Standardization of a device for continuous observation of local flow in tissue

As our experimental set-up for continuous recording of local blood flow in the cerebral cortex of a laboratory animal with chronically implanted miniature thermistors (based on the heat clearance principle) gave satisfactory results for routine tests of pharmacological agents during anoxia, hypoxia, hypercapnia, etc., experiments, we intended to standardize the apparatus, to increase accuracy, to facilitate calibration and to enhance flexibility with respect to the operator. The exponential aspect in the thermistor resistance/temperature characteristic is linearized by applying a logarithmic converter in the thermistor amplifier. Calibration to the centigrade temperature scale is performed by a three digit numerical adaptation of two thermistor constants determined in a thermostatic-cryostatic bath (zero and slope). A heating power measuring circuit is provided so that the dissipation constant of the thermistor implanted in tissue can be obtained and the thermal conductivity of the tissue can be estimated. Linearity of the relation between cooling of the heated thermistor and local flow, for small cooling values as they are registered in vivo, is still being investigated.