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Validity of the operational equation of the 14C-deoxyglucose method modified for changing arterial plasma glucose level.

The operational equation developed by Sokoloff et al. for measuring the local rate of cerebral glucose metabolism assumes that the arterial plasma glucose concentration (Cp) remains constant during the measurement. However, temporal changes in Cp may occur in actual studies. Savaki et al. proposed a modified operational equation for changing Cp. Their modified equation has frequently been used, although no systematic evaluation of its validity has been attempted. In this study, the derivation of Savaki et al.'s modified operational equation was examined to assess its validity from the theoretical standpoint. A few assump tions which are essentially incorporated in Savaki's and Sokoloff's original equations, are no longer upheld under conditions of changing Cp. One is that the net uptake of glucose from the blood equals the rate of glucose utilization in the tissue. If Cp changes, this assumption can not be valid because of no steady-state condition in terms of the glucose kinetics between the blood and tissue. Changes in the lumped constant which are expected to accompany changes in plasma glucose level may also complicate the situation. The deoxy glucose method based on Sokoloff's theory should be avoided under conditions of changing Cp, even though a modified operational equation for this situation does exist.