Introduction

The aorta is an elastic tube designed to withstand the high systemic arterial pressure so that it can facilitate the delivery of blood to all the capillaries from the tip of the head to the capillaries at the tip of the toes. During systole, the elastic nature of the aorta allows it to accommodate the ejection volume of the heart. During diastole the elastic components recoil, providing a steady forward force sustaining the flow toward the capillaries. The most common diseases that affect the aorta are atherosclerosis and aneurysmal disease. Aortic dissection is less common but has dramatic and potentially fatal consequences. The aorta is best imaged with contemporary multidetector Computed Tomography (CT) or by Magnetic Resonance Imaging (MRI). Aortography has taken a back seat since the evolution of the less invasive methods.

 

Structure

The unique structure of the aorta enables it to perform its essential function as a conduit for the transport of blood.   The aorta’s relative location, size, shape, and composition, all work in concert to receive and transport blood as it is pumped out of the heart guiding blood into the systemic circulation.

 

The aorta’s gross anatomy is sequentially divided into the ascending aorta, aortic arch, descending thoracic and abdominal aorta. Originating from the left ventricle, the aorta briefly arches above the plane of the heart. It then descends in the chest cavity, passes into the abdominal cavity through the diaphragm at the aortic hiatus, and terminates by bifurcating into the iliac arteries just above the pelvic cavity. Along its course, a series of arteries branch off, providing oxygenated blood to the heart, brain, upper limbs, airways, abdominal and pelvic cavities, and the legs. The largest branches arise from the top of the aortic arch. They include the brachiocephalic artery, the left common carotid and the left subclavian arteries.

The aorta is responsible for maintaining the pressure and velocity of the blood, both of which are accomplished as a result of the contractile forces of the heart, the prevention of backflow by the aortic valves, the elasticity of the wall, and the control of flow by the peripheral arterioles called the precapillary sphincters.  The contractile forces of the heart eject the blood via concentric contraction through the aortic valve.  The aortic valve lies at the base of the heart and consists of three fibrous leaflets which work in concert to prevent backflow into the heart.   The peripheral arterioles become muscular at the precapillary level and flow is controlled by neural hormonal and local factors depending on the need of the end organ.  The combined effect of the precapillary sphincters is the creation of a peripheral resistance.

In comparison with all the other arteries of the body, the aorta is by far the largest in diameter (approximately 3 cm at its origin). This trait allows the aorta to receive and transport a large volume of blood in order to efficiently supply the varied metabolic needs of each of the organs or structures.  If minute arterioles replaced the aorta as the receptor of cardiac output, there would be tremendous backflow into the left ventricle due to the excessive resistance the ejecting volume would be faced with, not to mention an entirely incompatible system for maintaining proper pressure and flow.

Aortic Wall

The aorta’s wall is composed of layered muscular and elastic tissue. Stretching of the aorta provides the potential energy needed to propel the blood forward and maintain a high mean pressure sufficient to enable forward flow throughout diastole. The wall is divided into the tunica intima, media and adventitia.  The tunica media comprises the majority of these elastic layers. Its discretely packaged fibers contribute greatly to the aorta’s distensible nature. Also, the tunica media, as well as the other layers, contributes to the aorta’s impressive radial strength. Such a property, accomplished by its unique makeup, helps mediate the rate of blood flow under high pressure conditions. Without such strong and flexible constituents, the aorta would simply be a large and rigid tube, unable to aid and adapt to oscillations in blood flow and pressure.

Applied Biology

Loss of elasticity is a normal result of aging and wear and tear of the aorta.  Atherosclerosis is an aging process and in general, causes a decrease in elasticity and compliance.  It is usual and normal for the diameter of the aorta to enlarge by about 1mm per year starting about the 6^th decade.  Aneurysmal disease is an accelerated progression of the weakening of the aortic wall.   In this entity the integrity of the aorta is challenged with progressive dilatation (about 2-3mms per year) and rupture becomes a significant risk and a life-threatening situation. Surgical repair usually is indicated when aneurysms reach a critical size. Occlusive, atherosclerotic disease is more common in the branches of the aorta though with severe atherosclerosis, occlusion of the distal abdominal aorta is possible.  It is usually well tolerated because it is a slow process and adaptive collateral flow through the inferior mesenteric artery, lumbar and iliac arteries maintain blood flow to the lower extremities.

Congenital abnormalities are usually of a stenotic nature.  Aortic valve stenosis due to a bicuspid aortic valve, when mild, is overcome by left ventricular hypertrophy.  As stenosis progresses, compensatory mechanisms reach a limit with consequent heart failure and pulmonary congestion.   Coarctation of the aorta is a congenital narrowing as well, usually occurring at the end of the arch in a segment of the aorta called the isthmus.  Left ventricular hypertrophy results to overcome resistance created by the narrowing, while collateral channels develop to bypass the narrowing.

The abdominal aorta is the most common site for aneurysmal disease which affects about 3-4% of the population.  The primary causative factor appears to be loss of wall elasticity.  The elastic tissue degenerates as a result of aging, but there are usually associated causative factors that accelerate the atherosclerotic process. As the wall starts to dilate the increased diameter will be subjected to the highest tension in the tube and this further accelerates the process.

This angiogram with overlay shows theoretical changes of the aortic wall during diastole (a) systolic expansion (b) and return to normal in diastole (c). In systole (b) the suprarenal artery is expanded by the pulse but is relatively decompressed by the low resistance and high flow renal arteries. The infrarenal aorta is relatively more expanded in systole (b) since the iliac arteries offer a relative resistance. This increased resistance causes the elastic tissue in the aorta to stretch (b) so that the recoil in diastole (c) results in a sustained forward moving force assisting the blood to get to their most distal destination – the feet. When the aorta starts to lose its elasticity, the recoil of systole gradually is weakened so that the aorta does not return to its normal diameter after each systolic expansion. Over many years this lack of recoil is progressive so that the resulting wall is weaker and dilated, until an aneurysm is formed. A small infrarenal fusiform aneurysm is seen in image d (diastole), image e, (systole) and image f, diastole. Some recoil is still present, as seen by a systolic dilatation (e) with slight decrease in diameter during diastole (f).

Courtesy of: Ashley Davidoff, M.D.

Principles

At its most basic, the aorta is a tube that transports fluid.

The aorta, of course, is more complex than a simple tube for many reasons. Its course is not straight but is curved.

