The validity of hydrogen clearance measurements of retinal blood flow

Simultaneous measurements of retinal and either choroidal or long posterior ciliary artery (LPCA) flow have been made in the isolated perfused dog eye by hydrogen clearance polarography. If measurement of retinal flow is made by an electrode placed preretinally within the vitreous, many of the assumptions required by the mathematical theory describing clearance measurements of flow are not satisfied. This paper investigates the extent to which contamination of the clearance curve by diffusion of H2 from the choroidal circulation, or inaccurate placement of the electrode at varying distances from the internal limiting membrane, affects estimates of flow. A non-equilibrium method of administrating H2 is used, as this allows higher flows to be measured than can be achieved by the equilibrium method, and also means that measurements can be repeated every 5 min. Measurement of flow with an electrode placed at the LPCA gave flow estimates which were larger than could be measured with an equilibrium method of clearance. LPCA flow was proportional to total ocular flow, as was choroidal flow. This technique may have potential use clinically for estimating uveal flow. The estimate of retinal flow was not significantly affected by H2 diffusing from the choroidal circulation, but was greatly reduced as the electrode was placed at increasing distances from the internal limiting membrane.