Magnesium, calcium, zinc, and nitrogen loss in trauma patients during continuous renal replacement therapy

Background: Whether standard nutrition support is sufficient to compensate for mineral loss during continuous renal replacement therapy (CRRT) is not known. Methods: Adult men with traumatic injuries were recruited; one-half of recruits required CRRT for acute renal failure. All urine and effluent (from CRRT) were collected for 72 hours. Urine, effluent, and dialysate were analyzed for magnesium, calcium, and zinc using atomic absorption spectrometry. Urea nitrogen in blood, urine, and effluent were determined by measuring conductivity changes after the addition of urease. Blood was analyzed for magnesium and calcium as part of routine care. Intake was calculated from orders and intake records. Results: Patients receiving CRRT (n = 6) lost 23.9 +/- 3.1 mmol/d (mean +/- SEM) of magnesium and 69.8 +/- 2.7 mmol/d of calcium compared with 10.2 +/- 1.2 mmol/d and 2.9 +/- 2.5 mmol/d, respectively, lost in patients not in acute renal failure (n = 6; p <.01). Zinc intake was significantly greater than loss in both groups (p <.03). Urea nitrogen excretion did not differ between groups. Serum magnesium was 0.75 +/- 0.04 mmol/L for CRRT patients, significantly lower than the 0.90 +/- 0.03 mmol/L for control patients (p <.01). Total blood calcium was below normal in both groups; ionized calcium was below normal in CRRT patients. Conclusions: CRRT caused significant loss of magnesium and calcium, necessitating administration of more magnesium and calcium than was provided in standard parenteral nutrition formulas. However, additional zinc was not required. CRRT removed amounts of urea nitrogen similar to amounts removed by normally functioning kidneys.