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W.-D. HEISS, M.D. THE FUNCTIONAL STATE of the central nervous system is dependent on adequate delivery of the main substrates. For the supply of sufficient amounts of glu­ cose and oxygen cerebral blood flow (CBF) has to be kept above a critical level. However, when perfusion drops below a certain threshold, in consequence, cere­ bral function becomes impaired. While the increase of blood flow above this threshold restitutes full cerebral function, a further decrease of the flow below a critical limit for certain period of time leads to irreversible functional impairment and may initiate destruction of the structural integrity of the tissue. The upper level was termed as the flow threshold for failure of neur­ onal function,1 the lower level as the flow threshold of membrane failure;2 the condition of an ischemic brain region perfused within the range of the two thresholds was called "ischemic penumbra."1 Methods for Determination of Flow Thresholds In recent years various experimental and clinical approaches were chosen to determine the threshold of brain function and of ischemic damage. Determination of functional impairment: While the development of neurological deficits cannot be fol­ lowed in man, animal experiments with chronic prep­ arations permit to relate certain flow values to partial or complete reversible or irreversible disturbances of function.3 More widely applicable are electrophysio­ logical techniques as indicators of disturbed function related to flow changes. During carotid surgery EEG activity was used as indicator.4,5 More accurate deter­ minations of the flow-function relationship were possi­ ble in animal experiments using evoked potentials,6,7 spontaneous activity of single cortical cells8 or cortical DC potentials.9 In most of these studies blood flow was measured with inert gas clearance techniques (133Xe in man, H2 in animals). Determination of morphological damage: When the

Flow thresholds of functional and morphological damage of brain tissue.