Pressure is generated by the heart’s cyclic pumping into the aorta, creating an arterial pressure wave that migrates down the aortic tube. The aorta’s ability to recoil in union with the heart’s contractions along with fluctuations in intrathoracic pressure stabilizes the necessary pressure difference for the movement of blood between two points in a tube. Blood’s velocity depends on such a difference as well as the elastic composition of the aortic walls and the shape of its path as it courses towards the upper and lower reaches of the systemic circulation.

The aortic arch and abdominal aorta give rise to several large branches, increasing the available sites for flow impact against the walls. This may seem like a serious flow issue, but in healthy individuals the effects of shear stress are not of issue. Due to the great muscular strength of the aorta and its viscoelastic properties, the tubular walls are able to withstand the stress caused by axial and transverse collision of blood flow. Its function as a tubular vessel is fulfilled by such properties.

Principles: Radius and Diameter

As blood passes from the ascending to the descending aorta, its pathway narrows as the aorta’s diameter decreases in size from about 3cms. to 2cms. The diameter’s progressive decrease along its course resembles the overall structure of systemic circulation. A diminishing tubular network allows blood to flow everywhere in the body. Blood then experiences increasing effects of fluid friction in addition to the effects from the varying elastic properties of the walls. Its rate of flow must decrease in order for efficient diffusion to occur across the endothelial cell’s permeable membranes. This gives enough time for oxygen to be transported into the adjacent tissues to be converted into metabolic processes.  When the red cells reach the capillaries, they are close to the size of the capillaries.

As a receptacle of cardiac output, the aorta must be large in diameter in order to accommodate the large volume of blood. Diseases which decrease the diameter of the aorta disrupt its receptive ability, causing an increase in flow turbulence as well as an increase in resistance.

Principles: Shape

As a tube among tubes, the aorta’s curved ascending portion and its arch distinguishes it from the rest. Its design has evolved to incorporate the upward push of blood from ventricular ejection in order to supply blood to the head in addition to the other lower regions of the body.  Considering the aorta’s curved geometry in comparison to that of a straight cylindrical tube, the aorta experiences collisions with the curving elastic walls.  It is the elastic recoil and overall compliance that, in general, allows for laminar flow.

Parts

he aorta is divided, in general, into the thoracic aorta and the abdominal aorta.  The thoracic aorta starts at the level of the aortic valve and continues as the ascending portion, the arch and the descending portion.  The main divisions of the abdominal aorta are the suprarenal and infrarenal portions, which are demarcated by the departure of the renal arteries from the aorta around the top of the second lumbar vertebra. This breakdown is commonly used by vascular surgeons when characterizing the locations of abdominal aortic aneurysms.

Parts: Thoracic Aorta

Parts: Ascending Aorta

The ascending aorta is the first segment or the most proximal portion of the aorta’s structural anatomy. It is located between the left ventricle and the aortic arch, starting at the base of the heart and ending at the first brachiocephalic artery at the origin of the aortic arch.

The ascending aorta is located behind the left half of the sternum and it passes upward, forward, and to the right, forming a slight curve.   It is contained in the pericardium and lies posterior and to the right of the main pulmonary artery and anterior and to the left of the superior vena cava.

The ascending aorta consists of a bulbous portion and a tubular portion and they are separated by the sinotubular junction. They are each characterized and named by their respective shapes. The tubular portion starts at the sinotubular junction and extends to the brachiocephalic artery.

Ascending Aorta: Aortic Sinus

The bulbous portion, also known as the aortic sinus or aortic bulb, houses the aortic annulus, aortic valve, three sinuses of Valsalva, and the coronary ostia. The left and right coronary arteries arise from their respective left and right coronary cusps. The sinotubular junction marks the border between the sinus portion of the aorta and the tubular portion. Diseases such as syphilitic aortitis, usually start at the sinotubular junction and progress distally to the tubular portion. There is no direct involvement of the sinus portion, but the aortic valve may become distorted by the aneurysmal disease with resulting aortic regurgitation.   Ankylosing aortitis on the other hand usually involves the valves, causing thickening with subsequent regurgitation. The tubular portion of the ascending aorta extends from the sinotubular junction to the brachiocephalic artery. It has no branches.

Ascending Aorta: Annulus

The aortic annulus is the ringed juncture of the outer wall of the aortic valve and ascending aorta.  It is characterized structurally by its circular geometry. It acts as the foundation for the attachment of the aortic valve. The most common disease is calcified aortic sclerosis. As a distinct structure, it is not easily appreciated by conventional imaging but when calcified, for example, in aortic sclerosis, it is easily seen by CT scanning.  In the case of a ring abscess caused by bacterial endocarditis, this entity is well seen by echocardiography. The abnormal sclerotic annulus itself is usually asymptomatic.  When aortic sclerosis progresses with aortic stenosis to become severe, then surgical treatment by aortic valve replacement is warranted.

Parts: Aortic Valve

The aortic valve is one of the four valves of the heart, located between the left ventricle and the base of the aorta. It is characterized structurally by the semilunar shape of its three leaflets. The root of the valve is attached to the aorta in a curvilinear manner. The one-way valve opens passively during systole and closes during diastole.

Common diseases include: calcific aortic stenosis, aortic regurgitation, bacterial endocarditis which can sometimes progress to ring abscess. Bicuspid aortic valve is one of the most common congenital cardiac abnormalities.  Most entities of the aortic valve are best and easily diagnosed by echocardiography.  CT (computed tomography) scans and MRI (magnetic resonance imaging) are useful as well.  Surgical treatment, particularly aortic valve replacement is common, with mechanical prosthetic valves or biological valves. The biological valve is commonly made of porcine valvular tissue.  The transplantation of the tissues of one species into another is called a xenograft.  In some cases the prosthetic valve is harvested from a human cadaver, in which case it is called a homograft.  St Jude valve is a commonly used mechanical prosthetic valve.

The aortic valve’s structure resembles that of a semilunar valve. It contains three crescent shape flaps of tissue, separated by dilations in the luminal layer.  The valve is slightly larger than a quarter.  Its design is very similar to the design of the pulmonary valve.  The semilunar cusps are larger, thicker, and stronger than those of the pulmonary valve. In the middle of the free margin of each leaflet is a small bulge or nodule called Arantius’ nodule. (aka: nodulus, corpus Arantii, nodulus valvulae semilunaris, Arantius’ nodule, Bianchi’s nodule, Morgagni’s nodule.)  The points where the attachments of two adjacent cusps are joined are called commissures. There are also thickened bands extending from nodules along the edges of the leaflets that enable coaption of the leaflets along the commissures. The parts of the valves between these arcs of the cusps’ free margins are called lunulae, due to their crescent shapes. They are more distinct than the lunulae of the pulmonary trunk. In addition, there are dilated pockets between the cusps and the aortic wall; each pocket is called an aortic sinuses (also know as the sinus of Valsalva, sinus aortae, Petit’s sinus). The coronary arteries arise from two of the three sinuses.

