Step I Train of four response decrease w/ phase 2, nondepolarzing?

[anbuitachi]

Does anyone know why in nondepolarizing and phase 2 nueromuscular block, the train of 4 has a fading pattern? Is it because of using up the Ach ? Thanks

 

[step1april2013]

Does anyone know why in nondepolarizing and phase 2 nueromuscular block, the train of 4 has a fading pattern? Is it because of using up the Ach ? Thanks

a test for measuring the level of neuromuscular blockade. Four consecutive stimuli are delivered along the path of a nerve, and the response of the muscle is measured in order to evaluate stimuli that are blocked versus those that are delivered. Four equal muscle contractions will result if there is no neuromuscular blockade, but, if nondepolarizing blockade is present, there will be a loss of twitch height and number, which will indicate the degree of blockade. This test is commonly used in intensive care units.

In phase I depolarizing the action potentials are in refractory period (they are so depolarized that the h gate is closed so the channels are inactivated). At this point any electrical pulse given will NOT result in any sort of contraction so the block is ABSOLUTE. And an action potential will only be able to fire once the channels in the muscles being used are repolarized. (example: the first pulse fires off and cells are still inactivated, second pulse fires off and cells are still inactivated, third fires and etc.)

In phase II and nondepolarizing the degree of muscle contraction just depends on the sodium channels or Ach receptors that are open and ready to fire. (example: first pulse fires off and this amount of channels are blocked, second pulse fires and this amount are blocked but now more are free of the drug, third fires and etc). Remember the block by nondepolarizing drugs is competitive, reversible.

 

[sharklasers]

So succinylcholine will initially have no response in the train of four study and then will show a gradually increasing response

while other neuromuscular blocking agents will only have a gradually increasing response.

is this right?