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Old 05-25-2012
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Drug Class I antiarrhythmic drugs

I've been told that class I drugs are furthre divided into Ia, Ib and Ic. Ia having moderate rate of association with activated/inactivated Na channels, moderate rate of dissociation from resting Na channels; Ib having faster rate of association/dissociation; Ic having slower rate. At the same time, Ia and Ic slow the rising of phase 0, ie slowing depolarisation rate, therefore slowing conduction of impulses. Ib has little/no effect on depolarisaiton rate and so it does not slow down the conduction. Can anyone relate/explain, if there's any relationship between the rate of association/dissociation and rate of depolarisation?
If it is real, how does drug Ib work? Does it still block Na channel or it's a weak blocker? And why it's more useful in ventricular arrhythmia but not atrial or junctional?
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Old 05-25-2012
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Default Have you read any text ?

What text have you read for this?
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Old 05-26-2012
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Lippincott's Pharmacology textbook. If it's wrong, can you please tell the differences between class Ia, Ib and Ic drugs?

Also in wiki, it says about Ib drug: "They have fast onset and offset kinetics, meaning that they have little or no effect at slower heart rates, and more effects at faster heart rates."
http://en.wikipedia.org/wiki/Sodium_channel_blocker
The graph at the side shows Ib have no effect on slowing depolarisation rate, so what does it do actually?
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Old 05-26-2012
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Correct Answer

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Originally Posted by hongyee View Post
Lippincott's Pharmacology textbook. If it's wrong, can you please tell the differences between class Ia, Ib and Ic drugs?

Also in wiki, it says about Ib drug: "They have fast onset and offset kinetics, meaning that they have little or no effect at slower heart rates, and more effects at faster heart rates."
http://en.wikipedia.org/wiki/Sodium_channel_blocker
The graph at the side shows Ib have no effect on slowing depolarisation rate, so what does it do actually?
Lippincott is really confusing at times.

1a acts preferentially on open/activated Na channels & increases their APD and thus Effective Refractory period.
1b acts only on inactivated Na channels & therefore has preference for partly Depolarized tissue. Remember Na channel remains inactivated once it is depolarized until it reaches resting state again.
1c acts on all types of Na channels open/resting/deactivated but it has limited use because it is very proarrhythmogenic.

Kaplan explains it really well. Just go through the video once.
Hope it helps.
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Old 05-26-2012
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Class 1a:
Affect atrial and ventricular arrhythmias. They block the INa, and slow conduction velocity in the atria, Purkinje, and ventricular cells. At high doses they also slow AV conduction. Reduction in ventricular conductions results in increased QRS. In addition they also block the IK and slow depolarization. Therefore, they increase the action potential duration and the effective refractory period in addition to slowing conduction velocity and ectopic pacemaker

Class 1b:
Selectively affects ischemic or depolarized Purkinje and ventricular tissue, and has little effect on atrial tissue; Lidocaine reduces the action potential duration in some cells, but because it slows recovery of sodium channels from inactivation it does not shorten, and may even prolong the effective refractory period.

In summary:
Class 1b drugs are the most selective for sodium channels in ischemic tissue with negligible effects on channels in normal tissue, whereas 1a and 1c cause reduction of INa in normal cells as well.

All class 1 drugs have sodium channel blockade. Subclasses of this action reflect effects on the action potential duration and the kinetics of sodium channel blockade. Drugs with class 1a action prolong the action potential duration and dissociate from the channel with intermediate kinetics; drugs with class 1b action shorten the action potential duration in some tissues of the heart and dissociate from the channel with rapid kinetics; and drugs with class 1C action have minimal effects on the action potential duration and dissociate from the channel with slow kinetics.

Last edited by Romaeus; 05-26-2012 at 01:26 AM. Reason: Better formatting and added a summary
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