Canadian Cardiovascular Society/Canadian Society of Cardiac Surgeons/Canadian Society for Vascular Surgery Joint Position Statement on Open and Endovascular Surgery for Thoracic Aortic Disease

Commonly used arterial cannulation sites include the distal ascending aorta, right axillary, innominate, and femoral arteries. Alternatives include the carotid artery, and left ventricular apex (acute type A dissection).^35,36 Right axillary artery or innominate artery cannulation have the distinct advantage of sequential use for antegrade cerebral perfusion (ACP). Special considerations regarding the arterial cannulation site should be made in patients with acute type A aortic dissection, because malperfusion is common and the site of arterial cannulation might improve overall perfusion or worsen malperfusion depending on flow dynamics.

ACP and retrograde cerebral perfusion were developed as adjuncts to hypothermia for brain protection to allow safer and longer duration of circulatory arrest. Deep hypothermic circulatory arrest (DHCA), defined by core temperature of 14°C-20°C³⁷ was pioneered by Griepp et al. in 1975, to improve safety of arch surgery.³⁸ A few landmark series have shown that DHCA alone yields “acceptable” rates (2%-7%) of permanent neurological deficit when circulatory arrest times are < 40-50 minutes.^39-41 Rates of transient neurological dysfunction have been shown to increase after 30 minutes of straight DHCA.⁴² A 2013 meta-analysis of observational studies reported significantly lower incidence of permanent neurological dysfunction (12.8% vs 7.3%; P < 0.001) with moderate hypothermia and ACP vs deep hypothermia alone. There was no significant difference in mortality in this meta-analysis (13.5% vs 11.1%).⁴³

ACP strategies, involving perfusion via the innominate and/or carotid arteries, were developed with the recognition that it was impossible to reduce cerebral metabolic consumption to 0 with hypothermia alone.^44-46 Right axillary artery cannulation,⁴⁷ through a separate infraclavicular incision, facilitates central perfusion and unilateral ACP without manipulating the ascending aorta.^47-50 Despite concerns about an incomplete Circle of Willis in some patients, bilateral ACP has not shown superiority to unilateral selective ACP strategies.⁵¹ ACP is typically carried out under varying degrees of hypothermia and lower body circulatory arrest at a flow rate of 10-15 mL/kg/min with a target right radial arterial pressure of 50-70 mm Hg.⁴⁵ Brain perfusion monitoring with continuous near-infrared spectroscopy, electroencephalography, and transcranial Doppler might guide adjustment in flows or need for bilateral ACP.

Adjunctive retrograde cerebral perfusion via the superior vena cava provides the theoretical benefits of cerebral cooling and removal of atheromatous debris during aortic arch surgery,^52,53 but might have limited nutritive benefit beyond DHCA alone.^54,55

Although deep hypothermia is protective, adverse effects on organ function, the coagulation system, and need for prolonged cardiopulmonary bypass time remain important limitations. With the advent of ACP, aortic arch surgery is increasingly being performed under lesser degrees of hypothermia with potentially less morbidity.^56,57

The goals of extended distal repair are to seal tears extending beyond the transverse arch and improve false lumen obliteration in the descending aorta. Theoretical benefits include prevention of early and late complications including reduced early malperfusion and late distal aortic dilatation, aortic reintervention, and mortality.

In the acute setting, patients who present with documented distal malperfusion have up to a fivefold increased mortality.⁶¹ Detection of malperfusion might be a challenging clinical diagnosis and the true incidence of malperfusion is likely underestimated.

In addition, up to 30% of patients might have a primary intimal tear distal to the ascending aorta.⁶² Resection of the intimal tear is one of the principles of surgical repair of dissection and these distal tears are not resected using standard “hemiarch” repair. Although resection of the intimal tear is thought to be of benefit, the evidence is equivocal, with a decreased repeat surgery rate observed in some reports,^60,63-65 but not in others.⁶⁶ Furthermore, despite intimal tear resection, most patients are left with a patent false lumen after standard hemiarch repair.^67,68 Patency of the residual false lumen might increase the risk of distal aorta repeat surgery and reduce long-term survival,^69,70 although this has not been consistently observed.⁷¹

