Anti-muscarinic and Prolong QT

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I'm reading about quinidine and I'm a bit confused with some things regarding the prolonged QT.

Now I understand there is a prolong QT with the actual blocking of the Na receptors themselves. Increased 0 phase = Increased Effective Refractory Period= QT prolongation.

Now with this anti-muscarinic deal...

quinidine is also anti-muscarinic which means there will be an increase in HR and an increase in AV node conduction (dec PR interval). Now why would that increase QRS and QT interval??

the ERP is increased by blocking K channels i thought (takes longer to repolarize). don't think antimuscarinic effects would effect the QT? not sure tho.

That does seem confusing. I believe a lot of the prolonged QT effect comes from the fact that quinidine also inhibits repolarization by K+ efflux. Also due to the known Na+ channel blocking effects.


The way I look at it is that all of those effects happen very rapidly. The antimuscarinic effects increase the HR, but they can still prolong the QT interval at the same time.

Sorry I'm not very good at explaining it, but basically increased chronotropy and increased QT don't have to be two separate events.