Magnesium sulfate (MgSO4) is widely utilized in the treatment of preeclamptic hyperreflexia. It is well known that magnesium enhances nondepolarizing neuromuscular blockade. Eclamptic convulsions are almost always prevented by magnesium in plasma concentrations of 4 to 7 mEq/L.

The effects of various concentration of magnesium on the potency and reversibility of pipecuronium were investigated in vitro rat phrenic nerve-hemidiaphragm. The phrenic nerve-hemidiaphragm was dissected and suspended in organ bath containing modified Krebs' solution. Forty samples were divided into 4 groups (n=10 in each group). Group I was studied at the physiologic magnesium concentration(2.4 mEq/L, control group). Group II, III, IV were studied at the concentration of 4, 5.5, and 7 mEq/L, respectively. In each group, we added pipecuronium until twitch height decreased more than 90% of initial level. To compare the recovery, we added neostigmine and calcium, and then, measured TOF ratio.

The amounts of added pipecuronium were 73.8±15.2 microgram (mean±S.D.) in Group I, 38.1±5.0 microgram in Group II, 33.0±4.1 microgram in Group III and 16.1±1.7 microgram in Group IV. The amounts of pipecuronium in Group II, III, IV were significantly less than Group I. After the addition of neostigmine, the values of TOF ratio were under 0.6 in all groups. But after the addition of calcium, all groups were recovered with TOF ratio over 0.85 except Group I.

This study indicated that the increased magnesium concentration potentiated pipecuronium-induced neuromuscular blockade and at higher level, it was more apparent. Neostigmine was not significantly effective to reverse the pipecuronium-induced neuromuscular blockade potentiated with magnesium. But calcium was significantly effective.