Objective . We had shown previously that preterm infants undergo three phases of fluid and electrolyte homeostasis; prediuretic, diuretic, and postdiuretic. The objectives of the present study were: (1) to determine whether infants even more immature and infants cared for under thermal environmental conditions different from those previously studied also undergo these three phases; and (2) to relate these phases to changes in renal function.Methods . Consecutive, timed urine collections were made during the first 5 days of life in 32 infants with birth weights of 1000 g or less. Infants were cared for in radiant warmers for 24 hours and then transferred to nonhumidified incubators. Diuresis was defined as urine flow rate (V) of 3 mL or more/kg per hour and weight loss of 0.8 g or more/kg per hour. The physiologic relationships among water and sodium balance, insensible water loss, arterial blood pressure, and renal function were made during the three phases.Results . Twenty-eight (87%) of the 32 infants underwent the three homeostatic phases. The median ages of onset and cessation of diuresis were 25 and 96 hours, respectively. There was no correlation between onset of diuresis and change of thermal environment. During the prediuretic phase, V averaged 1.6 mL/kg per hour, and 17 of 28 infants had at least one collection period in which V was less than 1 mL/kg per hour; urinary sodium excretion was 0.1 mEq/kg per hour; the glomerular filtration rate (GFR) was 0.22 mL/kg per hour; fractional excretion of sodium (FENa) was 6.2%; and urine osmolality was dilute (221 mOsm/kg). During the diuretic phase, V and sodium excretion more than tripled; GFR and FENa doubled; and there was no change in urine osmolality. During postdiuresis, V and Na excretion decreased to values intermediate between the prediuretic and diuretic phases, and FENa fell to prediuretic levels, but there was no change in GFR or urine osmolality. There was poor correlation between blood pressure and GFR. Insensible water loss was high and variable during all phases, exceeding 190 mL/kg per day in the smallest infants.Conclusions . Extremely low birth weight infants manifest three phases of fluid and electrolyte homeostasis, as do more mature infants, independent of thermal environment. Diuresis and natriuresis are the result of abrupt increases in GFR and FENa. We speculate that this may be the result of expansion of the neonatal extracellular space as fetal lung fluid is reabsorbed.
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机译：目标。先前我们已经证明，早产儿经历了液体和电解质稳态的三个阶段。利尿前，利尿和利尿后。本研究的目的是：（1）确定是否有更多未成熟的婴儿和在不同于先前研究的热环境下接受照料的婴儿也经历了这三个阶段; （2）将这些阶段与肾功能的变化联系起来。在出生后的头5天，对32例出生体重在1000 g或以下的婴儿连续进行定时尿液收集。将婴儿放在辐射加热器中护理24小时，然后将其转移到未加湿的培养箱中。利尿定义为尿流率（V）为每小时3 mL或以上，体重减轻为每小时0.8 g或以上。在这三个阶段中，建立了水和钠平衡，感觉失水，动脉血压和肾功能之间的生理关系。 32名婴儿中有28名（87％）经历了三个稳态阶段。利尿开始发作和停止发作的中位年龄分别为25小时和96小时。利尿发作与热环境变化之间没有相关性。在利尿前阶段，V平均为每小时1.6 mL / kg，在28个婴儿中有17个至少有一个采集期，其中V小于每小时1 mL / kg。尿钠排泄量为每小时0.1 mEq / kg;肾小球滤过率（GFR）为每小时0.22 mL / kg;钠（FENa）的排泄率为6.2％;尿渗透压被稀释（221 mOsm / kg）。在利尿阶段，V和钠的排泄量增加了两倍多; GFR和FENa加倍;尿渗透压没有变化。利尿后，V和Na的排泄量降低至利尿前和利尿期之间的中间值，FENa降低至利尿前的水平，但GFR或尿渗透压没有改变。血压与GFR之间的相关性较差。无知觉失水率很高，并且在所有阶段都有变化，最小的婴儿每天的失水量超过190 mL / kg。出生时体重极低的婴儿以及更多成熟的婴儿表现出液体和电解质稳态的三个阶段，而不受热环境的影响。利尿和利尿是GFR和FENa突然增加的结果。我们推测这可能是由于胎儿肺液被重吸收导致新生儿细胞外空间扩大的结果。
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