Aortic Valve: Leaflets

The three leaflets are named according to their position and their relationship to the coronary ostia.  Hence, there is a left coronary cusp, right coronary cusp and a noncoronary cusp.  The right cusp lies anterior and to the right, the left lies leftward and slightly posterior, while the noncoronary cusp is most posteriorly located and also the most inferiorly.

 

The aortic valve’s semilunar shape provides a complementary three-dimensional structure that can open and close without regurgitation. During systole, the valves are forced open by the ejection pressure created by the heart.  During diastole, as the pressure starts to fall, the blood fills back into the pockets of the sinuses of Valsalva causing them to distend enabling the leaflets to coapt perfectly so that there is no regurgitation back into the left ventricle.

The structure of the aortic valve and pulmonary valve are simpler than the atrioventricular valves (mitral and tricuspid) which contain chordae tendinae and papillary muscles. The semilunar valves of the great vessels operate in a more passive manner, opening and closing according to pressure differences between the left ventricle and the aorta.

Aortic Valve: Position and Relationships

The position of the aortic valve in relation to the mitral valve, its angular relationship to the LVOT (left ventricular outflow tract), and its position in relation to the pulmonary artery and pulmonary valve are all critical in terms of the embryology, physiology, and pathology.  The valve lies between the two low pressure atria, probably one of the most comfortable spots in the body as blood flows in waves, almost simulating the sound of the waves of the ocean.  It lays posterior and to the right of the right ventricular outflow tract and main pulmonary artery, and lies below and to the right of the pulmonary valve.  It is oriented toward the right shoulder, and tilted so that the left coronary ostium is slightly higher than the right.

The aortic cusps are anchored to other fibrous portions of the left ventricle. The left coronary cusp and part of the noncoronary cusp are in fibrous continuity with the anterior leaflet of the mitral valve. The right-sided portion of the non coronary cusp and the right coronary cusp are intimately related to the membranous portion of the ventricular septum. Thus, the ascending aorta is anchored down to the heart via strong fibrous attachments to the mitral valve, membranous septum and to the aortic annulus. In health, the intimate connections enable structural strength and functional integration. On the other hand, in disease they provide easy pathways for spread of the disease process.

 

Normal Aortic Valve in Fibrous Continuity with the Mitral Valve

The ascending aorta originates from the left ventricle, and together with its valve, is intimately attached to the anterior leaflet of the mitral valve, membranous ventricular septum, and aortic annulus. This diagram shows the intimate relationship between the anterior leaflet of the mitral valve, and the aortic valve. Courtesy Ashley Davidoff MD

The aortic valve lies posterior to, rightward of, and inferior to the pulmonary valve.  This relationship is as a result of important embryological changes that occur at the infundibular level.  Specifically, the subaortic infundibulum resorbs while the subpulmonary infundibulum grows.  This action elevates the pulmonary valve and positions it to the left of the aortic valve.

Parts: Sinuses of Valsalva

Each main coronary artery arises from its respective right and left coronary cusp. The noncoronary cusp lies inferior and posterior to each of the coronary cusps.   The sinuses become filled with blood during diastole, causing the valves to coapt with each other and prevent backward flow into the left ventricle (LV). During systole, the valve leaflets are forced open and the sinuses are functionally and virtually obliterated. The combined complex of valves and sinuses is called the bulb of the aorta, and its dimensions are just slightly larger than that of the ascending aorta itself.

The sinotubular junction marks the boundary between the bulbous portion of the aorta and the ascending proximal aorta. The junction becomes effaced in diseases such as sinotubular ectasia which occurs in Marfan disease and syphilitic aortitis, which is now rare.

Parts: Aortic Arch

The aortic arch is the segment between the ascending and descending thoracic aorta. Its course begins at the upper level of the second right sternocostal junction. The arch runs upwards in an anterior-superior curve towards the left in front of the trachea. It then proceeds downward passing the left side of the body of the fourth thoracic vertebra and then meets the proximal descending aorta. The aortic arch is characterized structurally by its tubular curvature and elastic properties. At the top of the arch are three arterial branches called as a group the brochiocephalic arteries, which carry blood to the head and upper limbs.

The three branches of the arch are the brachiocephalic artery, left common carotid and left subclavian artery. The brachiocephalic artery gives rise to the right subclavian and right common carotid arteries. There are normal variations of this pattern.

There are three branches that extend from the aortic arch. The first branch is the innominate artery, which rapidly branches again into the right common carotid and right subclavian arteries.  The next branch is the left common carotid artery, and the last branch off the arch is the left subclavian artery. The most common variation is when the brachiocephalic artery and the left common carotid have a common origin. Sometimes the vertebral arteries arise as independent branches off the arch. The aberrant right subclavian artery is one of the other common variations named when the right subclavian artery arises from the left side as the last branch of the arch, after the left subclavian artery. It thus, has to make its way from the left side to the right by crossing behind the trachea and esophagus.

There are numerous variations in just how many branches originate from the arch. In rare cases, the left carotid and left subclavian can originate from the innominate, making the innominate the only branch extending from the arch. On the other extreme, there have been cases where 5 or 6 branches extend from the arch. In these cases, the internal and external carotids arise directly from the arch (and therefore, the common carotid is absent).

Common diseases include atherosclerosis and aortic aneurysm. Dissection commonly involves the arch, often originating just beyond the left subclavian artery.

The arch starts just anterior to the trachea in a relatively rightward position and forms two arches. The first is the more easily understood vertical arch formed as it ascends up and over the left main stem bronchus. It is often described as being candy cane in shape.

However, when viewed in the frontal projection, the arch is not quite perfect.

Angle of the Arch in this Projection has an Acute Component

This angiogram of the normal thoracic aorta is taken in the left anterior oblique (LAO) projection. In this projection the shape is not quite a candy cane suggesting some complexity to the arch and known when seen as a three-dimensional structure. Image courtesy of: Ashley Davidoff, M.D.

