Page 344 - Libro 2

324 PART 5 — ABDOMINAL ANATOMY
The external iliac veins drain the lower extremities as a continuation of the common femoral veins. They begin at the inguinal ligament and course cephalad as they dive deep into the pelvis. The internal iliac veins drain the pelvic viscera and musculature and join the external iliac veins at the level of the sacro- iliac joints to form the common iliac veins. The IVC begins at the junction of the right and left common iliac veins at the level of the fifth lumbar vertebra. It ascends in the retroperitoneum to the right of the ab- dominal aorta and courses through the deep fossa on the posterior surface of the liver between the caudate lobe and bare area. After passing through the dia- phragm at the level of the eighth thoracic vertebra, the IVC terminates in the right atrium. The venous tributaries of the IVC are listed in Table 21-1.
The IVC receives blood from all organs and tis- sues inferior to the diaphragm. It returns the de- oxygenated blood to the heart for oxygenation and recirculation. The diameter of the IVC depends on the hydration status of the patient. In well-hydrated patients, the IVC appears distended and the mean di- ameter at the level of the renal veins is 17 to 20 mm.1 Megacava (excessively large IVC diameter) is a rare condition that tends to occur only in patients of very large stature or in patients with congestive heart fail- ure. Most large series reported that less than 3% of examined patients had an IVC larger than 28 mm in diameter.2 Dehydration causes collapse of the IVC, making it narrow and difficult to detect and evaluate with ultrasound. In these situations, having an as- sistant elevate the patient’s legs often increases the caliber of the vein, making it easier to visualize.3
ANATOMIC VARIANTS
Because several precursor veins contribute to its for- mation during embryogenesis, the IVC can display a wide spectrum of anatomic variation. Persistence of the left precursor of the IVC can result in paired venae
TABLE 21-1
IVC Venous Tributaries
Hepatic veins
Renal veins
Common iliac veins
Right adrenal vein
Right ovarian vein or testicular vein Inferior phrenic vein
Four lumbar veins Medial sacral vein
cavae or a left-sided IVC. In the case of paired venae cavae, the duplication typically terminates at the level of the renal veins when the left IVC drains into the left renal vein. A completely left-sided IVC can either terminate in the left renal vein or can extend cranially to drain into the azygos vein in the chest.4
Anomalies of the IVC may also involve its intrahe- patic portion. When the intrahepatic portion of the IVC is congenitally absent, blood is returned to the heart via the azygos or hemiazygos veins. Although ultrasound may not detect this anomalous collateral venous pathway, the diagnosis can be made by dem- onstrating direct drainage of the hepatic veins into the right atrium as well as absence of the intrahe- patic IVC. In membranous obstruction of the intra- hepatic IVC, ultrasound demonstrates a fibrous sep- tum in the IVC just cephalad to the insertion of the right hepatic vein. Flow may be reversed in the IVC, and flow in the distal hepatic veins is sluggish and continuous.
SONOGRAPHIC EXAMINATION TECHNIQUES
PATIENT PREPARATION
Extensive bowel gas represents the most common impediment to a successful sonographic examination of the IVC and iliac veins. The bowel gas prevents transmission of the ultrasound signal and precludes accurate identification of any deep abdominal struc- tures. Having the patient fast for 8 hours prior to the exam can decrease the likelihood of bowel gas; however, the exam should still be attempted even if the patient has eaten. Although morbid obesity can make abdominal imaging difficult due to the depth of penetration necessary, it rarely precludes ade- quate IVC visualization. In surgical patients, open abdominal wounds can encroach on the areas of the abdomen typically used to place the ultrasound probe. These patients may require alternative probe placement and the assistance of the nursing staff to achieve a complete ultrasound exam.
PATIENT POSITIONING
The exam begins with the patient in a supine posi- tion and with the sonographer standing to the pa- tient’s right side. The height of the bed or stretcher should be adjusted so that the level of the patient’s abdomen is slightly lower than the sonographer’s waist. In this configuration, the sonographer can ex- tend his or her scanning arm downward and ergo- nomically apply pressure to the patient’s abdomen. The ability to easily and comfortably apply pres- sure with the probe often allows the sonographer to