Changes

=====Extrahepatic Shunts=====

=====Extrahepatic Shunts=====

A ventral midline coeliotomy is performed and the shunting vessel is identified.  If pre-operative imaging revealed the location of the shunt, it should be found easily but intra-operative mesenteric portovenography can be performed by catheterising a jejunal vein and instilling contrast medium.  As a guide, the vena cava accepts blood from the renal veins and the small phrenico-abdominal veins before it reaches the hepatic hilus.  Any vessel that joins the vena cava bewteen the phrenico-abdominal and hepatic veins is abnormal.

A ventral midline coeliotomy is performed and the shunting vessel is identified.  If pre-operative imaging revealed the location of the shunt, it should be found easily but intra-operative mesenteric portovenography can be performed by catheterising a jejunal vein and instilling contrast medium.  As a guide, the vena cava accepts blood from the renal veins and the small phrenico-abdominal veins before it reaches the hepatic hilus.  Any vessel that joins the vena cava between the phrenico-abdominal and hepatic veins is abnormal.

Direct ligation of the shunt will cause a large rise in portal pressure, reducing the venous drainage of the organs contributing to the portal vein.  For this reason, a tributary to the portal vein (such as a jejunal vein) should be catheterised to allow the portal pressure to be measured directly during the procedure.  When the shunt is ligated, the portal pressure should not rise by more than 10 cm of water from its baseline value and it should not rise above an absolute value of 20-23 cm of water.  If the portal pressure rises above this level, the surgeon is likely to notice that the intestines are blanched (due to reduced perfusion), that the pulses in the mesenteric arteries are hyperkinetic, that the intestines show vigorous contractility and that the pancreas appears oedematous.  If any of these signs are observed, the shunt should be attenuated rather than completely ligated.

Direct ligation of the shunt will cause a large rise in portal pressure, reducing the venous drainage and perfusion of the organs draining into the portal vein.  For this reason, a tributary to the portal vein (such as a jejunal vein) should be catheterised to allow the portal pressure to be measured directly during the procedure.  When the shunt is ligated, the portal pressure should not rise by more than 10 cm of water from its baseline value and it should not rise above an absolute value of '''20-23 cm of water'''.  If the portal pressure rises above this level, the surgeon is likely to notice that the '''intestines are blanched''' (due to reduced perfusion), that the '''pulses in the mesenteric arteries are hyperkinetic''', that the '''intestines show vigorous contractility''' and that the '''pancreas appears oedematous'''.  If any of these signs are observed, the shunt should be attenuated rather than completely ligated.

For complete ligation, silk is often used as a suture material in dogs because its presence promotes a fibrous reactions which reinforces the ligature.  Although silk is often categorised as a non-absorbable suture material, its strength is lost after 6 months and it is often fully absorbed after approximately 2 years.  Better knot security can be achieved if the silk is wet when the ligatures are tied.  Since silk does not stimulate a strong inflammatory response in cats, it is preferable to use the synthetic non-absorbable monofilament polypropylene in this species.

For complete ligation, silk is often used as a suture material in dogs because its presence promotes a fibrous reactions which reinforces the ligature.  Although silk is often categorised as a non-absorbable suture material, its strength is lost after 6 months and it is often fully absorbed after approximately 2 years.  Better knot security can be achieved if the silk is wet when the ligatures are tied.  Since silk does not stimulate a strong inflammatory response in cats, it is preferable to use the synthetic non-absorbable monofilament polypropylene in this species.

If the shunt cannot be ligated, attenuation may be achieved by three major techniques:

If the shunt cannot be ligated, attenuation may be achieved by three major techniques:

*An '''ameroid constrictor''' consists of a ring of stainless steel with an collar of casein.  When the ring is applied around a vessel, the casein sheath swells gradually to occlude blood flow and it may also stimulate some fibrosis.     

*An '''ameroid constrictor''' consists of a ring of stainless steel with an inner collar of casein.  When the ring is applied around a vessel, the casein sheath swells gradually to occlude blood flow and it may also stimulate some fibrosis.     

*'''Cellophane banding''' involves the application of cellophane around the shunt.  This material is sutured in place to stimulate fibrosis around the vessel, resulting in gradual occlusion.  This material does not stimulate a strong inflammatory response in cats and should not be used in this species.

*'''Cellophane banding''' involves the application of cellophane around the shunt.  This material is sutured in place to stimulate fibrosis around the vessel, resulting in gradual occlusion.  Cellophane does not stimulate a strong inflammatory response in cats and should not be used in this species.

*'''Partial ligation''' is achieved by applying a loose ligature to the shunt so that some blood flow still occurs.  In a proportion of animals, a complete ligation will then have to be performed at a later date.

*'''Partial ligation''' is achieved by applying a loose ligature to the shunt so that some blood flow still occurs.  In a proportion of animals, a complete ligation will then have to be performed at a later date.