The lack of effect of routine magnesium administration on respiratory function in mechanically ventilated patients

We wished to determine if magnesium infusion would improve respiratory muscle function in long-term ventilated patients even in the absence of hypomagnesemia.

Prospective study of mechanically ventilated patients using a double-blind crossover design.

A combined medical-surgical ICU of a university teaching hospital.

Twenty-one separate admissions to the ICU in 20 patients were studied. Patients who were selected had been intubated and mechanically ventilated for at least 6 days with the admitting diagnosis of respiratory failure.

Twelve patients received 6 g MgSO4 intravenous (i.v.) infusion over 16 h on day 1 followed by placebo infusion on day 2. Nine patients received placebo on day 1 followed by MgSO4 (6 g i.v.) on day 2.

We measured vital capacity (VC), maximal inspiratory pressure (Pmax) and maximal expiratory pressure (Pemax) in all patients. There were no significant differences in Pimax (37 +/- 14 vs 42 +/- 20 cm H2O), Pemax (59 +/- 32 vs 61 +/- 38 cm H2O), and VC (850 +/- 460 vs 960 +/- 490 ml) comparing values before and after magnesium infusion. We could not find a subgroup of patients with a marked improvement in Pimax or Pemax.

In patients requiring mechanical ventilation for respiratory failure, magnesium infusion is not associated with increased respiratory muscle strength. Although a trial of MgSO4 administration may be considered for patients with difficulty weaning from mechanical ventilation, it is unlikely to result in clinical improvement.