Coadministration of propofol and midazolam decreases bispectral index value as a result of synergic muscle relaxant action on the motor system.

Midazolam and propofol have direct relaxant properties. Midazolam exhibits a well-known myorelaxant effect as a result of a block of inactivated Na channels in skeletal muscle fiber. Moreover, interaction of midazolam, at very low concentrations, with the nicotinic acetylcholine receptor leads to a substantial reduction of the current amplitude, which suggests an additional closed channel block responsible, to some extent, for the muscle-relaxing effects of midazolam. Propofol also provides some degree of muscle relaxation. At clinical concentrations, propofol acts on peripheral parts of the motor system, depressing spinal motor neuron excitability. Motor evoked potentials are also affected by propofol. Because of their muscle relaxation properties, propofol and midazolam are used in tetanus management not only for sedation but also for muscle relaxation. These data, taken together, widely indicate that in sedated patients coadministration of both propofol and midazolam may result in a synergic muscle relaxant action on the motor system and therefore in a decrease in BIS value.