Assessment of the in vivo recording of local cerebral blood flow using a thermistor device.

In order to obtain a continuous measurement of local blood flow in the cerebral cortex of a laboratory animal using chronically implanted sensors, we have developed a device based on the heat clearance principle. Flow information is obtained from temperature measurement by means of two thermistors one of them being heated at a defined level above ambient tissue temperature; as such, cooling of the heated thermistor caused by convection phenomena in its vicinity, can be related to local perfusion rate. In a first step "in vitro" measurements were performed in order to study the behaviour, sensitivity and reliability of the device; a physical model was established explaining the results. In this paper we describe "in vivo" tests in the rabbit's brain cortex with the miniature thermistors (0.5 mm diameter) chronically implanted (at the cortical surface). Results are correlated with oxygen tension measurements using (smaller) pO2 electrodes inserted into the cortical tissue. We have observed that all sensors are well tolerated by the animals who remain symptom free. Test experiments, inducing a well known physiological effect on local blood flow, such as arterial clamping, inhalation of CO2 gas mixtures, etc., are performed. The phenomena during induced anoxic anoxia are also shown. These preliminary investigations are essential in order to attempt by future experiments the establishment of a correlation between "in vivo" recorded flow signals and the "in vitro" measured characteristics.