Validity of endogenous creatinine clearance in low birthweight infants

Wild Bornean orangutans experience muscle catabolism during episodes of fruit scarcity

Pronounced temporal and spatial variation in the availability of food resources can produce energetic deficits in organisms. Fruit-dependent Bornean orangutans face extreme variation in fruit availability and experience negative energy and protein balance during episodes of fruit scarcity. We evaluate the possibility that orangutans of different sexes and ages catabolize muscle tissue when the availability of fruit is low. We assess variation in muscle mass by examining the relationship between urinary creatinine and specific gravity and use the residuals as a non-invasive measure of estimated lean body mass (ELBM). Despite orangutans having a suite of adaptations to buffer them from fruit scarcity and associated caloric deficits, ELBM was lower during low fruit periods in all age-sex classes. As predicted, adult male orangutans had higher ELBM than adult females and immatures. Contrary to expectation, flanged and unflanged males did not differ significantly in ELBM. These findings highlight the precarity of orangutan health in the face of rapid environmental change and add to a growing body of evidence that orangutans are characterized by unique metabolic traits shaped by their unpredictable forest environment.

Pediatric Renal Ontogeny and Applications in Drug Development

The clinical applications of renal ontogeny mainly include renal function evaluation and optimal dosing of renally eliminated drugs in pediatric patients, which rely on pharmacometric models and/or bedside estimated glomerular filtration rate equations. However, these applications in drug development are based on an understanding of renal function development, especially when considering premature infants, and the renal biomarkers that can be used for renal function assessment. This review provides a general overview on (1) renal function development, (2) the biomarkers that are used to assess renal function, and (3) the practical application of this knowledge to drug dosing for renally eliminated drugs during pediatric development. While pharmacometric approaches for estimating renal function during development have improved considerably, the number of drug development programs that have studied premature infants is small and suggests that caution should be taken in estimating doses for renally eliminated drugs during periods of rapid change in renal function.

Glomerular Filtration Rate Estimation Formulas for Pediatric and Neonatal Use

Renal function assessment is of the utmost importance in predicting drug clearance and in ensuring safe and effective drug therapy in neonates. The challenges to making this prediction relate not only to the extreme vulnerability and rapid maturation of this pediatric subgroup but also to the choice of renal biomarker, covariates, and glomerular filtration rate (GFR) estimating formula. In order to avoid burdensome administration of exogenous markers and/or urine collection in vulnerable pediatric patients, estimation of GFR utilizing endogenous markers has become a useful tool in clinical practice. Several estimation methods have been developed over recent decades, exploiting various endogenous biomarkers (serum creatinine, cystatin C, blood urea nitrogen) and anthropometric measures (body length/height, weight, muscle mass). This article reviews pediatric GFR estimation methods with a focus on their suitability for use in the neonatal population.

Impaired renal function and development in Belgrade rats

Belgrade rats carry a disabling mutation in the iron transporter divalent metal transporter 1 (DMT1). Although DMT1 plays a major role in intestinal iron absorption, the transporter is also highly expressed in the kidney, where its function remains unknown. The goal of this study was to characterize renal physiology of Belgrade rats. Male Belgrade rats died prematurely with ∼50% survival at 20 wk of age. Necropsy results indicated marked glomerular nephritis and chronic end-stage renal disease. By 15 wk of age, Belgrade rats displayed altered renal morphology associated with sclerosis and fibrosis. Creatinine clearance was significantly lower compared with heterozygote littermates. Urinary biomarkers of kidney injury, including albumin, fibrinogen, and kidney injury molecule-1, were significantly elevated. Pilot morphological studies suggest that nephrogenesis is delayed in Belgrade rat pups due to their low iron status and fetal growth restriction. Such defects in renal development most likely underlie the compromised renal metabolism observed in adult b/b rats. Belgrade rat kidney nonheme iron levels were not different from controls but urinary iron and transferrin levels were higher. These results further implicate an important role for the transporter in kidney function not only in iron reabsorption but also in glomerular filtration of the serum protein.

Technical note: variation in muscle mass in wild chimpanzees: application of a modified urinary creatinine method

Individual body size and composition are important variables for a variety of questions about the behavioral ecology and life histories of non-human primates. Standard methodologies for obtaining body mass involve either capture, which poses risks to the subject, or provisioning, which can disrupt the processes being studied. There are no methods currently available to assess body composition from living animals in the wild. Because of its derivation in muscle, the amount of creatinine that an individual excretes in 24 hours is a reliable and frequently used indicator of relative muscle mass in humans and laboratory animals. Although it is not feasible to collect 24-hour urine samples from wild primates, we apply here a simple method to approximate muscle mass variation from collections of spot urine samples. Specific gravity (SG), an alternative method for assessing urinary water content, is both highly correlated to creatinine and free of mass-dependent effects. Individuals with greater muscle mass should excrete more creatinine for a given SG. We examine this relationship in a dataset of 12,598 urine samples from wild chimpanzees in the Kibale National Park, Uganda. As expected from known differences in body composition, the slope of the relationship between SG and creatinine is significantly greater in adult males than adult females and in adults versus immature individuals. Growth curves generated through this method closely approximate published weight curves for wild chimpanzees. Consistent with the role of testosterone in muscle anabolism, urinary testosterone predicted relative creatinine excretion among adult male chimpanzees.

