Calculation of brain elastic parameters in vivo.

In an earlier study [Am. J. Physiol. 232 (Regulatory Integrative Comp. Physiol. 1): R27-R30, 1977], we defined the concept of brain elastic response in vivo as measured by a pressure-depth ratio (G0) derived from a graphic analysis of the elastic response tests. These tests have shown that brain elastic response in vivo is sensitive to changes in the intracranial system and that the response is nonlinear. In this study we identify a second parameter, G0, a second-order pressure-depth ratio that characterizes the nonlinear behavior and, along with G0, can be evaluated from a mathematical relation that models the experimental results obtained from the elastic response test. The equation is a logarithmic function relating the pressure and the subpial insertion depth. From this we obtain G0 and G0 as the slope and curvature of the response function at the subpial position. In animal experiments we correlated the changes in these parameters with those of cerebral hemodynamics during hemorrhagic and drug-induced hypotension. The calculated values of G0 and G0 are reproducible and reflect changes in cerebral blood flow and/or volume.