The kinetics of iodoamphetamine in dogs after colonic administration: Application to the noninvasive measurement of portasystemic shunt

The kinetics of /sup 123/I iodoamphetamine (IAMPH) absorbed from the bowel was investigated in dogs to evaluate its potential use in quantifying portasystemic shunt. The tracer was given transrectally into the colon. Both gamma camera imaging and samples from the portal and systemic blood were obtained. IAMPH was rapidly absorbed with a T/sub 1/2/ of 20-30 minutes. It was absorbed unchanged, as shown by paper chromatography of blood samples. Hepatic extraction of IAMPH, measured by rapid sequential imaging after bolus injection into the portal system, was >90%. Washout was slow, with a T/sub 1/2/ of about 60 minutes. Pulmonary extraction of the tracer was >90%, with a washout T/sub 1/2/ of 15-20 minutes. Animals with surgically created portasystemic shunts were studied to quantify the shunt fraction. The shunt was constructed by diverting part of the portal blood flow through a catheter in the splenic vein and returned by an extracorporal machine to the systemic circulation through the femoral vein. The chest and abdomen were imaged after IAMPH administration. Shunt fraction was calculated as lung activity/(lung + liver activity) and varied from 10-58%. Tc-99m labeled macroaggregated albumin microspheres injected into the mesenteric vein draining the colon served as the standard. Imaging over the chest and abdomen was performed to obtain shun fraction as above. Close agreement was seen in all cases, with less than 5% error. The authors conclude that IAMPH shows promise as a quantitative and noninvasive method for the measurement of the PPS