The venae cavae are the great veins of the body. They are the final common pathways for transport of deoxygenated blood collected from the body en route to the lungs via the right side of the heart. The superior vena cava (SVC), as its name implies, drains blood mostly from the superior aspects of the body including the head, upper limbs, and chest cavity, while the inferior vena cava (IVC) drains the abdominal cavity and lower limbs. The IVC is laden with the products of digestion, which have been processed and packaged for distribution by the liver. In this course, we will examine the anatomy of the venae cavae, and apply the anatomy to imaging these vessels in health and disease.
Hippocrates, the Father of Medicine, wrote, “The vessels which spread themselves over the whole body, filling it with spirit, juice, and motion,are all of them but branches of an original vessel. I protest, I do not know where it begins or where it ends, for in a circle there is neither a beginning nor an end.” Although Hippocrates did not have the opportunity to view the body with today’s imaging modalities, the assumptions that he made in 470 B.C. were fairly accurate. In the course, we analyze one portion of this mysterious circle – the vena cava.
Tubes, Branches, Circles and Cycles
The body is made of a series of tubes and a series of organs. The way the body is structured is a microcosm of our cities towns and villages. The tubes that transport fluids, secretions, metabolic products and neuroelectrical impulses, are the roads, sewers, cables and wires of our environment that enable us to function. Tubes are also structurally and functionally essential in all biological systems, including plant and animal life. Tubes are designed to transport material effectively, and the venae cavae are designed to transport deoxygenated blood effectively. The venous system in general, is slow in flow, low in pressure, and adaptable to changing volume.
Veins: General Principles About Veins
The following facts should be kept in mind when learning the names and locations of veins:
Veins are the ultimate extensions of capillaries, just as capillaries are the eventual extensions of arteries. Whereas arteries branch into vessels of decreasing size to form arterioles and eventually capillaries, capillaries unite into vessels of increasing size to form venules and eventually, veins.
Although all vessels vary considerably in location and number of branches (and whether they are even present), the veins are especially variable. For example, the median cubital vein in the forearm is absent in many individuals.
Many of the main arteries have corresponding veins bearing the same name and are located alongside or near the arteries.
The large veins of the cranial cavity, formed by the dura mater, are not usually called veins but are instead called dural sinuses, or simply, sinuses. They should not be confused with the bony, air-filled sinuses of the skull.
Veins communicate (anastomose) with each other in the same way as arteries. Such venous anastomoses provide for the collateral return blood flow in cases of venous obstruction.
The arteries do not have valves except where they take their origin from the heart. Most veins have valves. The superior vena cava does not have any valves and the inferior vena cava has an ineffective valve positioned at its entrance into the heart.
The veins are slow in flow, low in pressure, with variable capacitance to accommodate to changing volume needs. They act as a “storage” house for volume, and can hold onto unneeded volume, or can increase delivery of volume when required.
Flow of venous blood is maintained by multiple mechanisms including the push and pull actions of the heart, the changing intrathoracic and intrabdominal pressures caused by breathing mechanisms, and by the effect of skeletal muscle contraction.
General Considerations About the Venae Cavae
The venae cavae are the main receiving vessels for systemic blood returning to the heart from the various tissues and organs of the body. The superior vena cava (SVC) returns blood from the upper extremities and the brain, originating at the confluence of the left and right innominate veins (which are in turn formed by the internal jugular and subclavian veins). The SVC enters the upper back portion of the right atrium. The inferior vena cava (IVC) receives blood from the lower extremities and abdominal cavity. It is larger than the superior vena cava. The larger veins that enter the IVC include the iliacs, renals and hepatics, while the smaller veins include the lumbar, right gonadal, right adrenal, and inferior phrenic veins. The largest connecting vein that allows the two systems to communicate is the azygos vein.
The Superior Vena Cava: The Superior Vena Cava (SVC) – Big Blue from Above
The SVC measures about 7 cm. in length and is formed by the junction of the two innominate (brachiocephalic) veins. The SVC begins immediately below the cartilage of the right first rib close to the sternum, and, descending vertically behind the first and second intercostal spaces, ends in the upper part of the right atrium opposite the upper border of the third right costal cartilage. Half of the vessel is within the pericardium and there is no valve at the entrance to the atrium.
