What is the cause of tetany in patients with hyperventillation ?

Low levels of carbon dioxide cause tetany by altering the albumin binding of calcium such that the ionized (physiologically influencing) fraction of calcium is reduced; one common reason for low carbon dioxide levels is hyperventilation. Low levels of magnesium can lead to tetany Hypocalcemia is the primary cause of tetany. Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to sodium ion, causing a progressive depolarization, which increases the possibility of action potentials. This occurs because calcium ions interact with the exterior surface of sodium channels in the plasma membrane of nerve cells. When calcium ions are absent the voltage level required to open voltage gated sodium channels is significantly altered (less excitation is required). If the plasma Ca2+ decreases to less than 50% of the normal value of 9.4 mg/dl, action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscles. Hypocalcemia is not a term for tetany but is rather a cause of tetany Ref Davidson 23rd edition pg 369