The peripheral nerve stimulator can deliver different types of stimuli to elicit an assessment of the depth of neuromuscular blockade. This is an important monitor when using neuromuscular blocking agents (NMBAs) since there’s tremendous variability in how patients respond based on how concurrent comorbidities/pharmacotherapy that can shorten or prolong the duration of an NMBA.
As an anesthesiologist and intensivist, I consider the milliamps of current, the location of the electrodes (typically facial or ulnar nerve), and the duration/frequency of the stimulus. The popular train-of-four (TOF) assessment sends four stimuli over a two second period. The resulting “twitch” of the fourth stimulus is compared to the first stimulus. Nondepolarizing NMBAs exhibit a characteristic “fade” in their TOF. For example, a deep blockade may reveal no twitches whereas a resolving blockade may exhibit four strong twitches. In the latter case, the ratio of the fourth twitch to the first twitch (most commonly by visual or tactile assessment) would be ~1. Whenever I paralyze patients in the ICU, I typically want to see 2-3 twitches whereas in the OR, sometimes patients need deep paralysis to avoid catastrophic complications.
Newer technologies are using force transducers, electromyography, and accelerometry to assess the degree of residual paralysis. Remember that even with four strong twitches on TOF, 60-70% of the nicotinic acetylcholine receptors can still be occupied. This stresses the importance of adequate reversal!
As a side point, I’ve had multiple ICU nurses inquire about the difference between bispectral index (BIS) and TOF monitoring in paralyzed patients. BIS is a not-so-perfect monitor that assesses “awareness” through a transformed frontal EEG (see my previous post) whereas TOF monitors the degree of neuromuscular blockade. They are not interchangeable as they monitor completely different things.
Drop me a comment below with questions! 🙂