Cardiac question about Aherns/Prentice CCRN Prep

[ParrotHeadRN]

I just rec'd my Critical Care Certificiation Prep by Aherns & Prentice. I am studying the heck out of cardiac because I know that's the biggest portion and it's probably my weakest area. (I'm a neuro ICU nurse)

In reading the first chapter, which is cardiac, I am very confused about something. In reading about the 5 phases of cardiac cell activity, I came across something that I am either totally not understanding or the book is not right. I figure it's the former, not the latter.

For those of you that have the book, my question regards pages 8 & 9. Phase 0 states that potassium is inside the cell and sodium is outside the cell. When depolarized, postassium moves out of the cell allowing sodium in. Then in phase 2 it states that potassium leaving the cell balances the calcium entering the cell. At the end of that paragraph it states "Also, a small amount of potassium begins leaving the cell at this point." I was under the impression that potassium began leaving the cell in phase 0? In the paragraph on phase 3 it states that potassium loss from the cell is greatest at this point. "The potassium loss returns the cell to electronegativity. Sodium and calcium currents are completely inactivated." How can potassium loss return the cell to electronegativity when the original negative charge was from potassium IN the cell and sodium OUTSIDE the cell? To further confuse me, phase 4 states "the sodium/potassium pumps are the most active here in effecting an exchange of position of potassium for sodium across teh cell membrane. Potassium CONTINUES to leave the cell, and when electronegativity reaches -90mV, phase 0 starts again if a stimulus occurs."

This entire thing really confuses me because what I am reading is that potassium leaves the cell almost constantly, but never returns. If potassium is continuously leaving the cell, how can there be more potassium IN the cell at the beginning of the action potential? Am I completely misunderstanding? Actually, I know I am completely misunderstanding and if someone could clarify, I would really appreciate it. I've read the whole thing over and over and am still coming up w/ the same thing.

Thanks so much!

[babynursewannab]

If it reads just like that, it's a poorly written description (IMHO). I am a cardiac nurse and have never quite heard it put...uh...that way. I will not pretend to be an expert in ion-pump activity at depolorization so what I will say is this: find another source to read into this. Every author describes things differently. Go to your A&P book if you still have it. Google it. Check the Merck Manuals online. Often, online, you will get the added benefit of "action illustrations" (little cartoons that let you see what you're reading about). These tend to help me more than anything else when I find them.

Good luck.

-Alyssa

[ParrotHeadRN]

Thanks Alyssa. I actually have been looking around online and just haven't found anything yet that really helps me understand action potential. I think that's mostly because I'm so confused by the book. I am really hoping it isn't this confusing all the way through. I paid a pretty penny for this book. The first one I got was CCRN Review by LGV and I was disappointed to find that book was simply questions and answers and not a real study guide, which is what I need.

Thanks again.

[ParrotHeadRN]

Anybody else? I'd really like to hear from those of you that have this book.

[ParrotHeadRN]

Okay, I'm going to try this one more time in case someone that can help me missed it.