53. Type A Aortic Dissection- Clinical Scenarios

Heidi Reich, MD, and Dominic Emerson, MD

Adapted from 1^st edition chapter written by Armin Kiankhooy, MD, and Ravi Ghanta, MD

Concept

• Diagnosis and management of acute type A dissection
• Operative techniques and potential pitfalls
• Postoperative management

Chief complaint

“A 63-year-old man presents to the emergency room with sudden onset of severe tearing chest and back pain.”

Differential

This presentation warrants a high index of suspicion for acute aortic dissection. Up to 30% of patients with acute aortic dissection are initially thought to have another diagnosis. Two-thirds of all patients with aortic dissection present with involvement of the ascending aorta (Stanford type A) and one-third present with isolated involvement of the descending aorta (type B). Differential includes acute aortic syndromes (aortic dissection, intramural hematoma, and penetrating aortic ulcer), acute coronary syndrome, aortic aneurysm, acute aortic insufficiency, pulmonary embolism, pericarditis, and musculoskeletal pain.

History and physical

A focused history elicits the quality (ripping, migrating), location (mid-sternum or interscapular), and chronicity of the pain, as well as signs of malperfusion, i.e., stroke, paraplegia, abdominal pain, anuria/oliguria, and claudication. Family history of aortic dissection or aneurysm should be assessed. On exam, hypertension, hypotension, and a pulse differential or deficit may be apparent. Presence of a blowing, diastolic, decrescendo murmur at the left 3rd intercostal space suggests aortic regurgitation. Tamponade physiology, with characteristic JVD, pulsus paradoxus, and pericardial friction rub, suggests aortic rupture. Neurologic assessment, abdominal exam, and pulse exam should be clearly documented prior to surgery.

Pathophysiology

Medial degeneration, leading to compromised aortic wall structural integrity, intimal tear, and propagation of the intimal flap, is associated with development of aortic dissection. Hypertension is the key mechanical force contributing to dissection. The tear is typically > 50% of the circumference and usually occurs along the right anterior aspect of the ascending aorta and circumferentially spirals around the arch and into the descending thoracic and abdominal aorta on the left and posteriorly. Approximately 10% of the time, the dissection propagates retrograde to involve the coronary ostia. Forty percent of patients with acute type A aortic dissection die immediately. Thereafter, mortality is 1-2% per hour in the first 48 hours and, without surgical intervention, up to 90% at 30 days. If rupture does not occur, weakening of the outer media and adventitia results in aneurysmal dilatation. Early mortality results from 1) aortic rupture with or without pericardial tamponade, 2) acute aortic insufficiency and congestive heart failure, 3) acute myocardial infarction due to coronary involvement, and 4) stroke due to arch vessel involvement. Urgent surgery converts a 90% mortality risk to at least a 70-80% probability of survival.

Risk factors

The most frequent risk factor is hypertension. Other risk factors include connective tissue disorders, personal or family history of aortic aneurysm or dissection, aortic valve disease, pregnancy, recent aortic manipulation (catheter-based or surgical), and stimulant use.

Tests

• Labs:Establish baseline lab values (CMP, CBC, Coags). No ideal screening lab test exists for acute aortic dissection. D-dimer below 500 ng/mL has been proposed as an exclusionary test, but reduced sensitivity of D-dimer limits its reliability in certain scenarios, such as short-segment or subacute dissections.
• EKG: Ischemic changes, ranging from non-specific ST/T wave changes to ST-segment elevation, result from hypotension, coronary ostial involvement (most commonly inferior wall ischemia in RCA distribution), and pre-existing coronary artery disease. Initial misdiagnosis of acute coronary syndrome is common.
• CXR:Classic findings include widened mediastinum and pleural effusion, but over 20% of acute type A dissections present with normal mediastinal and aortic contour.
• CT angiography: Diagnostic study of choice due to widespread availability and high sensitivity/specificity. Delineates proximal and distal flap extent, aortic root and arch involvement, associated aneurysms, and pericardial effusion. An ECG-gated CT of the entire aorta, neck to pelvis through the femoral vessels, is ideal.
• TEE: Preferred diagnostic study for unstable patients and can be performed on-table in the operating room. Identifies the intimal tear, true and false lumens, aortic valve characteristics, aortic insufficiency, proximal coronary involvement, and pericardial effusion.
• If clinical suspicion is high and initial aortic imaging is negative, a second imaging study (CT, TEE, or MRI) should be performed.