Note that the angle between the arch and descending aorta is too acute.  This aberrancy is due to a second bed or arch in the aorta as it crosses the trachea. The arch of the aorta thus forms two curves, one with convexity upward and over the trachea and the second an almost horizontal bend around the trachea first to the left and then posterior.

Parts: Aortic Isthmus

Parts: Descending Thoracic Aorta

The descending thoracic aorta is the segment between the arch and abdominal aorta.  Within the posterior mediastinal space, the descending thoracic aorta begins at the distal aortic arch at the lower border, and on the left side of the fourth thoracic vertebra. It terminates in line with the lower border of the twelfth thoracic vertebra in front of the diaphragm at the level of the aortic hiatus.

The descending thoracic aorta is characterized structurally by its tubular shape and highly elastic composition. As this segment courses downward, it gives rise to the two bronchial arteries as well as most of the posterior intercostal arteries. The artery of Adamkiewicz is a very important small branch that usually arises from one of the left-sided posterior, intercostal arteries.  It supplies the lower two-thirds of the spinal cord via the anterior spinal artery.

The descending aorta is usually straight, but with advancing age dilates and becomes more tortuous.

Common diseases of the descending aorta include: atherosclerosis, aneurysmal disease, and aortic dissection. Diagnostic studies include: CT imaging, MRI, and aortography. Surgical treatment is most common. Percutaneous placement of endoluminal grafts are evolving as less invasive techniques but at this stage are more commonly used in the abdominal aorta.

Parts: Abdominal Aorta

The abdominal aorta is the last segment of the aorta, commencing at the end of the thoracic aorta and terminating by bifurcation into the common iliac arteries. The abdominal segment originates at the aortic hiatus of the diaphragm at the plane of the thoracic and lumbar vertebral junction. The aorta is surrounded by the right and left crura at this stage.  The abdominal aorta is structurally characterized as a tapering tube, composed of muscular and elastic tissue.

Along its course, the abdominal aorta gives rise to several large branches including the celiac axis, the superior mesenteric artery (SMA), and the inferior mesenteric artery (IMA) which arises 3 to 4 cm above the bifurcation.

The suprarenal aorta, as its name implies, is that portion of the abdominal aorta that lies above the renal arteries. The renal arteries arise from the lateral border of the aorta between the SMA and IMA branches.

The infrarenal aorta, as its name implies, lies below the origin of the renal arteries. It is located between the top of L2 and the bottom of L4.  The gonadal arteries (testicular and ovarian) are small branches that also arise from the lateral border of the abdominal aorta below the origin of the renals and above the IMA.

The abdominal aorta gives off many branches and diminishes rapidly in size from about 2.5cms to about 1.75 cm. in diameter, being slightly smaller in females and finally terminating into the two common iliac arteries.

Common diseases include abdominal aortic aneurysm, (AAA – “triple A”) and atherosclerosis. Diagnostic studies include CT imaging, MRI, aortography and ultrasound.  Treatment is usually surgical, with percutaneous techniques such as endoluminal graft insertion becoming more frequently utilized.

The abdominal aorta begins at the diaphragm in front of the last thoracic vertebra and descends in front of the vertebral column to the fourth lumbar vertebra, slightly to the left of the middle line.

Diseases of the Aorta

We will approach this section, relating to the diseases of the aorta, by reviewing in some detail the more common diseases including atherosclerosis, aneurysmal disease, and dissection.  Thereafter, we will review in lesser detail diseases that are less common, or diseases that are location specific.   We will follow the anatomic pattern of the parts described above by starting at the aortic valve and progressing around the aorta to the abdominal portion for the second section

Atherosclerosis

The most common disease of the aorta is atherosclerosis, a process in which lipid material, fibrous tissue, and calcium form deposits, called plaques, within the inner layer of the aortic wall (the intima). The earliest signs of atherosclerotic change are yellow fatty streaks in the intima. The fatty streaks are first observed in the descending thoracic aorta, adjacent to the departure of intercostal arteries.

The initiating event appears to be a breach of the endothelium (left image).

 

 

Lipoproteins from the blood in the lumen migrate through the area of injury into the intima and subintimal layer (right image).

A lipoprotein-proteoglycan complex forms, which traps the lipoprotein in the intima

Leukocytes migrate from the lumen into the intima evoking an inflammatory response

 

Monocytes transform into macrophages and phagocytosis of the fat complexes result in fat laden, foamy macrophages.

 

Smooth muscle cells from the media migrate into the intima and transform into fibrocytes. A fibrous capsule around the fatty complex is formed (right image).

Cell death and destruction of tissue results with associated formation of dystrophic calcification

There is growth of the atheromatous complex occurs and then starts to occupy space with impingement on the luminal space.

The smooth glistening endothelium of the aorta becomes replaced with yellow streaks and as the atheroma builds, the aorta eventually, in the worst case, looks like porridge or gruel- the Latin etymology of the word.
There is progressive enlargement of the plaque which has the potential of the complex to rupture.  Acute rupture of a plaque predisposes to superadded thrombosis.

As the disease progresses, dystrophic calcification of dead tissue results.

Aneurysmal Disease

An aortic aneurysm is defined as an enlargement of the aorta with a dimension greater than 1.5 times its normal diameter.

In the overwhelming majority, the cause of aneurysmal aortic disease is related to the atherosclerotic process which causes weakening and loss of elasticity of the wall.  In this disease, the primary disease is in the intimal layer.

Seventy five percent of patients with an abdominal aortic aneurysm (AAA) are asymptomatic. Symptoms include abdominal pain or tenderness, or back pain. Aneurysms become palpable when they are larger than 4 cm in diameter. The aneurysm is commonly identified on plain film and is easier seen on a lateral abdominal film, since 90% have mural calcification.

Aneurysms are classified based on their location in the aorta as, ascending arch, descending thoracic, thoracoabdominal, suprarenal, or infrarenal.

The majority (90%) of aortic aneurysms are found in the infrarenal abdominal aorta.  The ascending thoracic aorta is the area least prone to atherosclerosis.

An aneurysm is a true aneurysm when all three layers are part of the wall, whereas a false aneurysm is characterized by the absence of vessel wall and the wall of the aneurysm is created by surrounding thrombus and is contained by the thrombus and surrounding tissues.

A fusiform aneurysm is a spindle-shaped aneurysm implying that it is widest near the middle and tapers in toward both ends. Atherosclerotic aneurysms are classically fusiform.

