Properties and Performance of Membrane-covered Rapid Polarographic Oxygen Catheter Electrodes for Continuous Oxygen Recording in vivo

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Abstract
A theoretical analysis is presented concerning the characteristics of membrane-covered polarographic oxygen electrodes mounted on the tip of a catheter for application to continuous recording in gases and fluids in vivo, particularly with regard to flow dependency and response time. In the course of oxygen electrolysis the electrode uses up oxygen which has to be resupplied by diffusion. The electrode current is a linear function of the oxygen pressure of the medium but may be limited by the diffusion resistances of membrane and medium. In the gas phase the membrane presents the only limitation whereas in streaming liquids the formation of a hydrodynamic boundary layer causes an additional flow-dependent diffusion resistance. The resistances and capacitances of these two layers in series are responsible for a finite response time with transient changes. The equations derived for computing flow dependency and response time were compared with experimental determinations obtained with a modified version of catheter electrodes. It is shown that the best compromise is reached by using small cathodes of appropriate shape (ring cathode).

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