Surgical Techniques for Extra Vascular Occlusiion of tic Shunts

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Vol.18, No. 7 July 1996

Surgical Techniquesfor ExtravascularOcclusion of Intrahepatic Shunts

Washington State University

Karen M. Swalec Tobias, DVM, MS

University of Georgia

Clarence A. Rawlings, DVM, PhD

Intrahepatic portosystemic shunts are congenital vascular anomalies that arefound primarily in large-breed dogs.1 Surgical occlusion of portosystemicshunts is the therapy of choice for improving the quality of life and increas-

ing the life span of affected animals. Location of intrahepatic portosystemicshunts can be determined by exploratory laparotomy, ultrasonography, portog-raphy, or nuclear scintigraphy.2 Because of the location of the intrahepatic por-tosystemic shunt, direct ligation may be difficult. Other surgical options includeligation of the portal vein branch supplying the shunt, ligation of the hepaticvein branch draining the shunt, or temporary inflow occlusion and intravascularclosure of the shunt or associated hepatic vein.3,4 This article reviews the perti-nent anatomy and surgical approaches for extravascular occlusion of intrahepat-ic portosystemic shunts and the veins supplying or draining intrahepatic shunts.

ANATOMY OF THE LIVERThe canine liver consists of six lobes and three divisions (Figure 1). The left

lateral and left medial lobes make up the left division; the right medial and

quadrate lobes, which lie on either side of the gallbladder, compose the centraldivision; and the right lateral and caudate lobes form the right division. Thecaudate lobe is subdivided into the caudate and papillary processes, which re-ceive portal blood supply from the vessels of the right and left divisions, respec-tively.5–7 The right lateral and caudate lobes surround a portion of the caudalvena cava as it courses cranially in the dorsal abdomen.6 The liver is attached tothe diaphragm, primarily by the left triangular ligament (Figure 2); the righttriangular ligament is smaller and provides less support.7

Caudal to the liver, the portal vein is ventral to the caudal vena cava, epiploic

Continuing Education Article

V

FOCAL POINT

KEY FACTS

#Knowledge of anatomy of

the liver and its associated

vasculature is critical for locating

and isolating intrahepatic

portosystemic shunts.

I If not readily visible during surgery,

intrahepatic portosystemic shunts

may be located by palpation,

ultrasonography, catheterization via

the portal vein, or measurement of

portal pressure changes during

digital vascular occlusion.

IIntraoperative hepaticparenchymal hemorrhage is

decreased with blunt dissection

or use of an ultrasonic aspirator.

I Intrahepatic portosystemic

shunts of the left hepatic division

are occluded by direct ligation of

the portosystemic shunt or by

ligation of the left hepatic vein.

I Intrahepatic portosystemic

shunts of the central and right

hepatic divisions are oftenoccluded by ligation of the

associated portal vein branch.



foramen, and hepatic artery. Its tributaries, from caudalto cranial, include the cranial mesenteric, caudalmesenteric, splenic, and gastroduodenal veins. The por-tal vein branches are fairly consistent in number and lo-cation (Figure 1). The right main branch of the portalvein supplies the right division of the liver, except for

the papillary process of the caudate lobe.6,7

The rightmain branch may be partially or completely surround-ed by hepatic tissue when it divides to form the rightlateral and caudate portal branches. The larger leftmain branch gives off a central branch to the right me-dial lobe and a small papillary branch to the papillary lobe before dividing into left lateral, left medial, and quad-rate branches.7 Branches of the hepatic artery and bileducts are usually located on the ventral surface of the

portal vein, although some branches may befound dorsal to the portal vein.7

Dogs usually have six to eight hepaticveins that form a partial spiral around

the caudal vena cava6 (Figure 2).The left hepatic vein drains

the left division of the liverand is the largest, mostcranially located hepaticvein.5,7 The left hepaticvein enters the left later-al surface of the caudalvena cava near the vis-ceral surface of the di-aphragm.6 One third to

one half of the vein’s cir-cumference is in close con-

tact with hepatic parenchyma.The left hepatic vein can be

seen more readily by incisingthe left triangular ligament(Figure 3). The central divi-

sion of the liver may bedrained by one or two hep-atic veins. These veins enterthe ventral surface of the

caudal vena cava caudomedi-al to the left hepatic vein and may be completely surrounded by he-patic parenchyma at their insertions.7

