Quantitative requirements of the infant and child for water and electrolyte under varying conditions.

The participants in this symposium have described the principles underlying the physiology of the body fluids and their application to clinical medicine. These principles apply equally well throughout the age span from birth through old age. My assignment is to describe the quantitative aspects of fluid and elec­ trolyte therapy as they apply over the period of growth while body size and metabolic activity reciprocally change. The clinician working with children faces the problem of prescribing fluids for the smallest premature infant as well as the nearly adult-sized adolescent. How is this assessment to be made over such a wide range of body size? For the purpose of day-by-day planning of parenteral replacement therapy, the water and electrolyte requirements are best thought of in terms of 4 com­ ponents of loss. Water is continually lost from the skin and lungs in the interests of regulation of body temperature. Secondly, fluid must be continually lost as urine to insure regulation of the osmotic pressure and acid-base balance of the body fluids as well as in the interests of excretion of metabolites. Thirdly, in many pathologic states, water and salts are lost through abnormal gastrointestinal function. The fourth component that must be considered may be that of pre­ viously incurred deficit of water and electrolyte. The first 2 of these, and often the third, must be regarded as obligatory and continuous losses, while the fourth is static and, once restored, will not recur provided the obligatory losses are con­ tinually and adequately met. The prime factor dictating the obligatory fluid requirement of the individual is his metabolic activity in terms of heat production. The smaller the individual, the greater must be his heat production per unit of body weight. Table 1 indi­ cates the heat production in terms of body weight and surface area of the average 4-month-old infant and the average 70-Kg. young adult with comparable degrees of physical activity (approximately twice basal). The infant has one tenth of the weight and one fifth of the surface area of the adult. Each kilogram of the infant's weight presents twice the surface area of the adult and, hence, twice the heat-radiating surface. In order for thermal equilibrium to be maintained, twice as many calories must be expended for heat production. In terms of surface area the caloric requirements may generally be thought of as being approximately equal at both ages although the observational data shown in Table 1 indicate that this is not strictly true. Since water loss from the skin and lungs (insensible