9 Cardiovascular system

ANATOMY

HEART

Development of the heart

The heart begins to develop towards the end of the third week of gestation as a pair of endothelial tubes which fuse to become the primitive heart tube. This develops within the pericardial cavity from which it is suspended from the dorsal wall by a dorsal mesocardium.

The primitive heart tube develops grooves which divide it into five regions: the sinus venosus, atrium, ventricle, bulbus cordis and truncus arteriosus (Fig. 9.1). The arterial and venous ends of the tube are surrounded by a layer of visceral pericardium. The primitive heart tube then elongates within the pericardial cavity, with the bulbus cordis and ventricle growing more rapidly than the attachments at either end, so that the heart first takes a U-shape and later an S-shape. At the same time it rotates slightly anticlockwise and twists so that the right ventricle lies anteriorly and the left atrium and ventricle posteriorly (Fig. 9.1). Despite this, and an increase in the number of vessels entering and leaving, they still continue to be enclosed together in this single tube of pericardium.

Fig. 9.1 The development of the heart.

As the tube develops, the sinus venosus becomes incorporated into the atrium and the bulbus cordis into the ventricle. Endocardial cushions develop between the primitive atrium and ventricle. An interventricular septum develops from the apex up towards the endocardial cushions.

The division of the atrium is slightly more complicated. A structure called the septum primum grows down to fuse with the endocardial cushions, but leaves a hole in the upper part which is termed the foramen ovale. A second incomplete membrane develops known as the septum secundum. This is just to the right of the septum primum and foramen ovale. Thus a valve-like structure develops which allows blood to go from the right to the left side of the heart in the fetus (Fig. 9.2). At birth, when there is an increased blood flow through the lungs and a rise in the left atrial pressure, the septum primum is pushed across to close the foramen ovale. Usually the septa fuse, obliterating the foramen ovale and leaving a small residual dimple (the fossa ovalis). The sinus venosus joins the atria, becoming the two venae cavae on the right and the four pulmonary veins on the left (Fig. 9.1).

Fig. 9.2 The fetal circulation.

Development of the aortic arches

A common arterial trunk, the truncus arteriosus, continues from the bulbus cordis and gives off six pairs of aortic arches (Fig. 9.3). These curve around the pharynx to join to dorsal aortae which join together lower down as the descending aorta. These aortic arches are equivalent to those supplying the gill clefts of a fish. The first and second aortic arches disappear early, the third remains as the carotid artery, and the fourth becomes the subclavian on the right, and the arch of the aorta on the left, giving off the left subclavian. The fifth artery disappears early and the ventral part of the sixth becomes the right and left pulmonary artery, with the connection to the dorsal aortae disappearing on the right but continuing as the ductus arteriosus on the left connecting with the aortic arch.

Fig. 9.3 Development of the aortic arches.

In the early fetus the larynx is at the level of the sixth aortic arch, and when the vagus gives off its nerve to it this is below the sixth arch. However, as the neck elongates and the heart migrates caudally, the recurrent nerves become dragged down by the aortic arches. On the right the fifth and sixth absorb leaving the nerve to hook round the fourth (subclavian) in the adult, while on the left it remains hooked around the sixth arch (the ligamentum arteriosum) of the adult.

Fetal circulation

Before birth the circulation (Fig. 9.2) obviously differs from that in the adult because oxygen and food must be obtained from maternal blood instead of from the lungs and the digestive organs. Oxygenated blood from the placenta travels along the umbilical vein, where virtually all of it bypasses the liver in the ductus venosus joining the inferior vena cava (IVC) and then travelling on to the right atrium. Most of the blood then passes straight through the foramen ovale into the left atrium so that oxygenated blood can go into the aorta. The remainder goes through the right ventricle with the returning systemic venous blood into the pulmonary trunk. In the fetus the unexpanded lungs present a high resistance to pulmonary flow, so that blood in the main pulmonary trunk would tend to pass down the low resistance ductus arteriosus into the aorta. Thus the best-oxygenated blood travels up to the brain, leaving the less well-oxygenated blood to supply the rest of the body. The blood is returned to the placenta via the umbilical arteries, which are branches of the internal iliac artery. At birth when the baby starts to breathe, there is a rise in the left atrial pressure, causing the septum primum to be pushed against the septum secundum and thus to close the foramen ovale. The blood flow through the pulmonary arteries increases and becomes poorly oxygenated, as it is now receiving the systemic venous blood.

The pulmonary vascular resistance is also abruptly lowered as the lungs inflate, and the ductus arteriosus becomes obliterated over the next few hours or days. This occurs by a prostaglandin-dependent mechanism which causes the muscular component of the ductal wall to contract when exposed to higher levels of oxygen at the time of birth. Closure of the ductus arteriosus is less likely to occur in very premature babies or those with perinatal asphyxia. Ligation of the umbilical cord causes thrombosis and obliteration of the umbilical arteries, vein and ductus venosus. The thrombosed umbilical vein becomes the ligamentum teres in the free edge of the falciform ligament.

Congenital abnormalities of the heart and great vessels

Given the complex nature of the development of the heart, it is hardly surprising that there are a number of congenital abnormalities, which may be classified as follows.

Malposition

This includes dextrocardia, which is a mirror image of the normal anatomy, or situs inversus, where there is inversion of all the viscera. (Appendicitis may present as left iliac fossa pain in this condition.) These are very rare in normal life, but slightly more common in exams! In pure dextrocardia there is no intracardiac shunting and cardiac function is normal.

Left to right shunts (late cyanosis)

Right to left shunts (cyanotic)

Fallot’s tetralogy

The four features of this abnormality are VSD, a stenosed pulmonary outflow tract, a wide aorta which overrides both the right and left ventricles, and right ventricular hypertrophy. Because there is a right to left shunt across the VSD there is usually cyanosis at an early stage, depending mainly on the severity of the pulmonary outflow obstruction.

Obstructive non-cyanotic abnormalities

Coarctation of the aorta

This is a narrowing of the aorta which is normally just distal to the ductus arteriosus and is thought to be an abnormality related to the obliterative process of the ductus. There is hypertension in the upper part of the body, with weak delayed femoral pulses. Extensive collaterals develop to try and bring the blood down to the lower part of the body, resulting in large vessels around the scapula, anastomosing with the intercostal arteries and the internal mammary and inferior epigastric arteries. These enlarged intercostals usually cause notching of the inferior border of the ribs, which is a diagnostic feature seen on chest x-ray. This is another condition which used to require a major thoracic operation but now can frequently be treated by balloon angioplasty.

