Monday, April 8, 2013

Why does heart stop in diastole when plasma potassium level rises?

Normally, potassium has a tendency to move outside the cells due to the concentration gradient.

As plasma potassium rises, this concentration gradient is reversed. 
So potassium will move into the cells. 

The cell's resting membrane potential is very sensitive to changes in extracellular potassium ion concentration.
Elevated potassium, or hyperkalemia, causes the resting electrical potential of the heart muscle cells to be lower than normal (less negative). 

Without this negative resting potential, cardiac cells cannot repolarize. 

That means all your cells are depolarized.
This inactivates sodium channels. 

Inactivated sodium channels means the cells can not fire. 
The heart can not contract. 
That's why, heart stops in diastole. 

Hope you understand :)

This is the mechanism of execution by lethal injection, sudden depolarization of the cell without the ability to repolarize.

Increased extracellular potassium is also seen in rhabdomyoloysis, tumor cell lysis, hemolysis etc.

Cool fact:
In ischemic tissue, potassium in the surrounding interstitium rises. 
Wanna know why?
Ischemic tissue does not receive oxygen.
So there is lack of ATP.
Na+ - K+ ATPase doesn't work
K+ leaves the cell and no one can pump it back in.
(Special thanks to a friend for explaining this fact to me ^_^ )

Another cool fact:
Increased extracellular calcium tend to stop the heart in systole (contracted).
This is known as calcium rigor.

That's all!

-IkaN


6 comments:

  1. I am a nursing instructor. In order for me to teach I like to understand it myself down to the cellular level. It bugs me that in the text book it will say 'do not give potassium IV push'.....why? I want to know why? What happens? What does it do? So I appreciate this new forum I have found. Thanks!

    ReplyDelete
    Replies
    1. Hello!

      You don't wanna give rapid infusion of potassium because it can cause hyperkalemia which can lead to a number of adverse effects. The most important is the effect of potassium on the heart - cardiac arrhythmias, heart block and even cardiac arrest.

      Potassium is lethal and therefore, a slow infusion with ECG monitoring is preferred.

      Delete
  2. You say the concentration gradient becomes lower, inhibiting the ability to repolarize, this makes sense. Then you say the membrane potential gets lower, less negative, this makes no sense. If the concentration gradient is ruined because of extracellular potassium, the extracellular space have become even more positive and the membrane potential is wider, hence more negative? yes? no?

    ReplyDelete
    Replies
    1. I am sorry, I used confusing terminologies.

      If the membrane potential went from -30 to -20 because of potassium moving into the cell, it became less negative (More towards zero, or more positive).

      Delete
  3. Excuse me, can you explain me further why "without this negative resting potential, cardiac cells cannot repolarized"? Thanks ^^

    ReplyDelete

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