The role of serum and urinary urea in the evaluation of enteral protein intake in adequate and small-for-gestational-age very low birth weight infants

INTRODUÇÃO E OBJETIVOS: Os recém-nascidos de muito baixo-peso (RNMBP) têm necessidades nutricionais especiais. Existe uma tendência atual de se individualizar a oferta protéica para essas crianças. O objetivo do trabalho é determinar a utilidade da uréia sérica e urinária como indicadores da oferta protéica em RNMBP adequados (AIG) e pequenos para a idade gestacional (PIG). TIPO DE ESTUDO E LOCAL: Estudo prospectivo realizado no Berçário Anexo à Maternidade Instituto da Criança "Prof. Pedro de Alcântara" do Hospital das Clínicas, Departamento de Pediatria da Faculdade de Medicina, Universidade de São Paulo, Brasil. MÉTODOS: Setenta e dois RNMBP (oferta protéica média = 3,7 g/kg/dia) foram incluídos, em um estudo de coorte prospectivo, em dois grupos: AIG (n = 34) e PIG (n = 38). Amostras de sangue, coletas de urina de seis horas (Ur6h) e em amostras isoladas (AIUr) foram obtidas para determinação de uréia e creatinina após a 3ª semana de vida e duas semanas após. Análise estatística: teste t de Student, correlação de Pearson e regressão linear (p < 0,05). RESULTADOS: Não houve diferença entre os grupos quanto aos níveis de uréia sérica, uréia Ur6h e uréia AIUr, bem como entre as duas avaliações dentro de cada grupo. A uréia sérica correlacionou-se à uréia Ur6h nos RNAIG e nos PIG, bem como à uréia AIUr nos RNPIG. A uréia Ur6h correlacionou-se à uréia AIUr nos RNAIG e nos RNPIG. Não houve correlação entre a oferta protéica e a uréia sérica ou urinária. CONCLUSÕES: A uréia sérica e a urinária não refletiram a oferta protéica quando foram utilizadas ofertas médias de 3,7 g/kg/dia. Uréia AIUr pode ser tão confiável quanto uréia da urina coletada por períodos mais longos.

The role of serum and urinary urea in the evaluation of enteral protein intake in adequate and small-for-gestational-age very low birth weight infants.

CONTEXT AND OBJECTIVE: Very low birth weight (VLBW) infants have special nutritional needs. There is a current tendency to individualize their protein needs. The objective of this study was to determine the suitability of serum and urinary urea as indicators for protein intake in adequate-for-gestational-age (AGA) and small-for-gestational-age (SGA) VLBW infants. DESIGN AND SETTING: Prospective study in the nursery attached to the Maternity Ward of the "Prof. Pedro de Alcântara" Children's Institute, Hospital das Clínicas, Department of Pediatrics, Faculdade de Medicina da Universidade de São Paulo, Brazil. METHODS: Seventy-two VLBW infants (mean protein intake = 3.7 mg/kg/day) were enrolled in a prospective cohort study in two groups: AGA (n = 34) and SGA (n = 38). Blood samples, six-hour urine (6hUr) collections and urine sample tests (STUr) were obtained for urea and creatinine assays at three and five weeks of life. STATISTICAL ANALYSIS: Student's t test, Pearson correlation and linear regression (p < 0.05). RESULTS: There were no differences between groups for serum urea, 6hUr and STUr, or between two assessments within each group. Serum urea correlated with 6hUr in both AGA and SGA, and to STUr in SGA; 6hUr correlated with STUr in both AGA and SGA. There was no correlation between protein intake and serum or urine urea. CONCLUSIONS: Serum and urinary urea did not reflect protein intake when mean intakes of 3.7 g/kg/day were used. Sample tests of urinary urea can be as reliable as urea from urine collected over longer periods.

Potassium metabolism in extremely low birth weight infants in the first week of life.

OBJECTIVE: Nonoliguric hyperkalemia has been reported to occur in the first week of life in as many as 50% of extremely low birth weight (ELBW) infants. We studied potassium balance and renal function in the first 5 days of life to characterize potassium metabolism during the three phases of fluid and electrolyte homeostasis that we have described in ELBW infants and to elucidate the factors that contribute to the development of nonoliguric hyperkalemia. STUDY DESIGN: Plasma potassium concentration (PK), potassium intake and output, and renal clearances were obtained for the first 6 days of life in 31 infants with a birth weight of 1000 gm or less. Collection periods in which urine flow rate was greater than or equal to 3 ml/kg per hour and weight loss was greater than or equal to 0.8 gm/kg per hour were denoted to be diuretic. Prediuresis includes all collection periods before the first diuretic period; diuresis includes all collection periods between the first and last diuretic periods; postdiuresis includes all collection periods after the last diuretic period. Infants with a PK greater than 6.7 mmol/L on at least one measurement were denoted to have hyperkalemia. RESULTS: PK increased initially after birth--despite the absence of potassium intake- and then decreased and stabilized by the fourth day of life. Diuresis occurred in 27 of 31 infants. The age at which PK peaked was closely related to the onset of diuresis. PK decreased significantly during diuresis as the result of a more negative potassium balance, despite a significant increase in potassium intake. In fact, PK fell to less than 4 mmol/L in 13 of 27 infants during diuresis. After the cessation of diuresis, potassium excretion decreased even though there was a significant increase in potassium intake, potassium balance was zero, and PK stabilized. Hyperkalemia developed in 11 of 31 infants. The pattern of change in PK with age was similar in infants with normokalemia and hyperkalemia: PK initially increased (essentially in the absence of potassium intake) and then decreased and stabilized by the fourth day of life. However, the rise in PK after birth was greater in infants with hyperkalemia than in those with normokalemia: 0.7 +/- 0.2 versus 1.8 +/- 0.2 mmol/L (p < 0.001). No differences in fluid and electrolyte homeostasis or renal function were identified as associated with hyperkalemia. CONCLUSIONS: PK increases in most ELBW infants in the first few days after birth as a result of a shift of potassium from the intracellular to the extracellular compartment. The increase in the glomerular filtration rate and in the fractional excretion of sodium, with the onset of diuresis, facilitates potassium excretion, and PK almost invariably decreases. Hyperkalemia seems to be principally the result of a greater intracellular to extracellular potassium shift immediately after birth in some ELBW infants.