Magnesium and Electrolytes in Head Injury Cases

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 for 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 ? SD, 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, 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.

Magnesium (Mg) is a cofactor in numerous enzymatic reactions, and Mg ions are important for regulation of the sodium/potassium (Na/K) transport across membranes through activation of a Na-K ATPase pump [1] [2] . Loss of cellular Mg results in depletion of intracellular K and changes in resting membrane potentials of various cells, including cardiac and vascular smooth muscle cells [1] . Mg is a natural calcium channel blocking agent, and low levels of Mg are associated with hypocalcemia. Hypomagnesemia has been linked to cardiac arrhythmias associated with Q-T and P-R prolongation (including attacks of torsades de pointes), increased sensitivity to digoxin, sudden death, ischemic heart disease, hypertension, coronary vasoconstriction, transient ischemic attacks, strokes, seizures, neuromuscular irritability, preeclampsia-eclampsia, and diabetes [1] [2] [3] [4] [5] [6] [7] . Hypomagnesemia is associated with increased mortality both in the general ward [8] and in the intensive care unit (ICU) [8] [9] . Hypomagnesemia may result from gastrointestinal or renal losses (the latter because of primary renal Mg wasting) or from cellular redistribution of Mg. The plasma Mg concentration (0.8-1.1 mmol/L) is only equivalent to 1% of total body content. Thus, a Mg deficit may exist while plasma Mg concentration is normal, and the occurrence of a low Mg level usually indicates significant total body Mg deficiency [4] . However, measurement of plasma Mg, perhaps in combination with urinary Mg excretion, is the simplest way of testing for possible Mg deficiency [6] .

Phosphate (P) is a major intracellular anion and plays an important role in many biochemical pathways relating to normal physiologic functions, especially in maintaining muscle tone [7] [10] . Hypophosphatemia has been shown to be associated with muscle weakness, including weakness of respiratory muscles [11] [12] , and with respiratory infection [12] . Aubier et al. [13] found that hypophosphatemia impairs the contractile properties of the diaphragm in critically ill patients, leading to difficulties in weaning from the ventilator. Low P may also be associated with decreased cardiac output and with ventricular tachycardia after myocardial infarction [14] . Patients with hypophosphatemia may be asymptomatic or may experience weakness, malaise, anorexia, bone pain, and respiratory arrest [10] .

K is the major intracellular cation, with relatively low extracellular levels. Changes in K levels can affect muscle contraction and nerve conduction; hypokalemia is associated with cardiac arrhythmias (especially in patients with ischemic heart disease and left ventricular hypertrophy), muscle weakness (including weakness of respiratory muscles), rhabdomyolysis, renal failure, and hyperglycemia. Hypocalcemia is associated with neuromuscular irritability and muscle spasms, seizures, delayed ventricular repolarization, and cardiac failure.

Patients with severe head injury are at risk for developing polyuresis through a variety of mechanisms, including the syndrome of inappropriate antidiuretic hormone secretion, cerebral salt loss, and other, as yet unknown, mechanisms. In addition, patients with cerebral trauma are often treated with mannitol, which can induce or increase polyuresis. Thus, polyuresis is likely to occur in patients with severe head injury. We speculated that this excessive diuresis might lead to the loss of various important electrolytes, including Mg, P, K, and calcium (Ca). In addition, neurologic trauma in itself can induce severe arrhythmias [15] ; therefore, clinical effects of electrolyte disorders might be enhanced in these patients. Routine laboratory tests at admission in most hospitals and most ICUs do not include Mg and P; therefore, low levels of Mg and P are likely to remain undetected longer than frequently tested electrolytes, such as Na or K.

The aim of this study was to assess whether levels of the above-mentioned electrolytes were low in patients with severe head injury at admission to our ICU and to compare these levels with patients with no or only mild head injury.

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