Abnormalities of the valves Any of these may be imperfectly formed and tend to cause either stenosis or complete occlusion (atresia). The pulmonary and the aortic valves are more frequently affected than the other two.

Anatomy of the heart

Surfaces and borders

The heart (Fig. 9.4) is a muscular organ which pumps the blood around the arterial system. It consists of four chambers: right and left atria and right and left ventricles. When viewed from the front it has three surfaces and three borders. The anterior surface consists almost entirely of the right atrium and right ventricle with a narrow strip of left ventricle on the left border and the auricle of the left atrium just appearing over the top of this. It lies just behind the sternum and costal cartilages. The posterior surface consists of the left ventricle and left atrium with the four pulmonary veins entering it, and the right edge is visible. The inferior or diaphragmatic surface consists of the right atrium with the IVC entering it and the lower part of the ventricles.

Fig. 9.4 The heart and great vessels. anterior view. posterior view.

Source: Rogers, A W Textbook of anatomy; Churchill Livingstone, Edinburgh (1992).

The three borders are the right, the inferior and the left. The right is made up entirely of the right atrium with the SVC and IVC. This extends from the third to the sixth right costal cartilage approximately 3 cm from the midline. The inferior border consists of the right ventricle and the apex of the left ventricle. It extends from approximately 3 cm to the right of the midline at the level of the sixth costal cartilage to the apex which is in the fifth left interspace in the mid-clavicular line (approximately 6 cm from the midline). The left border extends from the apex up to the second left interspace approximately 3 cm from the midline (Fig. 9.5). The outline of the heart can be seen clearly on a chest x-ray (Fig. 9.6). The apex of the heart is the lowest and most lateral point on the chest wall at which the cardiac impulse can be felt. As the heart is in contact with the diaphragm, it moves with each respiration. However, the anterior fibres of the diaphragm are short, so that the central tendon on which the heart rests moves relatively less.

Fig. 9.5 The surface projections of the heart. A, P, T and M indicate auscultation areas for the aortic, pulmonary, tricuspid and mitral valves.

Source: Rogers op. cit.

Fig. 9.6 PA radiographs of the chest. inspiration. expiration.

Source: Rogers op. cit.

Chambers of the heart

The heart (Fig. 9.7) consists of a right side which pumps blood through the lungs and the left side which pumps it through the systemic circulation. The atria collect blood from the veins and pump it into the ventricles during ventricular relaxation (diastole). When the ventricles are full they contract (systole), the valves between the atria and ventricles close, and the ventricles discharge their contained blood into the appropriate great vessel.

Fig. 9.7 The interior of the right side of the heart.

Source: Rogers op. cit.

Right atrium

This receives blood from the SVC and IVC and from the coronary sinus. Running down between the venae cavae is a muscular ridge, the crista terminalis, which separates the smooth walled posterior part of the atrium, which is derived from the sinus venosus, from the rougher area due to the pectinate muscles derived from the true atrium. The interatrial septum has an oval depression (the fossa ovalis) which marks the site of the fetal foramen ovale (Fig. 9.7).

Right ventricle

The walls (Fig. 9.7) are much thicker than those of the atrium and there are a series of muscular thickenings, the trabeculae carnae. The tricuspid valve lies between the right atrium and right ventricle, and the three valve cusps are referred to as septal, anterior and posterior. The atrial surfaces are smooth, but the ventricular surfaces have a number of fibrous cords, the chordae tendineae, which attach them to the papillary muscles on the wall of the ventricle. These prevent the valve cusps from being everted into the atrium when the ventricle contracts.

The pulmonary valve lies just above the right ventricle at the beginning of the pulmonary trunk and consists of three semilunar cusps each with a thickening in the centre of its free edge. The pulmonary trunk has a dilatation or sinus alongside each of the cusps.

Left atrium

The left atrium (Fig. 9.8) also develops both from a combination of the fetal atrium and the sinus venosus. There are four pulmonary veins, two from each side. On the interatrial surface there is again an impression representing the site of the fetal interatrial foramen.

Fig. 9.8 The interior of the left side of the heart.

Source: Rogers op. cit.

Left ventricle

The walls of the left ventricle (Fig. 9.8) are three times thicker than those of the right ventricle because the vascular resistance of the systemic circulation is so much greater than that of the pulmonary vasculature. The mitral valve lies between the atrium and ventricle and has two large cusps which were thought by early anatomists to look like a bishop’s mitre. Chordae tendineae run from the ventricular surfaces and margins of these cusps to papillary muscles in the ventricular wall, as with the right ventricle.

The aortic valve is similar to the pulmonary valve but stronger to cope with the higher pressure. There are three cusps – right, left and posterior – and each also has a central nodule in the free edge and a sinus or dilatation in the aortic wall alongside each cusp. The left and right coronary arteries open from the left and right valves, respectively. In about 1% of the population the aortic valve is bicuspid, and these individuals are more likely to develop calcification and stenosis in later life.

Fibrous skeleton

The two atrioventricular orifices are bound together by a conjoined fibrous ring in the form of a figure of eight which acts as a fibrous skeleton to which the valves are attached and which also serves for attachment of the muscles of both the atria and the ventricles. This provides a tough yet flexible fibrous skeleton which helps to maintain the shape and position of the heart, but allows some change of shape during contraction.

Conducting system

Although cardiac muscle is similar to skeletal muscle in many ways, it does have certain differences. Cardiac muscle cells tend to be shorter and are frequently Y-shaped and are linked at each end to other muscle cells. At the sites of attachment there is an intercalated disc which, as well as anchoring the membranes of the cells, permits the spread of electrical activity. Cardiac muscle cells are able to contract both spontaneously and rhythmically, and indeed isolated cells in culture contract regularly. As all the cells are in contact with each other and can all contract spontaneously, those with the fastest rate of contraction will drive the others. These are situated in the wall of the right atrium at the upper end of the crista terminalis (Fig. 9.9) and are termed the sinoatrial node (SA node or ‘pacemaker of the heart’). From there the cardiac impulse spreads through the atrial muscles to reach the atrioventricular node, which lines the atrial septum close to the opening of the coronary sinus. From there the atrioventricular bundle (of His) passes through a channel in the fibrous skeleton of the heart to the membranous part of the interventricular septum, where it divides into a right and left bundle branch. The left bundle is larger than the right and divides into an anterior and posterior fascicle. These run underneath the endocardium to activate all parts of the ventricular musculature in such a way that the papillary muscles contract first and then the wall and septum in rapid sequence from the apex towards the outflow track, with both ventricles contracting together. The atrioventricular bundle is normally the only pathway through which impulses can reach the ventricles.