A saccular aneurysm is more focally eccentric and round. Saccular aneurysms tend to have a higher propensity for rupture. They occur between fixation points where there is focal loss of elastin and collagen and an increase in elastase activity.

Mycotic aneurysms are classically saccular.

 

 Thoracic Aneurysm

A thoracic aortic aneurysm is a condition characterized by the expansion of the aortic wall to the extent that the diameter of the vessel is 1.5 times larger than the normal.  Thus, a diameter of larger than 4-4.5cms is considered aneurysmal in the thoracic aorta.  Two to five percent of aneurysms occur in the thoracic aorta.

Patients are more commonly males between 60 and 70 years. Dilation can occur anywhere along the thoracic aorta and the most common sites are the arch and the descending aorta.  Rupture of an aneurysm is heralded by the dramatic onset of excruciating pain.   Aneurysms greater than 7 cm are more prone to rupture than smaller ones and symptomatic disease is more prone to rupture than asymptomatic aneurysmal disease.

Symptoms are secondary to impingement on adjacent structures.  They include:  stridor, hemoptysis, dyspnea, pneumonitis from compression of the tracheobronchial tree, hoarseness from compression of the laryngeal nerve, dysphagia from esophageal compression, superior vena cava (SVC) syndrome, and pain due to compression and erosion on musculoskeletal structures.

Elective resection is recommended for thoracic aneurysms greater than 6cm and in some institutions 7cm in diameter, two times normal diameter.  All symptomatic aneurysms require urgent elective surgery.   Major operative complications include paraplegia from inadvertent interruption of the spinal cord’s arterial blood supply (5%) and those secondary to associated arteriosclerosis (MI, CVA, renal failure).

Abdominal Aortic Aneurysm

An abdominal aortic aneurysm (AAA) is a focal widening of all three mural layers of the abdominal aorta, usually considered when the transverse dimension of the aorta is greater than 3 cm. or 1.5 times its normal diameter.

AAA is more commonly seen in males over 60 years, with a M:F (male:female) ratio of 5:1. The incidence in Western cultures has tripled over the last three decades, due to the larger numbers of people in the elderly population. Improved diagnostic techniques have also been a major factor in the apparent increased incidence.

Common iliac artery aneurysms occur in about 20%.  Popliteal aneurysms occur in about 8-10%.  Common femoral artery aneurysms also occur.  Many patients also have associated chronic obstructive pulmonary disease and cholelithiasis.

In most instances (90%), AAA is infrarenal and the aneurysm extends into the iliac arteries in about 2/3 of the patients. The normal aorta grows in diameter by 1mm per year. An established aneurysm will grow by 2-4mm per year. Half of aneurysms greater than 6 cm will rupture within a year, 25% of aneurysms 4-6 cm will rupture in a year, 15-20% of smaller aneurysms will rupture in a year.

Rupture occurs in 25% of patients. Factors that predispose to aneurysmal rupture include large aneurysm size, presence of hypertension and chronic obstructive pulmonary disease (COPD).

 

Aneurysm: Imaging

The aneurysm is commonly identified on a plain film x-ray and is easier seen on a lateral abdominal film, since 90% have mural calcification.

The study of choice when diagnosis is suspected is ultrasound (US), since it is accurate, does not involve radiation, and is relatively inexpensive.

Once diagnosed, an AAA is best followed by ultrasound at six monthly intervals. Ultrasound has an accuracy of 98% in assessment of the size of the AAA. Angiography, once the gold standard, is rarely used for diagnostic purposes. The largest diameter in any direction is used for evaluation, as long as it is orthogonal to the lumen. The aorta is considered aneurysmal when the transverse diameter is greater than 3cms or when a true cross-sectional diameter at any level is greater than 1.5 times that of the smallest diameter.

Surgery is considered when the size reaches 5.5cms, although in some centers 4cms is the cut off.  Aneurysmal rupture usually occurs when the aneurysm enlarges to greater than 6cms. Rapid expansion is an indication for surgery and is considered when the diameter increases by more than 0.5 cm over six months. Back pain in a patient with a known aneurysm is a strong indication for surgery.

CT Angiography (CTA) is the most commonly utilized method of evaluation since surgical questions relating to size, and position of the aneurysm with reference to the renal and iliac arteries can be best evaluated using MDCT.

Diseases of the Aorta: Stents

Both conventional surgery as well as endovascular stent-graft placement can be used in the treatment.

Mortality for elective surgery is 2%, for the AAA that is symptomatic it is 20% and for the ruptured AAA it is 50%.

 

Pseudoaneurysm

A pseudoaneurysm is a false aneurysm in that its walls do not contain intact components of the aortic wall.  The histopathology is characterized by labile walls constituted by hematoma or fibrous tissue rather than the expanded intima media and adventitia of a true aneurysm.

Causes include trauma and infection.  When the pseudoaneurysm is caused by infection, it is called a mycotic pseudoaneurysm.

The pseudoaneurysm is usually eccentric in location and has a relatively narrow neck.

There is a significantly higher predisposition to rupture and hence, treatment is considered urgent, particularly if there is pain and signs of inflammation such as a fever or elevated white count.

 Acute Aortic Syndrome

There is a group of diseases that clinically present as an entity called acute aortic syndrome that reflect advancing complications of severe, atherosclerotic aortic disease.  They are characterized clinically by an acute pain syndrome which is represented by acute, severe, shearing, back pain.  They include penetrating ulcers that bleed into the wall, mural hematoma, focal dissection of the wall, and frank rupture of the aorta.  They appear to be a continuum of the same process of advancing atherosclerotic disease.

Aortic Dissection

Aortic dissection is a sudden catastrophic disruption of the aortic wall caused by a shearing tear of the intima and secondary involvement of the media by the advancing dissection and hematoma. The event is characterized by a splitting of the arterial wall along the longitudinal plane of the aorta.

Aortic dissections usually occur as a result of hypertension and are formed after the intima of the aorta ruptures, leading to entry of blood into the aortic wall. Initiating tears are most frequent in the ascending aorta, though the ascending aorta is not the most commonly involved portion of the aorta regarding dissecting aneurysms. The proximal portion of the descending thoracic aorta is the second most common location of a primary intimal tear but because most ascending aortic dissections extend to involve the descending portion and because few primary tears in the descending aorta extend retrograde to the ascending aorta, the descending aorta is the most commonly involved portion of the aorta with dissecting aneurysms.

