Do Not Attempt to Reverse Neuromuscular Blockade if there are no Twitches
E. David Bravos MD
Paralysis through the use of neuromuscular blockers can provide optimal conditions for many surgical procedures and can help facilitate intubation for patients requiring mechanical ventilation. These drugs can be broadly classified as being depolarizing or nondepolarizing not only by their mechanism of producing paralysis but by their response to peripheral nerve stimulation and reversal of blockade.
Neuromuscular blockers are structurally similar to acetylcholine and produce their effects by interacting with acetylcholine receptors at the neuromuscular junction. Depolarizing neuromuscular blockers such as succinylcholine bind to the acetylcholine receptor and cause a conformational change resulting in activation of the receptor. This leads to depolarization of themuscle membrane and an action potential if the threshold potential is reached. This explains themuscle fasciculation seen with use of succinylcholine. These drugs remain bound to the receptor and prevent acetylcholine from binding, thus producing paralysis. Nondepolarizing agents bind the acetylcholine receptor as well; however, they do not cause a conformational change and therefore do not cause depolarization. Similarly, acetylcholine cannot bind to any bound receptor with resultant paralysis.
Watch Out For
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.
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.
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