Great vessels: Surgical access

Cardiopulmonary bypass

Venous blood is drained via a large-bore cannula, typically via a central point at the right atrium, although other options include a long femoral venous line with the tip positioned in the right atrium under transoesophageal echocardiography (TOE) guidance. Supplementary drainage may be acquired from the superior vena cava (SVC), either via direct cannulation or percutaneously via the internal jugular vein. Direct cannulation of the pulmonary artery may also be used for additional venous drainage. Arterial return may be via any decent-sized artery that will accept an arterial cannula or a Dacron graft. Often, this would be the ascending aorta or proximal arch, but commonly the axillary artery, brachiocephalic artery or femoral artery may also be used. The bypass configuration will typically also involve a vent into one of the chambers of the heart to help create a bloodless field and prevent cardiac distension, and a centrifugal pump, oxygenator and open reservoir are situated between drainage and return.

Left heart bypass

Left heart bypass is used during surgery on the DTA and thoraco-abdominal aorta. The circuit is essentially a shunt with cardiopulmonary function preserved. A clamp will commonly be placed around the LSA and either the diaphragm or infrarenal aorta. In order to provide distal perfusion to the limbs and visceral vessels, a shunt is created whereby blood is taken from either the left atrium (inferior pulmonary vein) or the aorta proximal to the proximal aortic cross-clamp, and pumped via a closed circuit to beyond the distal clamp. Side arms allow selective perfusion of the coeliac artery, superior mesenteric artery and renal arteries.

Hypothermic circulatory arrest

Very commonly, replacement of the aortic arch requires interruption of blood flow entirely in order to reconstruct epi-aortic arch vessels. Such a manœuvre obviously exposes the entire body to no-flow ischaemia. The basis of end-organ protection under these circumstances is hypothermia and, depending on surgeon preference, this may be between 28°C and 18°C. Often, this will be accompanied by selective cerebral perfusion either anterogradely or retrogradely, and early body reperfusion. Orchestration of this methodology in terms of perfusion priority for heart, brain and body is discussed later.

Clinical anatomy relevant to aortic arch surgery

The aortic arch is divided historically on an anatomical basis into proximal, tranverse and distal sections ( Fig. 44.3 ). The transverse aortic arch is the main arterial conduit positioned at the very epicentre of the superior mediastinum, channelling and distributing blood and serving as a reference point to juxtaposed structures. There is a surgical art to preparing the aortic arch for resection and the approach depends on the intended extent of surgical resection, anatomical variation and pathology.

The distinction between the distal ascending aorta and the proximal aortic arch is slim but has clinical significance. In effect, the proximal arch is a slither of aorta extending along two tangents from the proximal origin of the BCT – one extending perpendicular to the underside of the arch, the other extending under the epi-aortic vessels – and reaching the apex of the inferior aortic arch. For the surgeon, establishing cardiopulmonary bypass and applying a cross-clamp on the distal ascending aorta just proximal to the BCT define the resection as ascending. DHCA and removal of the cross-clamp allow the surgeon to choose to restrict the resection to the ascending aorta as an open distal anastomosis or to undercut the epi-aortic vessels resecting the proximal aortic arch.

The transverse aortic arch is defined by the length that gives rise to the epi-aortic vessels. Clinically, resection of the transverse aortic arch is often defined by the site of distal anastomosis and may involve one, two or three of the epi-aortic vessels (BCT, LCC and LSA).

The distal arch, in a similar way to the proximal arch, is a slither of aorta between the proximal DTA, the origin of the LSA and the apex of the underside of the aortic arch. A more contemporary regional definition of the aortic arch is shown in Fig. 44.1 ; this is based around an arbitrary definition using the epi-aortic vessels and it is certainly more useful clinically to define the extent of treatment, surgical or endovascular, performed.

A number of important nerves traverse the aortic arch (see Fig. 44.3 ). Knowledge of their course is crucial to the aortic arch surgeon in order to avoid significant postoperative morbidity, such as a hoarse voice from recurrent laryngeal nerve (RLN) injury or a paralysed diaphragm from phrenic nerve injury.

