SIGNIFICANCE OF CALCIUM
More than 99% of the body’s calcium is located in the skeletal system; it is a major component of bones and teeth. About 1% of skeletal calcium is rapidly exchangeable with blood calcium.
• Calcium plays a major role in transmitting nerve impulses and helps to regulate muscle contraction and relaxation, including cardiac muscle.
• Calcium is instrumental in activating enzymes that stimulate many essential chemical reactions in the body,
• and it also plays a role in blood coagulation.
The normal total serum calcium level is 8.5 to 10.5 mg/dL (2.1–2.6 mmol/L). It exists in plasma in three forms: ionized, bound, and complexes.
About 50% of the serum calcium exists in an ionized form that is physiologically active and important for neuromuscular activity and blood coagulation.
The normal ionized serum calcium level is 4.5 to 5.1 mg/dL (1.1–1.3 mmol/L) and is the only form that is physiologically and clinically significant. Less than half of the plasma calcium is bound to serum proteins, primarily albumin. The remainder is combined with non protein anions: phosphate, citrate, and carbonate.
Calcium is absorbed from foods in the presence of normal gastric acidity and vitamin D. Calcium is excreted primarily in the feces, the remainder in urine. The serum calcium level is controlled by PTH and calcitonin. As ionized serum calcium decreases, the parathyroid glands secrete PTH. This event then increases calcium absorption from the GI tract, increases calcium reabsorption from the renal tubule, and releases calcium from the bone. The increase in calcium ion concentration suppresses PTH secretion. When calcium increases excessively, the thyroid gland secretes calcitonin. It briefly inhibits calcium reabsorption from bone and decreases the serum calcium concentration.
The causes of hypocalcemia include the following:
• Vitamin D inadequacy or vitamin D resistance
• Hypoparathyroidism following surgery
• Hypoparathyroidism owing to autoimmune disease or genetic causes
• Renal disease or end-stage liver disease causing vitamin D inadequacy
• Pseudohypoparathyroidism or pseudopseudohypoparathyroidism
• Metastatic or heavy metal (copper, iron) infiltration of the parathyroid gland
• Hypomagnesemia or hypermagnesemia
• Sclerotic metastases
• Hungry bone syndrome postparathyroidectomy
• Infusion of phosphate or citrated blood transfusions
• Critical illness
• Drugs (eg, high-dose intravenous bisphosphonates)
• Fanconi syndrome
• Past radiation of parathyroid glands
• Tetany is the most characteristic manifestation of hypocalcemia and hypomagnesemia.
• Sensations of tingling may occur in the tips of the fingers, around the mouth, and less commonly in the feet.
• Spasms of the muscles of the extremities and face may occur. Pain may develop as a result of these spasms.
• Trousseau’s sign can be elicited by inflating a blood pressure cuff on the upper arm to about 20 mm Hg above systolic pressure; within 2 to 5 minutes, carpopedal spasm (an adducted thumb, flexed wrist and metacarpophalangeal joints, extended interphalangeal joints with fingers together) will occur as ischemia of the ulnar nerve develops.
• Chvostek’s sign consists of twitching of muscles supplied by the facial nerve when the nerve is tapped about 2 cm anterior to the earlobe, just below the zygomatic arch.
• Seizures may occur because hypocalcemia increases irritability of the central nervous system as well as of the peripheral nerves.
• Other changes associated with hypocalcemia include mental changes such as
And even hallucinations.
A prolonged QT interval is seen on the ECG due to prolongation of the ST segment; a form of ventricular tachycardia called torsades de pointes may occur.
- It is important to observe for hypocalcemia in patients at risk. Seizure precautions are initiated when hypocalcemia is severe.
- The status of the airway is closely monitored because laryngeal stridor can occur. Safety precautions are taken, as indicated, if confusion is present.
- People at high risk for osteoporosis are instructed about the need for adequate dietary calcium intake; if not consumed in the diet, calcium supplements should be considered.
- Also, the value of regular weight-bearing exercise in decreasing bone loss should be emphasized, as should the effect of medications on calcium balance. For example, alcohol and caffeine in high doses inhibit calcium absorption, and moderate cigarette smoking increases urinary calcium excretion.
- Additional teaching topics may involve discussion of medications such as alendronate (Fosamax), risedronate (Actonel), raloxifene (Evista), and calcitonin to reduce the rate of bone loss. Teaching also addresses strategies to reduce risk for falls.