Tolerability and outcomes of kinetically guided therapy with gentamicin in critically ill neonates during the first week of life: an open-label, prospective study

BACKGROUND

Aminoglycosides are bactericidal antibiotics used worldwide for the treatment of serious infections in critically ill patients, including neonates. Critically ill neonates constitute a unique challenge in dosing owing to the pathologic alterations that accompany severe illness and the rapidly changing conditions of these patients.

OBJECTIVES

The main objective of this study was to analyze the kinetically guided dosage adjustment of gentamicin in neonates critically ill during the first week of life based on plasma concentrations after the first dose and to identify the impact of covariates (eg, fluid intake, body fluid retention) with respect to gestational age (GA). Tolerability of therapy was also assessed.

METHODS

This 10-day, open-label, prospective study included neonates critically ill during the first week of life admitted to the neonatal intensive care unit of a children's hospital between January 2006 and July 2009. Hearing and renal assessments were conducted over a 24-month follow-up period. The patients were treated with gentamicin for suspected sepsis, proven sepsis, or pneumonia as an early sign of sepsis. The first and second doses of gentamicin 4 mg/kg were adjusted according to birth weight and GA: group 1 (GA < 34 weeks), 48-hour interdose intervals; group 2 (GA 34-38 weeks), 36 hours; and group 3 (GA > 38 weeks), 24 or 48 hours. Individual pharmacokinetic parameters were estimated after the first dose (given in 30-minute intravenous infusions) using 4 concentrations. Individual pharmacokinetic parameters were estimated by fitting the parameters of a 2-compartment model into 4 concentrations. The last 2 blood samples were taken 30 minutes before the fourth infusion (C(trough,3)) and 1 hour after its start (C(max,4)). Dosing was individualized to reach target ranges for the C(trough,3) (0.5-2.0 mg/L) and C(max,4) (6-10 mg/L) values. If needed, initial dosing was changed after the second dose by adjusting (reducing or increasing) the third and subsequent doses, or by adjusting (prolonging or shortening) the interdose intervals. C(trough,3) and C(max,4) were assessed to determine differences between predicted and assayed values. Fluid retention was registered as the difference between fluid intake and urine output at different intervals related to the first dose per kilogram of birth weight, and from the start of the first infusion (0 hour) to the day of the fourth infusion. The C(max)/minimum inhibitory concentration (MIC) ratio was determined for assessment of optimal response. Tolerability was evaluated during the 24-month follow-up period using renal sonography to screen for nephrocalcinosis and transient evoked otoacoustic emission recordings to evaluate hearing abnormalities.

RESULTS

A total of 84 neonates (all white; 53 males, 31 females; birth weight range, 0.8-4.56 kg; GA range, 24-42 weeks) were enrolled in 3 groups: group 1, GA < 34 weeks, n = 27; group 2, GA 34-38 weeks, n = 22; and group 3, GA > 38 weeks, n = 35. The C(max) value detected 1 hour after the start of the first infusion (C(max,1)) reached the target range of 6-10 mg/L in 66 of the 84 neonates (79%). After the initial dose, C(max,1) was variable (%CV, 29%); the failure rate to reach 6 mg/L was 13%. V(d) decreased with GA (r = -0.30, P < 0.01) and achieved mean (SD) rates of 0.51 (0.10), 0.48 (0.13), and 0.40 (0.15) L/kg in groups 1, 2, and 3, respectively. Neither C(max) nor V(d) was correlated with fluid intake relative to the first infusion. Mean gentamicin clearance measured after dose 1 (0.47 [0.23], 0.66 [0.26], and 0.76 [0.32] mL/min/kg) increased with GA (r = 0.45, P < 0.001). The interdose interval was prolonged after the second and subsequent infusions in 8 of 84 neonates (10%) or by decreasing the third dose and subsequent doses in 51 neonates (61%). The target C(max,4) and C(trough,3) values occurred in 63% (22 of 35) and 83% (29 of 35) of full-term patients (GA >38 weeks), respectively. In preterm neonates, the target range for C(max,4) was reached in 11 of 27 patients (41%) in group 1 and 11 of 22 patients (50%) in group 2; for C(trough,3), the target range was reached in 25 patients (93%) in group 1 and in 16 (73%) in group 2. C(trough,3) >2 mg/L was detected in 1 full-term neonate, and gentamicin was withdrawn. Suspected fluid retention within the time period of 0 hour to the day of the fourth infusion was well correlated with actual body weight (r = 0.58, P < 0.001), but it was negatively correlated with C(max,4) (r = -0.25, P = 0.02). Thirteen of the 84 neonates (15%) had confirmed sepsis. C(max)/MIC was >12 except for 2 resistant staphylococcal infections (C(max)/MIC = 0.4); amikacin and vancomycin were substituted for gentamicin in these cases. Clinical signs and laboratory data indicative of suspected sepsis disappeared in 5 to 10 days in 68 of 71 neonates. In 1 neonate, gentamicin was withdrawn after dose 4 because of a high C(trough,3) value. In the 3 remaining neonates, C-reactive protein was decreased >10 days without changing therapy. Two neonates died, 1 of severe hypoxic-ischemic encephalopathy as a consequence of perinatal asphyxia and another of stage IV intraventricular hemorrhage. Transient renal dysfunction attributable to gentamicin was detected in 1 case. No signs of late toxicity (nephrocalcinosis) were found during the second year of follow-up. Two neonates were diagnosed with unilateral hearing loss, a secondary phenomenon of hypoxic-ischemic encephalopathy thought to be related to the severe perinatal asphyxia.