Normal Transverse View
These are the great vessels of the superior aspect of the mediastinum. They remind me of a story – “Once upon a time there were three bears -a big papa bear, a mama bear and a little baby bear. Papa was usually the largest – but only by a bit – and walked on the left and always in front. Mama was always by his side but just behind and to the right, while baby was always behind and to the right. They went for a walk starting out from the quiet neck (of the woods) making their way toward the heart of activity…”
Nature of the Vessel
The SVC is part of a low-pressure system and its walls are thin. It is tubular but pliable, enabling it to accommodate changes in intravascular volume and pressure. With this ability, its shape varies depending on how “full” the system is, and hence its shape in cross section varies between round, oval, lenticular to slit-like. It is sometimes pushed around by the more powerful and high pressured aorta which sits leftward of it.
Landmarks – Branch Points
A good method of observing, remembering, and evaluating a structure is to identify and recognize certain landmarks. In the case of the SVC, there are four important landmarks, three of which are recognized by a “head and tail” configuration. These landmarks imaging from cranial to caudal include:
Azygos vein entry into the SVC
SVC positioning when the arm of the right pulmonary artery gives it a hug
SVC entrance into the right atrium
The following images detail the three “head and tail” landmarks (1, 2, & 4)and focus on the specific branch points. The “pulmonary artery hug,” (3) follows on the next page.
Landmarks – The Pulmonary Artery Hug
SVC – Origin
As previously detailed, the SVC extends caudally for 6-8 centimeters, terminating in the superior and posterior aspect of the right atrium, anterior to the right main stem bronchus. The SVC is joined posteriorly by the azygos vein as it loops over the right main stem bronchus and lies posterior to and to the right of the ascending aorta.
The next landmark is the “head and tail” view of the azygos vein’s entry into the SVC. The azygos vein takes its origin opposite the first or second lumbar vertebra and enters the thorax through the aortic hiatus in the diaphragm, passes along the right side of the vertebral column to the fourth thoracic vertebra, arches forward over the root of the right lung, and finally ends in the SVC superior to the pericardial attachment.
Right Pulmonary Artery Position
The third most superior landmark is the “pulmonary artery hug”. The pulmonary artery originates from the right ventricle of the heart and conveys venous blood to the lungs. The main pulmonary artery is short and relatively wide (about 5 cm. in length and 3 cm. in diameter). The pulmonary artery extends obliquely upward and backward, passing at first in front and then to the left of the ascending aorta, as far as the under surface of the aortic arch, and then divides into right and left branches of nearly equal size.
Junction With The Right Atrium
SVC – Review of the Four Landmarks
Abnormal SVC – Trouble at the Origin
Trouble at the Azygos
How does the blood get back to the heart? The azygos vein is the bridge between the SVC and IVC – and blood will find its way to the IVC via this collateral pathway and back to the right atrium
Trouble at the Right Pulmonary Artery
Trouble at the Right Atrium
The SVC is sometimes placed on the left side of the mediastinum, and as such is called a “left sided SVC” or “LSVC”. In the early stages of embryonic development, the great veins tend to be symmetrical structures, and with time, the right grows and the left side regresses. Occasionally bilaterality persists, or the left sided component grows while the right side regresses. When this occurs in the SVC, it is called an “LSVC”. The LSVC does not enter directly into the right atrium but rather via the coronary sinus.
Inferior Vena Cava (IVC) – Big Blue From Below
The inferior vena cava (IVC) returns blood to the heart from the tissues and organs below the diaphragm. The IVC is formed by the junction of the two common iliac veins, and ascends along the front of the vertebral column on the right side of the aorta.
The inferior vena cava (IVC) returns blood to the heart from the tissues and organs below the diaphragm. Below the diaphragm, the abdominal portion of the IVC receives blood from the common iliacs, lumbar, right gonadal, renal, suprarenal, inferior phrenic, and hepatic veins. The thoracic portion of the IVC is very short (about 2.5 cm.), and does not receive any additional veins prior to entering the right atrium.
Normal Frontal View
Normal Transverse View
This T1-weighted “out-of-phase” image through the abdomen reveals the normal lenticular shaped IVC in blue overlay. The IVC will normally change in shape depending on the phase of respiration and the intra-vascular volume. Image courtesy of Ashley Davidoff M.D.
The IVC perforates the diaphragm at T8 and eventually runs into the lower and back part of the right atrium. The thoracic portion of the IVC measures only 2.5 cm. in length, half of which is enveloped by the pericardium. The abdominal portion of the IVC begins at L5 and measures between 24 and 28 cm. The transverse diameter of the IVC is similar to that of the aorta measuring on average about 2.5cm.
