Hepatic vascular anomalies are rare in dogs and cats, but result in substantial morbidity and death. Congenital and acquired portosystemic shunts as well as arterioportal fistulae alter hepatic hemodynamics by routing portal blood flow away from the liver. The reduced portal blood flow causes microhepatia as well as neurologic and gastrointestinal clinical signs.
Hepatic perfusion has been studied in clinically normal dogs,1–6 naturally occurring portosystemic shunts,7 and surgically created shunts8 by use of a variety of invasive and noninvasive methods. To compensate for reduced portal blood flow, hepatic arterial, pancreatic, and small intestinal perfusion increases in dogs with portosystemic shunts and in people with portal hypertension.9,10 The hepatic arterial vascular resistance decreases with decreased portal flow, providing a 0.0025 mL/min•g−1 increase in arterial flow for every 0.01 mL/ min•g−1 decrease in portal flow.11 The techniques generally used to measure hepatic perfusion include nuclear scintigraphy and Doppler ultrasonography.
The increase in arterial contribution to hepatic blood flow can be measured by use of the hepatic perfusion index. Dogs have been evaluated with nuclear scintigraphy, where the hepatic perfusion index is defined as the ratio of the slopes of arterial and portal portions of the hepatic time-activity curve.5,7 Computed tomography has been used in people to measure the hepatic perfusion index, which is defined as the ratio of absolute arterial to total hepatic flow.12,13 Iodinated contrast medium can be used as a perfusion tracer because of the linear relationship between density and concentration in CT. Both of these methods have shown an increase in the hepatic perfusion index in conditions where reduced portal blood flow exists, whether congenital or acquired.7,10,13,14
To calculate a separate arterial and portal flow in the liver, the TAC generated from dynamic CT must be divided into arterial and portal portions. In people, the peak attenuation of the spleen is used to signal the end of hepatic arterial enhancement. In dogs, dynamic CT performed at the level of the porta hepatis rarely contains the spleen as a reference organ. The pancreas and stomach are usually included in the dynamic CT and are also supplied by the celiac artery. We hypothesized that these organs could be used in place of the spleen to separate the hepatic TAC into arterial and portal components.
The purpose of the study reported here was to compare hepatic perfusion index, hepatic arterial perfusion, hepatic portal perfusion, total hepatic perfusion, gastric perfusion, and pancreatic perfusion on CT scans of clinically normal dogs with those of dogs with portosystemic shunts.
Time attenuation curve
GE Prospeed, General Electric Co, Milwaukee, Wis.
Diatrizoate sodium-diatrizoate meglumine 76%, Hypaque-76, Nycomed Inc, Princeton, NJ.
Iohexol, Amersham Health Inc, Princeton, NJ.
Basama Perfusion, version 220.127.116.11, statistical software program, Available at www.basama.net/perfusion/. Accessed Apr 3, 2007.
Microsoft Excel 2002, Microsoft Corp, Redmond, Wash.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
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