TSRA Content:
Author: Stephanie N. Nguyen, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Ezequiel Molina, MD.
This section will cover aortic anatomy, pathology, and indications for operative intervention.
Anatomy
The Aortic Wall
The aortic wall is composed of three layers, from the innermost, the tunica intima, tunica media, and tunica adventitia. The intima is thin and composed of endothelial cells and their supportive tissue; the media is the thickest portion of the wall and is composed of elastic fibers, smooth muscle, and collagenous tissue. Finally, the tunica adventitia consists of mostly connective tissue, making it the strongest layer. It also contains small blood vessels called vasa vasorum that supply blood to the cells that make up the arterial wall.
The Aortic Root
The aortic root is the first segment of the aorta that connects the heart to the systemic circulation. It is composed of the aortic valve leaflets and its commissures, the annulus, sinuses of Valsalva, and the sinotubular junction (STJ) which defines the upper extent of the root. The left and right coronary ostia arise from their respective sinuses within the root.
The most common indication for proximal aortic replacement is an aneurysm, which commonly occurs in the setting of a bicuspid aortic valve (BAV) or connective tissue disorder (CTD). Generally, surgical intervention is indicated if the aortic diameter exceeds 5.5cm in a patient without CTD. This threshold is decreased to 4.5cm in patients with CTD or those undergoing cardiac surgery for another reason. Repair is also indicated if there is rapid expansion (>1 cm/year).
Root replacements may be performed in a valve-sparing fashion if the aortic valve is normal or with a valved conduit if the valve is diseased. While the technical details of the operations are beyond the scope of this section, aortic root replacements can be divided as follows:
Valve-sparing aortic root replacement (VSARR):
- Reimplantation (David procedure)
- Remodeling (Yacoub procedure)
Non-valve sparing aortic root replacement:
- Composite valve-graft (biological or mechanical Bentall procedure)
- Homograft root replacement
- Ross procedure
The Ascending Aorta
The ascending aorta comprises the segment between the STJ and the proximal aortic arch at the level of the innominate artery takeoff. Replacement of the ascending aorta is generally performed in the setting of aneurysm or an acute aortic syndrome. Size criteria for replacement are those mentioned in the above subsection.
The Aortic Arch
The branches of the aortic arch, in order, include the innominate artery, the left common carotid artery and the left subclavian artery. A less common pattern of branching consists of a common origin for the innominate and left common carotid artery (bovine aortic arch). Aneurysms of the aortic arch are replaced if >5.5 - 6 cm in diameter.
The 3 goals of aortic arch surgery in order of importance are: (1) protect the brain; (2) protect the heart; (3) restore anatomy. The major anatomical structure to avoid during dissection is the left recurrent laryngeal nerve, which lies next to the ligamentum arteriosum along the lesser curvature of the arch and can be injured during the dissection as it travels under the arch. During replacement of the aortic arch, a cross clamp cannot be applied, and the systemic circulation must be interrupted requiring circulatory arrest. This entails lowering the body temperature in order to protect the brain by decreasing its metabolism and oxygen consumption. Depending on the extent of aortic arch replacement (arch vs hemiarch) there are different strategies in terms of temperature management: deep versus moderate hypothermia. Different approaches are also available regarding cerebral perfusion techniques which could potentially decrease the risk of brain injury: (1) antegrade cerebral perfusion (cannulating right axillary artery to flow the head vessels, clamping innominate artery at its origin to prevent flow into the field) (2) retrograde cerebral (cannulating the SVC and having it attached to an arterial line with a snare around the SVC superior to the azygos vein origin), or (3) no brain perfusion (deep hypothermic circulatory arrest). Even with deep hypothermic circulatory arrest, most surgeons maintain some element of cerebral perfusion. With deep hypothermia at 18°C, the safe period is around 30-45 minutes of circulatory arrest time.
The Descending Aorta
The descending aorta is located distal to the takeoff of the left subclavian artery and extends to the level of the diaphragm. Aneurysms of the descending aorta >6 - 6.5 cm should be replaced (>6 cm with Marfan’s disease). The descending aorta is usually approached through a left posterolateral thoracotomy (6th intercostal space). The left parietal pleura is incised, the aorta is exposed, and the aneurysm is dissected free proximally and distally. The operation can be performed with the assistance of partial left atrial to femoral artery bypass or without bypass support if the expected ischemic time is short. Cross clamps are applied proximally and distally, and the aneurysm is resected and replaced with a tube graft.
The Thoracoabdominal Aorta
Thoracoabdominal aneurysms require extension of the thoracotomy incision towards the anterior abdominal wall, division of the diaphragm and exposure of the aorta in the retroperitoneal space. The abdominal branches of the aorta may need to be individually isolated, controlled, and re-anastomosed depending on the abdominal extension of the aneurysm. A devastating complication of descending thoracic and thoracoabdominal aneurysm repair is spinal cord ischemia and paraplegia. The available strategies that decrease the risk of neurologic damage include reimplantation of the artery of Adamkiewicz (a large spinal artery), CSF drainage via a lumbar drain, and avoidance of hypotension during the procedure and postoperatively.
Acute Aortic Syndromes
Acute Type A Dissection
An acute Type A dissection is caused by an intimal tear in the aortic wall proximal to the takeoff of the innominate artery. This results in separation of the intima from the media and creating a false lumen. Propagation of the dissection can proceed in antegrade or retrograde fashion from the intimal tear to cause obstruction of coronary ostia, aortic insufficiency, rupture, tamponade, or end-organ malperfusion. Type A dissection is a surgical emergency with the principal goal being complete resection of the intimal tear and exclusion of the false lumen.
Stanford Type A versus Type B Aortic Dissection
• Type A: involves the ascending aorta (tear in ascending or arch typically), surgical emergency
• Type B: ascending aorta is NOT involved, can be medically managed
Acute vs Chronic
• Acute: initiating event <2 weeks
• Chronic dissection >2 months ago
• Subacute: between 2 weeks and 2 months
Intramural Hematoma
Aortic intramural hematoma (IMH) is considered a precursor of dissection and is treated as a dissection when it occurs in the proximal aorta. IMH results from a ruptured vasa vasorum in the tunica media, resulting in an aortic wall infarct which may progress to a dissection.
Penetrating Atherosclerotic Ulcer
Deep ulceration of atherosclerotic plaques in the proximal aorta can lead to IMH, dissection, or rupture. Penetrating atherosclerotic ulcers (PAU) appear as an ulcer-like projection into an aortic wall hematoma on cross-sectional imaging. Endovascular stenting is emerging as a popular therapeutic modality in these cases.