CONCLUSIONS

The initial dose of gentamicin 4 mg/kg for these critically ill premature and mature neonates with sepsis during the first week of life was high enough to reach bactericidal C(max,1) within 6-10 mg/L. C(max,1) <6 mg/L occurred in 13% of neonates. The interdose interval modified according to the recommendation resulted in C(trough) values within the target range of 0.5-2.0 mg/L in all but 2 neonates. The kinetically guided maintenance dosing of gentamicin based on plasma concentrations after the first dose should be optimized, taking into account actual body weight. (EudraCT number: 2005-002723-13).

Urological emergency in neonates with congenital hydronephrosis

OBJECTIVE

It is well described that unilateral pelviureteric junction obstruction (PUJO) is a benign condition, because the dilatation resolves spontaneously and the function does not decrease in most of the kidneys. However, there is exceptional PUJO that requires emergent treatment in neonatal periods. The aim of this article is to report the urological emergency and management in neonates with PUJO.

MATERIALS AND METHODS

Nine children (seven boys and two girls) with PUJO who underwent neonatal emergent treatment during the last 13 years were reviewed. Renal function was evaluated according to decay curve of serum creatinine (SCr) levels corresponding to gestational age (GA) at delivery. Physical examination, ultrasonographic monitoring, and chest and abdominal plain radiographs were repeated in each neonate.

RESULTS

Eight patients were detected prenatally. In five patients, multicystic dysplastic kidney (MCDK) was demonstrated on the contralateral side. Three patients underwent percutaneous puncture of fetal hydronephrosis. Decrease of amniotic fluid was evident in three fetuses. Indications for emergent treatment included mass effect from hydronephrosis in three patients, renal dysfunction in five, and severe urinary tract infection in one. During neonatal periods, a percutaneous nephrostomy tube was placed in seven, and open nephrostomy in one with anorectal malformation. Repeated punctures of the dilated renal pelvis were done in one patient. Renal function after pyeloplasty was stable in eight patients, while it was moderately decreased in one who was associated with oligohydramnios in utero.

CONCLUSION

Indications for emergent treatment in neonates with PUJO included mass effect from giant hydronephrosis, renal dysfunction and severe urinary tract infection. At birth, respiratory and circulatory conditions must first be stabilized. In neonates with hydronephrosis of the solitary kidney or severe bilateral PUJO, serial SCr should be monitored to evaluate renal function. Decrease of amniotic fluid suggested renal functional compromise that would not recover after urological management.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Renal function in very low birth weight infants: normal maturity reached during early childhood

Development of glomerular and tubular renal function is delayed in preterm infants. To study the pattern of maturation during infancy and childhood, we re-evaluated renal function in 22 very low birth weight infants--in 14 of the infants at 18 months postconceptional age (9 months corrected age) and in the remaining 8 infants at 8 years of age. The glomerular filtration rate remained lower at 9 months corrected age than in term infants of the same postconceptional age: 82 +/- 23 versus 125 +/- 18 ml/min per 1.73 m2 (p < 0.001). At 8 years of age the glomerular filtration rate did not differ from that of healthy control subjects. Effective renal plasma flow, filtration fraction, albumin excretion, maximal concentrating ability, and kidney size determined by ultrasonography were all normal at 8 years of age. We conclude that renal function, which is markedly reduced during the neonatal period in very low birth weight infants, reaches normal maturity by 8 years of age but not by 9 months corrected age.