There is usually associated disease of the media in the form of cystic medial necrosis or the aging process. Familial entities such as Marfan syndrome and Ehlos Danlos syndrome are associated with a significant increase of risk for aortic dissection. Entities that increase wall stress such as congenital bicuspid aortic valve, coarctation and hypertension are other factors that contribute to an increase in the incidence of the dissection. Pregnancy in the third trimester also has an increase incidence in dissection presumably due to the changes that occur to collagenous ligaments in pregnancy. About 50% of aortic dissections that occur in women under 40 are in women who are pregnant. Aortic dissection affects men more than women (2:1) with peak incidence in 6th and 7th decades.

Ninety five percent of dissections occur within a few centimeters of the aortic valve in the ascending aorta or just distal to the left subclavian artery.

There have been two classification systems with the Stanford system being more universally used.

Stanford Classification:

A- Involvement of ascending aorta

B – Involvement of aortic arch and distal aorta

 

Debakey Classification

Type I – ascending aorta, extends beyond the arch (30%)

Type II – ascending aorta only (20%)

Type III – descending aorta involving the thoracic aorta (50%)

Therapy is aimed at halting progression of the dissection.   Initially, blood pressure should be reduced to <100-120 mm Hg with an intravenous (IV) agents, such as nitroprusside or trimethaphan.

Treatment options are based on the position of the dissection.  If the dissection involves the ascending aorta, then emergency surgery is indicated because of the propensity for occlusion of the coronary artery, associated dysfunction of the aortic valve, and rupture into the pericardium.

If only the descending aorta is involved and there are no signs of distal ischemia, then medical management is indicated.

Marfan Syndrome

Marfan Syndrome is a connective tissue disorder caused by a mutation of the fibrillin-1 gene, manifesting as an autosomal dominant abnormality with 25-30% being sporadic mutations. These patients are characteristically tall, with long, thin, spidery fingers and they have a number of associated abnormalities.  Because of their height, these patients often end up as basketball players.  It is extremely important to screen this group of players for the disease, since sudden death from associated cardiovascular complications is potentially preventable if the diagnosis is recognized early in its course.

There are a wide range of clinical presentations with variable involvement of the central nervous system, ocular abnormalities, cardiovascular disease, pulmonary disease and musculoskeletal disease.

The clinical examination may reveal aortic regurgitation or mitral regurgitation.  When a patient with Marfan syndrome presents with back pain, it is essential to exclude the diagnosis of dissection.

When aortic disease is suspected, changes are best evaluated with MDCT.  If the patient is hemodynamically unstable and has suspected mitral regurgitation, pericardial tamponade or aortic regurgitation, transesophageal echocardiography becomes the study of choice.

Ring Abscess

Ring abscess is a suppurative infection of the annulus caused most frequently by infectious endocarditis. Such an infection is fatal if left untreated. In many cases, more than one microbial agent is responsible. The most common bacterial agents are E. coli and staphylococci.  With degeneration of the endothelium overlying the annulus, deposition of platelets and fibrin occurs. This fibrous complex is nonbacterial at first, called nonbacterial thrombotic endocarditis. This structural defect can become colonized by bacteria and may progress to a ring abscess that requires drainage. A drained abscess may produce a false aneurysm of a sinus of Valsalva. A ring abscess is diagnosed by echocardiography. Surgical treatment involving excision and interposition grafting is a necessity to prevent further spread of infection and damage to the valve.

Aortic Sclerosis

Aortic sclerosis is a degenerative disease commonly seen in older patients. It results in the progressive calcification of the aortic annulus and/or aortic valve. The aortic annulus is prone to calcification because fatty acid deposits easily accumulate on the aortic surface at its junction with the valves. This disease also induces thickening of the arterial wall. It is considered a marker for atherosclerotic disease.  If the calcification induces stenosis, left ventricular pressure rises in compensation, causing increased workload for the left ventricle, and increasing the risk of myocardial ischemia. The detection of a calcified annulus by CT scan or transthoracic echocardiography indicates the diagnosis of aortic sclerosis. If severe, surgical treatment is required.

Bicuspid Aortic Valve (BAV)

Bicuspid aortic valve (BAV) is one of the most common congenital lesions of the cardiovascular system.  In this entity, only two aortic valve cusps are developed. Patients with bicuspid aortic valve commonly have aortic narrowing and regurgitation of variable severity.  Most are usually isolated mild lesions but are prone to progressive calcification and advancing aortic stenosis later in life. With time, the gradient increases and the left ventricle hypertrophies to accommodate the increased pressure and workload.  The structural distortion of the valve predisposes it to infection and the condition, bicuspid aortic valve and therefore can be complicated by bacterial endocarditis.

Heart murmurs result from either regurgitation or stenosis and are best diagnosed by echocardiography.

Surgery with replacement of the valve is indicated when the pressure gradients become high, usually with gradients that are greater than 50-60mmHg across the valve.

Aortic Stenosis

Aortic stenosis is a mechanical disorder of the aortic valve caused by narrowing of the valve resulting in a reduction of flow through the valve.

The disease is often caused by a congenital bicuspid valve as a result of fusion of the commissures. This entity is seen in 1-2% of population, with a dominance in males. It is commonly seen in association with coarctation of the aorta. Hypoplastic valve is also a cause of aortic stenosis. Acquired causes include aortic sclerosis and rheumatic heart disease.

Clinically, the patient presents with a systolic murmur and a plateau pulse, with signs of hypertrophy of the left ventricle. The diagnosis is best made with echocardiography, where estimates of the gradient and valve area can be made fairly accurately. A gradient of less than 40mmHg. is considered mild, and a gradient of 80mmHg. or greater is considered severe.

The need for aortic valve replacement is based on severity of the disease.

Echocardiography is the study of choice. Calcification of the aortic valve identified on CXR suggests significant stenosis, usually more than 50 mmHg. CT is more sensitive to the calcification and is more commonly not associated with a gradient unless there is heavy calcification. Gated CT can be used to quantify left ventricular hypertrophy (LVH) and ejection fraction.

Aortic Regurgitation

Aortic regurgitation is a malfunction of the aortic valve resulting in the improper closure of the leaflets. This results in blood leakage or backflow. Aortic regurgitation is caused by congenital abnormalities such as bicuspid aortic valve and/or infections of the heart. Rheumatic aortic valve disease and bacterial endocarditis are well-known causes of acquired aortic regurgitation. Aortic regurgitation causes volume overload of the left ventricle. Mild cases are treated with medication that regulate the heart’s rhythmic pumping and control body fluids in efforts to prevent edema. If severe, aortic valve replacement surgery is necessary.

Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is the incomplete formation of the chambers, valves, and/or vessels of the left side of the heart. Infants with HLHS are also born with additional defects including ventricular septal defect, patent ductus arteriosus, and narrowing of other parts of the aorta. With poor systemic supply of oxygenated blood from the left heart, the ductus arteriosus usually remains open so that blood, albeit deoxygenated blood from the pulmonary circulation, is able to provide some blood to the systemic circulation.  Mixing of deoxygenated and oxygenated blood therefore occurs.  Infants are born with extremely low oxygen saturation as well as respiratory distress. HLHS can be fatal within hours if untreated. Standard fetal ultrasound is used to diagnose HLHS. This defect is not entirely curable. Various surgical treatments exist in stages including cardiac transplantation if HLHS is severe. The Norwood procedure is commonly performed as the first stage followed by a second procedure which establishes adequate connection for the systemic and pulmonary circulations.

Right Aortic Arch

Right aortic arch is a congenital abnormality which has no specific known causes. It may be isolated or it may be associated with other CHD (congenital heart diseases).  It is associated with both Tetralogy of Fallot and truncus arteriosus.

The right aortic arch ascends and descends over the root of the right bronchus instead of over that of the left and passes down either on the right or left side of the vertebral column. When it descends on the right, the thoracic and/or the abdominal viscera are usually transposed and congenital heart disease tends to be more common. When the aorta descends on the left, it is not usually accompanied by transposition of the viscera.

The diagnosis can be made with a plain chest X-ray.

Cervical Arch

The cervical arch is a congenital abnormality characterized by a malposition of the arch which is too cranial or sometimes even within the soft tissues of the neck.  As a result, the distal arch and proximal descending aorta become buckled and presents as a pseudocoarctation.  Most cervical arches are right-sided.

The diagnosis should be considered as a rare cause of dysphagia since the abnormally positioned arch could impinge on and obstruct the esophagus.  A chest X-ray would suggest the diagnosis but CT or MRI would confirm the diagnosis.

Treatment for symptomatic patients is usually surgical.

Pseudocoarctation

Pseudocoarctation of the aorta is a narrowing of the aorta that has both congenital and acquired forms.

It is caused by kinking or buckling of an elongated thoracic aortic arch distal to the origin of the left subclavian artery without measurable gradient. Associated diseases include a cervical arch, bicuspid aortic valve, ventricular septal defect (VSD), patent ductus arteriosus (PDA), and it is also seen in middle-aged men with hypertension.

Diagnosis is made by angiography, CT or MRI where the kinking of the proximal thoracic aorta, in the absence of the measurable gradient is noted. Plain film may suggest the diagnosis where a “three” sign reminiscent of true coarctation in both the P-A and lateral can be seen. Rib notching is absent. There is no therapy needed since there is no functional abnormality.

 Hypoplastic Arch

Hypoplastic arch is usually part of the hypoplastic left heart syndrome, which is a congenital abnormality of the left-sided structures of the heart.  Poor forward flow through the left system caused, for example, by mitral stenosis during the development of the heart may result in hypoplasia of the left ventricle, aortic valve, ascending aorta, arch and isthmus.

Patent Ductus Arteriosus

Patent ductus arteriosus (PDA) is a congenital persistence of an embryologic shunt between the left pulmonary artery and aorta at the level of the isthmus.  This shunt is normal in the fetus allowing blood to be shunted away from the non-functioning lungs.  Blood is therefore directed from the pulmonary arterial circulation to the systemic circulation.  A PDA is often patent in the neonate and is normal up to the age of 10 days.

The abnormality may be an isolated abnormality or associated with other congenital defects.  When associated with transposition of the great vessels for example, it is an essential and life sustaining shunt.

When the PDA persists beyond the neonatal period, it results in a left to right shunt since in the postnatal period the pulmonary resistance and pressures are lower than the resistance and pressures of the systemic circulation.  With the shunt, there is thus a volume overload on the pulmonary circulation.

If left untreated, pulmonary hypertension ensues.  The diagnosis is best made by hearing a continuous murmur at the left upper sternal border.  Diagnosis is confirmed by echocardiography.

Treatment is first attempted using indomethacin.  Minimally invasive options include balloon occlusion.  Surgery is indicated if other methods fail or in the patient where a large shunt is present.

 Coarctation

Coarctation of the aorta is a relatively common congenital disorder of the aorta and it is characterized by a focal narrowing usually at the level of the isthmus.  It accounts for 5-8% of disorders.

The cause is unknown but there is a known genetic association with 30% of siblings known to have the abnormality.  Associated defects include ventricular septal defect (VSD) and patent ductus arteriosus (PDA).  Sixty five to seventy percent of the patients have a bicuspid aortic valve.  Typically, the structural defect consists of thickening of the media, or may appear as a membrane that occurs in a juxtaductal location. The abnormality results in high upstream pressure, and low downstream pressure and is compensated for by associated left ventricular hypertrophy and numerous collaterals of the internal mammary and intercostal vessels.

The diagnosis is suspected clinically when the upper limb pulse and pressures are higher than the lower limb findings.  Echocardiography is the study of choice both in the diagnosis of the structural defect as well as the assessment of the gradient.  CT Angiography (CTA) and MR Angiography (MRA) are excellent alternatives.

Treatment options include balloon angioplasty and open surgical repair.

Aortic Trauma Isthmus

Aortic trauma is usually caused by blunt decelerating injury such as occurs in a motor vehicle accident where the driver or passenger is thrown against a hard part of the car and as the isthmus is anchored by the ligamentum arteriosum, which is therefore held fixed as the deceleration caused by the impact results in a traumatic shear stress on the aorta.

The result may be a tearing, or laceration of the aorta commonly at the isthmus, but it can affect any part of the aorta. The laceration of the aorta which is characteristically a transverse tear usually originating at the insertion point of the ligamentum arteriosum (80%) on the inferior surface of the transverse arch. Other sites that sometimes are involved are the proximal ascending aorta and descending aorta at diaphragmatic hiatus.

The ligamentum arteriosum is located at the aortic isthmus, where the fixed, fibrous ligamentum arteriosum acts as an anchor and puts extra stress on the aorta as it is flung along the line of force.  As the patient’s body is reflected off a hard object, the combination of the decelerating force and anchoring forces create a shearing and tearing stress on the isthmus. Many patients die at the scene of the accident as a result of a ruptured aorta.  Some patients develop a pseudoaneurysm where the rupture is contained by a combination of remnants of the wall, hematoma and surrounding tissues.

