Train-of-four is the most common peripheral nerve stimulation test used in the ICU to monitor neuromuscular blockade in a mechanically ventilated patient who is receiving a neuromuscular blocker infusion.

The peripheral nerve stimulator delivers a small electric stimulus; the level of neuromuscular blockade is determined by observing the number of muscle twitches after stimulation. No twitch indicates that 100% of the nerve receptors are blocked, which exceeds the desired level of neuromuscular blockade. Two twitches out of four indicates approximately 85% neuromuscular blockade, the desired level for a mechanically ventilated patient.

The chart below shows the extent of neuromuscular blockade that can be determined based on the number of twitches.