Hypomagnesemia and hypophosphatemia at admission in patients with severe head injury

Objective: Low serum levels of electrolytes such as magnesium (Mg), potassium (K), calcium (Ca), and phosphate (P) can lead to a number of clinical problems in intensive care unit (ICU) patients, including hypertension, coronary vasoconstriction, disturbances in heart rhythm, and muscle weakness. Loss of these electrolytes can be caused, among other things, by increased urinary excretion. Cerebral injury can lead to polyuresis through a variety of mechanisms. We hypothesized that patients with cranial trauma might be at risk far electrolyte loss through increased diuresis. The objective of this 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 (group 1). As controls, we used 19 trauma patients with two or more bone fractures but no significant cranial trauma (group 2). Setting: University teaching hospital. Patients: Eighteen patients with severe head injury admitted to our surgical ICU (group 1) and 19 controls (trauma patients with no significant cranial 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 patients 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 levels, <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 risk for the development of hypomagnesemia, hypophosphatemia, and hypokalemia. 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 trauma. Mannitol administration may be a contributing factor. Intensivists should be aware of this potential problem. If necessary, adequate supplementation of Mg, P, K, and Ca should be initiated promptly.