Fig. 9.9 The conducting system of the heart.

Source: Rogers op. cit.

Blood supply to the heart

The arterial supply (Fig. 9.4) is of great clinical importance, as coronary occlusion is the chief cause of mortality in the western world. The right and left coronary arteries arise from the anterior and the left aortic sinuses, respectively, just above the aortic valve, and the main branches lie in the interventricular and the atrioventricular grooves.

Right coronary artery

This passes between the pulmonary trunk and the right atrium and runs along the atrioventricular groove around the inferior border to the diaphragmatic surface. It ends by anastomosing with the terminal branch of the left coronary artery. The main branches are an artery to the SA node and adjacent atrium, the right marginal artery and the posterior interventricular which really runs inferiorly and is often called by clinicians the posterior descending artery. This branch also supplies the AV node and bundle, and parts of the right and left bundle branches.

Left coronary artery

Arising from the left aortic sinus the left coronary artery (the left main stem) varies from 4–10 mm in length and is the most important artery in the human body, in that occlusion will invariably lead to rapid demise! If stenosis of this artery is diagnosed, urgent operation is required to bypass it. It continues passing to the left behind the pulmonary trunk, reaching the atrioventricular groove. It is initially under cover of the left auricle, where it divides into two branches of equal size: the anterior interventricular (left anterior descending) and the circumflex artery. The circumflex artery continues around the left surface of the heart in the atrioventricular groove to anastomose with the terminal branches of the right coronary artery. The left anterior descending (also known as ‘the widow maker’!) runs down to the apex of the heart in the anterior interventricular groove, supplying the walls of the ventricles down the interventricular septum. It gives off the diagonal branch and goes on to anastomose with the posterior interventricular artery. However the natural anastomosis is poor and unless there has been a gradual stenosis giving time for collaterals to develop, sudden occlusion of a mild stenosis from plaque rupture (see p. 271), is almost invariably fatal, hence it’s nickname.

There are some reasonably common variations. Firstly, the left coronary and circumflex artery may be larger and longer than usual and give off the posterior interventricular artery before anastomosing with the right coronary, which is smaller than usual. This occurs in approximately 10% of the population and is known as ‘left dominance’. Another 10% have ‘codominant’ coronary circulation with equal contribution to the posterior interventricular branch. In approximately one-third of individuals the left main stem may divide into three rather than two branches. The third branch, the intermediate, lies between the left anterior descending and the circumflex and may be of large calibre and supply the lateral wall of the left ventricle.

The blood supply to the conducting system is of clinical importance. In just under 60% of the population the SA node is supplied by the right coronary artery, while in just under 40% it is supplied by the circumflex (dual supply in 3%). The AV node is supplied by the right coronary artery in 90% and circumflex in 10%.

Venous drainage

There are three groups of veins of the heart (Fig. 9.4):

1. Some tiny veins that drain directly into the chambers of the heart (venae cordis minimae).

2. The anterior cardiac veins, which are small and open directly into the right atrium.

3. The coronary sinus, which is the main venous drainage; it lies in the posterior atrioventricular groove and opens into the right atrium just to the left of the mouth of the IVC. It has three main tributaries:

• the great cardiac vein, which ascends in the anterior interventricular groove next to the left anterior descending artery;

• the middle cardiac vein, which drains the posterior and inferior surface and lies next to the posterior interventricular artery; and

• the small cardiac vein, which accompanies the right marginal artery and drains into the termination of the coronary sinus.

Nerve supply to the heart

The sympathetic supply (cardio-accelerator) is from the upper thoracic segment of the spinal cord through the sympathetic trunk, and the parasympathetic supply is from the vagus (cardio-inhibitor), and the fibres of each go via the superficial and deep cardiac plexuses.

Pain fibres pass through sympathetic ganglia to spinal nerves via the white rami communicantes. The close proximity with the cervical and thoracic spinal nerves may explain the site of referred cardiac pain to the chest, neck and arm.

Pericardium

Fibrous pericardium

The heart and the roots of the great vessels are contained within the fibrous pericardium. It is fused with the adventitia of the great vessels. You will remember from the development of the heart that the pericardium surrounded the original primitive heart tube, which subsequently had two arteries and two veins at each end and then, as the heart enlarged, folded upon itself so that the arteries and the veins were close to each other. This still applies, and the two arteries become the aorta and the pulmonary trunk while the veins to the right atrium become the SVC and IVC and to the left the four pulmonary veins, and these latter two structures become incorporated into their respective atria. Thus, the SVC and IVC and the four pulmonary veins are all invested with the same layer of fibrous pericardium, while there is another layer investing the aorta and the pulmonary trunk, and the gap between the two becomes the transverse sinus while the blind end coming up between the four pulmonary veins and the IVC becomes the oblique sinus.

The fibrous pericardium can stretch very gradually if there is a gradual enlargement of the heart, but if there was a sudden increase in the volume of its contents, such as from bleeding, then it cannot stretch and will embarrass the function of the heart (cardiac tamponade).

Serous pericardium

This covers the heart and the origin of the great vessels and fuses with the fibrous pericardium at the sites around the great vessels just described. This is a very small space between the two layers, which normally has a small amount of fluid allowing lubrication for movement of the heart within the pericardium.

Clinical features

Cardiac arrest

Because the bulk of the heart and especially the ventricles is just behind the sternum, regular compression there can be used for external cardiac massage in cardiac arrest until more definitive treatment can be given.

Cardiac tamponade

A chronic pericardial effusion may be drained by inserting a needle just to the left of the xiphisternum, pointing upwards at an angle of about 45º and slightly laterally towards the tip of the left scapula. This is done under both electrocardiogram (ECG) and image intensifier control when a guidewire is passed through a needle and then a catheter over the guidewire, with minimal aspiration till the catheter is in place. This reduces the risk of the right ventricle expanding on to the needle, with the risk of potentially fatal myocardial laceration, if aspiration is done earlier before the needle has been removed. In an acute cardiac tamponade if there is not time to get imaging it is safer to make an incision just to the left of the xiphisternum and deepen the wound in the same direction as previously described, but using a combination of forcep and finger dissection. If the diagnosis is correct, a bulging pericardium will be felt; if it is not, no harm has been done.

