Use of hydrogen for measurement of regional cerebral blood flow: problem of intercompartmental diffusion

The extreme diffusibility of hydrogen, compared with xenon or krypton, may create serious artifacts when it is used to measure local blood flow with a tissue electrode. The errors are greatest when hydrogen is given by intra-arterial slug injection, and when the electrode is within 2mm of another tissue compartment, CSF, or air. These all appear to be a consequence of intercompartmental diffusion which can occur at rates of the same order of magnitude as clearance from the tissue by blood flow. No matter how small the electrode, the ultimate spatial resolution of the method appears to be about 2mm unless quantitative account is taken of diffusion. An important precaution in use of the method is to obtain homogeneous tissue saturation by prolonged inhalation administration.