INFERIOR VENA CAVA ULTRASOUND

THE INFERIOR VENA CAVA (IVC)
Ultrasound is highly successful in demonstrating the proximal IVC, by using the liver as an acoustic window, especially if the patient is turned right anterior oblique. The distal IVC may be obscured by overlying bowel gas and, unlike the aorta, is also susceptible to compression, making visualization difficult in some cases. The normal IVC has thinner walls and a more flattened profile than the aorta, and its lumen alters with changing abdominal pressure; for example, during respiration the lumen decreases on inspiration, or with the Valsalva manoeuvre. Its course becomes slightly anterior as it passes through the diaphragm, unlike the aorta which travels posteriorly at this point.

The main renal veins may be seen in TS, entering the IVC just below the level of the pancreas. Haemodynamically, the blood flow spectrum from the IVC alters according to the distance of the sample volume from the right atrium. The blood flow through the IVC and proximal hepatic veins is pulsatile, with reverse flow during right atrial systole. Pulsatility reduces in the distal IVC. The most common anomaly of the IVC is that of duplication. However this is infrequently picked up on ultrasound and is best demonstrated with CT or MRI. Transposition of the IVC may be seen in situs inversus.

Pathology of the IVC
Thrombus in the IVC may be due to benign causes, or the result of tumour. It is not usually possible to tell the difference on grey-scale appearances alone, but vascularity may be demonstrated on power or colour Doppler within tumour thrombus, and the clinical history is helpful. Tumour thrombus invades the renal vein and enters the IVC in around 10% of renal carcinoma cases. Tumour thrombus from hepatic or adrenal masses can also invade the IVC.

Coagulation disorders, which cause Budd - Chiari syndrome predominantly affect the hepatic veins, but may also involve the IVC. Patients may require the insertion of a caval filter, which is performed under X-ray guidance, but may be monitored for patency using ultrasound with Doppler.

Dilatation of the IVC is a finding commonly associated with congestive heart failure, and is frequently accompanied by hepatic vein dilatation. Compression of the IVC by large masses is not uncommon. This may be due to retroperitoneal masses, such as a large lymph node, or liver masses such as tumour or caudate lobe hypertrophy. Colour or power Doppler is particularly useful in confirming patency of the vessel and differentiating extrinsic compression from invasion. Insertion of metallic stents may be performed under angiographic control to maintain the vessel patency, particularly if the compression is due to inoperable hepatic metastasis. Tumours of the IVC are rare. Leiomyosarcoma is a primary IVC tumour, appearing as a hyperechoic mass in the lumen of the vein. This causes partial or complete obstruction of the IVC, resulting in Budd - Chiari syndrome. In partial occlusion, the hepatic veins and proximal IVC may be considerably dilated. Resection of the tumour, with repair of the IVC, is possible provided the adjacent liver is not invaded.