Cardiac surgery

Thoracotomy

The most common approach to the heart for cardiac surgery is the median sternotomy in which the sternum is split in the midline, the diaphragm detached and tissues behind dissected away carefully avoiding damaging the pleura, particularly on the right, as it may cross the midline a little. Other methods are lateral thoracotomy in which the approach is through the upper border of the chosen rib, trimming the periosteum off and thus avoiding damage to the intercostal nerve and vessels which run in a groove just below the rib.

Cardiopulmonary bypass

The superior and inferior venae cavae are cannulated through the wall of the right atrium to take blood to the bypass machine which will oxygenate the blood, and this will then be brought back through a cannula in the aortic arch, usually proximal to the brachiocephalic trunk.

Coronary artery bypass grafts

Traditionally the great saphenous vein has been used to anastomose from the ascending aorta to the relevant coronary vessel distal to the block. The ten-year results are that approximately one-third are normally patent, one-third are stenosed and one-third blocked. The internal thoracic (internal mammary) artery may be anastomosed directly to the relevant coronary artery with better results, so much so that if an extra graft is needed or the internal mammary is unavailable, then the non-dominant radial artery may be used, in which case it is obviously very important to check the ulnar blood supply to the hand first. Some cardiac surgeons are prepared to use both internal thoracic arteries which may compromise the blood supply to the sternum and impair healing and/or resistance to infection, should it occur. However the majority of patients with coronary artery disease who need intervention are treated with balloon angioplasty with or without stent. This leaves the cardiac surgeons to operate on the complex multi-vessel patients with arterial occlusion, frequently in high risk patients, where angioplasty has often been done as an emergency procedure.

Septal defects

The atrium is approached through its right border, thus avoiding the SA node, whereas the right ventricle can be incised vertically or transversely, avoiding any obvious arteries or veins. The left atrium can be incised behind the interatrial groove and in front of the two pulmonary veins in order to approach the mitral valve.

Transplantation

The patient’s heart is removed, incising through the right atrium, leaving the two venae cavae, the posterior wall of the atrium and the region of the SA node intact. The posterior part of the left atrium with the four pulmonary veins is also left in situ. The incision continues through the aorta and pulmonary trunk, and the donor heart is trimmed in a similar way and anastomosed along this line described.

AORTA AND GREAT VESSELS

Ascending aorta

This measures approximately 5 cm, and the whole of it is within the fibrous pericardium along with the pulmonary trunk. It starts at the aortic valve and goes up and slightly to the right, ending to the right of the sternum at the level of the second right costal cartilage.

Branches

The left and right coronary arteries are the only branches; these have already been described on pages 230 and 231.

The arch of the aorta

This is a continuation of the ascending aorta and travels first superiorly and to the left, and slightly posteriorly, crossing the anterior surface of the trachea and posteriorly over the root of the left lung, and finishing just to the left of the fourth thoracic vertebra where it becomes the descending aorta (Fig. 9.10). Its apex reaches the midpoint of the manubrium sterni.

Fig. 9.10 The relations of the arch of the aorta.

Source: Rogers op. cit.

Descending thoracic aorta

This is the continuation of the arch and starts opposite the lower border of the 4th thoracic vertebra and slightly to the left of it. It ends in the midline at the lower border of the 12th thoracic vertebra, where it passes behind the median arcuate ligament of the diaphragm.

Branches

These can be classified into three groups.

Lateral segmental branches

These are the posterior intercostal arteries that supply the lower nine of the eleven intercostal spaces. Each artery gives off a dorsal and a lateral cutaneous branch. The dorsal branch gives off a spinal branch to supply the spinal cord. The blood supply to the cord consists of the anterior and posterior spinal arteries, which descend in the pia from the intracranial part of the vertebral artery. They are reinforced by segmental arteries, and in the thoracic region these are the dorsal branches of the 2nd to 11th posterior intercostal arteries. These supply the radicular arteries to the spine, which are a very important contribution to reinforce the longitudinal vessels. As a consequence they are known as ‘booster’ or ‘feeder’ vessels. These are very variable in size and position. The largest one is known as the arteria radicularis magna (or artery of Adamkiewicz), which most commonly arises at the 10th or 11th thoracic level but may arise anywhere up to the 4th thoracic level. Operations on the thoracic spine or thoracic aneurysms may interfere with the parent stems of these radicular vessels, which may result in damage to the spinal cord, causing paraplegia.

Lateral visceral (bronchial)

These supply the bronchial walls and substance of the lung excluding the alveoli.

Midline branches

There are four or five oesophageal branches.

Relations

Anteriorly are the root of the lung, the pericardium of the left atrium, and below that the posterior fibres of the diaphragm. Anteriorly and to the right lie the oesophagus and trachea; lower down the oesophagus becomes anterior and then moves to its left as it descends. Posteriorly are the vertebral column and hemiazygos veins, to the right are the azygos veins and thoracic duct and pleura and lung, and on the left the pleura and lung.

Abdominal aorta

The abdominal aorta (Fig. 9.11) commences at the aortic opening of the diaphragm at the level of the 12th thoracic vertebra, descending to the 4th lumbar vertebra where it divides into the two common iliacs. It tapers as it gives off a number of large branches.

Fig. 9.11 The abdominal aorta and the IVC.

Source: Rogers op. cit.

Branches

Posterior lateral branches to the body wall

There are five-paired branches: the inferior phrenic artery and four lumbar arteries.

Paired to viscera

There are three-paired visceral arteries: the suprarenal, the renal arteries and the testicular or ovarian arteries.

Midline unpaired branches to the viscera

There are three such branches, as follows:

• The coeliac trunk supplies the foregut and its derivatives which are the stomach, duodenum, liver, gallbladder and part of the pancreas. The coeliac trunk arises from the aorta, immediately below the aortic opening in the diaphragm.

• Superior mesenteric artery supplies the midgut, i.e. from the middle of the second part of the duodenum to the commencement of the left third of the transverse colon, and it arises a centimetre below the coeliac trunk.

