Chapter Contents
Introduction 331
Postnatal alterations in body water distribution 331
Non-renal influences on water balance 332
Clinical implications of postnatal and developmental changes: constructing a fluid prescription 334
Monitoring fluid balance 336
Common clinical problems 337
Hypernatraemia and hyponatraemia 337
Appropriate and inappropriate antidiuretic hormone secretion 338
Common pharmacological influences on fluid balance 340
Introduction
This chapter will discuss the principles of newborn electrolyte and water balance and present a practical approach to clinical management. Renal function, clinical monitoring and renal disease are discussed in Chapter 35 part 1 .
Postnatal alterations in body water distribution
The newborn baby’s body is largely composed of water. The extracellular compartment forms around 65% of bodyweight at 26 weeks’ gestation, reducing to 40% at term and 20% by the age of 10 years ( Fig. 18.1 ) ( ). Superimposed on this gradual reduction with age is a more abrupt contraction that occurs shortly after birth ( ; ; ; ; ; ). This is due to loss of the interstitial fluid component of the extracellular fluid (ECF) compartment and is closely related to cardiopulmonary adaptation. The contraction of ECF accounts, at least in part, for early postnatal weight loss and occurs rapidly in healthy babies, but is delayed in babies with respiratory distress syndrome (RDS) ( ).
Several studies suggest that the contraction of the ECF is triggered by atrial natriuretic peptide released in response to increased atrial stretch as pulmonary vascular pressure falls ( ; ; ; ) and left atrial venous return increases. The intravascular compartment may also be acutely expanded during birth by the reabsorption of lung liquid and the effect of a variable placental transfusion. As the timing of the diuresis/natriuresis is to some extent related to the fall in pulmonary vascular pressure, it is not surprising that attempts to improve the course of RDS with diuretics have not shown benefit ( ).
A corollary of the isotonic loss of ECF is that net water and sodium balance in the first days after birth is negative ( Fig. 18.2 ) ( ). This is borne out by the observation that in newborn babies an increase in sodium intake leads to an increase in excretion ( ; ; ) until contraction of the extracellular compartment occurs. Sodium balance then becomes positive, commensurate with growth. However, preterm babies have a limited capacity to excrete a sodium load (see Ch. 35 part 1 ), so that, despite increasing excretion in response to an increase in intake, sodium retention readily occurs ( ; ). If there is concurrent restriction in the intake of water, these babies readily become hypernatraemic ( ). If a more liberal intake of water accompanies the intake of sodium, isotonic expansion of the extracellular compartment is the result, evidenced by weight gain at a time when weight loss is to be expected. In most babies, this cumulative positive balance is lost later, so that the normal postnatal change in body water distribution occurs but is delayed ( ). Delayed loss of extracellular water increases the risks and severity of respiratory illness in the newborn ( ; ; ; ; ) and weight gain in the first days after birth, in babies with RDS, is associated with an increased risk of developing chronic lung disease ( ).
Present-day management involving antenatal steroid therapy, postnatal surfactant, good intrapartum care and immediate postnatal stabilisation has modified the natural history of RDS so that cardiopulmonary adaptation generally occurs rapidly and the postnatal loss of ECF occurs imperceptibly, as in a full-term baby. In the pre-surfactant era, the abrupt onset of diuresis/natriuresis ( ) heralded the improvement in respiratory function.
Non-renal influences on water balance
Insensible water loss occurs through the skin, respiratory tract and in stool. Stool water loss is usually small and usually less than 5 ml/kg/24 h in the first days after birth. The upper respiratory tract warms and humidifies inspired gases, and full saturation (44 mg/l) is achieved by the mid-trachea. If the upper respiratory tract is bypassed with an endotracheal tube, respiratory water loss must be reduced by adequate humidification of inspired gases. Humidification is also advisable when delivering low-flow oxygen and during continuous positive airway pressure.
The skin is an important regulator of water balance in extremely preterm babies. Transepidermal water loss reflects skin immaturity and may be considerable, even exceeding urine volume. Sodium is not lost through the skin, because babies born below 36 weeks’ gestation do not sweat, though this develops within the first 2 weeks after birth ( ).
