Hypocalcemia is defined by an ionized calcium level of below 2.0 meq/L. In the presence of hypoalbuminemia, total calcium may be very low, yet ionized calcium remains normal. Some more common causes of hypocalcemia include shock, sepsis, renal failure, and pancreatitis. It is quite uncommon in ambulatory patients, except those with hypoparathyroidism (secondary to surgery) or in chronic renal disease.
ETIOLOGY (Tablei . . 23-10) Movement into "Sick" Cells The concentration of ionized calcium in the ECF is about 1.0 mmol/L or 10-3 M. The concentration of ionized calcium in the cytoplasm of most cells is about 10-7 M. This gradient of 104, or 10,000, to 1 is maintained by active metabolic processes. Any process that interferes with cell metabolism, such as shock or sepsis, will tend to reduce ionized calcium levels by allowing increased net movement of calcium across the cell membrane into the cytoplasm of the poorly functioning cells. Following trauma, serum calcium levels may be low, especially with the fat embolism syndrome, not only because of cell dysfunction and binding of calcium to free fatty acids, but also because of fatty inhibition of cell membrane calcium pumps.
TABLE 23-10 Etiology of Hypocalcemia
Pancreatitis Acute pancreatitis is an important cause of hypocalcemia. Pancreatic lipase breaks down fat into fatty acids and glycerol. The fatty acids combine with calcium to form insoluble calcium soaps and reduce serum calcium levels. The combination of necrotic fat cells plus calcium soaps makes up much of what is recognized as the fat necrosis of pancreatitis. In addition, as protein moves into the inflammatory exudate, the resultant hypoproteinemia may cause total calcium levels to fall. Pancreatitis can also reduce PTH secretion and the response of tissues to it. If total calcium levels fall below 7.0 or 8.0 mg/dL, there is an increased chance of severe complications from pancreatitis.
Drugs A large number of drugs can cause hypocalcemia (Xable ...23.Z11). One of the most frequently used of these is cimetidine. This histamine receptor-blocking agent apparently lowers serum calcium levels by decreasing the synthesis or secretion of parathyroid hormone.
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