Renal function in sick very low birthweight infants: 2. Urea and creatinine excretion

Plasma urea and creatinine concentrations and urea and creatinine clearances and excretion were measured in a sample of 40 infants of 25.5-33 weeks' gestation, birth weight 720-2000 g, between the ages of 0.5 and 33 days. Creatinine excretion rate was between 60 and 120 mumol/kg/day in the first five postnatal weeks (mean 90.5) and was independent of sex or growth retardation. This can be used in clinical practice to estimate instantaneous urine flow rate V, if the creatinine concentration is measured in a randomly voided urine sample, from the formula V = 90.5/urine creatinine, with 95% confidence limits +/- 39%. There is a wide range of plasma creatinine at all gestations and ages decreasing from range 75-130 mumol/l in the first two days to 35-80 mumol/l at 3 weeks of age. Plasma urea is a poor indicator of glomerular filtration rate (GFR) in sick preterm infants. GFR (ml/min/kg) can be estimated from plasma creatinine from the formula GFR = 69.2/plasma creatinine but this estimate is imprecise with 95% confidence limits +/- 46%. Urea:creatinine clearance ratio was usually less than 1.0 (range 0.18 to 1.5) and was lower when the urine flow rate was low. Urea excretion was up to 17 mmol/kg/day in the first two weeks, higher in the more immature infants. These high levels were paralleled by a high plasma urea concentration, up to 18 mmol/l. A high plasma urea is not necessarily associated with renal failure or dehydration.

A reappraisal of the measurement of glomerular filtration rate in pre-term infants

Thirty-seven single-injection polyfructosan-S (PF-S, Inutest) and 98 continuous-infusion PF-S/creatinine clearance studies were performed in 39 sick very low birth weight infants. The single-injection clearance method for measuring glomerular filtration rate has been shown to be a reliable technique if sampling is continued for 8 h or more and the PF-S (Inutest) assay is sensitive, accurate and precise. The continuous-infusion clearance method is also valid if the infusion is continued for more than 24 h and preceded by a loading dose in the form of a double-rate infusion for 8 h. Creatinine clearance is usually less than PF-S clearance, the mean ratio being 0.91, suggesting that there is some creatinine reabsorption in the renal tubule in sick very low birth weight infants.

Addition of metolazone to overcome tolerance to furosemide in infants with bronchopulmonary dysplasia

A decreased response to the loop diuretic furosemide develops within a few doses in young infants. We tested the hypothesis that the use of the thiazide-like diuretic metolazone, in combination with furosemide, would inhibit water and electrolyte reabsorption and overcome pharmacologic tolerance to furosemide alone. Infants with bronchopulmonary dysplasia of similar gestational and postnatal ages were randomly assigned to one of three groups. Group 1 (n = 6) received furosemide (1 mg/kg per dose) intravenously every 24 hours for a total of five doses. Group 2 (n = 8) received the same treatment as group 1, but in addition metolazone (0.2 mg/kg per dose) was given enterally with doses 3 and 4 of furosemide. Group 3 (n = 8) received metolazone (0.2 mg/kg per dose) enterally every 24 hours for five doses. Urine was collected before the first diuretic dose and throughout the study for determination of the urine flow rate; urinary excretion of sodium, chloride, and potassium; and creatinine clearance. Urinary flow rate and urinary sodium and chloride excretion increased after the first dose in all groups. In the infants treated with either furosemide or metolazone, urinary flow rate and urinary and chloride excretion returned to baseline values after the last three doses. In contrast, when furosemide was administered with metolazone, urinary flow rate and urinary excretion of sodium, chloride, and potassium were greater than the values for baseline and for the previous dose, as well as for the corresponding doses of furosemide in group 1 and metolazone in group 3. Tolerance to furosemide (group 1) and metolazone (group 3) appeared to be explained by compensatory increased sodium and chloride reabsorption without changes in creatinine clearance. We conclude that the administration of metolazone with furosemide enhances diuresis, natriuresis, and chloruresis and overcomes the rapid development of tolerance to furosemide in infants with bronchopulmonary dysplasia by blocking the compensatory increase in renal sodium and chloride absorption.

