Step I UWorld Question ID 1973

[kstreet]

This question provides an answer explanation that states this:

"Class IA IC III anti arrhythmic drugs prolong repolarization, as evidenced by prolongation of the QRS duration and QT interval on cardiac tracings."

Is this an accurate statement for class IC or is this an error?

I believe IC is only going to slow down phase 0 depolarization and would not change APD therefore would not prolong QRS or QT...

Can anyone back me up on this?

 

[indsha008]

Class IC drugs (eg. flecainide, propafenone, encainide) do prolong QRS duration but DO NOT prolong QT interval. (Source: Katzung's)

A factoid: QT interval is not prolonged by drugs which DO NOT affect normal cardiac tissue (1B, 1C, 4 and Adenosine).

Hope I was of help to you.

 

[da8s0859q]

I believe IC is only going to slow down phase 0 depolarization and would not change APD therefore would not prolong QRS or QT...

Can anyone back me up on this?

Agreed -- AP duration is "up, down, no change" for 1A, 1B, and 1C, respectively, and:

Class IC drugs (eg. flecainide, propafenone, encainide) do prolong QRS duration but DO NOT prolong QT interval. (Source: Katzung's)

this.

The short way to remember it is to just associate long-QT / predisposition to torsades with IA and III. If it's not either of those, it doesn't affect QTi.

Also, I want to say there was something in UW that suggested that amiodarone doesn't prolong the QT interval, but best I can tell from other sources, it does like you'd expect a K-blocker to do.