Hepatic veins draining the right division of the liver join the caudal vena cava on its right ventrolateral sur-face and are completely surrounded by hepatic tissue.6,7

GENERAL SURGICAL PRINCIPLESIdentification of the Shunt

Intrahepatic portosystemic shunts are approached via aventral midline celiotomy.Median or paramedian ster-notomy and incision of thediaphragm can also be per-formed to increase exposure.Intrahepatic portosystemicshunts may be seen if they arenot completely surrounded

by hepatic parenchyma (Fig-ure 2). Intrahepatic portosys-temic shunts and hepatic orportal vein branches that areassociated with the portosys-temic shunts are usually dilat-ed and have turbulent bloodflow. If the portosystemicshunt is not visible, lobes

Small Animal The Compendium July 1996

L I V E R A N A T O M Y I S U R G I C A L A P P R O A C H E S

Figure 1— Anatomy of the liver (visceral surface), hepaticartery, and portal vein in the dog. The six lobes of the liverare the caudate lobe, which is subdivided into the caudate(CC ) and papillary (PC ) processes; left lateral lobe (LL );

left medial lobe (LM ); quadrate lobe (Q ); right mediallobe (RM ); and right lateral lobe (RL ). The gallbladder(G ) lies between the quadrate and right medial lobes. a =artery, v = vein.

Key Facts AboutIntrahepatic Shunts

I Large-breed dogs

primarily affected

I Left side of liver

more often affected

I Postligation

complication rate =

77%

I Postligation

mortality rate =

11% to 25%



should be palpated to determine whether there is an easi-ly compressible area (typical of an aneurysm) associated

with an intrahepatic shunt.8 The abdominal viscerashould be monitored during this palpation because ve-nous distention and increased portal pressure can devel-op if the portosystemic shunt is obstructed.

Intraoperative ultrasonography has been used duringexploratory surgery to locate portosystemic shunts thatare not readily visible. A sterilized ultrasound transduc-er is gently rested on the liver surface and irrigated withphysiologic saline, as needed. A needle and suture may then be passed around the portosystemic shunt withultrasonographic guidance to avoid perforating theshunt.9

Another method of locating intrahepatic portosys-temic shunts is to place a purse-string suture in the por-tal vein and insert a large-bore catheter or tube throughthe purse-string into the vein and advance it throughthe shunt (Figure 4). The catheter can also be passed

through a splenic vein to avoid placement of the portalvein purse-string suture. Proper placement of thecatheter is verified by palpating the tip in the caudal

vena cava cranial to the liver. Palpation of the intravas-cular catheter will identify the intrahepatic location of the portosystemic shunt and the hepatic and portalveins draining and supplying the shunt, respectively.10

Identification of the shunt may be confirmed by measuring changes in portal pressure. A mesenteric or

portal vein is catheterized, and baseline portal pressureis measured with a water manometer zeroed to the levelof the portal vein or with a pressure transducer. Normalportal pressure is approximately 8 to 13 cm H2O (6 to10 mm Hg); portal pressure in dogs with portosystemicshunts may be 0 to 12 cm H2O.2,11

The suspected shunt or its associated portal veinbranch is digitally occluded without inhibiting flowthrough the portal vein or its remaining branches. Dig-ital occlusion of the left hepatic vein may be similarly attempted; the surgeon must be careful not to simulta-neously obstruct the caudal vena cava. A rapid rise inportal pressure occurs with occlusion of the portosys-

temic shunt or its associated portal vein branch or he-patic vein; minimal changes are seen with compressionof other portal branches or hepatic veins. Occasionally,

The Compendium July 1996 Small Animal

S H U N T I D E N T I F I C A T I O N I U L T R A S O N O G R A P H Y I P U R S E - S T R I N G S U T U R E S

Figure 2— Anatomy of the liver (diaphragmatic surface) and the hepatic veins. The hepatic veins form a partial spiral aroundthe ventral surface of the caudal vena cava near the diaphragm. After incising the left triangular ligament, the left mediallobe is retracted to the right and the interlobar area is examined for a portosystemic shunt draining into the hepatic vein of the left lateral or left medial liver lobe. RL = right lateral lobe, RM = right medial lobe, GB = gallbladder, LM = left mediallobe, Q = quadrate lobe, LL = left lateral lobe, Lig = ligament.



with blunt dissection.13 Severe hemor-rhage may be controlled temporarily

via occlusion of the portalvein, hepatic artery, andcaudal vena cava. Blunt dis-section of the parenchyma

will decrease hemorrhagefrom larger vessels; however,hemorrhage from smaller ves-sels may obstruct visualizationand result in increased opera-

tive time and morbidity.The ultrasonic aspirator

selectively removes hepaticparenchyma without dam-aging essential structures(such as nerves or vessels).