• Inferior mesenteric artery supplies the hindgut from the left third of the transverse colon down to the rectum, where it terminates as the superior haemorrhoidal arteries. It arises from the lower third of the abdominal aorta, and is a much smaller artery than the coeliac and the superior mesenteric. It anastomoses with the superior mesenteric via the marginal artery (see Chapter 17).

Terminal branches

These are two common iliacs and the median sacral.

Relations

To the right from above downwards are the right crus of the diaphragm, the cisterna chyli and the commencement of the azygos vein. From the level of the superior mesenteric artery downwards, the IVC is closely applied to the right side of the aorta, although it gradually becomes more posterior at the lower end so that the iliac veins lie behind the iliac arteries.

To the left is the left crus of the diaphragm, the fourth part of the duodenum, the duodenojejunal flexure and the left sympathetic trunk.

Posteriorly are the upper four lumbar vertebrae.

Anteriorly at the level of the coeliac trunk, the lesser sac of peritoneum separates the aorta from the lesser omentum and liver. Below that, the left renal vein crosses the abdominal aorta immediately below the origin of the superior mesenteric artery. This is at the level of the neck of the vast majority of abdominal aortic aneurysms. It is usually possible to get a clamp on just below the renal vein, but occasionally the aneurysm extends high up, stretching the renal vein like a ribbon across it. Because the left renal vein has tributaries from the left adrenal and from the left ovarian or testicular, the left renal vein can be divided providing it is sufficiently far to the right not to impair the entrance of these vessels, which can then act as venous collaterals. The inferior mesenteric vein also runs quite close to the aorta at this level. In an elective aneurysm this is not a problem, but when there is a large haematoma following a leak, it is possible to damage it if one is not aware of its presence. Also the third part of the duodenum may be adherent to an aneurysm, which may be a particular problem if it is an inflammatory aneurysm. When the anastomosis between a graft and aorta has been done, it is important to have some tissue between it and the duodenum (usually the wall of the aneurysm sac is used). If this is not done there is a small risk of a fistula developing between the anastomosis and the duodenum (aortoduodenal fistula) which is an uncommon but serious cause of haematemesis and melaena. The pancreas lies anterior to the aorta with the third part of the duodenum below. Below this lie the parietal peritoneum and peritoneal cavity with the line of attachment of the mesentery to the small bowel.

It should be noted that in a slim person the aorta and IVC are remarkably close to the anterior abdominal wall. The lumbar vertebrae have a large body, spinal canal and spinous process. These vessels are thus at risk, for example, when inserting a needle to obtain a pneumoperitoneum. It is also worth noting that the bifurcation of the aorta is approximately at the level of the umbilicus, so that aneurysms of the abdominal aorta are normally above this level (although they may, of course, involve the common iliacs).

Other great vessels of the thorax

These are systemic arteries, namely the brachiocephalic, left common carotid and left subclavian artery, and veins: right and left brachiocephalic vein and the SVC. In addition there are the pulmonary trunk, right and left pulmonary arteries and the four pulmonary veins which are the great vessels of the pulmonary circulation (see Chapter 11).

The brachiocephalic artery

This is the first and largest of the three great arteries arising from the aortic arch. It originates from the apex of the arch in the midline, travelling superiorly and posteriorly to the right, and it terminates behind the right sternoclavicular joint by dividing into the right subclavian and right common carotid artery.

There are normally no branches, though occasionally the thyroidea ima artery may arise from it, supplying the lower part of the thyroid. It lies behind the left brachiocephalic vein and in front of the trachea.

Right subclavian artery

This arises from the bifurcation of the brachiocephalic artery and courses to the outer border of the first rib where it becomes the axillary artery (Fig. 9.12). It arches laterally over the apex of the lung to reach the superior surface of the first rib, where it lies in a groove just behind the insertion of the scalenus anterior. It is divided into three parts by the scalenus anterior muscle. The first part is medial to it and gives off three branches.

Fig. 9.12 The subclavian and axillary arteries.

Source: Rogers op. cit.

• The vertebral artery is the most important branch of the subclavian. It crosses the dome of the cervical pleura and passes through the transverse foramina of the upper six cervical vertebrae. It then turns posteromedially over the posterior arch of the atlas through the foramen magnum, where it joins its fellow from the other side in front of the pons to form the basilar artery. The vertebral artery gives off the anterior and posterior spinal arteries and the posterior inferior cerebellar arteries.

• The thyrocervical trunk gives off the inferior thyroid artery, the transverse cervical and suprascapular arteries.

• The internal thoracic artery (formerly known as internal mammary, Fig. 9.13) runs anteriorly and downwards over the pleura to reach the anterior ends of the intercostal spaces, giving off anterior intercostal branches, a musculophrenic artery, and finishing as the superior epigastric artery. Thus it supplies the whole of the anterior body wall down to the umbilicus. This artery is clinically important, because it can be used for coronary artery bypass grafts by mobilising it and anastomosing it directly to the coronary arteries beyond a stenosis or block. It may also be damaged in stab wounds of the chest.

Fig. 9.13 The internal thoracic artery and vein and the anastomoses in the rectus sheath.

Source: Rogers op. cit.

The second part of the subclavian artery lies deep to the scalenus anterior muscle. This gives off the costocervical trunk which supplies the deep structures of the neck, and also the superior intercostal artery which gives off the first and second posterior intercostal arteries.

The third part is lateral to the scalenus anterior and normally has no branches.

Relations

It is closely related to the pleura at the apex of the lung, being separated from the lung by the suprapleural membrane. The right vagus crosses the anterior surface of the artery at its medial end and gives off the recurrent laryngeal nerve which loops under the artery, travelling posteromedially, and then back up to the larynx between the oesophagus and trachea initially, and closely behind the thyroid higher up. The cervical sympathetic chain also divides into two branches which loop around the anterior and posterior surface of the artery, reuniting on the other side.

Behind the scalenus anterior muscle, the artery is closely related to the lower trunk of the brachial plexus posteriorly, and the upper and middle trunks are superior to it. The phrenic nerve runs down in front of the scalenus anterior, crossing it from lateral to medial. In surgical exploration of the subclavian artery, the scalenus anterior is divided to expose the artery, the phrenic nerve initially being retracted medially.

