The body measured 158 cm in length and weighed 80 kg. The heart weighed 600 g. The pericardium was smooth and shiny. The epicardium had focal petechial hemorrhages on the anterior surface consistent with resuscitation maneuvers. The atria were of normal size without thrombi. The right and left ventricular walls measure 0.6 and 1.6 cm in thickness, respectively. The valve leaflets were pliable and the chordae tendineae were thin. The endocardium had foci of hemorrhage. The myocardium was grossly normal. The coronaries revealed mild atherosclerosis with no occlusions. The aorta showed aneurysmal dilatation extending from the celiac artery to the iliac bifurcation. The aneurysm measured 19 cm in length by 8 cm in diameter and had a large defect with ragged edges measuring up to 15 cm in length. The wall of the aneurysm was very thin and covered with atherosclerotic plaques and areas of necrosis. There was an aneurysmal dilatation of the aortic arch measuring 4 cm in greatest diameter. The brachiocephalic artery was also dilated, and was occluded by a recent thrombus.
This man presented to the hospital with shortness of breath and severe abdominal pain of acute onset. Although he had tachycardia (138/min) suggesting hypovolemia, he was hypertensive (188/110 mmmHg), most likely due to peripheral vasoconstriction. On physical examination, a pulsatile mass in the left lower quadrant was evident. A CT scan of the abdomen and chest revealed an aortic aneurysm from the thorax to the common iliac arteries associated with retroperitoneal blood clot. He became unresponsive and had a cardiopulmonary arrest, after attempting stabilization he was taken to the operating room with the presumptive diagnosis of a ruptured aneurysm. The surgeons used a combined thoraco-abdominal incision because of the length of the aneurysm. Despite blood and fluid replacement, he had a cardiac arrest on the operating table, and could not be resuscitated.
The mortality rate of a ruptured abdominal aortic aneurysm that leads to emergency surgery can be as high as 90%. The outlook has improved recently, but still the mortality remains greater than 50%. In contrast, elective surgical intervention performed prior to rupture has a completely different result, with greater than 90% survival. Therefore, a primary goal in dealing with patients at risk for abdominal aortic aneurysm is the early diagnosis, and appropriate intervention. The issues we will discuss include the risk profile of patients, the etiology, anatomy and pathophysiology of aneurysms with and without rupture, and the complications beyond free rupture and hypovolemic shock.
An aneurysm is an outward 'bulge' of a vascular structure involving the full-thickness of the vascular wall. For instance, one can have an aneurysm of the left ventricle (a muscular vascular tube) following a myocardial infarction, with replacement of the full thickness of ventricular myocardium by scar tissue. Aneurysm development in arteries can involve the entire circumference of the vessel, in which case there is a diffuse or fusiform (cylindrical) aneurysm (Figures 70 and 71); or a localized bulge of a segment of the vessel known as a saccular aneurysm. Either type is susceptible to rupture or the other complications we will discuss. The term aneurysm is often applied to dissection of a vessel, but this is incorrect. A dissection is a partial tear or separation of layers within the vessel wall. It results in a pseudo-aneurysm, where there is a localized vascular rupture partially enclosed by surrounding tissue. Pseudo-aneurysm also can develop with a complete rupture of a vascular structure, and the tear site and hematoma are maintained within the surrounding tissues and organs (e.g.
pleura, pericardium, peritoneum). By this definition, there could be a rupture of a true aneurysm of the aorta, with a secondary pseudo-aneurysm, if the rupture site was walled-off by peritoneum, mesentery, or the vertebral column (Figure 72). The other use of the term aneurysm is applied to mycotic aneurysm (Figure 73). Mycotic aneurysm is a pseudo-aneurysm secondary to infection of the vascular wall. In general, the rupture site is an abscess with organizing inflammation. Mycotic aneurysm also can develop within a true aneurysm, following secondary infection. Any of these aneurysms, or pseudo-aneurysms has the potential to rupture.
