LAUSR

We read with interest the results from Hammond and colleagues assessing the “rule of thumb” for intravenous magnesium replacement based on the fact that every 1 g (8 mEq) of magnesium administered will increase the serum magnesium concentration by 0.15 mEq/L for critically ill patients with mild-to-moderate hypomagnesemia (1-1.9 mEq/L) within 18 to 30 hours. Evaluation of empiric magnesium replacement strategies is important as it is a common practice in the critically ill but has limited studies to guide intermittent therapy until this important study. The practice of magnesium replacement is largely based upon acute myocardial infarction trials (Table 1) administering an initial magnesium bolus (eg, 2 g [16 mEq]) followed by continuous infusions up to 16 g (130 mEq) over 24 hours. The observed changes in serum magnesium concentrations have ranged from 0.07 to 0.11 mEq/L per gram of intravenous magnesium administered. Extrapolating these serum changes from continuous infusions of magnesium to lower doses (eg, 1-4 g) over 8 to 12 hours would be expected to have lower increases when rechecked up to 10 to 22 hours later (assuming 18-30 hours after dose). Although this study only included patients who met their empiric dosing strategy for mild-to-moderate hypomagnesemia, the dose of magnesium administered and how (eg, central vs peripheral venous access, diluent used, and duration of infusion) was not provided. Additionally, the serum creatinine was not provided, nor was the definition of chronic kidney disease to compare to prior magnesium infusion trials. The route of nutrition, whether an oral diet, enteral, or parenteral nutrition, was also missing in their description of this medical intensive care unit (ICU) patient population who had been hospitalized for approximately 2 weeks, as this could have also affected response to magnesium therapy. The use of proton pump inhibitors was not listed as an exclusion criteria and would be useful to know whether this may have affected magnesium response as it has been previously reported. Evaluating the responses to electrolyte replacement, such as this one for magnesium in the ICU, may prove better ways to conserve electrolyte supplies, especially in the age of drug shortages for patients most likely to benefit.