The stratum corneum of the skin consists of overlapping dead epidermal cells filled with keratin that form a barrier to water loss. Keratinisation begins at around 18 weeks’ gestation, but the fetal epidermis is still very thin at 26 weeks and the stratum corneum is barely visible. During the last trimester, the epidermis and stratum corneum thicken and keratinisation becomes more marked ( ). Each millilitre of water that evaporates from the skin is accompanied by the loss of 560 calories of heat, and so it is difficult to keep a baby with a high transepidermal water loss warm. This is the principle underlying the practice of placing extremely preterm babies in a plastic bag at delivery for thermal protection (see Ch. 15 ).
Skin maturation, unlike the maturation of renal function, is accelerated by birth, but not by antenatal steroid therapy ( ). Transepidermal loss falls exponentially with increasing gestational and postnatal age ( Fig. 18.3 ) ( ; ; ). After 32 weeks’ gestation, water loss through the skin is around 12 ml/kg/day ( ). Transepidermal loss is influenced by ambient humidity, skin integrity, environmental and skin temperature, air speed and radiant heat sources, including phototherapy ( Tables 18.1 and 18.2 ). Radiant heat sources can increase transepidermal water loss by a factor of up to 0.5–2 ( ). A high environmental humidity reduces transepidermal water loss, an effect that is most marked in the most immature infants ( Fig. 18.4 ) ( ). Insensible water loss can be reduced to less than 40 ml/kg/day in infants weighing less than 1000 g if ambient humidity is maintained above 90% ( ). Humidification is easier with an incubator, particularly if double-walled, but a low-cost stratagem is to use bubblewrap or other plastic sheeting and a humidified body box to achieve a high humidity microenvironment immediately around the baby. The degree of humidification that can be achieved without causing condensation (‘rain-out’) on the inner wall of the incubator is dependent on the temperature gradient across the incubator wall. Condensation predisposes to skin maceration and should be avoided. Draughts should be eliminated. Stripping of the stratum corneum and deeper abrasions of the skin can be reduced by using non-abrasive tape such as Micropore, neonatal electrodes and skin protectants, prior to affixing urine bags and electrodes. Water-impermeable barriers, such as soft paraffin, or topical ointments reduce transepidermal water loss ( ; ) but concern about increased infection and difficulty in fixing electrodes have limited their use. Adequate provision of fluid is also necessary, using a system that allows the intravenous glucose delivery rate to be altered independently of fluid volume in order to avoid hyperglycaemia ( ), osmotic diuresis and worsening of the situation. The extent to which transepidermal water loss is reduced, hypernatraemic dehydration and hyperglycaemia avoided and temperature stability maintained is an index of the quality of nursing and medical care and a useful measure for audit.
| INCREASED LOSS | DECREASED LOSS | INCUBATOR |
|---|---|---|
| Lower gestational age | Clothing | Body box |
| Lower postnatal age | High humidity | Plastic blanket |
| Denuded/broken skin | Good skin care | |
| Increased skin temperature | Topical ointments and emollients | |
| Increased activity | Humidification of inspired gases | |
| Increased environmental temperature | High humidity | |
| Radiant heat sources | ||
| Radiant warmers | ||
| Phototherapy units | ||
| Draughts | ||
| Excessive crying |
| GESTATIONAL | POSTNATAL AGE (days) | ||||||
|---|---|---|---|---|---|---|---|
| Age (weeks) | n | 0–1 | 3 | 7 | 14 | 21 | 28 |
| 25–27 | 9 | 129 ± 39 | 71 ± 9 | 43 ± 9 | 32 ± 10 | 28 ± 10 | 24 ± 10 |
| 28–30 | 13 | 42 ± 13 | 32 ± 9 | 24 ± 7 | 18 ± 6 | 15 ± 6 | 15 ± 6 |
| 31–36 | 22 | 12 ± 5 | 12 ± 4 | 12 ± 4 | 9 ± 3 | 8 ± 2 | 7 ± 1 |
| 37–41 | 24 | 7 ± 2 | 6 ± 1 | 6 ± 1 | 6 ± 1 | 6 ± 0 | 7 ± 1 |
Clinical implications of postnatal and developmental changes: constructing a fluid prescription
Didactic recommendations are unnecessary. With an understanding of the principles involved, a prescription tailored to the needs of the baby can be constructed. Fluid management in common situations in neonatal intensive care is summarised in Tables 18.3–18.5 .
