Hypocalcemia and Tetany

Ionized calcium is the free, biologically active calcium in extracellular fluid. It helps stabilize excitable nerve and muscle membranes. When ionized calcium falls, sodium channels open more readily, so peripheral nerves and skeletal muscle fire with too little stimulus. The patient first notices tingling; with further decline, muscles cramp and spasm. This state of involuntary neuromuscular excitability is tetany. Tetany often progresses in a recognizable sequence: - Perioral tingling and numbness of the fingers or toes - Muscle cramps, twitching, and stiffness - Carpopedal spasm: flexed wrist, thumb adduction, and extended fingers - Laryngospasm or bronchospasm - Seizure The myocardium is also affected. Hypocalcemia prolongs ventricular repolarization, producing QT-interval prolongation and increasing the risk of ventricular dysrhythmias. Severe deficiency can reduce myocardial contractility and contribute to hypotension. Total serum calcium is not always a reliable reflection of physiologically active calcium. Roughly half of circulating calcium is bound to albumin. A low albumin can produce a low total calcium even when ionized calcium is adequate. Conversely, alkalosis increases calcium binding to albumin and lowers ionized calcium; a patient who is hyperventilating...