Quinidine and potassium

[Dark tar]

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Hello everyone,
I finally got my head around the concept of coductance and read that hyperkalemia induces an increase in potassium conductance so you end up with a faster repolarization and hence the peaked T waves. At the same time, hypokalemia induces a decrease in potassium conductance and traps potassium in the cardiac cells making them depolarized and increasing the action potential duration and hence the prolonged QT interval.

Now I'm back to my kaplan pharm lect note and I read this in the antiarrhythmic chapter:
Hyperkalemia inhances the effects of Quinidine

As far as I know, Quinidine blocks the sodium channels of phase 0 and also blocks potassium channels of phase 3 so it increases the action potential duration and you endup with a prolonged QT interval much like hypokalemia
Could some one please clarify this for me I have put so much time and effort to understand this for kaplan to throw it in my face like that.

 

[pd1112]

Hyperkalemia paradoxically increases potassium efflux during phase 3, as you mentioned, increasing the speed of repolarization and causing peaked T waves. This effect is transient if present and is very poorly sensitive & specific for hyperkalemia. The most important effect of hyperkalemia is altered resting membrane potential due to the Nernst equation, which makes the resting potential less electronegative and more likely to depolarize initially in phase 0. However, the increased resting membrane potential results in a lack of complete repolarization in phase 3, such that not all sodium channels can transition out of the inactivated state and back into the closed state to be available for depolarization. The net effect is a progressive increase in the number of inactive sodium channels and decreased sodium conductance due to increased resting membrane potential (decreased phase 0 slope, widened QRS & P wave, prolonged P-R interval), which accentuates the effects of Quinidine, a sodium channel blocker.

For the sake of completeness, hypokalemia causes paradoxical decrease in potassium efflux during phase 3, as you mentioned, which results in prolonged action potential duration. Also, hypokalemia increases the resting gradient for potassium efflux in cardiac myocytes, resulting in decreased resting potential and increased threshold for depolarization, but all sodium channels are able to convert back to the closed state and are thus available to contribute to depolarization, unlike in hyperkalemia.

 

[Dark tar]

Hyperkalemia paradoxically increases potassium efflux during phase 3, as you mentioned, increasing the speed of repolarization and causing peaked T waves. This effect is transient if present and is very poorly sensitive & specific for hyperkalemia. The most important effect of hyperkalemia is altered resting membrane potential due to the Nernst equation, which makes the resting potential less electronegative and more likely to depolarize initially in phase 0. However, the increased resting membrane potential results in a lack of complete repolarization in phase 3, such that not all sodium channels can transition out of the inactivated state and back into the closed state to be available for depolarization. The net effect is a progressive increase in the number of inactive sodium channels and decreased sodium conductance due to increased resting membrane potential (decreased phase 0 slope, widened QRS & P wave, prolonged P-R interval), which accentuates the effects of Quinidine, a sodium channel blocker.

For the sake of completeness, hypokalemia causes paradoxical decrease in potassium efflux during phase 3, as you mentioned, which results in prolonged action potential duration. Also, hypokalemia increases the resting gradient for potassium efflux in cardiac myocytes, resulting in decreased resting potential and increased threshold for depolarization, but all sodium channels are able to convert back to the closed state and are thus available to contribute to depolarization, unlike in hyperkalemia.

You sir/ma'am are a bless on earth. THANK YOU. I know this doesn't serve you rightfully but it's all I can do. THANK YOU. Finally this kalemia thingy is done.