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

Whether standard nutrition support is sufficient to compensate for loss of minerals during continuous renal replacement therapy (CRRT) after traumatic injury is not known. 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 every hour for 72 consecutive hours. Urine, effluent and dialysate were analysed for magnesium and calcium using atomic absorption spectrometry. Amount of magnesium in dialysate was subtracted from the amount in the effluent to calculate loss from hemodiafiltration. Urea nitrogen in blood, urine and effluent was analysed by spectrophotometric assay after samples were treated with urease reagent. Blood was analysed for magnesium, calcium, albumin and pH as part of routine clinical care. Minerals provided in total parenteral nutrition formulas and intravenous boluses were calculated from physician orders and nursing bedside records.; Mineral loss did not differ between those receiving hemofiltration (n = 2) versus hemodiafiltration (n = 4). 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 from CRRT and urine compared to 10.2 ± 1.2 mmol/d of magnesium and 2.9 ± 2.5 mmol/d of calcium in urine from patients not in acute renal failure (n = 6; p < 0.01). Urea nitrogen excretion did not differ between groups. Serum magnesium was 0.75 ± 0.04 mmol/L for patients in the CRRT group, significantly lower than the 0.90 ± 0.03 mmol/L for control patients. Total blood calcium was below lab normal for both groups and ionized calcium was below lab normal for the CRRT group.; CRRT causes significant loss of magnesium and calcium, necessitating administration of more magnesium and calcium than is provided in standard parenteral nutrition formulas to support blood concentrations. CRRT can remove amounts of urea nitrogen similar to amounts removed by normally functioning kidneys. Therefore, protein intake does not need to be overly restricted during CRRT.