Mineral balance studies in sick preterm intravenously fed infants during the first week after birth. A guide to fluid therapy

Mineral balance studies were performed in 61 sick preterm infants given parenteral fluids only. Their gestational ages varied from 24 to 35 weeks, and 50 required mechanical ventilation. Two consecutive balance studies were performed; the first from admission to 48 hours in all babies given maintenance fluids of 10% Dextrose, and the second from 48 hours to 7 days in those babies given intravenous feeding (IVN). At the beginning and end of each balance period, the baby was weighed and an arterial blood sample taken for blood gases, electrolyte, urea, creatinine and protein determinations. During the balance period all urine was collected and analysed for electrolyte, urea, and creatinine composition, and all fluid intake was recorded. The balance of a mineral was calculated as the difference between parenteral intake and urine output. Infants requiring IVN were allocated alternatively to regimen X or regimen Y, which had the same calcium content of 9.5 mmol/L, but different phosphate contents, regimen X containing 7.3 mmol/L and regimen Y 11.6 mmol/L. In those infants requiring prolonged IVN, 12-24 hour balance studies were performed at weekly intervals after day 10. 1. Phosphate deficiency developed in infants given regimen X, who had higher urine calcium excretion, lower percentage calcium retention and lower plasma phosphate levels than those given regimen Y. These differences were apparent by day 7 and persisted after day 10. In infants given regimen Y, mean calcium retention from admission to day 7 was 3.9 mmol/kg, and after day 10 was 0.9 mmol/kg/day. 2. In the first 48 hours, urine output and creatinine clearance varied widely and were lower in infants with higher oxygen requirements at 48 hours. Ten babies had severe oliguria with outputs less than 10 mL/kg/day. Creatinine clearance was directly related to gestational age, mean arterial blood pressure, and plasma protein concentrations on admission. After 48 hours, urine output and creatinine clearance increased considerably. 3. In the first 48 hours, metabolic acidosis was produced by increased plasma non-protein metabolisable acid concentrations, which were associated with low creatinine clearances, and were thought to be due to lactic acid accumulation in response to decreased tissue perfusion. At 7 days, metabolic acidosis was of similar severity but was produced by decreased plasma non-metabolisable base concentrations, caused by increased urine loss of net base, and not directly by IVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Furosemide pharmacokinetics in very low birth weight infants

The pharmacokinetics of furosemide were studied longitudinally during long-term administration in 10 very low birth weight infants with bronchopulmonary dysplasia. Mean birth weight of the infants was 829 +/- 217 g, mean gestational age at birth was 26.6 +/- 2.9 weeks, and mean postnatal age at the start of therapy was 2.4 +/- 1.0 weeks. Serial determinations of furosemide pharmacokinetic parameters were performed during 2 weeks to 3 months of long-term therapy. Plasma half-life was prolonged in infants less than 31 weeks postconceptional age (gestational + postnatal age), frequently exceeding 24 hours. All infants less than 29 weeks postconceptional age whose dosing schedule was once every 12 hours accumulated furosemide to potentially ototoxic levels. Furosemide renal clearance increased and plasma half-life decreased in association with increasing postconceptional age. Furosemide secretory clearance was very low in patients less than 31 weeks postconceptional age, resulting in a reliance on glomerular filtration to deliver drug to its main site of action within the lumen of the loop of Henle. Thus elevated plasma levels may be required to ensure adequate luminal delivery and adequate diuresis in these infants with low secretory clearance. Nevertheless, the current dosing schedule (once every 12 hours) of furosemide should be modified to once every 24 hours in infants of low postconceptional age to avoid possible toxic effects.

The use of plasma creatinine concentration for estimating glomerular filtration rate in infants, children, and adolescents

The formula GFR = kL/Pcr can be used to estimate GFR in infants, children, and adolescents who have grossly normal body habitus and are in steady-state condition. GFR is expressed in ml/min per 1.73 m2 BSA, L represents body length in cm, Pcr represents plasma creatinine concentration in mg per dl and k is a constant of proportionality that reflects the relationship between urinary creatinine excretion and units of body size. The value of k varies as a function of age and sex being 0.33 in preterm infants, 0.45 in full-term infants, 0.55 in children and adolescent girls, and 0.70 in adolescent boys. The advantages of rapid determination, reasonable accuracy, and the avoidance of urine collection justify the use of this formula in pediatric patients.

Renal handling of calcium in the early newborn period

Urinary calcium excretion was studied in two matched groups of 28 clinically-well preterm and fullterm infants between 24 and 48 hours of life. In the developmental period from 28 to 40 weeks gestation, urinary calcium excretion was positively correlated to the gestational age (r = 0.583, P less than 0.01), serum calcium levels (r = 0.512, P less than 0.01), the rate of glomerular filtration (r = 0.715, P less than 0.001), and urinary cyclic AMP excretion (r = 0.717, P less than 0.001). Urinary calcium excretion was independent of sodium and calcium intake, and urinary sodium and phosphate excretion. The mean fractional total calcium excretion and fractional ionized calcium excretion in babies less than or equal to 32 weeks was less than 1.0%, compared to greater than 2.5% for sodium. Serum calcium levels were positively correlated with gestational age (r = 0.803, P less than 0.001), and serum phosphate levels (r = 0.85, P less than 0.001). Whole blood ionized calcium levels were positively correlated to gestational age (r = 0.625, P less than 0.001), and serum total calcium levels (r = 0.440, P less than 0.05). The newborn kidney in babies less than or equal to 32 weeks gestation, did not have impaired conservation of calcium as for sodium; and neither sodium nor calcium intake appeared to affect urinary calcium excretion in the well newborn infant. Probably neither excess excretion of calcium nor serum phosphate levels contribute to early neonatal hypocalcemia.