No specific symptoms or signs reflect this injury and further investigation by CT scan is warranted based solely on high clinical suspicion. Angiography is also an excellent method and is specific and sensitive.

Treatment is emergency surgery.

Takayasu’s Arteritis

Takayasu’s arteritis is an inflammatory disease of the aorta and its first order branches. It is caused by intimal proliferation and fibrosis along with fibrous scarring and degeneration of the elastic fibers of the media of the aorta and large arteries.  The male/female ratio is 1:8.  Typical age of onset is in teenage years.

As a result of the inflammatory process, the adventitia becomes thickened and the vaso vasorum are destroyed.  Localized aneurysm formation, post-stenotic dilatation and calcification in the arterial walls are late complications.  The process most often involves the arch and its major branches

The clinical diagnosis is suspected when a teenage patient presents with loss of pulses or ischemic parasthesias. Non specific findings include fever, malaise, night sweats, fatigue and occasionally pain and tenderness over the affected arteries. Imaging is best accomplished with CTA or MRA and for the smaller vessels angiography is still a useful modality.

Treatment options which include glucocorticoids may relieve systemic symptoms and surgical treatments may be needed for late complications Morbidity and mortality depend on the presence or absence of severe complications such as retinopathy, aortic regurgitation, secondary hypertension or aortic aneurysms There is a 97% survival over 7 years in uncomplicated disease and 59% in patients with complications.

Acute Aortic Occlusion

Acute aortic occlusion is most commonly caused by an embolus, thrombosis or stenotic atherosclerotic disease or by extension of an aortic dissection.  Acute occlusion with atherosclerotic disease is also called Leriche’s syndrome.  Other causes include iatrogenic occlusion due to catheter manipulation, hypercoaguable disease, and malignancy.

The result is acute ischemia of the distal organs, most commonly the lower limbs, so that pain, parasthesias, pallor, pulselessness and paralysis (5p’s) are clinical manifestations.

Leriche’s syndrome is characterized by absence of femoral pulses, pain in the legs or buttocks, and impotence and is characteristically seen in young males.

Diagnosis is usually suspected by the clinical presentation and is confirmed by ultrasound or CT scan.

Treatment is urgent and emergency embolectomy is usually the quickest and most effective method.  Thrombolytics can also be employed if the clinical situation is not deemed emergent.

Chronic Aortic Occlusion

Chronic occlusion of the aorta is usually caused by progressive stenotic and atherosclerotic disease of the distal abdominal aorta.

The slow progressive nature of the disease usually allows collateral supply from mesenteric intercostal and lumbar vessels to develop so that the result is relatively benign and may even be an incidental finding.

The diagnosis may be suspected clinically by reduced femoral pulses but CT scan or MRI would confirm the clinical suspicion.

Retroperitoneal Fibrosis

Retroperitoneal fibrosis (RPF) is a fibrogenic inflammatory process of the connective tissue of the retroperitoneum that has acute, subacute and chronic manifestations.

The cause of the disease in most cases is unknown, but it seems to be commonly associated with atherosclerosis.  It has been suggested that it is an autoimmune response to a lipid that originates from the atherosclerotic process arising in the wall of the aorta.  The lipid is known as ceroid.

Other associated causes include inflammatory diseases such as post irradiation, drugs, (methysergide chemotherapy) retroperitoneal infection, or malignancy.

The result is progressive fibrosis of retroperitoneal connective tissue, with the dominant result being the stenosis and subsequent obstruction of the ureters.

Treatment options include surgical release.  The use of steroids is controversial.

Lymphoma

Lymphoma of the aorta is most commonly due to lymphomatous involvement of lymph nodes around the aorta.  It is rare that the aortic wall is invaded by the disease.

The cause of most lymphomas is not known but it is most probably a manifestation of mutations in certain genes called the oncogenes which fail to inhibit uncontrolled growth.   Specific causes that have been linked to the disease include Epstein Barr virus, immune deficiency syndromes, autoimmune diseases, and exposure to chemicals such as pesticides, herbicides and water contaminated with nitrates.

Progressive enlargement of lymph nodes is a characteristic result.

The diagnosis is suspected clinically with complaints of night sweats or unexplained fevers, and the enlarged glands or enlarged spleen may be felt on examination. The studies of choice include CT and/or Positron Emission Tomography, PET-CT.  Biopsy and flow cytometry confirm the diagnosis as well as characterize the subtypes of the disease.

Treatment plans are based on the type and extent of the disease, and may involve either chemotherapy and/or radiation therapy.

References

1.  Hayter RG, Rhea JT, Small A, Tafazoli  FS, Novelline RA. Suspected Aortic Dissection and Other Aortic Disorders: Multi-Detector Row CT in 373 Cases in the Emergency Setting. Radiology 2006; 238:841-852.

2.  Nienaber CA, von Kodolitsch Y, Nicols V, Siglow V, Piepho A, Brockhoff C, Koschyk DH, Spielmann RP. The Diagnosis of Thoracic Aortic Dissection by Noninvasive Imaging Procedures. NEJM 1993; 328:1-9.

3.  Gavant ML, Menke PG, Fabian T, Flick PA, Graney MJ, Gold RE. Blunt Traumatic Aortic Rupture: Detection with Helical CT of the Chest. Radiology 1995; 197: 125-133.

4.  Yoshida S, Akiba H, Tamakawa M, Yama N, Hareyama M, Morishita K, Abe T. Thoracic Involvement of Type A Aortic Dissection and Intramural Hematoma: Diagnostic Accuracy – comparison of Emergency Helical CT and Surgical Findings. Radiology 2003; 2

General References

Atlas of Human Anatomy Frank Netter 4^th Edition Saunders Elsevier Philadelphia 2006

Diseases of the Aorta Lindsay Joseph Jr. Lea & Feibiger Philadelphia 1994

Histology A Text and Atlas Ross Michael H Romrell Lynn J and Kaye Gordon I Williams and Wilkins Baltimore 1995

Pathologic Basis of Disease Robbins and Cotran Elsevier Saunders 7^th Edition Philadelphia 2004

Textbook of Medical Physiology Guyton Arthur C and Hall John E  W B Saunders Co Saint Louis, Missouri, U.S.A. 1995

The Ciba Collection of Medical Illustrations Volume 5  The Heart Ciba Publications Department New Jersey 1971

Web References

http://www.emedicine.com/