Coronary Artery Bypass Grafting
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An average adult human body contains approximately 1,100 g of calcium, 99% of which is located in the skeleton. The rest of the calcium is attached to proteins in the blood, free or ionized (charged). Calcium is also linked to bicarbonate, phosphate, citrate, and lactate. Only the ionized calcium is physiologically active. In addition to its role in forming bone structure, calcium has many other vital roles in cell communication, enzyme activation, muscle contraction, platelet aggregation, and membrane stability. The average level of calcium in the blood is 8.5–10 mg/dL. Several hormones are essential for regulating calcium metabolism. Because only the free (ionized) form of calcium is physiologically active, it is that when measuring calcium levels, the ionized calcium concentration is measured directly or the calcium level is adjusted to the protein (albumin) level in the blood. There are several disorders that can cause the level of calcium to be either too high (hypercalcemia) or too low (hypocalcemia). Hypocalcemia can result from either increased loss of calcium or decreased intake. Excess calcium may be removed from the circulation with increased bone deposition, increased loss in the urine, or increased binding in the serum. The clinical features of hypocalcemia depend on the degree and rate of the development of hypocalcemia. Patients with chronic hypocalcemia can tolerate hypocalcemia better than those patients who develop hypocalcemia over a shorter time course. Hypocalcemia can be associated with nerve problems, numbness, weakness, muscle cramps, and ultimately tetany (muscle locking). A variety of central nervous system (CNS) abnormalities associated with hypocalcemia include altered/depressed mental status, irritability, or seizures. Severe hypocalcemia can also harm the heart. When the blood phosphate levels are high, calcium may be bound to phosphate and removed from the circulation thereby causing hypocalcemia. This condition is associated with renal failure, extensive muscle damage (rhabdomyolysis), pancreatitis, and phosphate supplementation. Hypocalcemia can also occur when the free calcium binds to substances in the blood stream such as citrate, lactate, foscarnet, and EDTA. Because stored blood products contain citrate, patients
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Springer-Verlag Berlin Heidelberg 2008
receiving massive blood transfusions or plasma exchange, can develop hypocalcemia. Vitamin D deficiency is another common cause of hypocalcemia. This may be caused by poor intake or malabsorption. Both renal and liver diseases cause decreased vitamin D production, and therefore, hypocalcemia. Decreased parathyroid hormone production (PTH) or hypoparathyroidism, may also cause hypocalcemia. One of the most common causes of decreased PTH secretion involves recent parathyroid glands removal. Also patients having a near total or total thyroidectomy for thyroid cancer develop hypoparathyroidism, which can be temporary or permanent. Magnesium disorders, sepsis, burns, and certain drugs may lower PTH production. Various chemotherape
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