A simple estimate of glomerular filtration rate in low birth weight infants during the first year of life: noninvasive assessment of body composition and growth

The management of the preterm infant often requires rapid assessment of glomerular filtration rate (GFR). We sought to develop a screening test using GFR = kL/Pcr, where GFR is expressed as ml/min/1.73 m2, L is body length in centimeters, Pcr is plasma creatinine concentration, and k is a constant that depends on muscle mass. The value for k in 118 appropriate for gestational age preterm infants (0.34 +/- 0.01 SE) was significantly less than that of full-term infants (0.43 +/- 0.02, P less than 0.001). There was no difference between 12- to 24-hour single-injection inulin clearance and either 0.33 L/Pcr or creatinine clearance in preterm infants. We compared the body habitus of preterm and full-term infants using the assessment of muscle mass from urinary creatinine excretion (UcrV) and from upper arm muscle area (AMA) and volume (AMV), and that of fatness from the sum of five skinfold thickness measurements. During the first year of life, premature infants were found to have a lower percentage of muscle mass than term infants did. On the other hand, they took on a relatively greater amount of subcutaneous fat. There was a very good correlation between AMA or AMV and urinary creatinine excretion (r = 0.91 and 0.94, respectively) in 68 infants with heterogeneous body composition during the first year, indicating the validity of the urinary creatinine measurement. Absolute GFR (ml/min) was also well estimated from AMA or AMV factored by Pcr. We conclude that GFR can be well estimated from 0.33 L/Pcr in preterm infants. The lower value for k reflects the smaller percentage of muscle mass in preterm versus term infants. As a screening test, 1.5 X k or 0.05 L/Pcr predicted low values of GFR with an efficiency of 73%, specificity of 67%, and sensitivity of 88%.

Evaluation of renal function and acute renal failure in the neonate

The diagnosis of renal dysfunction in the neonate can be a challenging problem for the practicing pediatrician. Although there are real differences in renal function between term and preterm infants, overall function is quite adequate in both groups when fluid intake and environmental conditions are carefully controlled. When confronted with an infant with a pathologic decrease in urine output, the clinician must provide adequate fluid resuscitation for the infant with prerenal oliguria without inducing fluid overload in the infant with established, intrinsic renal failure. In addition, the infant with obstruction to urine flow must be distinguished. This requires careful assessment of physical findings and a few key laboratory determinations. Once the diagnosis of renal failure is made, frequent clinical monitoring with anticipation of potential complications is critical. Long-term management of renal failure in infancy and intervention for suspected urinary tract malformation in the fetus have emerged as difficult medical and ethical problems as our technology has advanced.

Beta 2-microglobulin clearance in neonates: index of tubular maturation

Serum and urinary beta 2-microglobulin (beta 2M) were studied by enzyme immunoassay in 28 normal neonates at day 1 and day 4 of life in relation to gestational age (GA) and postnatal age (PNA). The infants were grouped according to GA; 10 with GA ranging from 32 to 35 weeks (mean 33.5 weeks) and 18 with GA ranging from 36 to 41 weeks (mean 38.3 weeks). Serum beta 2M varied directly with both GA and PNA. When values for serum beta 2M were related to conceptional age (CA), a significant positive correlation was present for all the infants studied (r = 0.68, P less than 0.01). Fractional excretion of beta 2M (FE beta 2M) decreased as a function of both GA and PNA. When a comparison of FE beta 2M was made in infants of all CA, a significant inverse correlation was noted for infants with CA less than or equal to 35 weeks (r = -0.89, P less than 0.001). The fall in FE beta 2M reached a plateau by 36 weeks. The highest FE beta 2M (33%) was observed in infants of 32 weeks CA who had the lowest filtered beta 2M (F beta 2M). No statistically significant relationship between changes in FE beta 2M and fractional urine flow rate was observed within each of the CA categories (infants less than or equal to 35 weeks, r = 0.21, P = 0.28; infants greater than or equal to 36 weeks, r = 0.25, P = 0.18).(ABSTRACT TRUNCATED AT 250 WORDS)