| GESTATIONAL AGE (weeks) | BIRTHWEIGHT (kg) | APPROXIMATE TRANSEPIDERMAL WATER LOSS (ml/kg/24 h) | ALLOWANCE FOR URINE OUTPUT (ml/kg/24 h) | ESTIMATED INTAKE RANGE (ml/kg/24 h) | SUGGESTED STARTING VOLUME (ml/kg/24 h) † |
|---|---|---|---|---|---|
| <27 | <1.0 | 120 | 30–60 | 150–180 | 150 ‡ |
| 27–30 | 1.0–1.5 | 40 | 30–60 | 70–100 | 90 |
| 31–36 | 1.5–2.5 | 15 | 30–60 | 45–75 | 60 |
| >36 | >2.5 | 10 | 30–60 | 40–70 | 60 |
* At higher ambient humidities, transepidermal water losses will be reduced and requirements will be lower.
† Once sustained weight loss of at least 5% is achieved, proceed to the intravenous volume necessary to support nutritional goals without stepwise increments.
‡ A cautious approach, commencing at the lower end of the estimated requirement, is recommended.
| GOAL | MEANS OF ACCOMPLISHING GOAL |
|---|---|
| Minimise insensible water loss | Provide high humidity around infant using incubator or body box and plastic sheeting; minimum handling; meticulous skin care; eliminate draughts |
| Facilitate loss of extracellular fluid | Minimise intravenous sodium administration until postnatal diuresis/natriuresis marked by weight loss is underway |
| Maintain glucose homeostasis | Use a variable glucose delivery system if glucose tolerance is unstable ( ) |
| Optimise nutritional support | Commence parenteral and minimal enteral nutrition on day 1; stepwise increments are unnecessary in stable infants |
| Maintain renal perfusion | Monitor blood pressure, core–peripheral temperature gap, capillary refill time, urine output, cardiac function and central venous pressure; use volume and inotropic support as necessary |
| PROBLEM | ACTION |
|---|---|
| Respiratory distress syndrome | Initial infusion volume determined by anticipated insensible water loss; delay maintenance intravenous sodium administration until postnatal diuresis/natriuresis marked by weight loss is underway |
| Patent ductus arteriosus | Routine fluid restriction inappropriate as this will compromise nutrition; fluid-restrict if there is evidence of heart failure; indometacin/ibuprofen toxicity is exacerbated by dehydration |
| Severe birth asphyxia at term | Anticipate possibility of renal failure; 0.9% NaCl may be necessary for initial resuscitation and restoration of renal perfusion; restrict maintenance intake to 20–30 ml/kg/day until renal function can be assessed; central vascular access likely to be necessary for infusion of hypertonic glucose |
| Chronic lung disease | Avoid prolonged periods of fluid restriction; poor nutrition will worsen prognosis; diuretic-induced chronic sodium depletion will further compromise growth |
| Necrotising enterocolitis | Intestinal fluid loss may be considerable; interpretation of changes in body weight is difficult; profound intravascular volume depletion may be present without weight loss; monitor toe–core temperature gap; provide isotonic volume replacement (0.9% NaCl) if temperature gap exceeds 3°C |
| Preoperative | Fluid intake should not be reduced preoperatively; the infant should be well hydrated |
| Intraoperative | Minimise transepidermal water loss; administer glucose 5% or 10% in 0.9% NaCl during surgery |
| Postoperative | Unrecognised hypovolaemia is common and may contribute to postoperative hyponatraemia; infuse glucose 5% or 10% in 0.9% NaCl for at least 12 hours following surgery |
The first days after birth
The goal of fluid management during the period of postnatal adaptation is to permit an isotonic contraction of the extracellular compartment and a brief period of negative sodium and water balance.
