[Investigations into neostigmine-induced inhibition of neuromuscular transmission]

Fade of the muscle contraction evoked by indirect tetanic nerve stimulation shows residual neuromuscular block. Anticholinesterases can reverse the partial block; however, they may also inhibit normal neuromuscular transmission and can cause fading responses by misuse of these drugs. The aim of this study is to investigate how neostigmine acts on normal neuromuscular function.

In cats, we observed a series of 8 consecutive muscular compound action potentials (mCAPs; M1-8) of the gastrocnemius muscle evoked by repetitive sciatic nerve stimulation at 100 Hz and calculated the M8/M1 amplitude ratio as an index of fading phenomenon. Neostigmine 0.05 mg x kg(-1) repetitively every 5 minutes before neuromuscular blocking agent had been administered, or after the complete recovery from vecuronium-induced block had been obtained.

Neostigmine caused dose-dependent fade in the mCAPs. The mean doses (SD) of neostigmine for depressing M8/M1 ratio to 50% of baseline were 0.087 (0.029) mg x kg(-1) before use of neuromuscular blocking agent and 0.161 (0.070) mg x kg(-1) after the recovery from neuromuscular block. The fading responses induced by neostigmine were paradoxically reversed by small doses of vecuronium.

Therapeutic doses of neostigmine administered during normal neuromuscular function cause fade of the repetitive muscle contractions. Neuromuscular monitoring should be used before the reversal with neostigmine.