Calcium homeostasis in the critically ill patient.

The serum concentration of ionized calcium is the physiologically active circulating calcium fraction, and its level is influenced by protein binding, pH and free fatty acid levels. Hypocalcemia is common in critically ill patients and primarily results from abnormalities in the parathyroid-vitamin-D axis and circulating chelators. Hypercalcemia is less common and primarily results from malignancy, hyperparathyroidism and posthypocalcemic hypercalcemia. Mild degrees of hypocalcemia and hypercalcemia are well tolerated. However, severe hypocalcemia may cause cardiovascular compromise and impair drug action. In ischemic and shock states, hypercalcemia may be detrimental, and calcium channel blockers may be useful.