Historical Background
Acute dissection of the thoracic aorta was first described by Nichools (1699-1778) in referring to the autopsy findings of King George II. Traditionally, acute uncomplicated type B dissection has been managed medically with an effective medical regimen directed toward lowering blood pressure and dP/dt first introduced by Palmer and Wheat in 1967. The 30-day mortality for acute uncomplicated type B dissection decreased from 40% in the 1960s to less than 10% at present. However, between 20% and 30% of patients remain at risk for developing a delayed aneurysm, with catastrophic aortic rupture in approximately 18% of patients. Dialetto and associates reported that 28.5% of uncomplicated type B dissections managed medically progress to aneurysmal dilatation at a mean of 18.1 ± 16.9 months. Risk factors for aneurysmal degeneration were patency of the false lumen and distal aortic diameter of 40 to 45 mm or larger.
The first attempt at surgical repair was reported in 1935 by Gurin and colleagues, who created a distal reentry point in the iliac artery to decompress the false lumen. Similar to early attempts to contain abdominal aortic aneurysms by cellophane wrapping, this technique was attempted by Abbott in 1949. In 1955 DeBakey and co-workers introduced a revolutionary surgical treatment that involved excision of the intimal tear, obliteration of the false lumen, and either direct reanastomosis or insertion of a prosthetic graft. Nevertheless, open surgery for symptomatic type B thoracic aortic dissection carries a mortality rate of 20% to 80%, especially in the presence of bowel ischemia. This motivated the development of less invasive techniques, such as fenestration and endovascular stent-graft therapy. The first series that described thoracic endovascular aortic repair (TEVAR) for treatment of type B aortic dissection was reported by Dake and associates in 1999.
Indications
Medical management of uncomplicated type B thoracic aortic dissection remains the current standard of care, although the role of endovascular grafting for such patients is an active area of clinical investigation. Indications for intervention include symptomatic type B dissection that manifests with aortic rupture or end-organ ischemia to the limbs or the gut ( Box 21-1 ). Endovascular intervention, when feasible, is preferred over open surgical repair for symptomatic disease due to its lower risk of mortality and morbidity. However, an extraanatomic axillofemoral or femoral-femoral bypass remains an option for the treatment of isolated lower extremity ischemia in the presence of type B thoracic aortic dissection. Persistent chest pain, despite adequate blood pressure management, has been considered an indication for intervention. However, wide variation exists in the definition of persistent pain from 48 hours to 14 days. TEVAR should be used in patients with syndromic connective tissue disorders only when other options are not practical. In contrast to type B dissections, all type A dissections are managed by immediate referral for direct surgical repair.
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Rupture
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Limb ischemia
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Visceral ischemia
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Acute false lumen enlargement
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Penetrating ulcer
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Aortic diameter >4 cm at site of thoracic dissection
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Refractory hypertension with back pain
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Persistent back pain despite adequate blood pressure control
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Partial false lumen thrombosis
Preoperative Preparation
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Intensive care unit monitoring. Suspicion of acute aortic dissection should involve immediate admission to the intensive care unit with blood pressure control and diagnostic evaluation.
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Considerations in the differential diagnosis. Other causes for chest and back pain need to be excluded, such as acute myocardial infarction (electrocardiogram or cardiac enzymes) and pulmonary embolism (chest computed tomography [CT] angiogram).
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Pain and blood pressure control. Immediate pain and blood pressure control can be achieved with morphine sulfate and intravenous beta-blockers (labetalol, metoprolol, or esmolol). Refractory hypertension requires vasodilating drugs (sodium nitroprusside) or angiotensin-converting enzyme inhibitors. Medical therapy should remain the first line of treatment in patients with uncomplicated acute type B dissection. Abdominal pain in the presence of type B dissection carries the diagnosis of bowel ischemia until proven otherwise.
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Preoperative hydration. The risk of renal dissection, presence of hypotension, and use of contrast for preoperative and intraoperative imaging can result in acute renal failure. Special attention to adequate hydration is mandatory.
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Imaging. A transesophageal echo can confirm the diagnosis of aortic dissection and discern between types A and B dissections. Nonetheless, a multidetector CT scan provides the best assessment of the thoracic aorta, with 64-slice multidetector technology capable of providing a CT angiogram from neck to groin in about 1 minute. CT imaging confirms the diagnosis, differentiates between type A and type B dissection, and identifies branch vessel involvement and possible contained rupture.
