Once neuromuscular blockade has occurred, the depth of blockade can be measured using peripheral nerve stimulation. Different responses to nerve stimulation can be seen depending on the type of neuromuscular blocker used, as well as the pattern of stimulation used. The most common patterns of stimulation are the train of four (TOF) and tetany. With depolarizing neuromuscular blockade, TOF stimulation produces a decreased response to successive stimulation known as fade. This is thought to be due to exhaustion of acetylcholine with each successive stimulation in the TOF (and therefore there is less acetylcholine available to overcome the antagonistic block). With tetanic stimulation, fade is seen as well. Depolarizing neuromuscular blockers do not produce fade with a single TOF. However, fade may be seen with successive TOFs. Rather, a constant but diminished response
may be seen if more than one twitch is present. The percent of receptor blockade can be roughly estimated by the number of twitches present. Three twitches is roughly a 75% block; two twitches, an 80% block; and one twitch, a 90% block.