The aspirator emulsifies andaspirates soft tissue with high-

water content (such as tumors,hepatic parenchyma, or prostatic

tissue) and leaves elastic structures(such as nerves and blood vessels) in-tact.14–17 In hepatic resections, ultra-

sonic aspirators reduce perioperative hemorrhage, thenumber of transfusions, postoperative complications,and duration of the opera-tion.14–16 Ultrasonic surgicalaspirators are particularly useful for dissecting aroundhepatic veins, which may beeasily disrupted by conven-tional dissection tech-niques.18

PortosystemicShunt Ligation

In a recent study, 73% of intrahepatic portosystemicshunts were partially ligatedto avoid development of fa-tal portal hypertension.1

Ligation limits for portosys-temic shunts are often deter-

mined by measuring portalpressures before and afterligation.2,10,12 Limits for maxi-mum postligation portal pres-sure range from 17 to 24 cmH2O,1,8,12 with a maximumincrease of 9 to 10 cm H2Oover baseline.11,12 Measure-ment of portal pressure


P O R T A L P R E S S U R E C H A N G E S I H E P A T I C H E M O R R H A G E I U L T R A S O N I C A S P I R A T O R S

Figure 3— Appearance of the caudal vena cava cra-

nial to the liver and caudalto the diaphragm. The lefttriangular ligament has been in-cised. The left hepatic vein is evi-dent entering the caudal vena cava. A portion of the central hepatic vein is also visible. Thehepatic veins of the right division are completely encasedin hepatic parenchyma. The dashed line indicates the ap-proximate location of the division between the left medialand left lateral lobes. CVC = caudal vena cava, LHV = lefthepatic vein, LM = left medial lobe, LL = left lateral lobe,lig = ligament, v = vein.

portal pressure changes minimally with shunt occlu-sion; this variation may be caused by increased splanch-nic compliance and pooling of blood in the intestinesand spleen.12

Isolation of the ShuntIsolating hepatic veins and intrahepatic portosystemic

shunts for ligation may be difficult.6 Hepatic veins arebroad and short and, except for the left lateral hepaticvein, are usually completely surrounded by hepaticparenchyma.6 Intrahepatic shunts may be tortuous andthin-walled; ligation of the portal branch supplying the

shunt or the hepatic vein draining the shunt may benecessary because of the difficulty in isolating the shuntitself.1

Hemorrhage occurs frequently during isolation of in-trahepatic portosystemic shunts.10 Traditionally, hepatichemorrhage has been decreased by providing adequate ex-posure, mobilizing the liver, isolating vascular structures,controlling hemorrhage from small vessels with electro-cautery or ligation, and separating hepatic parenchyma

Indicators ofIntraoperative PortalHypertension During

Shunt Ligation

I Postligation portal

pressure >17–24

cm H2O

I Increase in portal

pressure >10 cm H2O

I Decrease in central

venous pressure

>1 cm H2O

I Subjective signs,

including blanchingof the intestines;

pancreatic cyanosis;

intestinal hypermotility;

and distended,

pulsating jejunal

vessels



changes is not always reliable for preventing postopera-tive mortality. In a recent report, all dogs that died aftershunt ligation had postligation portal pressure less than19 cm H2O and increases in portal pressure of less than10 cm H2O after ligation.1

Central venous pressure may be monitored during

shunt ligation; a decrease in central venous pressure of greater than 1 cm H2O during shunt occlusion hasbeen associated with development of postoperative por-tal hypertension.12 In addition, viscera may be observedfor subjective signs of intraoperative postligation portalhypertension, including blanching of the intestines;pancreatic cyanosis; distended, pulsating jejunal vessels;and hypermotility of the small intestine.19 If objectiveor subjective signs of intrahepatic portal hypertensionare apparent, the ligature should be loosened to a pointat which these signs are no longer present.

