A Combination Isothermal-Hydrogen Clearance System for the Measurement of Local Tissue Flow

A miniature probe has been developed which combines the isothermal and hydrogen clearance methods for measurement of local tissue flow. In vitro testing demonstrated the stability and reproducibility of the results with each system. In vivo tests in the brain, spinal cord, and kidney of the cat showed that changes in the isothermal measurement correlated well with changes in the hydrogen clearance measurement. The hydrogen clearance system produced measurements in close agreement with literature values. A comparison between measurements in kidney tissue and renal artery flow measured with an electromagnetic flowmeter was linear up to a renal flow rate of 130 ml/ 100 g/min. Above that level measurements in the system were constant. The use of the isothermal system for continuous measurement with intermittent calibration using the hydrogen clearance system provides a method for quantitatively monitoring changes in local tissue flow.