Index scenario (additional information)

“This patient has < 24 hours of pain and CT angiography reveals a type A dissection. There is no aortic root or arch involvement. How would you like to proceed?”

Treatment/management

This patient meets the criteria for acute type A aortic dissection without arch or root involvement. Surgery is indicated to prevent life-threatening complications including aortic rupture. The goal is to prevent death and irreversible end-organ damage. Pre-operative anti-impulse (dp/dt) treatment with short-acting beta-blockers (goal HR ≤ 60) and peripheral vasodilators (“permissive hypotension” and goal SBP 90-120 mmHg) is critical and reduces transmural aortic wall stress and sheer stress. Pain control is also important. The presence of type A aortic dissection is an indication for immediate repair. Only minor delays for CT imaging in a stable patient can be justified. Hemodynamic instability demands immediate surgery. Focal neurologic deficits mandate a head CT. Relative contraindication to surgical repair include coma or dense neurologic deficits, age over 80 years, and significant comorbidities. For the most part type A dissections will be operative without delay.

Operative steps

Acute type A dissection repair

The primary goals of surgery are to save the life of the patient and, when feasible, to provide more definitive treatment to reduce the need for future aortic surgical interventions. The extent of aorta requiring replacement depends on the location and extent of the tear and aneurysm. The most common operation is supracoronary replacement of the ascending aorta and hemi-arch replacement. Considerations regarding the extent of surgery include patient age, comorbidities, clinical stability, presence of malperfusion, and extent of the aortic dissection.

• Anesthesia & monitoring
• General endotracheal anesthesia with central venous access, PA catheter, arterial lines (right +/- left radial and femoral), TEE, and bladder and esophageal temperature probes.
• Neurologic monitoring with electroencephalography and near-infrared cerebral oximetry.
• Neck, chest, abdomen, and bilateral lower extremities are prepped and draped widely.

• Cannulation Strategies
• Arterial Cannulation: It is crucial to achieve arterial cannulation of the true lumen. If high line pressures are encountered, this may be the result of false lumen cannulation. Stop the pump and obtain arterial cannulation at a different site.
• Right axillary artery: Prior to opening the chest, an 8-10mm graft is sewn end-to-side to the right axillary artery exposed infraclavicular or in the deltopectoral groove. Avoid in presence of axillary extension of the dissection. Disadvantages include additional time required for exposure and risk of axillary dissection.
• Ascending aorta: Direct cannulation of the ascending aorta is performed using Seldinger technique under TEE guidance to ensure true lumen cannulation. This strategy remains controversial.
• Femoral artery: Preoperative CT and pulse exam delineate if the femoral artery is perfused by the true lumen. Femoral cannulation is desirable due to ease of rapid access, which makes femoral cannulation a reasonable option for unstable patients. Disadvantages include the risk of differentially pressurizing the false lumen and resultant malperfusion. Arterial flow is switched to the aortic graft sidearm after circulatory arrest to ensure true lumen perfusion while rewarming.
• Venous cannulation: Bicaval or dual-stage venous cannulation are used depending on whether retrograde cerebral perfusion is planned.
• Initiate CPB and begin systemic cooling with ice placed around the patient’s head. A retrograde cardioplegia cannula and LV vent are inserted. Monitor closely for LV distension.

• During cooling, perform proximal dissection. When the heart fibrillates, carefully cross clamp the mid-ascending aorta, monitoring for distal malperfusion, and divide the aorta. Administer retrograde cardioplegia with or without direct ostial antegrade cardioplegia. Inspect aortic root and aortic valve. Perform as much proximal work (see below) as able while cooling.
• Cool to below 20° C with electrocerebral silence or empirically for at least 45 minutes, place the patient in Trendelenburg and initiate circulatory arrest. Administer selective antegrade or retrograde cerebral perfusion (see Circulatory Arrest chapter for details). Circulatory arrest times not exceeding 30-45 minutes with deep hypothermia are considered safe.
• Perform an open distal anastomosis under circulatory arrest. This ensures true lumen perfusion, facilitates inspection of the arch for intimal tears or other pathology that would change the planned operation, and allows the cross-clamp site to be resected. The distal extent of resection (ascending aortic replacement, hemiarch, total arch) depends on the location and extent of the intimal tear, as well as surgeon and center experience. The goal, at minimum, is to completely resect the intimal tear and replace the ascending aorta. The false lumen is obliterated, and the distal anastomosis is reinforced with strips of felt and pledgeted sutures. Resume cardiopulmonary bypass with central perfusion by switching arterial cannulation to the graft sidearm if using direct aortic or femoral cannulation initially, deair, and clamp the graft.
• Prior to the proximal anastomosis, confirm the root and coronaries are not involved. If the sinotubular junction is involved, reapproximate the dissected layers between internal and external strips of felt (“sandwich technique”). Trim the Dacron tube graft to an appropriate length and perform the proximal anastomosis. Wean bypass, deair, aggressively correct coagulopathy, and decannulate.