Creatinine and urea clearances compared to inulin clearance in preterm and mature babies

Simultaneous clearances of inulin, urea and creatinine were compared in 41 babies of 26-40 weeks gestation on 122 occasions during the first month of life. In each case creatinine was measured by a reaction rate method, and in thirty specimens it was also measured after adsorption onto resin. Urea clearance averaged only 62% of inulin clearance (P less than 0.001), and was a poor marker of glomerular filtration. Creatinine clearance measured by resin adsorption equalled inulin clearance, but the assay is manual and not suitable for routine clinical use. Creatinine clearance measured by reaction rate analysis underestimated inulin clearance by a quarter (P less than 0.01) because this automated method overestimated plasma creatinine by an average of 22 mumol/l. Urinary creatinine excretion was 72 +/- 17 nmol/kg per min (mean +/- S.D.) during the first week of life, and 66 +/- 13 nmol/kg per min in weeks two to four, and was not influenced by gestation or body size. Using these values, glomerular filtration rate, urine flow, and the urinary excretion rates of substances may be estimated from measurements made on plasma and untimed urines. Although these estimates are imprecise, with 9% confidence limits of 62-161%, they are useful in clinical practice because they avoid the need to make accurately timed collections of urine.

Composition of postnatal weight loss and subsequent weight gain in small for dates newborn infants

Using a sucrose and deuterium oxide dilution technique body water compartments and solids were serially determined in small for dates newborn infants at birth, at the moment of maximum postnatal weight loss and on recovery of birth weight. Compositions of weight loss and subsequent weight gain were calculated from the differences in body water compartments and solids between the first and the second and the second and the third study, respectively. Birth weight of the infants was 1.55 +/- 0.46 kg (mean +/- SD) (N = 7), gestational age was 35.7 +/- 3.1 weeks. Results show that despite changes in extra- and intracellular water volumes during weight loss, total body water volume and solids per unit of body weight remained remarkably constant throughout the study. Compositions of weight loss and subsequent weight gain were similar to body composition. This suggests that in small for dates newborn infants postnatal weight loss is the result of catabolism rather than dehydration and subsequent weight gain is the result of growth rather than rehydration.

Tobramycin sulfate elimination in premature infants

The elimination pharmacokinetics of tobramycin sulfate was studied in 25 newborn infants of birth weight 0.7 to 4.7 kg during 31 treatment episodes. The peak serum concentrations after a 2.5 mg/kg dose were usually within the therapeutic range of 5 to 10 micrograms/ml; however, the serum predose trough values were elevated above the theoretical safe limit of 2 micrograms/ml. Because of the prolonged serum elimination half-lives, a calculated extended dosage interval, sometimes greater than 24 hours, was necessary to obtain a predose trough of less than or equal to 2 micrograms/ml. The serum elimination half-lives inversely correlated with gestational age, extrauterine age, birth weight, and creatinine clearance. The very low ratio of tobramycin renal clearance to creatinine renal clearance was virtually constant and indicated a probable tubular reabsorption of tobramycin. A general dosage schedule based on birth weight was derived from the data. An alternative formula was derived to enable prediction of the tobramycin elimination half-life based on a combination of birth weight, gestational age, and extrauterine age for an infant younger than 7 days of age.

Sodium homeostasis in term and preterm neonates. I. Renal aspects

Eighty five 24 hour sodium balance studies and creatinine clearance measurements were performed in 70 infants of gestational age 27-40 weeks and postnatal age 3-68 days. The kidney's capacity to regulate sodium excretion was a function of conceptional age (the sum of gestational age and postnatal age) and an independent effect of postnatal age was also observed--extrauterine existence increased the maturation of this function. The sodium balance was negative in 100% of infants of less than 30 weeks' gestation, in 70% at 30-32 weeks, in 46% at 33-35 weeks, and in 0% of greater than 36 weeks, and the incidence of hyponatraemia closely paralleled that of negative sodium balance. Despite a low glomerular filtration rate (GFR) urinary sodium losses were highest in the most immature babies but fractional sodium excretion (FENa) was exponentially related to gestational age. An independent effect of postnatal age could be identified on FENa but not in GFR. These findings indicate that in infants of greater than 33 weeks' gestation sodium conservation is possible because of a favourable balance between the GFR and tubular sodium reabsorption, but that below this age GFR exceeds the limited tubular sodium reabsorption capacity. The rapid increase in sodium reabsorption in the first few postnatal days seems to be due to maturation of distal tubular function, probably mediated by aldosterone. We suggest that the glomerulotubular imbalance for sodium is a consequence of the immaturity of the tubuloglomerular feedback mechanism, and we estimate that the minimum sodium requirement during the first 2 weeks of extrauterine life is 5 mmol (mEq)/kg/day for infants of less than 30 weeks' gestation and 4 mmol (mEq)/kg/day for those born between 30 and 35 weeks.

