A comparison of renal blood flow results obtained in the intact animal by the nitrous oxide (derived Fick) method and by the para-amino-hippurate (direct Fick) method.

Abstract

The nitrous oxide method as originally conceived was applied to the cerebral circulation and found to provide an accurate clinical means of determining cerebral blood flow (1). The use of an inert gas as a test substance which reaches a blood-tissue diffusion equilibrium made possible the application of a derived Fick equation and circumvented the need for direct measurement of the rate of gas uptake by the tissue. Because of this advantage, a technically modified version of the original method was recently advocated (2) as a new approach to the measurement of renal blood flow in the intact subject, human or animal, with a particular application to the anuric state, since other renal flow methods in clinical use then become totally inadequate. In that communication, a comparison of results obtained by the nitrous oxide method with those obtained by the simultaneous use of a bubble flow meter was made and the agreement found under the prevailing conditions was felt to be quite good; approximately + 15 per cent (95 per cent limits). Following that study an opportunity has arisen during the course of other experiments to test further this method in intact dogs under more varied conditions (excluding anuria) at the same time as para-amino-hippurate (PAH) Fick renal blood flow measurements were being made. Since the PAH Fick method is recognized as the most accurate one clinically available for measuring renal blood flow, and since it too was previously found to show a good agreement with bubble flow meter measurements (3), an evalua