Relationship between single twitch depression and train-of-four fade: influence of relaxant dose during onset and spontaneous offset of neuromuscular blockade.

The characteristics of the train-of-four (TOF) response were studied electromyographically during onset and spontaneous offset of neuromuscular blockade with bolus doses of vecuronium (ED95, and ED95 X 2). During onset of blockade there was less fade with the larger than the smaller dose of vecuronium, demonstrating a variable and dose-related relationship between the ratio of height of the initial twitch, T1, and fourth twitch, T4. With both doses TOF fade was more pronounced during recovery than during onset of block, but at the same T1 values during offset, both doses were associated with similar degrees of fade during recovery. Thus with bolus doses of vecuronium the T4 ratio during recovery bears a fixed relationship to initial T1 depression that is independent of dose.