The conventional alternative to a “hemiarch” anastomosis is total arch replacement with or without elephant trunk repair (Fig. 2). More recently, hybrid extended distal repairs involving the arch and descending aorta at the time of type A dissection have been developed. They consist of combined open surgical repair and endovascular aortic repair that eliminates any residual dissection in the arch and treats a varying amount of the descending aorta. These emerging techniques are depicted in Figure 3 and may be grouped into 3 categories that consider the extent of arch replacement, location of inflow to head vessels (orthotopic or extra-anatomic), and the method in which the stent graft is deployed (during circulatory arrest or after weaning from cardiopulmonary bypass):

• (1)
  Total arch replacement with orthotopic arch vessel reimplantation and antegrade stent graft placed in the descending aorta at time of circulatory arrest.^71-74
• (2)
  Hemiarch replacement with antegrade stent graft placed in the descending aorta at time of circulatory arrest.^75-79 The arch vessels and a short segment of the native arch are left in situ.
• (3)
  For arch debranching extra-anatomic proximal rerouting of arch vessels with ascending aortic replacement and stent graft deployment off pump with intraoperative fluoroscopy is used to determine landing zones.^80-84 In this approach the stent graft may be deployed antegrade or retrograde.

There are no prospective trials on extended distal aortic repair compared with standard hemiarch replacement in acute type A dissections. All studies are small and rely on surgeon preference in the extent of distal replacement. Furthermore, these studies include a variety of repair techniques and a heterogeneous group of patients.^64,65,85-88 Long-term outcomes, particularly the decision to repeat surgery on progressive pathology in the distal aorta are similarly surgeon-dependent, which makes comparisons prone to selection bias. Supplemental Table S4 shows a summary of the contemporary results of extended distal repair for type A dissection and shows results that compare favourably with standard therapy. Contemporary registry data, mostly of standard “hemiarch repair,” reveals significantly improved mortality over a 17-year period (1995 to 2013) from 31% to 22%.⁸⁹ In 2012, the German Registry for Acute Aortic Dissection Type A (GERAADA) registry retrospectively compared 518 patients who underwent hemiarch replacement with 140 patients who underwent total arch replacement. All patients had a primary intimal tear in the ascending aorta. Because there was no significant difference in surgical mortality (18.7% vs 25.7%) and new postoperative stroke (13.6% vs 12.5%),⁹⁰ the authors concluded that a more aggressive approach to the arch can be carried out without increased perioperative mortality, with long-term benefit to be determined.

A dissection is considered complicated when there is evidence of leak, rupture, rapid aortic expansion, or malperfusion, which might involve the spinal cord, mesenteric and renal branches, or peripheral limb ischemia. Persistent intractable pain and refractory hypertension are also considered features of a complicated dissection, which can usually be controlled with medical therapy. Ischemia in any distal perfusion bed can occur from a dynamic or static obstruction.^130-132 Untreated complicated type B aortic dissection usually results in irreversible end organ damage or death.^133-136

Historically, results with open surgery via thoracotomy were poor and open aortic replacement was only attempted in cases of leak or rupture.¹³⁷ End organ ischemia was dealt with circuitously with a variety of procedures to create communications between true and false lumens and extra-anatomic bypasses, all aimed at avoiding open thoracic aortic surgery,^133,138 although open fenestration via thoracotomy remained an option.¹³⁹

Endovascular repair has now become first-line therapy for acute complicated type B dissections.¹⁴⁰ Goals of TEVAR in this context are to cover the primary intimal tear, reverse malperfusion syndromes, and promote thrombosis of the false lumen with subsequent aortic remodelling. Mossop and Nixon and colleagues¹⁴¹ introduced the idea of a distal uncovered (“dissection”) aortic stent to scaffold the remaining distal true lumen and promote aortic healing without compromising branch vessel flow. A multicentre¹³⁸ single-arm prospective study, the Study for the Treatment of Complicated Type B Aortic Dissection Using Endoluminal Repair (STABLE) trial,¹⁴² reported favourable clinical and anatomical results with this strategy. The population studied was mixed, with a number of cases considered chronic complicated dissections. Two-year follow-up showed a 30-day mortality of 5.5% in patients who presented acutely. All-cause mortality was 12% at 1 year and 15% at 2 years. Gradual expansion of the true lumen and contraction of the false lumen was observed.

In the absence of complicating features, an acute type B dissection is considered “uncomplicated.” Some have suggested that “uncomplicated” is a misnomer, because of the longer-term natural history showing aortic expansion and late complications. There is ongoing debate regarding the ideal initial management of this entity.

