Calcium imbalance due to immobility

[Jedi of Zen]

When a patient experiences prolonged immobility, would that patient be more likely to have elevated levels of calcium in the blood?

The reason I'm asking is b/c I'm assuming that with bone resorption (loss), which occurs with immobility, calcium as part of that loss bone begins circulating throughout the blood (for example, leading to renal calculi and other nasty problems).

However, in the outline for this lecture that my teacher gave us, it says (right in the middle of the discussion of this info) that you should "check lab values for decreased calcium". Now I'm wondering - why would calcium be decreased? Does that mean it would be lost from the bone? If so, how would a lab report tell you that? I was thinking rather that a lab report (presumably of the blood) would show elevated calcium levels - not decreased levels - again, all due to bone loss. Does anyone else agree?

[al7139, ASN, RN]

Hypocalcemia is a total serum Ca++ below 9.0mg/dl. Most calcium is stored in bone, and only a small amount of total body calcium present in extra-cellular fluid. The normal blood level of calcium is low, so any change in calcium levels has major effects on function.

Calcium is an excitable membrane stabilizer, regulating depolarization, and generation of action potentials. Calcium decreases sodium movement across excitable mambranes, decreasing the rate of depolarization. Low serum calcium levels increase sodium movement across excitable mambranes, allowing depolarization to occur more easily and at inappropriate times. Hypocalcemia is caused by many chronic and acute conditions and medical and surgical treatments.

Actual calcium loss occurs when the absorption of calcium from the GI tract slows or when calcium is lost from the body. relative calcium loss causes total body calcium levels to remain normal, while serum calcium levels are low. This type of hypocalcemis occurs when the free or ionized calcium in the body is reduced or when parathyroid gland function is decreased.

No the calcium is not lost from the bone, but rather from the free calcium in the rest of the body.

[Jedi of Zen]

Hypocalcemia is a total serum Ca++ below 9.0mg/dl. Most calcium is stored in bone, and only a small amount of total body calcium present in extra-cellular fluid. The normal blood level of calcium is low, so any change in calcium levels has major effects on function.

Calcium is an excitable membrane stabilizer, regulating depolarization, and generation of action potentials. Calcium decreases sodium movement across excitable mambranes, decreasing the rate of depolarization. Low serum calcium levels increase sodium movement across excitable mambranes, allowing depolarization to occur more easily and at inappropriate times. Hypocalcemia is caused by many chronic and acute conditions and medical and surgical treatments.

Actual calcium loss occurs when the absorption of calcium from the GI tract slows or when calcium is lost from the body. relative calcium loss causes total body calcium levels to remain normal, while serum calcium levels are low. This type of hypocalcemis occurs when the free or ionized calcium in the body is reduced or when parathyroid gland function is decreased.

No the calcium is not lost from the bone, but rather from the free calcium in the rest of the body.

Hello, and thank you for elaborating on that.

However, I'm still not entirely sure I understand this from the perspective of immobility.

[ridgeway]

Hi, I don't normally post things, but I had to on this one because the prior comments left out one major point. In terms of my knowledge of the subject, I received my Ph.D. in endocrine physiology and pharmacology a few years ago where my major point of study was hormonal control of calcium metabolism, more specifically post-menopausal osteoporosis and parathyroid hormone.

First of all, immobility for long periods of time, or being weightless for long periods of time does result in bone loss that can be severe. It is well documented that astronauts in space for extended periods of time lose significant amount of bone mass as a result of being in a weightless environment. Same goes for immobility issues for extended periods of time.

Now as to the question. I will try answer it using the astronaut example mentioned above. Given that astronauts lose bone matrix (read calcium) after extended stays in space, does that result in hypercalcemia? Not necessarily. Why? Calcium is one of the most closely regulated ions in the body. Too high, you die. Too low, you die. Given the normal range of calcium varies by around +/- 0.75 mg/dL, it takes a lot of samples to statistically detect a 0.5 mg/dL increase/decrease in calcium levels in the same person. This sort of repeated sampling on the same person usually never happens, except in clinical studies. You can have "normal" blood calcium levels and still be losing calcium from the bone. You can have "normal" blood calcium levels and be gaining calcium into the bone. Why? The answer to both situations is the bodies homeostatic mechanisms to control blood calcium mask what is really going on at the bone level. Calcium going into and out of bone is a slow process (not taking into account the ECF fluid that surrounds bone, but that is a relatively small amount compared to that found in bone).

Just because you are losing NET calcium from the body doesn't mean your blood calcium levels will be noticeably increased. Why? Your kidneys do a great job of retaining or removing calcium from the blood stream based on input from the parathyroid glands. Any by "noticeably", most people don't have 15-20 "normal" blood samples taken over a period of weeks to establish their statistical "norm". Hence the difficulty in seeing a real increase or decrease in blood levels versus the random fluctuations in blood calcium levels.

My bottom line is this. Just because somebody is losing calcium from bone as a result of weightlessness, doesn't mean you will see hypercalcemia. You will see increased calcium in the urine. That's where you need to look.