A somewhat subjective aspect of aneurysms is how much dilatation of the vessel wall is required before it is called an aneurysm. With a focal bulge it is relatively straightforward; a saccular outpouching is easy to recognize and discriminate from the remaining vessel wall. In contrast, a diffuse or fusiform aneurysm may involve a segment of the vessel, or it may be a continuation of a generalized dilatation. The latter, known as ectasia, is a common phenomenon in elderly patients over the age of 75 years. The aorta dilates diffusely, and may even become tortuous. As a rule, it is preferred to retain the term aneurysm for focal bulges, whether saccular or fusiform, that are more than 25% over the adjacent vessel circumference.
Although aneurysms can occur anywhere in the vascular tree (e.g. intracerebral arteries, thoracic aorta, etc), abdominal aortic aneurysms have the highest incidence, and are the most likely to have a fatal rupture. Even the familiar shorthand term for abdominal aortic aneurysms, AAA or 'triple A', reflects how common it is. The location of the aneurysm, and the profile of the patient often provides significant clues as to the etiology of the condition. AAAs are predominantly the result of severe atherosclerosis of the aorta. Thus, the risk factors for AAA should be the same of those for atherosclerosis. If this is the case, why is it that some individuals with severe atherosclerosis develop coronary or cerebral complications, while others develop aneurysms of the aorta? Are the risk factors for atherosclerosis a sufficient explanation for the development of AAA? Recent information strongly suggests the answer is a resounding no!
Although aneurysms can develop anywhere in the aorta, the ones due to atherosclerosis are far more common in the abdominal aorta, particularly distal to the renal arteries. They may extend into the iliac arteries as a continuous lesion, or there may form separate iliac aneurysms. Atherosclerosis of the abdominal aorta tends to increase in severity from proximal to distal. The explanation for this is uncertain, although it has been suggested that it is a phenomenon of human upright posture and gravity. The increased vascular wall injury develops secondary to the effects of the pressurized column of blood within the aorta (until the time when humans could live permanently in low gravity conditions, this hypothesis will not be tested). However, what is definitely known, is that there is a significant association of AAA with systemic hypertension, smoking, diabetes mellitus and older age (aneurysms are rare before the 6th decade, and are mos common between the 7th and 9th decades). Even though these are all risk factors for atherosclerosis, there is a disproportionate ratio of male gender, and a known familial predisposition. The presence of an AAA in a sibling or parent, significantly increases the risk for that individual. This familial tendency and male predominance strongly suggests that additional unrecognized entities are interacting with the more common and generally modifiable risk factors, which contribute to the development of atherosclerotic AAA.
Molecular defects that impact on the structural integrity of the vessel wall might explain the known link between aneurysms and several relatively rare genetic disorders. Osteogenesis imperfecta, a disease of the skeleton due to a defect in collagen type I (the primary fibrillar component of collagen), is associated with aortic aneurysms. The basis for Ehlers-Danlos type IV syndrome is a defect of collagen type III (the reticular component of collagen), and it presents with skin, connective tissue and cardiovascular system manifestations, including aortic aneurysms. On the other hand, defective collagen resulting from inadequate cross-linking, due to the effects of (the active ingredient in sweat-pea that has been associated with a clinical and experimentally-induced condition known as lathyrism), typically leads to aortic dissection in man and laboratory animals. Therefore, although collagen abnormalities may be associated with aneurysms, it is unclear whether genetic, acquired or both types of collagen defects explain the pathogenesis of aortic aneurysms. It is instructive to recall that for more than half a century the etiology of Marfan's syndrome and aortic dissection was thought to be defective collagen, until the defect was identified in a microfibrillar glycoprotein component of elastic tissue, called fibrillin.
There are two other pathogenic considerations that have received interest recently, but before we discuss them we should address the pathological changes that occur within aneurysms. The media of the aorta is an elastic structure composed of approximately 40 stacked layers of elastic tissue fibers, smooth muscle cells, and collagen, called lamellae. There are also vessels in the outer third of the tissue (vasa vasorum), that provide blood supply to the inner media (diffusion of oxygen from the lumen through the tissue is insufficient). Atherosclerosis is initially an intimal disease. However, with increasing severity, it extends into the media where it replaces the lamellar structure with plaque, debris, fibrous tissue, calcification, and inflammation. The loss of elastic lamellae leads to a weakening of the tensile strength of the aorta. If there is only localized atherosclerosis, there may be no adverse consequences; but when it becomes diffuse and involves the full aortic wall thickness, there may be aneurysm formation. The disruption of the tissue by plaque, can also affect the vasa vasorum and their ability to supply blood to the media, thereby causing further damage to the wall. The degenerative process resulting from atherosclerosis is the primary cause of aneurysms; but as we have seen, there appear to be other elements that contribute to the breakdown of the media, and the weakening of the aortic wall.
