SEVERAL algorithms for the calculation of ionized intracellular magnes
ium concentration from the chemical shifts of MgATP were compared, usi
ng in vivo P-31 NMR data obtained from swine brain during and followin
g hypoxic ischemia plus i.v. MgSO4 infusion. This analysis reveals tha
t both the absolute ionized intracellular magnesium and relative chang
es in magnesium may vary widely between algorithms used. The calculate
d intracellular pH, used in algorithms to determine ionized magnesium
concentration was found to be a critical parameter that governs the ex
tent of these differences.