All cases of aortic dissection require optimal medical treatment (OMT), usually in a critical care setting, which includes arterial blood pressure and pain control, and frequent imaging surveillance of the aorta. Although there is some debate regarding the optimal pharmacological agents and hemodynamic targets,^143-145 a blood pressure < 120/80 mm Hg, achieved using β-blockers as first-line agents, is widely accepted because of the theoretical benefit of reducing aortic wall shear stress. Early repeat imaging surveillance, typically at 48-72 hours and then again at 1-4 weeks, is an important component of medical therapy. The Investigation of Stent Grafts in Patients with Type B Aortic Dissection (INSTEAD) trial, which randomized 140 patients with uncomplicated type B aortic dissection in the subacute and chronic phases (2-52 weeks; mean, 10-12 weeks) to OMT or OMT with TEVAR, showed no difference in all-cause or aorta-related mortality between groups (96% vs 89%) at 2 years, but was underpowered for this end point.¹⁴⁶ Aortic remodelling was greatly improved in the stented group with false lumen thrombosis or expansion of true lumen occurring in 91% vs 19% in the TEVAR with OMT vs OMT alone groups. A longer-term landmark analysis (Investigation of Stent Grafts in Aortic Dissection With Extended Length of Follow-up [INSTEAD-XL]) showed a greater incidence of aortic dilatation (66% vs 21%), reduction in false lumen thrombosis (22% vs 91%), and poorer aortic remodelling (10% vs 80%), in the OMT-alone group after 5 years.¹⁴⁷ The International Registry of Acute Aortic Dissection (IRAD) registry used observational data to show significantly improved aorta-related survival in stented vs OMT alone at 5 years¹⁴⁸ even though other studies have failed to show any advantage.¹⁴⁹ The Acute Dissection: Stent Graft or Best Medical Therapy (ADSORB) trial randomized 61 patients to OMT or OMT with TEVAR with covered stent, and showed early benefits to stenting with respect to aortic remodelling.¹⁵⁰

Optimal management of uncomplicated acute type B aortic dissections might require a tailored approach to identify those at risk for late aneurysmal degeneration of the thoracic aorta. In a review of 18 studies that reported on predictors of aortic growth,¹⁵¹ younger age was associated with an increased growth rate.¹⁵² Less aortic growth was observed in the Asian population.¹⁵³ An increase in the level of fibrinogen degradation products was predictive of growth, but has not been widely studied.¹⁵⁴ Multiple studies consistently identified an initial aortic diameter of > 4 cm as predicting eventual further enlargement^149,155,156 although others have shown the opposite.¹⁴⁸ An initial false lumen diameter of > 22 mm,¹⁵⁷ a convex appearance (concave true lumen),¹⁵² or a proximal saccular shape were all predictors of poor outcome. The number¹⁵⁸ and size¹⁵⁹ of entry tears were also predictive of growth; with a single large tear¹⁵⁹ (≥ 1 cm) being most ominous.

Type B aortic dissections are considered chronic at > 90 days after their onset. As the aorta remodells, in a subset of patients, the aorta and particularly the false lumen continues to expand. The most common indication for intervention is aneurysmal degeneration or rapid growth in an asymptomatic patient. Some patients might present with pain related to further extension, compression of adjacent anatomical structures, malperfusion, or rupture.

The classical treatment for these patients has been open surgery via thoracotomy, clamshell incision, or a thoracoabdominal approach. A number of large open surgical series from major centres of excellence have provided acceptable results, but typically reflect heterogeneous populations of mixed acuity and extent of aortic disease.^160-169 In a series by Pujara et al.,¹⁶⁶ excellent results were realized with a 30-day mortality of 8% and an overall incidence of spinal cord injury of only 2.4%. They noted that greater initial preoperative aortic diameter, and a greater extent of disease were both predictive of worse outcome, making the case for earlier intervention in chronic type B dissections. Although open surgery is effective, it carries a significant risk of perioperative mortality and morbidity.

More recently, endovascular approaches have been attempted in selected patients with chronic type B dissection, with the primary goal of sealing off the main entry tear and major re-entry tears, thereby promoting aortic false lumen thrombosis and facilitating aortic remodelling. The INSTEAD trial^146,147 discussed previously, largely treated patients in the subacute phase but did include some chronic patients (up to 52 weeks). A number of observational cohorts have shown technical feasibility of TEVAR or hybrid (open debranching with TEVAR) procedures in selected patients with favourable anatomy.^170-175 Persistence of false lumen perfusion from distal re-entry tears remains an important limitation. There is a paucity of data on open vs endovascular treatment of chronic type B aortic dissections.