Pitfalls and Danger Points
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Visceral ischemia. Abdominal pain in the presence of type B dissection carries the diagnosis of bowel ischemia until proven otherwise. Patients need an initial CT diagnosis, followed by stent-graft exclusion of the entry point in the thoracic aorta, and may require exploratory laparotomy. If gut hypoperfusion persists despite proximal stent-graft exclusion and true lumen enlargement, stenting of the visceral vessel into the true lumen is recommended ( Fig. 21-1 ). Visceral bypass may be required when extensive dissection into the branch vessel has occurred. In the presence of dead bowel, resection and a second look laparotomy is the preferred approach.
Figure 21-1
Proximal thoracic stent-graft exclusion of the entry tear with additional stents placed into the superior mesenteric artery and celiac axis for intestinal ischemia. Note the location of the visceral stents, extending across the false lumen into the true lumen.
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Differentiating between the true and the false lumen of the thoracic aorta. Deployment of the stent graft in the true lumen with exclusion of the entry tear is axiomatic in TEVAR of dissection. The operator should understand the relative orientations of true and false lumen as depicted on the CT angiogram and verify this orientation intraoperatively with intravascular ultrasound (IVUS). The true lumen is typically smaller than the false lumen and darker in contrast ( Fig. 21-2 ). During initial guidewire manipulation, if the wire is located in the false lumen, it typically does not easily follow the arch to the ascending aorta but instead curls just distal to the left subclavian artery. However, the absence of this sign should not be used as a confirmation of true lumen catheterization. Reliable guidewire positioning in the true lumen includes careful assessment of the CT angiogram to select the appropriate access vessel, intraoperative verification by IVUS imaging, and right brachial access if retrograde transfemoral passage does not reliably afford true lumen wire position. On the arch angiogram, contrast first fills the true lumen and typically fills the false lumen a few seconds later ( Fig. 21-3 ). It is vital to confirm that the guidewire is located in the true lumen from the femoral artery to the arch, and IVUS is the most reliable predictor of the guidewire’s course. Inability to adequately differentiate true from false lumen can result in a variety of complications, including rupture, creation of large reentry tears, development of type A dissection, and acute true lumen occlusion ( Fig. 21-4 ).
Figure 21-2
IVUS catheter located in the false lumen (arrow). Typically, the false lumen is the larger of the two lumens and is lighter in tone.
Figure 21-3
Diagnostic angiogram demonstrating rapid early filling of the relatively compressed true lumen, with delayed blush seen in the false lumen.
Figure 21-4
Acute true lumen compression with inadvertent stent-graft deployment in the false lumen and acute type A conversion.
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Arterial access. Large-diameter devices require large access sheaths. The most frequently encountered complication in thoracic stent-graft trials has involved iliofemoral access-related complications. Vascular trauma or thrombosis of the iliac vessels was noted in 14% of patients in the W.L. Gore-sponsored TEVAR multicenter trial. Femoral access complications most commonly are encountered when using a 24-Fr sheath in patients with borderline iliac diameters. Smaller iliac diameters, combined with calcification and tortuosity, further increase the risk of iliac dissection or rupture. Preoperative planning should involve CT imaging to the level of the common femoral artery. Both contrast and noncontrast CT scans need to be evaluated to assess the smallest iliac diameter and extent of calcification. Tortuosity is best evaluated with a three-dimensional reconstruction or conventional angiogram. As a rule, if a conduit is thought to be necessary, it is almost always necessary. It is also important to watch the sheath move into the aorta at all times using fluoroscopy. The sheath is ideally introduced over a stiff Lunderquist or Meier wire. Most iliac artery injuries are commonly noticed during sheath withdrawal. Hence, if excessive force is required to introduce a sheath, despite the sheath moving forward, an iliac tear may be encountered during sheath withdrawal. If the sheath is not easily withdrawn, it may be advisable to plan for an iliac artery exposure with the sheath in place or to have a contralateral femoral sheath (12 Fr) and an aortic Reliant or Coda occlusion balloon readily available for emergency control of the aorta. It is also important not to lose wire access, because iliac artery injuries may be temporarily tamponaded by reintroducing the sheath and dilator until a definitive repair is undertaken.