Care must be taken during partial ligation of porto-systemic shunts to avoid overtightening the ligature

when the second throw is tied to set the first knot. Over-tightening may be avoided by placing a catheter or sec-tion of red rubber tubing alongside the portosystemicshunt and including the tubing within the ligature. Oncethe knot is tightened, the tubing is removed, thus pro-viding a set diameter for the encircling ligature. Alterna-tively, a bulldog clamp is placed across a portion of theshunt, and pressures and vis-cera are evaluated for portalhypertension. Once thed e s i r e ddegree of attenuation isobtained, the clampedportion of the shunt istransfixed with fine su-ture on a cardiovascular

needle.PORTOSYSTEMIC SHUNTSOF THE LEFT DIVISION

In the embryo, the ductus venosus connects the cra-nial anastomosis of the right and left vitelline veins

with the left umbilical vein.5 Thus, intrahepatic por-

tosystemic shunts (patent ductus venosus) are morelikely to be found in the left division of the liver, drain-ing into the left hepatic vein. Intrahepatic shunts of theleft division are often occluded at the level of the lefthepatic vein because this vein is readily accessible inmost dogs. Complete ligation of the left hepatic veinresults in transient hepatic congestion but no long-standing effect on hepatic structure, circulation, orfunction in healthy dogs.20

PORTOSYSTEMIC SHUNTS OF THECENTRAL AND RIGHT DIVISIONS

Because most hepatic veins of the central and right di-

visions are completely encircled with hepatic tissue, por-tosystemic shunts of these divisions are often treated by ligation of the supplying portal vein branch (Figure 5).Ligation of a portal vein branch in healthy dogs resultsin atrophy of the liver lobes supplied by the branch anddecreased bile flow and biliary excretion of these lobes.

Blood flow to the remaininglobes increases, althoughportal pressure remainsunchanged, and these

lobes hypertrophy


P O R T A L H Y P E R T E N S I O N I O V E R T I G H T E N I N G I S H U N T C H A R A C T E R I S T I C S

Figure 4—Location of a portosystemic shunt in the right medial liver lobe. A purse-string suture has been placed in the por-tal vein and a catheter has been inserted through the purse-string into the vein and advanced through the shunt. Thecatheter is palpated to determine the location of the shunt. In the illustrations, the right medial branch of the portal veinbecomes an aneurysmal dilatation at the site of the portosystemic shunt. CVC = caudal vena cava, GB = gallbladder, PV =portal vein.



so that overall hepatic function remainsnormal.21,22

An alternative technique for ligationof intrahepatic shunts is to “sandwich”the shunt and surrounding liver tissue betweenpieces of mesh placed on the diaphragmatic andvisceral surfaces of the affected lobe. a Suture ispassed through the mesh and liver parenchyma sothat the ligature surrounds the shunt and rests on themesh above and below the shunt. The ligature is tight-ened until an appropriate rise in pressure is noted. Al-though extensive dissection and hemorrhage are avoid-ed, this technique is not effective for shunts thatformed as windows between the portal vein and hepaticvein.

PROGNOSIS FOR INTRAHEPATICSHUNT LIGATION

Postoperative complications may be seen in 77% of dogs after portosystemic shunt ligation.23 The mostcommon complication is abdominal distention, whichmay not require treatment if it is the only clinical ab-normality seen.23 Survival rates of dogs undergoingintrahepatic portosystemic shunt ligation range from75% to 89%.1,8,23 Causes of death include peritonitis,thrombosis of the portal vein, and cardiovascular col-lapse secondary to fatal portal hypertension.1,3,8,23 Dogs

that have clinical signs of portal hypertension aftersurgery, such as severe abdominal pain, delayed recov-ery from anesthesia, or cardiovascular collapse, shouldbe treated with supportive therapy and immediately re-turned to surgery for ligature removal.

SUMMARYLigation of intrahepatic portosystemic shunts is tech-

nically demanding and can be associated with a highrate of intraoperative complications. Familiarity withthe anatomy of the liver and its vasculature is necessary to locate and isolate intrahepatic shunts. Shunt catheter-ization, dissection with ultrasonic aspirators, and othertechniques that improve identification and isolation of intrahepatic portosystemic shunts may help to decrease

surgical time and intraoperative and postoperative com-plications of intrahepatic portosystemic shunt ligation. With proper diagnostic, anesthetic, surgical, and criticalcare expertise, surgery is an excellent option for treat-ment of intrahepatic portosystemic shunts.