Cervical rib

A cervical rib is a common abnormality occurring in approximately 1 in 200 of the population, and in half of these it is bilateral. However, they only rarely cause symptoms. These may be neurological, arising from pressure on the lowest trunk of the brachial plexus, resulting in paraesthesia along the ulnar border of the forearm and wasting of the small muscles of the hand (T1). This tends to occur with smaller cervical ribs and fibrous bands. When there is a large cervical rib with a bulbous end, this may cause pressure on the subclavian artery. This may result in poststenotic dilatation. The dilated part may develop thrombi in the wall and these may break off and occlude the distal vessels of the arm and hand, sometimes with very serious consequences.

Left common carotid artery

The left common carotid artery is the second branch of the aortic arch arising slightly to the left of the midline. The trachea lies posteriorly, and the artery ascends to the thoracic inlet, passing behind and slightly to the left of the sternoclavicular joint, from where it continues up into the neck. There are no branches in its thoracic course.

Left subclavian artery

This is the third and most posterior branch of the arch of the aorta. It ascends posterior and to the left of the common carotid artery to the thoracic inlet, where it arches over with similar course and relations to those of the right subclavian artery, which have previously been described. There are no branches in the thoracic part of the left subclavian.

The great systemic veins of the thorax

The SVC which carries blood into the right atrium is formed from the union of the right and left brachiocephalic veins (Fig. 9.10). These receive blood from the head and neck and upper limbs as well as from the upper half of the body wall of the trunk.

Left brachiocephalic vein

The vein starts behind the medial end of the left clavicle by the union of the left subclavian and internal jugular veins. It runs obliquely downwards and to the right to join the right brachiocephalic behind the first right costal cartilage. Thus the left brachiocephalic vein is considerably longer than the right.

Tributaries

These are the same as for those on the right but in addition the superior intercostal veins drain into it.

Relations

At its origin it lies anterior to the cervical pleura. As it passes to the right it lies anterior to the left internal thoracic artery, the left phrenic nerve, the left subclavian artery, the left vagus nerve and the left common carotid artery and then the trachea and the brachiocephalic artery. The manubrium sterni and the remnant of the thymus gland lie anteriorly, with the aortic arch inferiorly.

Superior vena cava

This starts behind the first right costal cartilage by the union of the two brachiocephalic veins. It passes inferiorly to enter the right atrium behind the third right costal cartilage. It is important to be aware of these landmarks when inserting a central venous pressure line since the end should lie in the SVC, and this should be checked on x-ray. The lower part of the SVC is within the fibrous pericardium. It receives one other major tributary, which is the azygos vein, into which most of the venous drainage from the thoracic and abdominal walls drains (Fig. 9.14).

Fig. 9.14 The veins of the posterior chest and abdominal wall.

Source: Rogers op. cit.

Relations

Anteriorly are the right lung and pleura, the right internal thoracic artery and the medial ends of the upper two intercostal spaces. Posteriorly are the trachea, the right vagus and lung and pleura lateral in the upper part. Laterally are the right phrenic nerve and right pleura and lung, and medially is the ascending aorta.

BLOOD SUPPLY TO THE TRUNK

Arterial supply to the body wall

This comes from three sources: firstly the intersegmental branches from the aorta, secondly the branches from the subclavian and axillary arteries, and thirdly the branches from the external iliac artery.

Branches from the subclavian and axillary arteries

The internal thoracic artery has already been described and is shown in Fig. 9.13.

Branches of the external iliac artery

These are:

• the inferior epigastric artery, which arises from the external iliac just above the inguinal ligament, and medial to the deep inguinal ring enters the rectus sheath to supply the rectus abdominis muscle and to anastomose with the superior epigastric artery; and

• the deep circumflex iliac artery.

The superior and inferior epigastric vessels have a good anastomosis. They can each be used as the basis for plastic procedures. The so-called transverse rectus abdominus myocutaneous flap (TRAM flap) is sometimes used for breast reconstruction following mastectomy. A flap of upper rectus abdominis muscle and a transverse elliptical piece of skin attached to it are swung up to fill the defect in the breast region, being kept alive by blood from the internal thoracic artery and vein. Similarly the inferior epigastric artery and vein are such good vessels that a ‘free flap’ of the lower part of the rectus abdominus muscle and the overlying skin can be excised and moved to another part of the body, providing there is a suitable artery and vein to which they can be anastomosed using microvascular techniques.

Venous drainage of the body wall

This consists of the following.

Intersegmental veins

These are equivalent to the arteries described.

Azygos veins

These are three longitudinal veins lying on the bodies of the thoracic vertebrae (Fig. 9.14). There is a single azygos vein on the right, while on the left there are the hemiazygos and the accessory hemiazygos.

Vertebral venous plexus

This lies in the external surface of the vertebrae and is also known as Batson’s plexus.

Veins in the anterior chest abdominal wall

These are equivalent to the arterial supply.

Iliac arteries

See Fig. 9.11.

Common iliac artery

The aorta divides into the common iliac arteries to the left of the midline, at the level of the body of the 4th lumbar vertebra. They pass downwards and laterally to bifurcate into internal and external iliac in front of the sacroiliac joint at the level of the sacral promontory. The ureter passes just in front of the common iliac artery at its bifurcation. This is an easy site at which to identify the ureter at operation. There are normally no branches of the common iliac artery.

External iliac artery

This is a continuation of the common iliac artery which has travelled downward and laterally to reach the mid-inguinal point, where it passes deep to the inguinal ligament to enter the thigh as the femoral artery.

The branches are the inferior epigastric and the deep circumflex iliac artery. Remember it is the inferior epigastric which runs medial to the deep inguinal ring, so that a hernia lateral to it is an indirect hernia, whereas one medial to it is a direct hernia.

Internal iliac artery

This runs inferiorly to end opposite the upper margin of the greater sciatic notch by dividing into an anterior and posterior trunk. These supply the pelvic organs, perineum, buttock and anal canal. The internal iliac vein lies posteriorly and the ureter anteriorly.

In the fetus the internal iliac arteries are large, and each anterior trunk gives off an umbilical artery. These fibrose shortly after birth and subsequently become the medial umbilical ligaments, which are fibrous cords running up to the umbilicus.

Iliac veins

The external iliac veins (Fig. 9.11) run at first medially and, as they ascend and become common iliac veins, they run posterior to the iliac arteries. They join at the level of the fifth lumbar vertebra behind the right common iliac artery. Thus the left iliac vein is longer than the right. The tributaries of the internal and external iliac veins are equivalent to those of the arteries. The common iliac veins lie behind and slightly to the right of the common iliac arteries, to which they are very closely related. In aortoiliac operations, when the iliac arteries need to be clamped, great care is needed in dissecting to avoid damage to the iliac veins.

