As mentioned in a previous post, neuromuscular blocking drugs are used in anesthesia to ensure paralysis during surgery. The degree of neuromuscular block is assessed using nerve stimulation, where two electrodes impose a pulse of current on a peripheral nerve (e.g., ulnar n., facial n., posterior tibial n.) and induce muscle twitches which can then be monitored through the surgery. There are a few different ways to do nerve stimulation :
Tetany: A sustained stimulation (5 s)
Train-of-four (TOF): Four pulses in rapid succession
Double-burst stimulation (DBS): A series of 3 pulses followed after a pause by 2 or 3 pulses.
Post-tetanic potentiation: When a pulse is sent after a tetanic stimulation, it will bring on a stronger twitch than at first.
With non-depolarizing muscle blockers, there is a fade phenomenon where twitch amplitude decreases from the first stimulation. For instance, in a TOF each twitch is weaker than the last; the last twitch is the first to disappear with non-depolarizing blockade, while the first twitch is the last to disappear. This non-depolarizing fade is also seen in DBS and tetany, though there is still post-tetanic potentiation.
With a depolarizing muscle blockade, no fade will be seen. Instead, all twitches in response to stimulation will be uniformly decreased, and there is no post-tetanic potentiation. This pattern is known as a Phase I block. But, if there is a ton of succinylcholine or the blockade is of a long duration, the pattern of response will look like a non-depolarizing block. This would be a Phase II block.
Recovery of neuromuscular function
Throughout a surgery, the TOF ratio is often mentioned as a means of assessing neuromuscular blockade on an ongoing basis. This means dividing the amplitude of the fourth (and most influenced by neuromuscular blockers) twitch in a TOF by the amplitude of the first (which is the least affected). In normal people, the 4:1 amplitude is the same, for a TOF ratio of 1. In a Phase I depolarizing block, the TOF ratio is also 1. The TOF ratio will be less than 1 in a non-depolarizing block (remember the fade?). It is commonly mentioned that a TOF ratio of 0.7 represents an full recovery of neuromuscular function, but these days it is thought that a TOF ratio of at least 0.9 is needed before extubation.
It is very hard to tell what the TOF ratio is by sight or feel alone! DBS ratio is more sensitive than TOF ratio for assessing neuromuscular block, and it’s easier to gauge by tactile evaluation than the TOF ratio. So, quantitative monitoring by electomyography (EMG), mechanomyography (MMG), or accelerometry is ideal!
- Fuchs-Buder T. 2010. Neuromuscular monitoring in clinical practice and research. Springer.
- McGrath CD, Hunter JM. 2006. Monitoring of neuromuscular block. Continuing Education in Anesthesia, Critical Care & Pain; 6:7.
- Neuromuscular blocking agents. 2006. In: Clinical Anesthesiology (Eds: Morgan GE, Mikhail MS, Murray MJ). Lange.
- Viby-Mogensen J. 2005. Neuromuscular monitoring. In: Miller’s Anesthesia (Eds: Miller RD, Erikkson LI, Fleisher LA, Wiener-Kronish JP, Young WL). Elsevier.