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Iliac artery pseudoaneurysm. During sheath withdrawal an intimal flap may be noted adherent to the sheath. After the closure of the femoral arteriotomy, a completion pelvic angiogram may identify an iliac artery pseudoaneurysm that may result in delayed rupture. If two or more devices are required, an introducer sheath is advisable to prevent trauma to the iliac or femoral artery during each subsequent introduction.
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Vascular tortuosity. The descending thoracic aorta can have acute angles that may prevent the device from easily tracking across the curvatures. Various maneuvers may help overcome this problem. First, use of a stiffer Lunderquist wire placed proximally in the aortic root may provide more support. If this does not help, the use of a stiff “buddy wire” from the contralateral femoral access may help straighten the descending thoracic aorta. Lastly, the use of a brachiofemoral wire (“body floss” technique) may prevent buckling of the wire in the descending thoracic aorta. Because most patients with an acute type B dissection have minimal atherosclerotic arch disease, the risk of arch embolization may be lower than in patients with thoracic aneurysms. Right brachial artery access via a percutaneous sheath allows easy cannulation of the true lumen, especially in patients with difficult true lumen catheterization via the femoral route. Significantly, this technique frequently allows the device to track across a tortuous descending thoracic aorta by preventing the wire from buckling.
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Avoiding the generation of a de novo type A dissection. Routine ballooning of endografts in patients with dissection may not be advisable, especially in the proximal location, because this carries the risk of producing a cerebrovascular accident or type A conversion ( Fig. 21-5 ). Ballooning should be reserved for treatment of an endoleak or inadequate graft expansion. Mean blood pressure below 70 mm Hg also helps prevent proximal extension of the dissection flap during the endograft procedure, as well as reduce the risk of distal graft migration.
Figure 21-5
Proximal extension of the dissection into the arch as a complication of balloon dilatation of the proximal seal zone.
Endovascular Strategy
Preoperative Imaging
CT angiography is the initial imaging modality of choice. Magnetic resonance angiography is valuable in patients with severe iodinated contrast allergy, renal failure, or pregnancy. Magnetic resonance angiography has less spatial resolution than CT angiography. In addition, magnetic resonance angiography is sensitive to the presence of metal, with surgical clips and stents producing significant image artifacts. Patients with pacemakers, defibrillators, and certain cardiac valves cannot undergo a magnetic resonance angiography. CT imaging is generally preferred for evaluation of branch vessels and obtaining measurements for stent-graft placement. Echocardiography is often limited to the unstable patient or unreliable CT or magnetic resonance imaging because of excessive patient movement. Intraoperative transesophageal echocardiography can help exclude type A dissection after stent-graft placement.
Differentiating Between Type A And Type B Dissections
Type A dissection remains treatable solely by open surgery, and it is rare that type A dissection manifests with an isolated entry point in the descending aorta. Stent grafts inserted in the descending thoracic aorta for type A dissection can often be fatal because of abrupt closure of the exit point, resulting in rapid pressurization of the false lumen in the ascending aorta with attendant aortic valve insufficiency and aortic rupture into the pericardial space.
Defining the Extent of Treatment
The extent of coverage depends on the indication. Generally, coverage of the entry tear promotes false lumen thrombosis and aortic remodeling ( Fig. 21-6 ). Most tears occur a few centimeters distal to the left subclavian artery. This portion of the thoracic aorta is usually highly angulated, because it represents the transition from the arch to the descending thoracic aorta, and it is best to avoid landing stent grafts in this angle. The stent graft is preferably landed just distal to the origin of the left common carotid artery. This allows the proximal portion of the stent graft to have circumferential wall opposition. Landing the stent graft distal to this location may result in inadequate inner curve opposition that may result in stent-graft compression or collapse, acute type A conversion, or anchor wire flips ( Fig. 21-7 ). The portion of the aorta just distal to the left common carotid artery and proximal to the distal end of the left subclavian artery provides the most stable landing zone. This requires a carotid-subclavian bypass in certain patient subsets ( Box 21-2 ). Thoracic stent grafts 15 cm in length are often adequate, and it is best to use a stent graft that is less than 20 cm in length. The only exception to this rule is free rupture, where coverage is advisable from a point distal to the left common carotid artery to the celiac axis. This posture relates to the fact that the extent of coverage in patients with aortic dissection has been correlated with an increased incidence of paraplegia because of the presence of multiple open intercostal vessels.