One potential pathogenic mechanism may be related to the activation of lytic enzymes in the aortic atherosclerotic plaque. Similar to the transformation of a stable coronary artery atherosclerotic plaque into an unstable one, with plaque rupture and coronary thrombosis, the atherosclerotic AAA tissue breakdown can be associated with enzyme activity derived from inflammatory cells in the plaque. In particular, collagenases and elastases may play a significant role in media destruction and mural weakening. There appears to be variability in the degree of inflammation in atherosclerosis regardless of site. Some patients seem to have more inflamed plaques than others, making them more susceptible to atherosclerotic complications. Whether this inflammatory activity is genetically determined is unknown. Another potential etiology is related to the possibility that atherosclerotic plaques may become sites of secondary infection. Bacteria can lead to an inflammatory response and a secondary release of collagenases and elastases from inflammatory cells. Furthermore, they can also elaborate their own collagenases with direct degradation of tissue, which may lead to bacterial spread and abscess formation. Gram negative bacteria, and particularly Salmonella species, have a predilection for the debris and surface thrombus associated with atherosclerotic plaques in the aorta (Salmonella have been cultured from surgically repaired aortic aneurysms). Whether, any or all of these potential etiologies is working separately or together to cause aneurysms is unknown. But, since a number of these mechanisms may be prevented or modified by treatment, medical intervention to avoid aneurysm expansion and possible rupture may be feasible in the future.
Size and expansion is the critical issue with abdominal aortic aneurysms. The diameter of the aneurysm is directly proportional to its likelihood of rupture. Also, whether it is stable or progressing over time, determines its rupture potential. Since both of these parameters can be assessed non-invasively with ultrasound, CT scan, or magnetic resonance imaging (MRI), patients can be followed longitudinally to determine when, or if, it is appropriate to intervene therapeutically. What is the critical size? The normal aorta has a diameter of 2.5-3.0 cm. An aneurysm of 4.0-6.0 cm is at a critical size; beyond that, the potential for rupture increases significantly each year. Furthermore, an unstable aneurysm that expands progressively at a rate greater than 0.5 cm per year is at risk for rupture. A 4.0-6.0 cm aneurysm is generally palpable in most individuals; therefore physical examination is an important screening test. The size of the aneurysm and its growth are determined by the mechanisms discussed above. However, there are also physical factors that impact on the likelihood of rupture. As the aneurysm expands, the increased diameter affects the wall stress and potential for rupture. The relationship is explained by LaPlace's law:
Wall Stress = Pressure X Radius 2 X Wall Thickness
Thus, the radius or diameter of the aneurysm is directly proportional to the wall stress (a wall that is already weakened by atherosclerotic degeneration). Essentially, the larger the balloon, the more likely it is to burst. It is critically important to intervene before this occurs, for reasons that became evident in this case.
There are complications of aneurysm in addition to fatal rupture. Aneurysms frequently thrombose and the thrombus may fragment and embolize into the distal circulation of the lower extremities leading to acute ischemia. If the aneurysm is fusiform, and the thrombus continues to aggregate, there may be partial or complete obstruction of the lower abdominal aorta. This can cause lower extremity ischemia, intermittent claudication, or gangrene. When there is lower extremity ischemia involving the thigh and below, in association with impotence, the Leriche syndrome (obstruction of the distal aorta) must be considered. In the absence of significant thrombosis, the ulcerative, atheromatous surface of an aneurysm may be the source of cholesterol and plaque emboli. These usually affect smaller vessels in the distal extremities, but may lead to toe and foot gangrene, or the so-called 'trash foot'.
