Great vessels

Relations

The pulmonary trunk is located entirely within the pericardium, enclosed with the ascending aorta in a common sheath of the visceral layer of serous pericardium (see Figs 57.1 , 58.1 ). The fibrous pericardium gradually blends with and disappears within the adventitia of the pulmonary arteries. Anteriorly, it is separated from the sternal end of the left second intercostal space, and/or the third costal cartilage, by the pericardium, pleura, left lung and the superior part of transversus thoracis. Posterior relations of the proximal part near to its origin are the ascending aorta and left coronary artery, and superiorly, the left atrium. Moving superiorly, the ascending aorta spirals around the pulmonary trunk and comes to lie to the right of its more superior part. The atrial auricles and left and right coronary arteries lie on each side of its origin. The superficial cardiac plexus lies between the branching of the pulmonary trunk and the aortic arch (see Fig. 57.61 ). The tracheal bifurcation and subcarinal lymph nodes are superior and to the right (see Fig. 52.7 ).

Variations and congenital anomalies

The pulmonary trunk is a relatively constant structure and there are minimal variations in healthy individuals. Congenital anomalies include pulmonary atresia and truncus arteriosus (common truncus).

The coronary arteries usually originate from the aortic sinuses, but occasionally may arise from ectopic locations. Most commonly, an ectopic left coronary artery arises from the pulmonary trunk or one of its branches (Anomalous Left Coronary Artery from the Pulmonary Artery (ALCAPA) or Bland–White–Garland syndrome) ( Fig. 58.2 ; Commentary 7.2 ). This potentially fatal condition requires urgent surgical correction because the myocardium is supplied with deoxygenated pulmonary blood instead of systemic blood ( ). Infantile symptoms include pallor, fatigue, irritability, weak cry, cough, dyspnoea, and signs of ischaemia and cardiac failure precipitated during or after feeding, or during defecation or crying. The electrocardiogram may reveal deep narrow Q waves, left ventricular hypertrophy and left axis deviation. Radiologically, cardiomegaly is present with left ventricular and left atrial enlargement. Colour flow imaging can identify the anomalous origin of the left coronary artery. The pulmonary trunk may also give rise to the right coronary artery, the anterior interventricular branch of the left coronary artery, or even both coronary arteries. Typically, there is an extensive development of collateral vessels in the heart ( Fig. 58.3 ). FLOAT NOT FOUND FLOAT NOT FOUND

Pulmonary atresia is caused by a complete obstruction of pulmonary outflow and may be due to an absence or defect of the pulmonary valvular leaflets. It is associated with a blind-ending pulmonary trunk that causes right ventricular hypoplasia. Reduced flow may render the pulmonary trunk atretic, small or even normal in size, making diagnosis challenging. A diminished pulmonary flow is supplied through a patent ductus arteriosus ( ), and a concomitant ventricular septal defect may permit outflow from the right ventricle. Surgical repair is necessary to allow adequate oxygenation of blood throughout the body.

In truncus arteriosus (common truncus), a single common arterial trunk exits the heart and subsequently divides into the pulmonary trunk and the ascending aorta. Early neonatal life is possible because there is usually a coexisting ventricular septal defect; expedited surgical repair is necessary to avoid congestive heart failure, failure to thrive and death.

Relations

The ascending aorta lies within the fibrous pericardium, enclosed in a tube of visceral serous pericardium together with the pulmonary trunk (see Figs 58.1 , 57.1 , 57.4 ). The conus arteriosus of the right ventricle, initial segment of the pulmonary trunk and right auricle are anterior to its lower part. Superiorly, it is separated from the sternum by the pericardium, the pleura and the anterior border of the right lung, loose areolar tissue and the thymus or its remnants. The left atrium, right pulmonary artery and right main bronchus are posterior. The superior vena cava (also partly posterior) and the right atrium are to the right. The left atrium and, more superiorly, the pulmonary trunk are to the left. At least two para-aortic bodies (aortopulmonary paraganglia) lie between the ascending aorta and the pulmonary trunk. Relative to the ascending aorta, the inferior para-aortic body is located on its anterior surface near to the heart, and the middle para-aortic body is located on its right side.

The aortopulmonary window is a space between the pulmonary trunk and aortic arch, bordered by the ascending aorta anteriorly, the descending aorta posteriorly, the mediastinal part of the parietal pleura laterally and the left main bronchus medially (Deutsch and Savides 2005) (see Fig. 58.1 ; Fig. 58.4 ). It contains lymph nodes, fatty tissue, the ligamentum arteriosum, the superficial cardiac plexus, cardiac nerves and the left recurrent laryngeal nerve.

