The riddle of the high Pcr levels in term and particularly in preterm newborns seems to be solved. Once the umbilical cord is severed, the perfect intrauterine maternal-fetal biochemical balance is disturbed. Thereafter, the already transferred exogenous, adult-level creatinine will rapidly disappear in the first urine specimens passed by the now autonomous newborn infant. A new steady state is achieved in due time, based on independent neonatal factors. One of these factors is the unusual occurrence of tubular creatinine reabsorption. We hypothesize that this latter temporary phenomenon is attributable to back-flow of creatinine across leaky immature tubular and vascular structures. With time, maturational renal changes will impose a barrier to creatinine. From that point onwards, total body muscle mass, glomerular filtration rate, and tubular secretion will in health determine the Pcr level of the individual. plasma creatinine, tubular handling of creatinine, newborn, premature infants.

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Acute renal failure (ARF) is a frequent clinical condition in neonatal intensive care units. Plasma creatinine concentrations should be used with some caution for ARF diagnosis in the first days of life. An intravenous fluid challenge allows differentiation of prerenal failure and intrinsic renal failure. All clinical conditions associated with hypovolemia, hypoxemia, and hypotension in the newborn infant may lead to renal insufficiency, the leading causes being perinatal anoxia-ischemia and sepsis. The initial treatment mainly relies on correction of hypotension, acidosis, and hypoxemia, in order to reduce renal vasoconstriction and improve renal perfusion. If necessary, the main renal replacement therapy is usually peritoneal dialysis even if skilled medical and nursing personnel are available in some neonatal intensive care units to perform hemofiltration and hemodiafiltration safely.

This review discusses new aspects of normal and abnormal renal development that expand insight into the adaptation of the neonatal kidneys to the stress of extrauterine life. Highlighted are some pitfalls in measuring glomerular filtration rate in the neonate mainly caused by postnatal fluctuations in serum creatinine levels. Serum creatinine levels are correlated with the authors' recent finding of tubular reabsorption of creatinine in the immature neonatal kidney. Renal maldevelopment in premature and small-for-date babies has been shown related to serious medical problems in adult life, including hypertension. This finding presents the pediatrician with a new role in the time-honored vocation of preventing disease. Mutations in several genes may be responsible for most cases of congenital or hereditary renal aberrations. Two renal disorders, congenital nephrotic syndrome and neonatal acute renal failure, and one form of treatment modality of newborn infants, renal replacement therapy, are discussed in detail. These conditions are rare in general pediatric practice, but they illustrate some of the new developments in the renal care of the newborn. A word of caution is offered about the use of nonsteroidal anti-inflammatory drugs during pregnancy and the newborn period. All nonsteroidal anti-inflammatory drugs administered indirectly to the unborn fetus and directly to the young newborn impair renal structure (fetus) and function (both fetus and newborn). The new data have been obtained with genetic and molecular biology techniques and with established methods of developmental renal physiology. A better understanding of the pathogenesis of neonatal renal disorders will result in new diagnostic procedures and improved preventive and therapeutic possibilities relevant to the neonate with a renal disorder.

The healthy term, and particularly the premature infant, is born with a very low glomerular filtration rate (GFR), controlled by a delicate balance of intrarenal vasoconstrictor and vasodilator forces. Vasoactive disturbances can easily further reduce the already low GFR. The newborn infant is thus prone to develop vasomotor nephropathy (VMNP) or acute renal failure (ARF). The main causes for ARF at this young age are prerenal mechanisms, and include hypotension, hypovolemia, hypoxemia perinatal asphyxia, and neonatal septicemia. Other causes include the administration of angiotensin converting enzyme inhibitors, indomethacin and tolazoline. The most-important factors governing the ultimate renal prognosis are the severity of the underlying disorder, the rapidity of an accurate diagnosis, prompt treatment, and avoidance of severe iatrogenic complications. The immediate treatment is of particular importance in VMNP, i.e., prerenal ischemic ARF, and consists of correcting abnormalities in fluid homeostasis and reduction of the complications of the acute azotemic state (uremia, hyperkalemia, acidosis, and hypertension). In severe and prolonged (established) ARF, temporary dialysis therapy may be indicated. Prerenal ARF with oliguria or anuria warrants immediate volume resuscitation. Special attention should be given to infants with congestive heart failure (CHF). The sick neonate with persistent oliguria and CHF should be treated with intravenous dopamine. Furosemide (FM) is the second line of therapy for babies with indomethacin-induced ARF. In most other conditions, the therapeutic effect of FM is limited to a transient increase in urine flow, without improving basic renal function. The special conditions of the maturing kidney have to be appreciated in order to protect babies from undue renal injury. With the increasing knowledge of the mechanisms governing the development of ARF, progress has been made in the development of new treatment modalities. For example theophylline, calcium antagonists, ATP-MgCl2, thyroxine, and a variety of cytokines may in the near future be used to prevent or ameliorate VMNP and/or recently established ARF. With a combination of time-honored and new therapeutic strategies, there may well be a brighter future for neonates with vasomotor, prerenal, ischemic ARF.

The plasma creatinine concentration is elevated at birth and decreases concomitantly with the rapid increase in glomerular filtration rate that occurs in the first postnatal weeks. The velocity of these changes was assessed during the first 3 wk of life of 66 term and preterm infants. The plasma creatinine concentration, creatinine clearance, and sodium fractional excretion were measured serially at weekly intervals, starting 1-4 d after birth [mean = 1.5 -c 0.8 (SD) dl. Premature infants were separated into three groups according to their birth weight: group 1, 1001 to 1500 g; group 2, 1501 to 2000 g; and group 3, 2001 to 2500 g. Group 4 included 28 term infants (mean birth weight = 3165 ? 78 g). Mean gestational ages in the preterm groups were 31.3, 32.8, and 34.4 wk in groups 1, 2, and 3, respectively. The plasma creatinine concentration on d 1.

The acute renal effects of hypoxemia and the ability of theophylline to prevent these effects were assessed in anesthetized and mechanically-ventilated newborn and adult rabbits. Renal blood flow (RBF) and glomerular filtration rate (GFR) were determined by the clearance of para-aminohippuric acid and inulin, respectively. Each animal acted as his own control. In 14 newborn rabbits (group 1), hypoxemia was significantly associated with a fall in GFR (-22 +/- 6%) and filtration fraction (-17 +/- 3%) and with an increase in renal vascular resistance (+13 +/- 6%). Hypoxemia also induced a significant decline in GFR (-27 +/- 6%) and RBF (-29 +/- 6%) in 7 adult rabbits (group 3). Intravenous theophylline (0.5 mg/kg) completely prevented the hypoxemia-induced changes in GFR, filtration fraction (FF) and renal vascular resistance (RVR) in 8 newborn rabbits (group 2). An intravenous dose of 2.4 mg theophylline given after the induction of hypoxemia partially reversed the drop in GFR in adult rabbits (group 3). Separate renal functions were studied in 8 additional adult rabbits (group 4). Low-dose theophylline (27 micrograms/min) infused intra-arterially in the left kidney partially prevented the hypoxemia-induced decline in urine flow rate, GFR, RBF, FF as well as the increase in renal vascular resistance. The beneficial effects of theophylline could be mediated by its adenosine antagonistic properties.

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