Water should be provided in an intake sufficient to allow the excretion of a relatively small initial renal solute load ( ) and, in very preterm babies, to maintain tonicity in the face of initially high, but rapidly falling, transepidermal losses. Estimate the likely magnitude of insensible water loss using the information presented in Figures 18.3 and 18.4 and Table 18.2 . A rational initial intravenous fluid volume would be the sum of an allowance for urinary water of 30–60 ml/kg/day plus estimated insensible water loss ( Table 18.3 ) ( ). Given present-day care and an ambient humidity in excess of 50%, this usually equates to around 100 ml/kg/day for babies below 1500 g. If insensible water losses are predicted to be high, for example during the period immediately after delivery when the baby is being stabilised, start with a high intake but reduce this once a high-humidity environment is established. As it is not possible to predict precisely transepidermal water loss, the integrity of renal function or the timing of the postnatal natriuresis/diuresis, the adequacy of the estimate must be assessed within 6–8 hours.
The immediate administration of maintenance sodium in parenteral fluid is unwarranted and adversely affects respiratory outcome even in infants exposed to antenatal steroids. compared sodium restriction for 5 days after birth with immediate administration of 3–4 mmol/kg/day in a blind trial. Water was prescribed independently. In the latter group sodium balance was positive on the first day after birth and there was a significantly higher incidence of bronchopulmonary dysplasia (BPD). , in a blind, controlled trial, randomised infants born at 25–30 weeks’ gestation to receive a parenteral sodium intake of 4 mmol/kg/day from the first day after birth or when a weight loss of 6% had occurred. ECF volume was measured at birth and on day 14. A significant reduction in ECF volume was observed in the group who received the delayed sodium intake, in contrast to the early-intake group, in whom no reduction was seen. By the end of the first week, 35% of babies in the delayed-intake group and 8.7% of the early-intake group, and by 28 days after birth 40% of the delayed-intake group compared with 18% of the early-intake group, no longer required additional oxygen. There was no difference between the groups in the rate of reduction in pulmonary artery pressure ( ), suggesting that the poorer respiratory outcome in the early-intake group was not attributable to delayed cardiopulmonary adaptation but rather to persistent expansion of the extracellular compartment and delayed clearance of pulmonary interstitial fluid.
Intravenous sodium in maintenance fluid should be avoided until the physiological postnatal diuresis/natriuresis ( ; ) is underway, marked clinically by the onset of weight loss ( ; ; ). There is no ‘correct’ figure for postnatal weight loss, as hydration at birth is variable ( ) and birthweight does not correlate closely with extracellular water volume ( ). Early postnatal weight loss reflects both the loss of body water and the loss of body solids. With improved nutritional support for preterm babies, early postnatal weight loss is diminished, though body water loss remains the same. In a study comparing healthy preterm babies with a group with RDS, during the first week after birth, both groups lost an identical amount of body water, namely 10% of the total body water content at birth. However, the healthy babies lost a maximum of 5.9% of birthweight, in contrast to a loss of 8.6% in the RDS group. This was because the healthy babies received a higher energy intake and gained solids to a significantly greater extent ( Fig. 18.5 ) ( ).
Glucose delivery should commence at around 7 mg/kg/min. Increasingly this is delivered with amino acids as parenteral nutrition immediately after birth. When glucose requirements are unstable the use of 5% and 50% glucose solutions delivered through a Y connection allows the glucose delivery rate and the volume infused to be readily altered independently ( ) ( Table 18.6 ).
| GLUCOSE DELIVERY RATE (mg/kg/min) | 5% GLUCOSE INFUSION RATE (ml/kg/h) | 50% GLUCOSE INFUSION RATE (ml/kg/h) | TOTAL VOLUME (ml/kg/24 h) |
|---|---|---|---|
| 7 | 2.3 | 0.6 | 70 |
| 5.5 | 0.3 | 140 | |
| 5 | 2.6 | 0.3 | 70 |
| 5.8 | 140 |
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