Relations

The left mediastinal part of the parietal pleura is located anterior and to the left of the aortic arch. Deep to the pleura, the arch is crossed, in anteroposterior order, by the left phrenic nerve, left lower cervical cardiac branch of the vagus nerve, left superior cervical cardiac branch(es) of the sympathetic trunk, the left thoracic cardiac branches of the vagus nerve, and the left vagus nerve (see Figure 56.6, Figure 56.7 , 57.61 ). As the latter crosses the aortic arch, its recurrent laryngeal branch hooks inferior to the vessel, to the left posterior aspect of the ligamentum arteriosum, and then ascends on the right of the aortic arch. The left superior intercostal vein ascends obliquely anteriorly on the arch, anterolateral to the left vagus nerve and posteromedial to the left phrenic nerve and pericardiacophrenic vessels. The left lung and pleura separate all these structures from the thoracic wall. Posterior and to the right are the trachea and the deep cardiac plexus, the left recurrent laryngeal nerve, the oesophagus, thoracic duct and vertebral column. Superiorly, the brachiocephalic trunk, left common carotid and left subclavian arteries arise from its convexity (from right to left), and are crossed anteriorly near their origins by the left brachiocephalic vein. The branching of the pulmonary trunk (into pulmonary arteries), left main bronchus, ligamentum arteriosum, superficial cardiac plexus and the left recurrent laryngeal nerve are typically inferior.

Pneumomediastinum and aortic nipple

Pneumomediastinum is an overarching term that describes the presence of air in the mediastinum. It may arise from a wide range of pathological conditions or physiological states, e.g. penetrating trauma, ruptured major airways or oesophagus, hyperventilation or distressed ventilation such as acute asthma, periparturition or diabetic ketoacidosis.

The aortic nipple is the radiographic term used to describe a lateral nipple-like projection located on the aortic knuckle. It corresponds to the end-on appearance of the left superior intercostal vein coursing anteriorly and is not observed in all individuals. It may be mistaken radiologically for lymphadenopathy or an intrapulmonary nodule or neoplasm. Despite their relative independence, the aortic nipple is defined by new contours in cases of pneumomediastinum, taking on an ‘inverted aortic nipple’ appearance ( Fig. 58.5 ). In this position, the inverted aortic nipple may facilitate radiographic discrimination of pneumomediastinum from similar conditions. The presence of a particularly prominent aortic nipple has also been shown to precede pathological conditions such as venous obstruction in the superior and inferior venae cavae or left brachiocephalic vein, and has been identified in conditions where venous flow through the left superior intercostal vein is increased (e.g. portal venous hypertension and certain congenital venous anomalies). FLOAT NOT FOUND

Variations of the aortic arch

The most superior level of the aortic arch is typically about 2.5 cm inferior to the jugular notch, although this may vary. It is located more superiorly, closer to the jugular notch, in infants and again in senescence when it is a result of vascular ectasia.

In a right-sided aortic arch, the aorta curves over the hilum of the right lung and descends to the right of the vertebral column: this is usually associated with transposition of the thoraco-abdominal viscera (situs inversus) ( ). Less often, after arching over the hilum of the right lung, the descending thoracic aorta may pass posterior to the oesophagus to gain a left-sided position, and this is not accompanied by visceral transposition. The presence of a right-sided aortic arch is of relevance when planning the repair of oesophageal atresia in neonates.

The aorta may divide into ascending and descending trunks, the former dividing into three branches to supply the head and upper limbs. Alternatively, it may divide near its origin, producing a double aortic arch; the two branches soon reunite and the oesophagus and trachea usually pass through the interval between them. Entrapment of the oesophagus and/or trachea by a double aortic arch, or by aberrant great vessels (arteries), is referred to as a vascular ring.

There are several variants of this arrangement. The most common are: when a right-sided aortic arch and the upper part of the descending thoracic aorta pass anterior to the oesophagus and trachea, and the ductus arteriosus passes posterior to the oesophagus into an aortic diverticulum; a right-sided aortic arch and the upper part of the descending thoracic aorta pass anteriorly, and the ductus arteriosus is inserted into the left subclavian artery arising as a fourth branch from the aortic arch; a left-sided descending thoracic aorta is attached to a left-sided ductus arteriosus anterior to the oesophagus, and a right-sided aortic arch passes posteriorly; the right superior part of the descending thoracic aorta wraps around the oesophagus and the ductus arteriosus is right-sided; the right upper part of the aorta wraps around the oesophagus and the ductus arteriosus is left-sided ( ).