ACKNOWLEDGMENTThe authors thank Kip Carter of Educational Re-

sources at the College of Veterinary Medicine, University

The Compendium July 1996 Small Animal

A B D O M I N A L D I S T E N T I O N I S U R V I V A L R A T E S I C A U S E S O F D E A T H

Figure 5—Isolation of the right lateral branch of the portalvein (PV ). An ultrasonic aspirator was used to dissect he-

patic parenchyma from around the vessel. CVC = caudalvena cava, v = vein.

aPersonal communication: Department of Companion Animaland Special Species Medicine, College of Veterinary Medicine,North Carolina State University, Raleigh, NC, 1996.



of Georgia, Athens, Georgia, for drawing the illustra-tions for this article.

About the AuthorsDr. Tobias is affiliated with the Department of Veterinary

Clinical Sciences, College of Veterinary Medicine, Wash-ington State University, Pullman, Washington. Dr. Rawl-

ings is affiliated with the Department of Small Animal

Medicine, College of Veterinary Medicine, University of

Georgia, Athens, Georgia. Drs. Tobias and Rawlings are

Diplomates of the American College of Veterinary Sur-

geons.

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2. Swalec KM: Portosystemic shunts, in Bojrab MJ (ed): Dis- ease Mechanisms in Small Animal Surgery , ed 2. Philadelphia,Lea & Febiger, 1993, pp 298–305.

3. Breznock EM, Whiting PG: Portosystemic shunts andanomalies, in Slatter DH (ed): Textbook of Small Animal Surgery . Philadelphia, WB Saunders Co, 1985, pp 1156–1173.

4. Martin RA, August JR, Barber DL, et al: Left hepatic veinattenuation for treatment of patent ductus venosus in a dog.

JAVMA 189:1465–1468, 1986.5. Payne JT, Martin RA, Constantinescu GM: The anatomy

and embryology of portosystemic shunts in dogs and cats.Semin Vet Med Surg (Small Anim) 5:76–82, 1990.

6. Bjorling DE, Prasse KW, Holmes RA: Partial hepatectomy in dogs. Compend Contin Educ Small Anim Pract 7:257–265,1985.

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182:798–805, 1983.9. Wrigley RH, Macy DW, Wyles PM: Ligation of ductus

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12. Swalec KM, Smeak DD: Partial versus complete attenuationof single portosystemic shunts. Vet Surg 19:406–410, 1990.

13. Quattlebaum JK, Quattlebaum JK Jr: Technique of hepaticlobectomy. Ann Surg 149:648–650, 1959.

14. Fasulo F, Giori A, Fissi S, et al: Cavitron ultrasonic surg-ical aspirator (CUSA) in liver resection. Int Surg 77:64–66,1992.

15. Storck BH, Rutgers EJ, Gortzak E, et al: The impact of theCUSA ultrasonic dissection device on major liver resections.Neth J Surg 43:99–101, 1991.

16. Little JM, Hollands MJ: Impact of the CUSA and operativeultrasound on hepatic resection. HPB Surg 3:271–278,1991.

17. Tobias KM, Barbee D, Pluhar GE: Use of intraoperativesubtraction angiography and an ultrasonic aspirator to im-prove visualization of an intrahepatic portosystemic shunt.

JAVMA 208:888–890, 1996.18. Millat B, Hay JM, Descottes B, et al: Prospective evaluation

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19. Mathew K, Grofton N: Congenital extrahepatic portosys-temic shunt occlusion in the dog: Gross observation duringsurgical correction. JAAHA 24:387–394, 1988.

20. Payne JT, Martin RA, Moon ML, et al: Effect of left hepaticvein ligation on hepatic circulation, function, and micro-anatomy in dogs. Am J Vet Res 52:774–780, 1991.

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11:285–292, 1976.22. Siman J, Payer J, Stojkovic J, et al: Hemodynamics of theliver. II. Hemodynamics of the dog liver after resection of 70% of the liver and ligation of the left branch of the portalvein. Z Exp Chir 12:113–118, 1979.

23. Komtebedde J, Forsyth S, Breznock EM, et al: Intrahepaticportosystemic venous anomaly in the dog. Perioperativemanagement and complications. Vet Surg 20:37–42, 1991.


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Surgical Techniques for Extra Vascular Occlusiion of tic Shunts