Class IC anti arrythmics uworld vs first aid confusion

[teehees]

Little bit confused with uworld vs first aid

First aid states class IC increased Effective refractory period but minimal effect on Action potential duration

Uworld says it increased QRS duration.

How can you increase the QRS duration without increasing action potential duration? I feel like theres some fundamental understanding here that im missing.


Uworld says its rate dependent (QID 8869) At first I thought the rate dependency had something to do with the increased ERP but refractory period shouldnt effect duration. It goes on to explain it has to do with slow disassociation

Anyone clear this up for me?

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[zhopv10]

Action potential can be thought of roughly as the QT segment. So an increase in the QRS with a corresponding decrease in the ST would lead to an unchanged AP length. This can be remembered because it is why drugs that increase the QT (AP) length lead to TdP


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[teehees]

Ah I just came across another question that said the exact same thing although your ST segment part made it clearer

So QRS is increased in Class 1a/1c but not 1b..even though it affects the Na channels like the other two. Why would that be? Uworld did mention it rapidly dissociates but I don't feel thats the reason.

I'm starting to feel im thinking too much about this to be honest