The transient thermal clearance method for regional blood flow measurement--the influence of tissue heat conduction.

When heat is supplied to or retrieved from living tissue, the heat in or out is either conducted by means of the tissue or convected by blood. In order quantitatively to assess blood flow by thermal clearance the contributions of conduction and convection have to be separated, and various methods have been developed for this purpose. The various methods are reviewed and it is shown that the adequacy of each depends on the specific thermal clearance method used. The review concentrates on aspects of the role of thermal conduction in the non-invasive transient thermal clearance method. It is shown that by applying correct methodology, for regional blood flow of around 0.0025 ml blood per ml tissue per second the predominant contribution to the heat transfer is by convection. However, if the regional blood flow is much lower, the effect of heat conduction cannot be ignored.