Surgical Management of Extrahepatic Shunts
Pertinent Anatomical Structures: The Caudal Vena Cava
In a normal dog, there are no large vessels entering the caudal vena cava between the renal and hepatic veins. In an animal with a portocaval shunt, the caudal vena cava will appear dilated at the level of the shunt terminus and turbulent flow will be visible within the vein and near the terminus of the large, shunting vessel, which usually has a diameter of 1 cm.
Frequently, extrahepatic PSS terminate on the caudal vena cava at the level of the epiploic foramen, which is found to the right of midline at the base of the mesoduodenum. The boundaries of the epiploic foramen are formed dorsally by the caudal vena cava, ventrally by the portal vein and hepatic artery, cranially by the caudate lobe of liver, and caudally by the celiac artery. To locate the epiploic foramen, gently retract the duodenum ventrally and to the left, exposing the right kidney and caudal vena cava. Extrahepatic PSS may not be immediately obvious; it is important to gently retract the celiac artery caudally to see the PSS terminating on the caudal vena cava.
Portoazygos shunts, which are commonly seen in Yorkshire terriers and dachshunds in our clinic, often traverse the diaphragm at the level of the crura or aortic hiatus, and are obscured by overlying viscera. To improve detection of and access to these shunts, it may be necessary to open the omental bursa. To enter the omental bursa, return the duodenum to its normal position on the right side. Tear a hole in the superficial, ventral leaf of the greater omentum and retract the stomach cranially and the intestines caudally and laterally to view the portal tributaries and left border of the caudal vena cava. Check the caudal and dorsal bursal recesses for any vessel that is larger than the portal or splenic vein, has turbulent flow, or crosses the diaphragm. Normal vessels that traverse the diaphragm include the azygos and hemiazygos veins and aorta through the aortic hiatus and the caudal vena cava through the caval foramen. Any other vessel of significant size (portal or splenic vein in size) that penetrates the diaphragm is likely to be a portoazygos shunt.
Portoazygos shunts that traverse the diaphragm through the esophageal hiatus may be easier to approach outside of the omental bursa by retracting the liver and stomach to the right so that the cardia and esophagus are visible. Some shunts may be visible along the diaphragm, inserting on the left phrenic or left hepatic vein.
Occasionally a shunt which has not been identified by exploration of the cranial abdomen can be located by thorough examination of the caudal vena cava in the caudal abdomen. Thorough exploration is warranted in all dogs with single congenital PSS because of the possibility, though rare, of a second shunt. When a shunt is not found, the surgeon should obtain a liver biopsy to rule out other hepatic diseases such as hepatic microvascular dysplasia and perform intraoperative mesenteric or splenic portography to definitively rule out a PSS. If the dog has gastrointestinal disease and increased bile acids, intestinal biopsies should also be taken, since inflammatory bowel disease can be associated with hepatic veno-occlusive disease.
Once a PSS is located, it is critical to verify that the portal vein is complete and that it terminates within the liver, since PSS ligation may cause death in these dogs. The portal vein is located caudal to the liver and ventral to the caudal vena cava and travels along the ventral border of the epiploic foramen. The hepatic artery is often found lying against the dorsal or lateral border of the portal vein. In an average size dog, the portal vein may be about 1 cm in diameter at the porta hepatica where it bifurcates; however, an animal with a PSS will probably have a smaller portal vein.
Dissection and Occlusion of Extrahepatic PSS
Once the PSS is identified and presence of a prehepatic portal vein is verified, shunt occlusion can be attempted. Ligate the shunt as close to its insertion site as possible so that all tributaries of the shunt are upstream from the occlusion. Portocaval shunts should be occluded at their terminus on the caudal vena cava. Portoazygos shunts can be occluded at the abdominal side of the diaphragm. Thorough examination is warranted before ligature placement as portoazygos shunts frequently have small branches from gastric veins that enter the PSS just before it traverses the diaphragm. The diaphragm may be opened if more exposure is needed.
If suture is to be used to ligate the shunt, then a small opening is made through the fascia around the shunt by dissecting adjacent to the PSS at its terminus. Silk suture (2-0) is frequently used because of ease of handling and knot security. The shunt should be temporarily occluded for 5-10 minutes while the surgeon evaluates the viscera for evidence of portal hypertension, including pallor or cyanosis of the intestines, increased intestinal peristalsis, cyanosis or edema of the pancreas, and increased mesenteric vascular pulsations.
Additionally, the surgeon can measure portal and central venous pressures. To measure portal pressure, a catheter is placed directly into a jejunal vein or through the splenic parenchyma and into a splenic vein. The catheter is secured in place with gut suture and is attached to an extension set, 3-way stopcock, and syringe. A water manometer is attached to the 3-way stopcock, which is rested on the inguinal region of the patient to provide consistent readings during portal pressure measurements.
Recommendations for postligation pressures are to limit the maximum portal pressure to 17 to 24 cm H2O, maximal change in portal pressure to 9-10 cm H2O, and maximal decrease in central venous pressure to 1 cm H2O.
