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Hey guys, in this lesson we will talk about the electrolytes involved in cardiac conduction.
Before we get started, please refer to Fluid and Electrolytes lesson for more detailed information regarding electrolytes, this presentation is specifically for the heart’s conduction and the electrolytes that affect it. So with that being said, let’s get started! Sodium, Potassium, Calcium and Magnesium are the major electrolytes involved in creating electricity so the heart can contract. Without a perfect balance of these electrolytes our hearts will have arrhythmias.
So let’s break down these 2 electrolytes first. Sodium is the major extracellular positive ion, it lives outside of the cell in the intercellular or intravascular space. Anything outside of the cell is extracellular, whether it’s in the vascular space or in between the cells. Normal sodium levels in the plasma are 135-145 mEq/L, this is the amount of sodium outside the cell, there is very little sodium inside the cell. Calcium also lives outside of the cell with sodium and with a positive charge. Normal plasma levels of calcium are 8.4-10.2mg/dL.
Now let’s talk about these two electrolytes. Potassium is the most important intracellular electrolytes in the body, it is positively charged and has a plasma concentration of 3.5-5.5mEq/L. It has a very high concentration inside the cell, so the concentration is less outside of the cell because potassium lives inside the cell. Magnesium is also positively charged with a concentration of 1.6-2.6mg/dL which is also inside the cell.
Now let’s break it down a little further and I am going to explain how the electrolytes actually work. Its starts when an action potential occurs, which is the movement of ions across the cell membrane. Here we have a cell, the cell is full of Potassium, , outside of the cell is Sodium and Calcium which are also positively charged. There are more positively charged cations outside of the cell versus inside of the cell. So the negativity inside the cells initiates an action potential by opening the sodium and calcium channels. This allows sodium to enter rapidly while potassium is exciting the cell, causing the cardiac cells to depolarize or contract. As the calcium enters the cell, it increases the strength of the contraction so ensure the heart pumps out all of the blood in its chambers. So after sodium and calcium have entered the cell and potassium has exited, the threshold has been reached and the heart has finished depolarizing or contracting. Potassium can begin to enter the cell again with the help of Magnesium – it inhibits potassium channels, meaning it prevents potassium from leaking out. When potassium reenters the cell, repolarization occurs. This is seen as the T wave on an EKG waveform, when there is an excessive amount of potassium the heart does not repolarize as easily, so the T wave is elevated on an EKG. So if you see an elevated T wave make sure you know what the potassium values are. As a little side note, think of a calcium channel blocker. It will block calcium from entering the cell, so it will decrease the workload of the heart and dilate arteries because when calcium enters the cell it constricts the arteries. If the channels are blocked,the calcium will not enter the cell and the arteries will dilate. This is why calcium channel blockers are given to people with hypertension and arrhythmias because it slows down the heart’s conduction, workload/oxygen demand, and dilates the arteries.
Key points to remember about these electrolytes, sodium enters the cell and initiates action potential for contraction. It is the main extracellular cation that lives outside the cell.
Calcium is also extracellular and enters the cardiac cell to increase the strength of contraction, by doing so it constricts arteries.
Potassium is the main intracellular electrolyte that exits and re-enters the cell to produce depolarization and repolarization, it also creates the T waves on an EKG
Magnesium is an intracellular cation that assists with repolarization, if mag levels are low it can produce ventricular arrhythmias, usually potassium levels are low as well because it cannot allow the potassium to stay in the cell since it cannot inhibit potassium channels. So potassium leaks out. Mag sulfate is given IV, usually with potassium replacement as well. But need to have mag first so the potassium channels are closed and potassium stays in the cells.
Make sure to check out our other lessons and resources regarding fluid and electrolytes or any other topic you may need additional help with, and as always, go out and be your best selves today and happy nursing!