Branches and variations

From right to left, the brachiocephalic trunk, left common carotid and left subclavian arteries arise from the convex aspect of the aortic arch (see Figs 58.1 , 57.4 ,). These vessels may branch from the beginning of the aortic arch or the superior part of the ascending aorta. The distance between their origins varies, the most frequent being apposition of the left common carotid artery to the brachiocephalic trunk. Multiple variants of their branching pattern have been observed; other branches may arise from the aortic arch, including the inferior thyroid, thyroidea ima, thymic, left coronary and bronchial arteries ( ). The left vertebral artery may arise between the left common carotid and the left subclavian arteries.

The left common carotid artery may arise from the brachiocephalic trunk, when it is known as a bovine arch (7%), although this does not represent the morphology seen in bovines. Rarely, the left common carotid and subclavian arteries may arise from a left brachiocephalic trunk or from the right common carotid artery, with the right subclavian artery arising separately ( ). The right common carotid and subclavian arteries may arise separately, in which case the right subclavian artery often branches from the left part of the aortic arch distal to the left subclavian artery, and usually passes posterior to the oesophagus as an aberrant right subclavian artery (sometimes referred to as a lusoria artery). A diverticular outpouching (diverticulum of Kommerell) may be present at the origin of an aberrant left or right subclavian artery from a right or left aortic arch, respectively ( Fig. 58.6 ). It can become aneurysmal and cause fatal haemorrhage during endoscopy and can form a vascular ring around the oesophagus and trachea, presenting in the neonate as a feeding disorder with failure to thrive. FLOAT NOT FOUND

Rare avian forms of branching have been reported. For example, the right common carotid and right subclavian arteries arise from the aortic arch, and the left common carotid and left subclavian arteries arise from the descending thoracic aorta ( ). Alternatively, two common arterial trunks arise from the aortic arch, the right trunk branching into the left and right common carotid arteries, and the left trunk branching into the left and right subclavian arteries. Another rare order of the branching from the aortic arch is (from right to left): the right subclavian artery, left subclavian artery, followed by the right common carotid and left common carotid arteries, the latter two arising close together and almost forming a common carotid trunk.

The aortic arch may curve posterior to the oesophagus and trachea, instead of passing anteriorly, and may show variation in its branching. For example, another rare avian form reported is with both carotid arteries originating from the same common trunk, and a left subclavian artery originating from the aortic arch, while the right subclavian artery arises from the descending thoracic aorta. One or more of the ductus arteriosi may remain patent.

Very rarely, the external and internal carotid arteries arise separately from the aortic arch and the common carotid artery is absent on one or both sides, or both carotid arteries and one or both vertebral arteries can be separate branches. When a right aortic arch and descending thoracic aorta occur, the arrangement of its three branches is reversed and the common carotid arteries may have a single trunk. Other arteries may branch from it; most commonly, these are one or both bronchial arteries and the thyroidea ima artery.

Coarctation of the thoracic aorta

The aortic lumen is occasionally partly or completely obliterated, either above (preductal or infantile type), opposite or just beyond (postductal or adult type), the entry of the ductus arteriosus. In the preductal type, the length of coarctation is variable, aortic arch hypoplasia is common and the left subclavian and even the brachiocephalic trunk may be involved. Severe forms of infantile coarctation and its extreme form (aortic interruption) may be patent ductus arteriosus-dependent, as there is no time for effective collateral circulation to develop. Prostaglandin infusion prior to transfer, and surgery at a tertiary centre, often provide a good mid- to long-term outlook for such infants.