Potential questions/alternative scenarios

“The patient’s CT angiography demonstrates a type A aortic dissection with arch vessel involvement. How would you proceed?”

Hemiarch replacement is often sufficient to completely resect the intimal tear, exclude the false lumen, and re-establish arch vessel perfusion. Under circulatory arrest, the resection is extended as a tongue along the concavity of the arch. Tears in the arch can be oversewn. More extensive resection is warranted for arch rupture and large arch aneurysms (> 5.5 cm). Alternatively, total arch repair with frozen elephant trunk can be performed with reimplantation of the arch vessels individually using a trifurcated graft or as a Carrel patch.

 
“The patient has an unknown connective tissue disorder and CT angiography reveals the aforementioned findings as well as a widened aortic root. TEE demonstrates additional findings of retrograde propagation involving the aortic root and severe aortic regurgitation. How would you proceed?”

Patients with aortic dissection, particularly if under the age of 40 years, should be evaluated for underlying connective tissue disorders, including Marfan syndrome, Loeys-Dietz syndrome, and Turner syndrome (female patients only), as well as non-syndromic connective tissue disorders or annuloaortic ectasia. When connective tissue disorders are known or suspected, replacement of the entire aortic root is indicated. Options include root replacement with a mechanical valved conduit or handsewn pericardial composite root and coronary reimplantation (modified Bentall), or in experienced centers only a modified David procedure. Yacoub-type repairs should be avoided. Future reinterventions are most frequently due to failure to replace the entire root, failure to replace the entire ascending aorta, or aneurysm of the aortic wall at the coronary artery button.

In the absence of a known or suspected connective tissue disorder, if the regurgitation is secondary to commissural detachment and the valve leaflets appear normal, commissural resuspension with pledgeted sutures and closure of the proximal false lumen can be performed. This will often provide adequate repair with a competent aortic valve and allows the sinuses to be preserved.

“The initial CT angiography demonstrates intramural hematoma involving the ascending aorta.” Would this change your surgical management?”

The spectrum of acute aortic syndromes includes aortic dissection, intramural hematoma (IMH), and penetrating aortic ulcer. The hallmarks of IMH are the absence of an intimal tear and absence of false lumen flow. While 10% of IMH will resolve, the majority will not and may convert to a classic dissection or the aorta may enlarge and rupture. Generally, IMH in the ascending aorta is considered equivalent to full proximal dissection and should be treated alike. Select patients with aortic diameter (including hematoma) < 5 cm, IMH thickness < 1 cm, and no pericardial effusion may be managed expectantly. Repeat imaging is obtained within 48 hours of presentation to assess for progression.

“Upon opening the aorta, you are surprised to find that this patient has dissection of both coronary ostia. How would you address the coronary dissections?”

Coronary malperfusion in type A dissection may result from obstruction of the coronary ostia by the intimal flap or dissection of the coronary ostia. Compromise of the RCA is more frequent than the left. The surgical approach must ensure adequate myocardial protection reliant upon retrograde cardioplegia and the extent of the coronary dissection dictates the extent of the repair. Local repair techniques include mobilization of coronary buttons and reapproximating the dissected layers, which is reserved for dissection that reaches the coronary ostium without tear of the coronary itself. If the coronary itself is dissected and not amenable to repair, bypass or interposition with saphenous vein is performed.

“The patient demonstrates a decrease in the femoral pressure after initiation of bypass.”

Distal malperfusion is caused in most cases by false lumen compression on the true lumen. Most likely, either the femoral arterial line is in the false lumen or the arterial cannula is in the false lumen. To clarify the later check the radial pressure reading. If it is normal, then the femoral line is unreliable or a regional malperfusion event has occurred due to a distal re-entry tear. If the radial line is dampened, then stop the pump and switch to arterial cannulation at another site. Due to re-entry tears in the dissection flap and changes in distal flow dynamics it is possible to have malperfusion events during initiation of CPB, cross clamping, or after circulatory arrest.

