The Lack of Effect of Routine Magnesium Administration on Respiratory Function in Mechanically Ventilated Patients
Therapy directed toward improving respiratory muscle strength has been advocated for patients with respiratory muscle weakness and respiratory failure once their conditions have been stabilized by initiation of mechanical ventilation. Along with malnutrition, hypomagnesemia is a recognized cause of both skeletal muscle weakness and of respiratory muscle weakness. Patients with chronic obstructive pulmonary disease admitted to an ICU have been found to have reductions of intracellular magnesium levels when compared with control subjects. Although hypomagnesemia is frequent in patients admitted to ICUs, serum magnesium levels do not accurately reflect tissue magnesium levels. A prolonged course of mechanical ventilation has been found in patients with low intracellular magnesium levels despite normal serum levels. Finally, magnesium repletion has been shown to improve parameters of respiratory muscle strength in patients with hypomagnesemia.
Based on all these prior studies, it would be reasonable to obtain values of tissue magnesium in all patients having difficulty weaning from mechanical ventilation and then replete those with tissue evidence of magnesium deficiency. Although measurements of tissue magnesium may be of greater clinical importance in determining the need for magnesium repletion, the tissue samples (eg, muscle biopsy specimens) are not available on a routine basis. Methods to measure serum magnesium have been proposed for clinical use, but the physiologic significance of these measurements has not been fully elucidated. Since measurement of the pertinent tissue levels is not currently feasible, we elected to test whether the routine administration of magnesium, even to patients with normal serum levels, would result in a demonstrable clinical improvement in mechanically ventilated patients. We speculated that there would be a subgroup of patients with low levels of intracellular magnesium who would respond to the magnesium infusion by improving respiratory muscle function. Our results suggest that a trial of magnesium infusion is unlikely to be of immediate benefit in patients experiencing difficulty weaning from mechanical ventilation.