Partial ligation should be performed if evidence of portal hypertension is noticed during surgery. Objective pressure measurements should not be used as the sole criteria for degree of shunt attenuation, since blood pressures can vary with depth of anesthesia, hydration status, phase of respiration, degree of splanchnic compliance, and other systemic factors. To perform partial ligation, choose a cylinder ( a piece of tubing, steel pin, or rod) that is the approximate diameter that you wish to achieve during shunt occlusion. Place the cylinder next to the shunt and wrap the ligature around the shunt and the cylinder. Tie the ligature and remove the cylinder, then recheck portal pressures and evaluate the color of the viscera.
Abrupt occlusion and partial ligation of PSS have been associated with serious postoperative complications, including perioperative death in 14 to 21%, seizures in 7.5-11%, recurrence of clinical signs in 40-41%, and development of multiple PSS in 7%.
Ameroid Constrictors and other methods of gradual occlusion
Use of ameroid constrictors (Research Instruments NW Inc., Sweethome Oregon, 97386. 541-367-1855) for gradual, complete shunt ligation has recently been described in the literature. An ameroid constrictor is an inner ring of casein that is surrounded by a stainless steel sheath. Casein is a hygroscopic substance that swells as it slowly absorbs body fluid. The stainless steel sheath forces the casein to swell inwardly, eventually closing the ring and obliterating the shunt. Ameroid constrictors gradually close over 4-5 weeks. Time to occlusion of the vessel is dependent on the size of the vessel and constrictor and the rigidity of the outer ring. Closure is most rapid during the first 3-14 days after implantation; rate of closure declines thereafter. Ameroid constrictors are gas sterilized and therefore should not be used until 12 to 24 hours after sterilization to allow residual ethylene oxide to be released from the casein.
Ameroid constrictors come in various sizes, with internal diameters ranging from 3.5 to 9 mm; constrictors with 3.5 and 5 mm internal diameters are most frequently used for PSS ligation. The choice of ameroid constrictor size for PSS occlusion is based on shunt diameter; therefore, the surgeon should have a selection of sizes available at each surgery. To avoid postoperative portal hypertension, choose a constrictor that does not compress the shunt after placement. In cases where larger constrictors are not available, portal pressures can be measured during partial shunt occlusion and viscera can be evaluated subjectively for signs of portal hypertension to determine whether a smaller constrictor could be used.
Before constrictor placement, the "key", a small column of casein that completes the constrictor ring, is removed from the ameroid constrictor and set aside in a dry cup. The ameroid constrictor is held securely by a pair of Allis tissue forceps, which prevent rotation of the casein inside of the stainless steel ring. Dissection of the supporting fascia around the PSS should be kept to a minimum when placing an ameroid constrictor to prevent postoperative movement of the ring and acute obstruction of the shunt. Once an opening has been made through the fascia around the PSS, the shunt is flattened by elevating it with open right angle forceps or two silk sutures. The constrictor ring is slipped over the shunt and, with a hemostat, the key is replaced within the constrictor to complete the circle. If the key is difficult to place, then a small amount of casein can be shaved off of one of its ends. If the key is lost or unusable, the inner casein ring can be rotated so that its opening faces in the opposite direction from that of the stainless steel ring.
Complications reported after ameroid constrictor placement include death from portal hypertension in 14% and development of multiple acquired portosystemic shunts 3 months after surgery in 17% (2/12). Acute postoperative portal hypertension may be avoided by careful, limited dissection to prevent shunt vasoconstriction and shifting of the ring and by choosing the correct constrictor ring size. Complications are uncommon once experience in constrictor selection and placement is gained.
Gas sterilized strips of cellophane have been used to provide partial occlusion of shunts in over 4 dogs. Because the strips are flexible, they are easier to place around intrahepatic shunts. The strips are wrapped once around the shunt, compressing it to 2-4 mm in diameter, and are held together with surgical clips. Inflammation caused by the cellophane results in complete occlusion of most shunts in dogs in less than 4-6 weeks.
Surgery for intrahepatic shunts is much more difficult, and these cases are usually referred to a specialist. If not readily visible during surgery, intrahepatic PSS may be located by palpation, ultrasound, catheterization via the portal vein, or measurement of portal pressure changes during digital vascular occlusion. To catheterize the shunt, insert a long jugular catheter through the spleen and into the portal vein, and thread it through the shunt and into the caudal vena cava. Alternatively, place a purse string suture in the portal vein; make an incision into the vein within the purse-string suture, and thread a red rubber catheter into the portal vein and through the shunt. In most dogs it will be necessary to incise the diaphragm to feel the catheter in the caudal vena cava. Once the catheter is in place, the various portal vein branches, liver lobes, and hepatic veins are palpated to locate the origin and insertion of the shunt.
Intrahepatic PSS of the left hepatic division are occluded by direct PSS ligation or by ligation of the left hepatic vein. To ligate the left hepatic vein, divide the left triangular ligament, then gently dissect the hepatic vein branch just before its junction with the caudal vena cava.
Intrahepatic PSS of the central and right hepatic divisions are often occluded by ligation of the associated portal vein branch. Alternatively, blood flow into and out of the liver can be occluded temporarily and the shunts can be approached intravascularly through the caudal vena cava or portal vein. The shunt is located by catheter placement; once the caudal vena cava or portal vein is opened, the shunt is identified with the catheter running through it and is ligated transmurally. Inflow occlusion should be limited to 20 minutes to prevent death.
Recently thrombogenic coils have been placed via catheters through the jugular vein to gradually obstruct the shunts. Multiple coil placements are often required.