The inferior vena cava is a large, valveless, venous trunk. The IVC is a deep vein that accompanies the aorta through the abdomen, wrapped in the same outer sheath. As we have so often stated, the physiology of the IVC demands that it is pliable to accommodate breath-to-breath and beat-to-beat changes in volume and pressure. The IVC therefore does not have a single shape but is characterized by the low pressure, low volume lenticular or oval shape on the one hand, and the more rounded higher volume higher pressure shape on the other. The esophagus and vagina share this cross-sectional shape when in the “relaxed” or non-distended state, and will become rounded in the distended state.
The legs and most of the pelvis are drained through the paired common iliac veins, which merge to form the origin of the IVC to the right of the fifth, or last lumbar vertebra. The IVC will span the entire, more cranial, portion of the abdominal cavity and will end its abdominal portion at the diaphragm as it enters the thoracic cavity for its short, 2.5cm. course. The IVC is a retroperitoneal structure, and more specifically, lies in a special ensheathed portion of the anterior pararenal space referred to by some as the central retroperitoneum. “Big blue” and “big red” are vital transport systems and have to be protected. The adventitial layers and retroperitoneal fibrous sheath are fine but strong, intimate protective layers. More global protection is provided as a consequence of its position in front of the bony spine and behind all the organs of the abdominal cavity.
As previously mentioned, the IVC is sometimes placed on the left side of the aorta, as high as the left renal vein, and, after receiving the renal vein, crosses over to its usual position on the right side. Sometimes the IVC may be placed altogether on the left side of the aorta, and in such a case, the abdominal and thoracic viscera, together with the great vessels, are all transposed.
The pliable nature of the low-pressure great veins (-1 to +5mmHg range) is in sharp contrast to the rigid yet elastic nature of the aorta, which has to deal with the higher systemic pressures (120 mm Hg.). The last image revealed the importance of applying this fact to enable the distinction between artery and vein.
As with the SVC, a good method of observing, remembering, and evaluating a structure is to identify and recognize certain landmarks. In the case of the IVC, there are 5 important landmarks. These landmarks include: 1.) IVC origin: Confluence of the common iliacs 2.) Left renal vein entry into the IVC 3.) Right renal vein entry into the IVC 4.) Confluence of the hepatic veins 5.) Junction of the IVC and the right atrium The following pages will define these landmarks.
At the level of the inguinal ligament (which is at the anterior, diagonal border between the trunk and the thigh), the femoral vein becomes known as the external iliac vein. The external iliac unites with the internal iliac vein to form the common iliac vein. The internal iliac vein drains the pelvic walls, viscera, external genitalia, buttocks, and a portion of the thigh. The legs and most of the pelvis are drained through the paired common iliac veins, which merge to form the origin of the IVC to the right of the fifth (and last) lumbar vertebra.
Junction With The Left Renal Vein
The renal veins are fairly large in size and are positioned in front of the renal arteries. The left renal vein, which is longer than the right, passes in front of the aorta just below the origin of the superior mesenteric artery. The left renal vein receives the left gonadal and left inferior phrenic veins, and, generally, the left adrenal vein (in some cases, the left adrenal may join the left inferior phrenic prior to joining the renal). The left renal vein usually opens into the inferior vena cava at a slightly higher level than the right. NB: The right adrenal vein usually drains directly into the IVC – take a glance at the sword fight at Bunker Hill in the adrenal section.
Junction With The Right Renal Vein
The right renal vein – IVC confluence either presents as a “head and tail” structure, or as a “two headed” structure – one small (right renal) and one large (IVC).
Confluence Of The Hepatic Veins
The hepatic veins are formed in the liver as the central vein of the liver lobule where the sinusoids become confluent. The main branches of the hepatic veins are the right, middle, and left. The middle and left hepatic veins often join together to form a common trunk before entering the IVC. In addition, there are several short venous segments that drain the posterior surface of the liver directly into the IVC. We have mentioned that most veins run in conjunction with their counterpart, the artery. In the liver, the hepatic veins run without a neighboring artery. The hepatic artery runs with the portal vein and bile duct in the portal triad of the liver.
