Measurement of total body potassium in premature infants by means of a whole-body counter.

This paper describes a whole-body counter (WBC) specially designed to measured total body potassium (TBK) infants under 4,500 g. The counter is a "shadow shield" design and consists of a single 10 cm X 10 cm X 45 cm NaI(Tl) crystal, positioned lengthwise and shielded from environmental background radiation by a minimum of 10 cm of lead. The standard error of counting for a 2,000-s counting period is 19.9% for a 1,000-g infant and 11.9% for a 2,000-g infant. TBK of stillborn pigs, measured by the WBC, agreed to an average of 3% of TBK determined by carcass analysis in the same animals. A total of 118 measurements of TBK have been made in 50 premature infants ranging in weight between 1,100 and 3,600 g and in age between 2 and 75 days. The observed relationship of TBK with weight is described by the equation: TBK (mEq) = 0.0433Wt (g + 1.57 r = 0.92. Potassium retention per gram weight gain is estimated to be 0.043 mEq. The obtained TBK values agree well with values published by other workers but extend the range of measurement to 1,100 g.