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CLINICAL INSIGHTS JULY/AUGUST 2018 65 Extrahepatic portocaval shunts typically originate from the splenic, right gastric, or left gastric vein and enter the caudal vena cava between the phrenicoabdominal vein and the diaphragm. They are usually identified as a tortuous vessel with hepatofugal flow as noted using color or pulsed-wave Doppler. Hepatofugal flow is flow within the portal vein or one of its afferents that is directed away from the liver, contrary to normal dogs and cats, in which the portal vein flows toward the liver (hepatopetal flow). The vena cava, portal vein, and porta hepatis region should be scanned from the diaphragm to the level of the kidneys in search of an anomalous branching vessel entering the vena cava or travelling dorsally through the diaphragm toward the azygous vein adjacent to the aorta. The point of entry of portocaval shunts into the caudal vena cava can be identified by recognition of turbulent flow with color and pulsed-wave Doppler ( FIGURE 8 ). Acoustic windows include the subxiphoid and right caudal intercostal spaces of the last 3 ribs, relatively close to the spine, where the liver is visualized cranial to the right kidney. Using these intercostal windows, the ultrasonographer can image in short axis the adjacent aorta, caudal vena cava, and portal vein ( FIGURE 9 ). The shunting vessel can be identified coursing between the vena cava and portal vein ( FIGURE 8 ). Portal vein size can also be measured in the 12th or 11th dorsal intercostal window. The size of the portal vein cranial to the shunt is generally reduced in diameter. When the portal vein is measured cranial to the gastroduodenal-portal confluence, a portal vein: aortic diameter ratio of ≤ 0.65 is predictive for the presence of an extrahepatic shunt, and a value of ≥ 0.80 excludes it ( FIGURE 9 ). 20 If the ratio is ≥ 0.80, other types of disease, such as microvascular dysplasia, intrahepatic shunt ( FIGURE 10 ), and portal hypertension attributable to chronic liver disease with a secondary extrahepatic PSS, could still be present. The sensitivity and specificity of ultrasonography for the detection of extrahepatic PSSs have been reported to be 80.5% and 66.7%, respectively; a greater sensitivity of 100% was seen for intrahepatic PSSs alone. 21 In addition, microhepatia, bilateral renomegaly, nephrocalcinosis, nephroliths, and cystoliths attributable to urate crystals 22,23 or stones may be identified. 24 Acquired Portosystemic Shunts Multiple acquired extrahepatic PSSs develop secondary to chronic portal hypertension and result in reduced FIGURE 9. Short-axis view of the portal vein and aorta in the region of the porta hepatis in a dog. The portal vein:aortic diameter ratio is 0.84, which indicates that this dog likely does not have an anomalous extrahepatic portosystemic shunt. FIGURE 11. Long-axis view (2-dimensional with color Doppler overlay) of the left abdomen near the left kidney at the level of the aorta and caudal vena cava in an 8-year-old poodle with multiple extrahepatic portosystemic shunts (colored circular vessels) secondary to hepatic cirrhosis from sago palm toxicity. FIGURE 10. Short-axis view of the left division of the liver in a 5-month-old golden retriever with a single intrahepatic portosystemic shunt (A) with and (B) without color Doppler. Note the cystic, rounded, anechoic structure representing the portal vein (PV) in the left of the image. A large, distended vessel connects with the caudal vena cava. Aliasing and turbulence can be seen in the portal vein, shunt vessel, and caudal vena cava in A.

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