Hypomagnesemia and hypophosphatemia at admission in patients with severe head injury

Objective: Low serum levels of electrolytes such as magnesium (Mg), potassi um (K), calcium (Ca), and phosphate (P) can lead to a number of clinical pr oblems in intensive care unit (ICU) patients, including hypertension, coron ary vasoconstriction, disturbances in heart rhythm, and muscle weakness. Lo ss of these electrolytes can be caused, among other things, by increased ur inary excretion. Cerebral injury can lead to polyuresis through a variety o f mechanisms. We hypothesized that patients with cranial trauma might be at risk far electrolyte loss through increased diuresis. The objective of thi s study was to assess levels of Mg, P, and K at admission in patients with severe head injury. Design: We measured plasma levels of Mg, P, K, Ca, and sodium at admission in 18 consecutive patients with severe head injury admitted to our ICU (gro up 1). As controls, we used 19 trauma patients with two or more bone fractu res but no significant cranial trauma (group 2). Setting: University teaching hospital. Patients: Eighteen patients with severe head injury admitted to our surgica l ICU (group 1) and 19 controls (trauma patients with no significant crania l trauma; group 2). Main Results: Electrolyte levels at admission (group 1 vs. group 2; mean +/ - so, units: mmol/L) were as follows. Mg, 0.57 +/- 0.17 (range, 0.24-0.85) vs. 0.88 +/- 0.21 (range, 0.66-1.42 mmol/L; p <.01) P 0.56 +/- 0.15 (range, 0.20-0.92) vs. 1.11 +/- 0.15 (range, 0.88-1.44; mmol/L; p <.01). K, 3.54 /- 0.59 (range, 2.4-4.8) vs. 4.07 +/- 0.45 (range, 3.6-4.8 mmol/L; p <.02). Ca, 2.02 +/- 0.24 (range, 1.45-2.51) vs. 2.14 +/- 0.20 (range, 1.88-2.46; p = NS). In group 1, 12/18 patients had Mg levels <0.70 mmol/L vs. 2/19 pat ients in group 2 (p <.01); in group 1, 11/18 patients had P levels below 0. 60 mmol vs. 0/19 patients in group 2 (p < .01). Moderate hypokalemia (K lev els, <3.6 mmol/L) was present in 8/18 patients in group 1 vs. 1/19 patients in group 2 (p <.01). Severe hypokalemia (K levels, less than or equal to 3 .0) was present in 4/18 patients in group 1 vs. 0/19 patients in group 2 (p <.05). Conclusion: We conclude that patients with severe head injury are at high r isk for the development of hypomagnesemia, hypophosphatemia, and hypokalemi a. One of the causes of low electrolyte levels in these patients may be an increase in the urinary loss of various electrolytes caused by neurologic t rauma. Mannitol administration may be a contributing factor. Intensivists s hould be aware of this potential problem. If necessary, adequate supplement ation of Mg, P, K, and Ca should be initiated promptly.