There are several other complications associated with localized rupture. With progressive atherosclerosis, the abdominal aortic aneurysms often develop inflammation and fibrosis of the adventitia. The inflammatory response may induce the aorta to adhere to the surrounding tissues or organs, which include the intestine, vena cava, and vertebral column. The abdominal aorta is a retroperitoneal structure. It lies immediately posterior to the 3rd and 4th portions of the duodenum, proximal to the ligament of Treitz. The duodenum in this location crosses the aorta at the level of the aortic bifurcation into the iliac arteries. Accordingly, this site, which is the most prevalent for atherosclerotic aneurysms, is susceptible to the rupture of an aneurysm into the intestine. This is known as an aorto-enteric fistula. It is virtually always fatal, with exsanguinating gastrointestinal hemorrhage. Rarely, it may produce a slow leak, and be a cause of chronic anemia. If the aortic aneurysm becomes adherent along its lateral wall, it can rupture into the vena cava, causing an arteriovenous fistula, and high output congestive heart failure. Finally, if the aneurysm extends posteriorly, either with or without rupture, it can lead to vertebral column erosion and severe back pain.
Several comments about diagnosis and treatment. As noted above, aneurysms at a critical size (over 5.0 cm) can be palpated with careful examination of the abdomen at the level of the umbilicus. Palpation of a mass, with characteristic lateral pulsations, is strongly supportive of the diagnosis. It also is noteworthy that aneurysms susceptible to rupture often have slow leakage over days to weeks prior to massive rupture. This can lead to inflammation and irritation of nerves in adventitial tissues, thereby causing back or flank pain. When aneurysms do rupture, the blood generally tracks in the retroperitoneal tissues, causing flank ecchymoses, scrotal hematoma, and inguinal hematoma. Most often, patients with these signs will also be in shock. If the aneurysm ruptures anteriorly, it can 'break through' the peritoneal membrane, and cause massive hemoperitoneum. This is rapidly fatal.
Treatment is dependent on the size and the rate of growth of the aneurysm, and the presence of complications such as thrombosis and embolization. The approach for the last 5 decades has been prosthetic graft bypass of the aneurysm with interposition of a woven, artificial vessel between the aorta and the iliac or femoral arteries. The graft material is usually Dacron, and it functions very effectively with minimal risk of thrombosis. Even though it is a prosthetic material, which is potentially thrombogenic, its large diameter, and the ingrowth of the patient's own cells through the porous spaces (a de novo endothelial lining) between the weave fibers prevent thrombosis. For many years the aneurysm was actually resected; currently, the aneurysm is opened and the graft is placed over the aneurysm bed. The major complications of surgery are related to the time required to cross-clamp the aorta. Since most aneurysms occur below the renal arteries, the potential for renal ischemia is minimized. Clamping above the renal vessels puts the kidney at risk for ischemic injury. Other rare ischemic complications include spinal cord damage with hemiplegia due to interruption of vessels supplying the cord, and left colon ischemia due to lack of flow in the inferior mesenteric artery. Both are unusual, because of collateral blood supply that efficiently develops over time, triggered by the longstanding atherosclerotic occlusion of the larger vessel. Other standard surgical complications are bleeding and infection. Moreover, a major cause of morbidity and mortality, is related to cardiac and cerebral ischemia, since patients with AAA, invariably have systemic atherosclerosis.
As a final point, it should be mentioned that a new approach to aneurysm 'repair' is becoming prevalent in selected patients. Aneurysms without significant atherosclerotic stenosis of iliac arteries can be treated with endovascular grafting. Tubular, telescoped grafts can be introduced into the aorta from the femoral artery under radiographic control, where they can be extended from the normal aorta superior to the aneurysm to the iliac arteries. They function as a conduit across the aneurysm, thereby eliminating the risk of aneurysm rupture, and any of the surgical complications associated with repair. It is likely that as with the growing use of stents to maintain coronary patency following angioplasty, endovascular aortic grafting will rapidly become a popular approach to dealing with the common, and dangerous AAA.
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Figure 71 (right). Fusiform aneurysm of the abdominal aorta. The aneurysm has been opened to reveal a large thrombus.
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