Junction With The Right Atrium
Abnormal IVC – Trouble at the Origins
Trouble at the Renals
Trouble at the Hepatics
Trouble at the Right Atrium
Ultrasound of the Vena Cava: SVC – Ultrasound
The superior vena cava by itself is quite difficult to image with ultrasound, but its tributaries, such as the subclavian and jugular veins, are superficial and are well within the reach of probe. Evaluation for thrombosis and patency of stents is both easily accomplished technically and very helpful clinically.
IVC – Ultrasound
The ability to examine the IVC in its entirety is sometimes limited by the patients’ body habitus and by the presence of intervening bowel gas. The intrahepatic portions are almost always accessible, using the liver as a window, and the hepatic confluence is well seen. Color and spectral Doppler are very useful in evaluating the patency of the hepatic veins. In cirrhotic patients the hepatic veins become small and distorted and US is frequently the only study, short of venography, which can identify these diminutive hepatic veins.
CT of the Vena Cava: SVC – CT
The SVC is easily identified and imaged by CT, but the presence of an artifact caused by the mixing of unopacified with opacified blood is a frequent occurrence. Imaging in the early phase (first pass) of vascular enhancement can also be limited by misregistration artifact. This occurs when contrast is densely concentrated and has not had time to diffuse and mix with the blood pool at large. If the study is dedicated to SVC evaluation, then a two-phase study will enable better mixing and better visualization of the vessel. In the setting of SVC obstruction in the oncologic patient, CT is very helpful in determining whether the obstruction is caused and/or originates from the lumen or from the extrinsic pressure of metastases.
IVC – CT
The IVC is also an easy target for CT, but is wrought with artifacts, particularly in the region of the renal vein confluence. The reason for these artifacts – the kidneys receive nearly 20% of the cardiac output and circulation from the renal arterial to the renal venous phase is very rapid. In contrast, the circulation from the lower limbs is comparatively slower. Hence in the first and sometimes even in the second pass, opacified blood from the kidneys mixes with the unopacified blood from the lower body, resulting in the artifactual appearance of a filling defect. However the IVC is consistently seen in CT, and multiphase imaging enables us to evaluate the IVC in its later and more homogeneous phases.
MRI of the Vena Cava: SVC and IVC – MRI
Imaging the great veins with MRI has the advantage of multiplane imaging. Primary imaging in the coronal, sagittal, oblique or axial planes is now routine. T1-weighted, or SPGR, imaging provides the ability to see the blood as black or white, each having their respective advantages and disadvantages. However, because of the slow flow of the venous system, MRI is wrought with flow artifacts that often limit its use.
Venography of the Vena Cava: SVC and IVC – Venogram
Venography continues to remain the gold standard for great vein imaging, but it has lost its place as the primary imaging modality, which it held for so many years. In difficult cases where the lumen of the vessel needs to be imaged it still remains the final word, and in cases where intervention is required it is untouched by other modalities. The problem with venography is that it does not have the ability to “see” beyond the lumen. As a result, extrinsic masses can only be implied by the shape of the lumen but not primarily imaged. For adequate SVC imaging, both arms need to be injected or otherwise a mixing artifact precludes optimal enhancement. Adequate IVC imaging requires Valsalva maneuver to allow visualization of the iliac renal and hepatic branches.
Conclusion: Final Thoughts
The great veins are slow in flow, low in pressure and have the ability to accommodate changing intra-vascular volumes. They can, of course, only transport that which they receive, and in the end the net outflow from the left heart has to equal the net return to the right heart. One of the functions of the venous system is to regulate blood volume, and by virtue of the capacitance of the venous system, it can “hold” on to blood by dilating, or it can supply the system with volume, by contracting. The great veins have been designed as pliable tubes to accommodate and transport these changing volumes. Thrombosis is the most common disease plaguing the great veins, particularly as venous access devices are used in increasing frequency. Fortunately, in most cases of thrombosis, only one of the great veins is affected. The ability of veins to bypass obstructions is unequalled in the body and in the case of the great veins, the main bridge and collateral between the two, is the azygos system. US for the veins is a powerful multifunctional tool in that when the vessel is accessible it can measure functional aspects such as the velocity, direction, and the nature of flow, together with structural detail such as size and shape. Since both great veins are deep and are protected and surrounded by structures that may be difficult to penetrate with US, other modalities are frequently called upon to assist. CT and MRI have a global perspective and can view the innards as well as the outer aspects of the vessels. Venography may be called upon to be a final arbitrator, but in the end, also has the final and only word in acute therapeutic intervention.