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.
Om
ental bur
sa
Porto
azygos 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.
Oth
er sites
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.
Portal vein
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.
Intrahepatic Shunts
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.