The courses of the right and left phrenic nerves (see Fig. 44.3 ) are important in relation to the aortic arch, left internal thoracic artery (internal mammary artery) and pericardium, all of which may be mobilized with the inherent risk of injury. Both phrenic nerves origin­ate from C3, 4 and 5. The right phrenic nerve enters the thorax between the subclavian artery and origin of the right brachiocephalic vein. It passes along the lateral aspect of the SVC and along the pericardium over the right side of the heart, anterior to the hilum of the lung, lying close to the right internal thoracic artery and ending distally at the diaphragm. The left phrenic nerve passes lateral to the LCC and LSA, between the LSA and the subclavian vein, over the arch and down the lateral aspect of the pericardium anterior to the hilum of the lung, lying close to the left internal thoracic artery ( Video 44.2 ).

The courses of the right and left vagus nerves are important as waymarkers to the RLNs. The right vagus passes anterior to the subclavian artery and posterior to the right brachiocephalic vein. Importantly, the nerve then usually gives off the RLN, which hooks under the subclavian artery. This is key for those aortic surgeons chasing the origins of the right common carotid artery and right subclavian artery in aortic arch surgery. Risk of left RLN injury is variable in aortic surgery (0–20%), and knowledge of the course of the right RLN will certainly reduce the risks of bilateral RLN injury and ensuing issues ( Chs 17 and 18 ). The left vagus passes over the transverse arch between the LSA and LCC, giving off the left RLN, which passes under the transverse arch lateral to the ligamentum arteriosum.

Blood supply to the spinal cord and posterior circulation

Monitoring and managing the blood supply to the spinal cord and posterior circulation to the brain are crucial to avoiding stroke and paraplegia. Fig. 44.4 outlines the complex network that the aortic surgeon must understand, particularly when managing the LSA.

The left and right subclavian arteries give off vertebral arteries in their proximal portions. Each vertebral artery passes through the foramina in the transverse processes of all of the cervical vertebrae except the seventh, curves medially behind the lateral mass of the atlas, enters the cranium via the foramen magnum, and joins its fellow to form the basilar artery at the lower pontine border (see Fig. 44.4 ). The largest branch of the vertebral artery is the posterior inferior cerebellar artery. Asymmetry due to arterial hypoplasia, complete absence or unilateral termination of the posterior inferior cerebellar artery is very common. Vertebral arteries are described as left-dominant (approximately 45%), right-dominant (approximately 30%) or co-dominant (approximately 25%). Branches from the basilar artery include the anterior inferior cerebellar and superior cerebellar arteries. The basilar artery ends by dividing into two posterior cerebral arteries, which are joined by the distal branches of the internal carotid arteries via the posterior communicating arteries, forming the arterial circle of Willis. The blood supply of the spinal cord is derived in part from the anterior and posterior spinal arteries, fed segmentally in the cervical region by branches of the vertebral arteries and subsequently by intercostal arteries and lumbar arteries, forming a so-called paraspinal network. Preoperative imaging of the cerebral circulation before considering aortic arch surgery is critical to understanding the blood supply and planning surgery.

Sternotomy

Approaching aortic arch surgery begins with planning the sternal incision. The key questions relate to the proximal extent up on to the neck and placement of the incision median or oblique to the left or to the right. The traditional proximal extent of the incision is between the sternal notch and the manubriosternal junction, depending on the surgeon's preference. The decision is informed by the expected operative extent on the epi-aortic vessels. The proximal extent and form of incision should be planned carefully; extending the incision intraoperatively in a fully heparinized patient should be avoided. Extensive exposure is required to chase resection of a dissected epi-aortic vessel; usually a midline incision up to the inferior aspect of the cricothyroid membrane allows dissection to the left and right, aided by an inverted Travers retractor . Care needs to be exercised in undermining the incision up to the proximal trachea, as there is a significant risk that such patients will require tracheostomy, and communication between the mediastinum and potential tracheostomy needs to be avoided with its inherent risk of infection. Familiarity with the anatomy relating to the epi-aortic vessels is essential and is described below. The inferior aspect of the incision is the xiphisternum. The midline is identified by the intercostal spaces. Sternotomy is standard. Traditional anatomy books place a heavy emphasis on relating structures within the superior mediastinum to the manubrium and sternoclavicular joint; this does not apply here because the sternum is split and retracted, and aortic pathology will distort structures ( Fig. 44.5 ).