Hypoparathyroidism and QT prolongation

[chromuffin]

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Anyone want to explain how this exists?

I get hypoparathyroidism is going to cause hypocalcemia.
How would hypocalcemia cause QT prolongation? I would expect it to cause a decrease in contractility and a decrease in the depolarization of SA and AV nodes, but I don't see how this would cause QT prolongation.

To my understanding, QT prolongation happens with delayed repolarization via hypokalemia. I would think hypocalcemia would simply decrease the amplitude of the action potential or, if it is severe enough, even prevent reaching threshold.

Clearly my knowledge about action potentials translating to EKGs is flawed.

EDIT: I also understand that calcium is involved in the plateau phase where, at that time, Ca influx occurs. I assume this is ultimately the explanation for QT prolongation, but I guess I need more knowledge about the Ca channel itself. How does it work? Hypocalcemia would prevent Ca influx, but wouldn't this shorten the plateau phase and actually cause QT shortening?

 

[funtertaining15]

so I read on some site, and basically it said that because you have low calcium, Ca, which is responsible for the plateau phase in the ventricular/myocyte AP, phase 2- you would have a slower influx of calcium going into the cell because you have less, and since the QT interval is the time of depolarization to the end of repolarization, it would take longer for the cell to depolarize + repolarize= long QT interval... think about it- calcium is trying to get the myocyte to it's membrane potential, so if you have a lot of calcium, it'll get there faster, vs if you have less calcium, it'll take longer. Also I think w/ hypokalemia, again, you are trying to get the cell to become it's membrane potential (- something, like -90 mv, idk) and so if you have low K+, it'll take longer for the K to reach it's membrane potential, because you need K to keep coming out of the cell to make it more negative, aka to repolarize= QT prolonged. That's just my reasoning- I could be wrong, but literally just thought of this and and had a question about ions and potentials so this came to mind ^^

 

[chromuffin]

so I read on some site, and basically it said that because you have low calcium, Ca, which is responsible for the plateau phase in the ventricular/myocyte AP, phase 2- you would have a slower influx of calcium going into the cell because you have less, and since the QT interval is the time of depolarization to the end of repolarization, it would take longer for the cell to depolarize + repolarize= long QT interval... think about it- calcium is trying to get the myocyte to it's membrane potential, so if you have a lot of calcium, it'll get there faster, vs if you have less calcium, it'll take longer. Also I think w/ hypokalemia, again, you are trying to get the cell to become it's membrane potential (- something, like -90 mv, idk) and so if you have low K+, it'll take longer for the K to reach it's membrane potential, because you need K to keep coming out of the cell to make it more negative, aka to repolarize= QT prolonged. That's just my reasoning- I could be wrong, but literally just thought of this and and had a question about ions and potentials so this came to mind ^^

Yeah I guess that makes sense. Hypokalemia lengthens phase 3 of the action potential and hypocalcemia lengths phase 0 of the action potential. Pretty simple. Probably good enough for step 1 haha.