Inferior vena cava

From its origin at the level of the 5th lumbar vertebra to the right of the midline and behind the right common iliac artery, the IVC ascends vertically through the abdomen, piercing the central tendon of the diaphragm to the right of the midline to empty into the right atrium (Fig. 9.11). It is larger than the aorta, and as it ascends, is related anteriorly to the small intestine, the third part of the duodenum, the head of the pancreas with the common bile duct and then the first part of the duodenum. It lies in a deep groove in the liver before piercing the diaphragm. It receives the right and left hepatic veins from the liver. Sometimes these fuse to give one trunk going into the vena cava, but occasionally the central hepatic vein opens separately. In partial liver resections or in operations for transplantation, it is obviously important to know the precise anatomy prior to surgery. See Chapter 17.

Lymphatics

The lymphatics (Fig. 9.15) from the abdomen and lower limbs drain into the cisterna chyli, which lies between the abdominal aorta and the right crus of the diaphragm. It passes through the aortic opening to become the thoracic duct, ascending behind the oesophagus. At the level of T5 it inclines to the left of the oesophagus and runs upwards behind the left carotid sheath. It then passes around and over the left subclavian artery and drains into the commencement of the brachiocephalic vein. The left jugular, subclavian and mediastinal lymph trunks, draining the head and neck, the left upper limb and the thorax, respectively, usually join the thoracic duct shortly before it enters the brachiocephalic vein, although they may open directly into it. The equivalent lymph vessels on the right join to become the right lymphatic duct which enters the origin of the right brachiocephalic vein.

Fig. 9.15 The cisterna chyli, thoracic ducts and right lymph duct.

Source: Rogers op. cit.

It is important to be aware of the thoracic duct in operations on the neck in this area, particularly block dissection of the neck. If the thoracic duct is damaged and not ligated, then a troublesome chylous lymph-atic leak will result. Damage to the thoracic duct in the thorax may occasionally occur from fractures of the thoracic spine, or at surgery, and may result in a chylothorax.

Blood supply of the head and neck

The brachiocephalic artery and the left common carotid artery in the chest have already been described (pp. 235–237). Each common carotid artery enters the neck (Fig. 9.16), from behind the sternoclavicular joint, and thereafter on both sides they have a similar course and relationships. They ascend in the carotid fascial sheath with the internal jugular vein lying laterally and the vagus nerve between and somewhat behind them. The cervical sympathetic chain ascends immediately posterior to the carotid sheath, while the sternocleidomastoid muscle is superficial to it. The carotid sheath is crossed superficially by the omohyoid muscle. At the level of the upper border of the thyroid cartilage the common carotid artery bifurcates into the internal and external carotid artery. There are no other branches of the common carotid.

Fig. 9.16 The carotid arteries and the internal jugular vein after removal of the sternomastoid muscle.

Source: Rogers op. cit.

Internal carotid artery

This commences at the bifurcation of the common carotid artery, and at its origin is dilated into the carotid sinus which acts as a baroreceptor. In the bifurcation is the carotid body, a chemoreceptor. Both are supplied by the ninth cranial nerve. At first the internal carotid lies lateral and slightly more superficial to the external, but it rapidly passes medial and posterior to it, as it ascends to the base of the skull between the side wall of the pharynx and the internal jugular vein. The upper part of the internal carotid artery and the internal jugular vein are closely related to the last four cranial nerves (Fig. 9.17). The internal carotid is separated from the external in the upper part by the styloid process, the stylopharyngeus muscle, and the glossopharyngeal nerve and pharyngeal branch of the vagus.

Fig. 9.17 The cranial nerves related to the carotid arteries.

Source: Rogers op. cit.

At the base of the skull the internal carotid enters the petrous temporal bone in the carotid canal, and subsequently gives off the ophthalmic artery, the anterior and middle cerebral arteries and the posterior communicating artery. There are no branches of the internal carotid in the neck.

It should be noted that atheromatous emboli may arise from stenoses at the origin of the internal carotid. When they do so, they may cause transient attacks of blindness (amaurosis fugax) on the same side if emboli travel to the ophthalmic artery. However, if they go to the cerebral cortex, they will cause transient ischaemic attacks (sensory or motor) on the opposite side of the body due to the decussation of the nerve pathways.

External carotid artery

The external carotid (Fig. 9.18) extends from the upper border of the thyroid cartilage to a point midway between the angle of the mandible and the mastoid process. At its origin it is anteromedial to the internal carotid but, as it ascends, it becomes more superficial. Almost immediately it gives off two branches: the ascending pharyngeal and the superior thyroid. Shortly above, it gives off the lingual artery, and then the facial and occipital artery, with the hypoglossal nerve crossing the external carotid just beneath the occipital branch. It then gives off the posterior auricular artery and terminates by dividing into the maxillary and superficial temporal artery.

Fig. 9.18 The branches of the external carotid artery and related nerves.

The common carotid artery is sometimes ligated for an intracranial aneurysm arising from the internal carotid. The external carotid artery is occasionally ligated for severe bleeding from the nose or the tonsillar bed. The level of the bifurcation of the carotid does vary, and at its lowest end the internal carotid is more accessible than the external, although within a centimetre or so the external becomes more superficial. The external carotid is the only one of the three that has any branches in the neck. To be sure of ligating the correct vessel the external carotid should be identified by finding the lowest one or two branches.

Venous drainage

External jugular vein

Superficial drainage of the head and neck is via the external jugular vein, which is formed from the junction of the superficial temporal and maxillary vein and posterior auricular vein. It runs obliquely downwards and backwards superficially over the sternomastoid muscle, piercing the deep cervical fascia 2.5 cm above the clavicle to enter the subclavian vein.

Internal jugular vein

This is formed at the jugular foramen and is a continuation of the sigmoid sinus. It lies behind the internal carotid artery but, as it descends, it become lateral to the lower part of the internal and to the common carotid artery, with the vagus nerve lying between the vein and the artery. It receives some pharyngeal veins, the common facial vein, the superior and middle thyroid veins, and the lingual vein. It then joins the subclavian vein to become the brachiocephalic vein; the left and right brachiocephalic veins then merge to form the SVC. The deep cervical chain of lymph nodes is closely applied to the internal jugular vein.