“The patient complains of severe abdominal pain. CTA demonstrates occlusion of the SMA by the false lumen. How would you proceed?”

Malperfusion in type A dissection results from severe true lumen compression, branch vessel involvement, or both. Prompt recognition of myocardial, cerebral, iliofemoral, renal, mesenteric, innominate or spinal malperfusion is crucial. Distal malperfusion associated with a type A dissection will often resolve by re-establishing flow to the true lumen. If there is a question of the integrity of the bowel, a laparotomy should be performed at the time of dissection repair. Percutaneous fenestration, true lumen stenting, and branch vessel stenting may be indicated. Of note, iliofemoral malperfusion generally requires additional revascularization with axillofemoral or femorofemoral bypass and a low threshold for fasciotomies. After the operation, distal complications can be monitored through clinical examination, CT scan, and duplex as needed.

“The patient is unable to move his left upper and lower extremities. How would you proceed?”

For patients with type A dissection, treatment strategies when a neurologic deficit is noted on presentation present a dilemma. Comatose patients or those with dense neurologic deficits have poor prognosis, but patients with less severe deficits have similar outcomes to patients who present neurologically intact. Paraplegia is not an absolute contraindication to surgery and may reflect a hemispheric infarct or spinal cord malperfusion. Surgical repair of the type A dissection is warranted.

 “A 63-year-old male patient undergoes a CT chest for evaluation of a solitary lung nodule. Incidentally a 5.6 cm ascending aortic aneurysm is noticed with signs of chronic dissection. How would you like to proceed?”             
Chronic type A dissection is uncommon (4-31%), as most patients do not spontaneously heal acute type A dissections, but when they present in the chronic phase they usually present asymptomatically as an incidental finding. Surgery is indicated for symptomatic patients, large associated aneurysm, eccentric expansion, rapid expansion (> 1 cm per year), or aortic insufficiency. Operative mortality for chronic type A dissection ranges 4-17%, with a 4% risk of stroke. Rate of reoperation if native aortic valve preserved is 20%. Annual imaging follow-up with CT or MRI is recommended.

Operative steps are like acute type A dissection; however, the rate of native aortic valve preservation is less due to the chronicity of the disease. The distal false lumen is intentionally not obliterated, as many distal vascular beds may dependent on false lumen flow. A staged elephant trunk procedure is required in rare instances when the chronic type A dissection with aneurysm dilation extends from the ascending aorta through the arch and into the descending thoracic aorta.

Long-term management

• After type A dissection surgical repair, most patients on follow-up reveal distal false lumen perfusion. This places patients at risk for aneurysmal dilatation and potential rupture. Therefore, blood pressure control (< 120 mmHg) is critical to prevent late death from rupture and chronic dissection.
• CT or MR imaging of the aorta, plus TTE, prior to discharge and at 1, 3, 6- and 12-months post dissection is recommended, with annual imaging thereafter if stable.
• The majority (60-70%) of patients are free from reoperation at 10 years, however routine echocardiographic monitoring of the aortic valve and diagnostic imaging of the aortic diameter are still warranted. The rate of reintervention may be much higher in young patients and patients with connective tissue disorders: 70 – 90% at 10 years.

Pearls/pitfalls

• High index of suspicion
• Diagnosis with CT angiography
• Be comfortable describing alternative cannulation
• Open distal anastomosis under circulatory arrest
• Hemiarch most common repair (full arch rarely needed)
• Resuspend commissures, if possible, to preserve the native valve
• Replace the root if known connective tissue disorder
• Permissive hypotension preoperatively with strict dp/dt control pre/post-op
• Monitor for malperfusion

Suggested readings

• Hiratzka LF, et al. 2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/STS/SVM Guidelines for the Diagnosis and Management of Patients With Thoracic Aortic Disease. Circulation. 2010;121:e266-e369.
• Hussain ST and Svensson LG. Acute Type A Dissection Repair. In: Stanger OH, et al. (Eds).  Surgical Management of Aortic Pathology: Current Fundamentals for the Clinical Management of Aortic Disease. 2019:837-83.
• Desai ND and Bavaria JE. Chapter 58: Aortic Dissection. In: Kaiser LR, Kron IL, Spray TL (Eds). Mastery of Cardiothoracic Surgery, 3^rd ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2014: 563-74.