect and replace the entire thoracoabdominal aorta or the entire ascending aorta and transverse aortic arch, a logical question is: Why isn't this done in cases of extensive aortic dissection? Most cardiac surgical groups limit the length of descending aorta replaced to reduce the chance of paraplegia [10-12,28]. In cases of type A dissections, essentially all the aorta between the innominate artery and sinuses of Valsalva is replaced; if concomitant aortic valve replacement (AYR) is performed, then the sinuses are also replaced [10,11,28,34]. To attempt simultaneous replacement of the entire aortic arch is not prudent as a routine procedure due to the high mortality rate of such a formidable operation [10-12,24,34,35,40].

Despite these technical and theoretical limitations, it should be apparent that the customary conservative surgical procedures obviate the most common causes of death in cases of type A as well as type B dissections without leaving the patient at an unacceptable risk of late serious sequelae [10-12]. This suggests that the mechanism by which long-term survival is proffered by surgical treatment is resection of the segment of aorta most severely damaged (containing the primary intimal tear if possible), rather than permanent obliteration of blood flow in the distal false lumen [28]. This point also underscores the necessity for indefinite careful medical follow-up and treatment in all operative survivors, with particular attention on detection of late false aneurysm formation prior to rupture [29].