CaxP and Ca/P in the parenteral feeding of preterm infants.

Preterm infants requiring prolonged intravenous feeding frequently develop pathologic fractures and rickets. Infants who receive large amounts of calcium have fewer fractures. This observation led us to determine the maximal amounts of calcium and phosphate that can be added to parenteral nutrition solutions without the precipitation of calcium phosphate and to determine the optimal ratio of calcium to phosphate in these solutions. Clinical observations and in vitro experiments indicate that the product of calcium x phosphate (CaxP) in the dextrose-amino acid solution should not exceed 75 square millimolar (square millimole per square liter) to prevent calcium phosphate precipitation in barium-impregnated silicone rubber catheters and should not exceed 100 square millimolar in solutions administered through peripheral veins. Seven intake and output studies were performed in preterm infants to determine the ratio of calcium to phosphate (Ca/P) in the total parenteral nutrition solutions that minimized urinary losses. A Ca/P ratio of 5.0 minimized the sum of the calcium plus phosphate losses in the urine. However, experience with long-term total parenteral nutrition in preterm infants, awareness of the acute and life-threatening effects of body phosphate depletion, and an unmeasured endogenous enteric calcium secretion all suggest that a Ca/P ratio of approximately 3.0 provides a safer compromise between the acute and serious complications of phosphate deficiency and the chronic problems of fractures and rickets due to calcium deficiency.