Do intracellular, extracellular or urinary magnesium concentrations predict renal retention of magnesium in critically ill patients?

Plasma and red blood cell concentrations of zinc, copper, selenium and magnesium in the first week of paediatric critical illness

BACKGROUND & AIMS

Critically ill children are at risk of micronutrient deficiencies, which might lead to poor clinical outcomes. However, the interpretation of micronutrient concentrations in plasma is complicated due to age-dependent and critical illness-dependent changes. Certain red blood cell (RBC) concentrations might reflect the overall body status more reliably than plasma levels in the presence of systemic inflammatory response. This study longitudinally examined micronutrient concentrations in both plasma and RBC in critically ill children.

METHODS

This secondary analysis of the PEPaNIC RCT investigated the impact of early versus late initiation of parenteral macronutrient supplementation in critically ill children. All children received micronutrients when EN was insufficient (<80 % energy requirements). Blood samples were obtained on days 1, 3, 5 and 7 of Paediatric Intensive Care Unit (PICU) admission. Inductively coupled plasma mass spectrometry was used to measure zinc, selenium, and copper in plasma and selenium, copper, and magnesium in RBCs. Plasma magnesium was measured with colorimetric detection. Micronutrient concentrations were compared with age-specific reference values in healthy children and expressed using Z-scores. Changes in micronutrient concentrations over time were examined using the Friedman and post hoc Wilcoxon signed-rank tests.

RESULTS

For 67 critically ill children, median (Q1; Q3) age 9.5 (5.5; 13.2) years, PIM3 score -2.3 (-3.1; -0.8), samples were available at various time points during their PICU stay. For 22 patients, longitudinal samples were available. On day 1, the median plasma Z-score for zinc was -5.2 (-5.2; -2.9), copper -1.6 (-2.9; -0.2), selenium -2.6 (-3.8; -1.0), magnesium -0.2 (-1.6; 1.3), and median RBC Z-score for copper was 0.5 (-0.1; 1.3), selenium -0.3 (-1.1; 0.7), magnesium 0.2 (-0.4; 1.3). In the longitudinal analysis, plasma zinc was significantly higher on day 5 (Z-score -3.2 (-4.6; -1.4)) than on day 1 (Z-score -5.2 (-5.2; -3.0), p = 0.032), and plasma magnesium was significantly higher on day 3 (Z-score 1.1 (-0.7; 4.0)) than on day 1 (Z-score -0.3 (-1.6; 0.5), p = 0.018). Plasma copper and selenium remained stable, and the RBC concentrations of all micronutrients remained stable during the first five days.

CONCLUSIONS

Most patients had low plasma zinc, copper and selenium concentrations in the first week of their PICU stay, whereas they had normal to high RBC concentrations. More research is needed to examine the relationships between micronutrients and clinical outcome.

Challenges in the Diagnosis of Magnesium Status

Magnesium is a critical mineral in the human body and is involved in ~80% of known metabolic functions. It is currently estimated that 60% of adults do not achieve the average dietary intake (ADI) and 45% of Americans are magnesium deficient, a condition associated with disease states like hypertension, diabetes, and neurological disorders, to name a few. Magnesium deficiency can be attributed to common dietary practices, medications, and farming techniques, along with estimates that the mineral content of vegetables has declined by as much as 80⁻90% in the last 100 years. However, despite this mineral's importance, it is poorly understood from several standpoints, not the least of which is its unique mechanism of absorption and sensitive compartmental handling in the body, making the determination of magnesium status difficult. The reliance on several popular sample assays has contributed to a great deal of confusion in the literature. This review will discuss causes of magnesium deficiency, absorption, handling, and compartmentalization in the body, highlighting the challenges this creates in determining magnesium status in both clinical and research settings.

Magnesium status and magnesium therapy in critically ill patients: A systematic review

PURPOSE

Magnesium is frequently measured and administered in general intensive care unit patients. However, magnesium status, its association with outcomes, and therapeutic utility in such patients are unclear. We performed a systematic review of the relevant literature to define current knowledge in this field.

MATERIALS AND METHODS

We searched MEDLINE, CENTRAL, and EMBASE from 1975 to July 2014 for adult English language articles excluding obstetric, non-intensive care unit based, and specific population (poisoning, cardiothoracic, and neurosurgery) studies. We identified articles on magnesium measurement, associations, and therapy. We calculated pooled effect estimates from reported adjusted risk estimates.

RESULTS

We identified 34 relevant studies. Total serum total magnesium was the most commonly measure of magnesium status. Risk of mortality was significantly increased with hypomagnesemia (odds ratio, 1.85; 95% confidence interval, 1.31-2.60). No consistent associations existed between magnesemia or magnesium administration and any other outcomes.

CONCLUSIONS

Total serum magnesium levels are generally used to estimate magnesium status in critical illness. Hypomagnesemia appears associated with greater risk of mortality, but the efficacy of magnesium administration is open to challenge.