The postductal type of coarctation has been attributed to abnormal extension of tissue of the ductus arteriosus into the aortic wall, stenosing both vessels as the duct contracts after birth. This form may permit years of normal life, allowing the development of an extensive collateral circulation to the aorta distal to the stenosis ( Figs 58.7 – 58.8 ). High vascularity of the thoracic wall is important and clinically characteristic. Many arteries that arise indirectly from the aorta proximal to the region of coarctation anastomose with vessels that are connected to the aorta distal to the region of coarctation; these vessels form a bypass route and become greatly enlarged. Thus, in the thoracic wall, the thoraco-acromial, lateral thoracic and subscapular arteries (from the axillary artery), the suprascapular artery (from the subclavian artery) and the supreme intercostal artery (from the costocervical trunk) anastomose with other posterior intercostal arteries and the internal thoracic artery and its terminal branches (e.g. the musculophrenic artery) anastomose with many of the posterior intercostal arteries and the inferior epigastric arteries (from the external iliac artery). Posterior intercostal arteries are always involved, and enlargement of their dorsal branches may eventually groove (notch) the inferior margins of the ribs. The radiographic shadow of an enlarged left subclavian artery is also increased. Enlargement of the scapular vessels and anastomoses may lead to widespread interscapular pulsation which is palpated easily with the palm of the hand, and is sometimes heard on auscultation. FLOAT NOT FOUND FLOAT NOT FOUND

Aortic aneurysm formation

Degeneration of the aortic tunica media and intimal dissection play a major role in the pathogenesis of aneurysms affecting the ascending aorta and aortic arch. Smooth muscle cells are lost and elastic fibres degenerate, producing cystic spaces in the media that then fill with mucoid material. The loss of these structural cells leads to weakening of the wall with progressive dilation. Ageing and hypertension are major predisposing factors to cystic medial degeneration and there is a strong link with cigarette smoking. Thoracic aortic aneurysms are categorized by their location. A particular pattern of aortic root involvement, anulo-aortic ectasia, is seen in Marfan syndrome ( ). Descending aortic aneurysms are generally caused by atherosclerosis (90%) and the remainder result from mycotic disease or trauma.

Associations also exist between thoracic aortic aneurysm and other connective tissue diseases such as homocysteinuria and Ehlers–Danlos syndrome. A genetic relation is seen in familial thoracic aortic aneurysm syndrome ( ). Rarely, aneurysms may occur as a result of Takayasu’s arteritis, or infections within the aortic wall. Some aortic aneurysms are incidental findings on chest films or CT studies. Symptomatic cases present with breathlessness, unbearable chest and back pain, hoarse voice, cough and haemoptysis. Early diastolic murmurs caused by aortic regurgitation may be audible. Repair is carried out in patients with symptoms or fusiform dilation measuring more than 5 cm in diameter. Aneurysms may also occur in an aberrant right subclavian artery and may involve the diverticulum of Kommerell, leading to dysphagia and even tracheal compression. Inadvertent rupture may occur during endoscopy because of the retro-oesophageal location.

Aortic dissection

Aortic dissection occurs as a result of degeneration of the tunica media of the aortic wall and is associated with senescence, persistent hypertension or collagen vascular diseases such as Marfan syndrome. Many patients with aortic dissection have a pre-existing aneurysm. Other associations include aortic coarctation, Turner syndrome, cocaine abuse (<1%) and trauma during surgical procedures. There is a higher prevalence in males than females, but after 75 years of age there is no sex difference in prevalence ( ). In a classic aortic dissection, an intimal tear may occur, producing a split into the tunica media that creates a false lumen ( Fig. 58.9 ). If another tear occurs, connection can be made once again with the true lumen (double-barrel aorta). Additional aetiopathologies include penetrating atherosclerotic ulceration, where an atherosclerotic plaque ruptures into the aortic tunica media, and aortic intramural haematoma, where the vasa vasorum haemorrhage into the wall of the aorta ( ). FLOAT NOT FOUND

Aortic dissections are classified as types I, II, IIIa and IIIb (DeBakey classification), or types A and B (Stanford classification). Dissections that include the aorta proximal to the left subclavian artery, with or without distal extension, are classified as type I, II or A. Type I involves up to the entire aorta (ascending, arch and descending), whereas type II involves the ascending aorta only. Dissections that are distal to the left subclavian artery are classified as type IIIa (extend to the respiratory diaphragm), IIIb (extend below the respiratory diaphragm) or B. These cases present acutely with severe retrosternal, neck or interscapular chest pain. Depending on the extent of the dissection, they may be associated with neurological signs, diarrhoea or leg weakness. Extension into the pericardium causes cardiac tamponade and circulatory collapse. Diagnosis is established by echocardiography and on contrast-enhanced CT or magnetic resonance imaging (MRI). Medical management is possible for descending aortic dissections, while surgical repair is essential for ascending aortic or aortic arch dissection.