Simple estimate of creatinine clearance from plasma creatinine in neonates

Thirteen newborn infants, 8 preterm and 5 term, with either mild or serious neonatal asphyxia were studied. From the first 24 hours of life to day 13, glomerular filtration rate (GFR) estimated by creatinine clearance was compared with the values obtained using Schwartz's formula: GFR (ml/min per 1.73 m2) = 0.55 x length (cm)/plasma creatinine (mg/100 ml). Both in term and preterm infants, values of formula-calculated creatinine clearance were slightly higher than values obtained by the classical method; nevertheless the data show significant correlations, respectively r = 0.867 and r = 0.795 (P less than 0.001). This formula provides an adequate estimation of neonatal creatinine clearance (a marker for GFR) directly from plasma creatinine provided that body length is taken into consideration. The necessity for urine collection and the associated problems are thus obviated. The simplicity of Schwartz's formula permits easy monitoring of renal function, especially in neonatal asphyxia in which the risk of developing renal failure is great.

Postnatal development of renal function in pre-term and full-term infants

This study has been designed to examine the effect of gestational age (GA) on the postnatal development of renal function and has been performed in pre-term (PT) infants (GA=30-34 weeks) and in full-term (FT) infants (GA=39-41 weeks). Postnatal age has ranged from 1-35 days. From 8 hour urine samples collected after spontaneous voiding and a capillary blood sample, determinations have been made of the clearance of creatinine (CCr), the fractional excretion of beta 2-microglobulin (FE beta 2) and the fractional excretion of sodium (FENa). In some infants receiving fluid parenterally, simultaneous determinations were made of the clearance of creatinine and inulin. As judged from this study, CCr is a reliable indicator of the glomerular filtration rate (GFR). GFR was almost the same in newborn PT and FT, but from 0.3--1 week of age GFR increased significantly more rapidly in FT than in PT. From 1--5 weeks of age GFR increased at approximately the same rate in PT and FT infants. The absolute value for GFR in 3--5 weeks old infants was lower in PT than in FT. FE beta 2 was higher in PT than in FT infants during the entire first month of life and FENa was higher in PT than in FT infants during the first week of life, suggesting a glomerular tubular imbalance at least at the level of the proximal tubule in PT infants. It is concluded that different stages of maturation will alter the preconditions for the renal adaptation to extrauterine life during at least the first month of life. Therefore special attention must be paid to the limited renal function in PT during their entire first month of life.

Acute effects of albumin infusion on blood volume and renal function in premature infants with respiratory distress syndrome

The purpose of this study was to determine the acute effects of albumin infusion on blood volume and renal function in preterm infants with RDS and low total serum protein values. Ten infants (gestational age 28 to 36 weeks, body weight 0.88 to 2.46 kg) were given albumin 1 gm/kg (as 25% iv solution) over a ten-minute period. Within ten minutes after infusion was completed, total serum protein concentration, colloid osmotic pressure, and blood volume rose significantly while hematocrit fell from their preinfusion levels (P < 0.0005). Mean arterial blood pressure showed a smaller and less clear-cut increase (P < 0.05). Creatinine clearance rose significantly with infusion; even though preinfusion clearances correlated poorly with gestational age (r = 0.43), postinfusion clearances correlated well (r = 0.92). No significant rises in urinary flow rate Uosm/Posm, or free-water clearance were observed. These results indicate that albumin infusion acutely increases both blood volume and glomerular filtration in premature infants with RDS.

Effect of fluid administration on the development of symptomatic patent ductus arteriosus and congestive heart failure in premature infants

We studied 170 premature infants with birth weights between 751 and 2000 g in a randomized sequential trial comparing "high" and "low" volumes of fluid intake. Beginning on the third day of life, the low-volume group received only enough water to meet average estimated requirements, and the high-volume group received an excess of at least 20 ml per kilogram of body weight per day (mean excess, 47 ml per kilogram per day). Sequential analysis showed that the risk of patent ductus arteriosus with congestive heart failure was greater in infants receiving the high-volume regimen. Thirty-five of 85 infants in the high-volume group acquired murmurs consistent with patent ductus arteriosus, and 11 of these 35 had congestive heart failure. Only nine of 85 infants in the low-volume group had murmurs consistent with patent ductus arteriosus, and two of these nine had congestive heart failure. More cases of necrotizing enterocolitis also occurred in the high-volume group. We conclude that limitation of fluid intake to amounts estimated to meet requirements for excretion, insensible loss, and growth can reduce the risks of patent ductus arteriosus and congestive heart failure in premature infants.