Antiarrhythmia drug question

[juddson]

The class IIB antiarrhythmics are Na channel blockers (lidocaine, etc.,). All of my books say they work by speeding up phase 3 (repolarization) and (therefore) reduce the action potential duration. What gives? Phase 3 repolarization consists of outward K conductance, not inward Na. Why would an Na channel blocker affect phase three repolarization times? I'm sure I'm forgetting something fundamental - but i can't figure it out.

Class IA and IC work just as I would expect Na channel blockers to work. What gives with the IB class?

Judd

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[seasurfer]

The class IIB antiarrhythmics are Na channel blockers (lidocaine, etc.,). All of my books say they work by speeding up phase 3 (repolarization) and (therefore) reduce the action potential duration. What gives? Phase 3 repolarization consists of outward K conductance, not inward Na. Why would an Na channel blocker affect phase three repolarization times? I'm sure I'm forgetting something fundamental - but i can't figure it out.

Class IA and IC work just as I would expect Na channel blockers to work. What gives with the IB class?

Judd

Class IB drug block more Na channel than the other classes do.

Points to know:
1. Class 1A drug blocks Na channel MORE readily when they are open or inactivated, IN CONTRAST, class 1B drug block both activated and inactivated Na channels. Put it simple, more Na channels are blocked with IB.

2. Even during phase 3, there are still some Na channels working, of course, it is not significant when compared to K channels. If these Na channels are also blocked, the flowing out of K through K channels are less counterbalanced by the in flow of Na through the Na channel. Therefore, AP will be shorter.

 

[mjl1717]

The class IIB antiarrhythmics are Na channel blockers (lidocaine, etc.,). All of my books say they work by speeding up phase 3 (repolarization) and (therefore) reduce the action potential duration. What gives? Phase 3 repolarization consists of outward K conductance, not inward Na. Why would an Na channel blocker affect phase three repolarization times? I'm sure I'm forgetting something fundamental - but i can't figure it out.

Class IA and IC work just as I would expect Na channel blockers to work. What gives with the IB class?

Judd

**Class II is beta blockers which diminish phase 4 thus depressing automaticity and decreasing heart rate eg. (propanolol, esmolol.)

 

[PELE#10]

**Class II is beta blockers which diminish phase 4 thus depressing automaticity and decreasing heart rate eg. (propanolol, esmolol.)

Who said anything about class II.

Bottom line with class Ib is if your not blocking K efflux you will hyperpolarize (get more negative) quicker than the Ia's because they block the K efflux (stay more positive(depolarized)longer).

Hope it helps.

 

[AlternateSome1]

Who said anything about class II.

The OP did, though likely as a typo.

 

[Krazykritter]

Just a guess...but possibily affect the speed w/ which the H gate of the fast channel opens, thus returning the Na channel to the resting state (and out of the Relative Refractory Period).

Please let me know if I am way off.