anti-arrhythmics.. FA mistake?

[MissMedicine]

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On pg 321 of FA, do they mean a decreased slope of phase 0 depolarization for Class I... isnt phase 4 just the RMP... the slope of "phase 0" (not phase 4) is decreased bc of the block of NA channels?

 

[AlexRusso]

Actually that is correct. Class I are Na+ blockers. Remember that the depolarizing current in phase 4 of PACEMAKER CELLS is a Na+ current (If) Hence blocking this current would reduce the slope of depolarization in pacemaker cells (which is what FA says)

This was alos brought up earlier....check out this thread

http://forums.studentdoctor.net/showthread.php?t=123924

 

[MissMedicine]

yeah i know that but how come the picture underneath doesnt show a nodal AP profile?! im confused.

 

[AlexRusso]

Yea i agree with you it'd be a lot less confusing if they put a second diagram in there. I'm guessing its because the main effect is not on nodal cells (hence no diagram of those) but they mentioned the nodal effect for completeness.

 

[MissMedicine]

on a related note, if qt elevation causes torsades, how come only quinidine and sotalol cause torsades.... argh, as you can tell, I HATE antiarrythmics

 

[MissMedicine]

thanks for your help alex!

um, im trying to read lippincott pharm and understand if there is any sort of relationship bw all these (QT, ERP, AP duration, PR etc), it doesnt really explain them very well. for ex, how come with class iv you have an increase in ERP, increase in PR but no QT elevation... and then later in the toxicities it lists torsdaes for bepridil (in FA). I would like to understand these without just trying to memorize them.... any help would be appreciated.

 

[AlexRusso]

I'd love to hear this explained. I gave up on trying to understand these myself a looong time ago. I'm basically resorting to memorization and god ol' praying that I dont get any q's on this. (watch my test be heavy on antiarythmics )

 

[PACtoDOC]

I swear to you guys that the pharm was very typical and easy. I came home and reviewed page by page the Lippincott pharm book to count the questions and see if I could figure out which ones I had missed. I counted dozens of questions on pharm, and maybe only 3-4 that I got wrong. Pharm is the life saver...worry more about the crazy molecular bio in case you get it!!