Validation of transit-time ultrasound flow probes to directly measure portal blood flow in conscious rats

Direct measurement of portal venous blood flow is technically difficult, yet crucial for accurate assessment of liver hemodynamic and metabolic functions. The aim of this investigation was to assess the feasibility of implanting transit-time ultrasound (TTUS) perivascular flow probes on the portal vein of the rat and to validate this technique as a means of directly measuring portal blood flow in conscious rats. A TTUS flow probe was implanted on the portal veins of 10 rats. One week later, portal flow was measured under basal conditions in these rats by TTUS probes and after pharmacological manipulation of portal flow by intravenous injections of Glypressin or infusions of adenosine while the rats were conscious. Portal flow was simultaneously measured in the same rats using radioactive microspheres. Basal systemic hemodynamics, regional blood flows to splanchnic organs, and portal blood pressure were not significantly modified by the presence of the probe on the portal vein compared with a control group of rats not instrumented with flow probes. Basal portal flows measured by the TTUS and microsphere techniques were not different (20.6 +/- 2.6 and 17.6 +/- 1.3 ml/min). After Glypressin, portal flows measured by the TTUS and microsphere techniques were 12.3 +/- 2.9 and 9.3 +/- 1.9 ml/min and, in response to adenosine, increased to 27.2 +/- 3.4 and 31.3 +/- 4.1 ml/min. There was no significant difference between the TTUS and microsphere flows. Both the relationship between absolute flows and the relationship between changes in flows measured by the two techniques were linear with slopes approaching 1.0. Thus TTUS flow probes can be used to directly measure portal flow from the portal vein in conscious rats. This methodology is as effective as the standard technique of radioactive microspheres. More importantly, the TTUS technique allows for continuous direct measurement of portal flow and eliminates the hazards and sources of error associated with the radioactive microsphere technique.