In the operation of carotid endarterectomy, the sternomastoid muscle is dissected and retracted backwards, and the common facial vein is then doubly ligated and divided. When this has been done and the internal jugular is also dissected backwards, the common and internal carotid arteries are exposed.

The lymphatics of the head and neck are described in Chapter 13.

BLOOD SUPPLY OF THE UPPER LIMB (Fig. 9.19)

Axillary artery

The axillary artery is the continuation of the subclavian artery, extending from the outer border of the first rib to the lower border of teres major. It is divided into three parts by the pectoralis minor muscle. Surgical division of this muscle displays the axillary artery, which may be helpful in operations such as axillo-femoral bypass. The muscle should be divided as close as possible to its insertion into the coracoid process as the blood supply comes from below, thus reducing bleeding from the cut muscle and avoiding leaving necrotic muscle made necrotic by ischaemia. It is enclosed in the axillary sheath along with the axillary vein and the components of the brachial plexus. The vein lies medial to the artery, and the cords of the brachial plexus are arranged around the artery. The pectoralis major covers it apart from its distal end. It conveniently has one branch on the first part, two from the second and three from the third. These are:

• first part – superior thoracic artery;

• second part – acromiothoracic and lateral thoracic artery; and

• third part – subscapular artery, anterior circumflex humeral and posterior circumflex humeral.

Fig. 9.19 The arteries of the upper limb.

Source: Rogers op. cit.

There is a rich arterial anastomosis around the scapula, which may be an important collateral channel in cases of obstruction of the distal subclavian artery.

Brachial artery

This is a continuation of the axillary artery commencing at the lower border of teres major and running along the medial borders of coracobrachialis and biceps accompanied by venae comitantes. At its lower end it runs under the bicipital aponeurosis dividing into the radial artery and ulnar artery at the level of the neck of the radius. It is crossed at the level of the middle of the humerus by the median nerve, which passes superficially from its lateral to medial side. Its branches are the profunda brachii, a nutrient artery to the humerus, and the superior and inferior ulnar collateral arteries.

The lower part of the brachial artery is susceptible to damage in supracondylar fractures of the humerus, particularly in children. Despite the anastomosis around the elbow, intense spasm of the arteries lower down may occur and if uncorrected may result in ischaemic damage of the forearm muscles (Volkmann’s ischaemic contracture).

Radial artery

This commences at the level of the neck of the radius lying on the tendon of biceps. It travels down the forearm, and distally it may be found lying superficially between brachioradialis and flexor carpi radialis, and it is between these two tendons that it may be palpated at the wrist. It then passes distally, giving off a branch to assist in the formation of the superficial palmar arch before winding round the lateral border of the wrist to reach the ‘anatomical snuffbox’. It then pierces the first dorsal interosseous muscle and enters the palm to form the deep palmar arch with a deep branch of the ulnar artery.

The ulnar artery

The ulnar artery extends from the bifurcation of the brachial artery to the superficial palmar arch in the hand. It accompanies the ulnar nerve, and together they descend along the lateral border of the flexor carpi ulnaris. It becomes palpable at the wrist and crosses superficial to the flexor retinaculum with the ulnar nerve on its medial side. It divides into a superficial and deep branch with the larger superficial branch forming the superficial palmar arch.

The radial artery is normally selected for insertion of a cannula for measuring intra-arterial pressure. There is a small risk of thrombosis of the artery, and it is, therefore, important to check for the integrity of the palmar arches, and in particular the ulnar inflow, before inserting the arterial line. This is done by Allen’s test in which both arteries are occluded by the examiner’s firm finger pressure whilst the patient clenches their fist a few times to exsanguinate it. The pressure on the radial artery is maintained, while that on the ulnar is removed; if the palmar arch is satisfactory, it will rapidly flush again. Integrity of the radial artery input can be checked in the same way.

VENOUS DRAINAGE OF THE UPPER LIMBS (Fig. 9.20)

Superficial veins

The veins in the digit drain into a dorsal venous arch on the back of the hand. Two veins are formed from the dorsal and venous arch: the cephalic and the basilic.

Fig. 9.20 Variations in the patterns of venous drainage of the upper limb.

Source: Rogers op. cit.

Cephalic vein

This starts in the anatomical snuffbox and courses upwards along the lateral aspect in front of the forearm. At the elbow it is lateral to the biceps tendon, and continues up the arm along the lateral border of the biceps and along the deltopectoral groove. It then pierces the clavipectoral fascia and drains into the axillary vein.

The cephalic vein at the wrist is the most popular site for intravenous cannulation. It should be noted, however, that it is also the most useful vein for creating an arteriovenous fistula for haemodialysis, because of its proximity to the radial artery. In patients with chronic renal failure who may require a fistula, it is appropriate to try to cannulate other veins to avoid thrombophlebitis occurring in the cephalic vein, which would make creation of a fistula difficult.

Basilic vein

This runs upwards on the posteromedial aspect of the forearm, passing to the anterior aspect of the arm just below the elbow. Above the elbow it continues along the medial border of the biceps. It pierces the deep fascia in the middle of the arm, ascending along the medial aspect of the brachial artery. At the lower border of teres major the basilic vein joins the venae comitantes of the brachial artery to form the axillary vein.

There are a number of veins in the cubital fossa, but it is best to avoid these for intravenous injection, as the brachial artery is close to them and separated only by the bicipital aponeurosis. An inadvertent injection of the artery can have disastrous consequences.

Deep veins

These run along the arteries as paired venae comitantes. At the lower border of teres major they are joined by the basilic vein to form the axillary vein, which continues up medial to the axillary artery.

The axillary lymphatics are described in Chapter 15. Suffice it to say that in block dissection of the axilla, one of the early steps is to divide the pectoralis minor muscle as high as possible. This exposes the axillary contents and in particular the axillary vein, which has to be dissected clean of lymph nodes.

BLOOD SUPPLY OF THE LOWER LIMBS

Femoral artery

This is a continuation of the external iliac artery after it has passed deep to the inguinal ligament at its midpoint (Fig. 9.21). The upper part lies in the femoral triangle and the lower part in the adductor canal. Anatomists talk about the whole artery from the inguinal ligament to the popliteal fossa as being ‘the femoral artery’. However, vascular surgeons and radiologists describe the first inch or so as being ‘the common femoral artery’, which gives off two branches: the deep femoral or profunda femoris artery, and the superficial femoral artery which is the main artery entering the adductor canal. The main branches are shown in Fig. 9.21.