Calcium is a major cation in ECF. Almost all of the calcium in thebody (99%) is stored in bone, where it can be mobilized, if neces-sary. When dietary intake isn’t enough to meet metabolic needs, calcium stores in bone are reduced.
Extracellular calcium exists in three forms—it’s bound to plasma protein (mainly albumin); complexed with such substances as phosphate, citrate, or sulfate; and ionized. Ionized calcium is the physiologically active form and plays a role in cellular functions.
Chronic insufficient calcium intake can result in bone demineral-ization. Calcium is replaced orally or I.V. with calcium salts, such as:
· calcium carbonate
· calcium chloride
· calcium citrate
· calcium glubionate
· calcium gluceptate
· calcium gluconate
· calcium lactate.
Oral calcium is absorbed readily from the duodenum and proxi-mal jejunum. A pH of 5 to 7, parathyroid hormone, and vitamin D all aid calcium absorption.
Absorption also depends on dietary factors, such as calcium bind-ing to fiber, phytates, and oxalates, and on fatty acids, with which calcium salts form insoluble soaps.
Calcium is distributed primarily in bone. Calcium salts are elimi-nated unchanged primarily in stool; the remainder is excreted in urine.
Calcium moves quickly into ECF to restore calcium levels and reestablish balance. Calcium has several important roles in the body:
§ Extracellular ionized calcium plays an essential role in normal nerve and muscle excitability.
§ Calcium is integral to normal functioning of the heart, kidneys, and lungs, and it affects the blood coagulation rate as well as cell membrane and capillary permeability.
§ Calcium is a factor in neurotransmitter and hormone activity, amino acid metabolism, vitamin B12 absorption, and gastrin secretion.
§ Calcium plays a major role in normal bone and tooth formation.
Calcium is helpful in treating magnesium intoxication. It also helps strengthen myocardial tissue after defibrillation (electric shock to restore normal heart rhythm) or a poor response to epi-nephrine during resuscitation. Pregnancy and breast-feeding in-crease calcium requirements, as do periods of bone growth during childhood and adolescence.
The major clinical indication for I.V. calcium is acute hypocal-cemia (low serum calcium levels), which necessitates a rapid in-crease in serum calcium levels, as in tetany, cardiac arrest, vita-min D deficiency, parathyroid surgery, and alkalosis. I.V. calcium is also used to prevent a hypocalcemic reaction during exchange transfusions.
Oral calcium is commonly used to supplement a calcium-deficient diet and prevent osteoporosis. Chronic hypocalcemia from such conditions as chronic hypoparathyroidism (a deficiency of parathyroid hormones), osteomalacia (softening of bones), long-term glucocorticoid therapy, and plicamycin and vitamin D defi-ciency is also treated with oral calcium.
Calcium has few significant interactions with other drugs.
· Preparations administered with digoxin may cause cardiac ar-rhythmias.
· Calcium replacement drugs may reduce the response to calcium channel blockers.
· Calcium replacements may inactivate tetracyclines.
· Calcium supplements may decrease the amount of atenolol available to the tissues, resulting in decreased effectiveness of the drug.
· When given in total parenteral nutrition, calcium may react with phosphorus present in the solution to form insoluble calcium phosphate granules, which may find their way into pulmonary ar-terioles, causing emboli and possibly death. (See Adverse reac-tions to calcium.)