Introduction

Children can present for various intracranial surgeries, which include intracranial tumors, vascular malformations [arteriovenous (AV) malformations, Moyamoya disease], and spine surgeries (e.g., scoliosis, release of tethered cord, laminectomies, etc.). Neonates and infants can present for craniosynostosis correction, congenital hydrocephalus requiring shunt surgeries, etc.

It should be kept in mind that children are not “small adults.” Their body physiology and response to fluids and blood loss is manifested in an entirely different pattern compared with adults.

The differences between children and versus adults are as follows:

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  Body fluid compartments : Water accounts for 50–80% of the human body by weight. The variation in water content depends on the tissue type: adipose tissue contains only 10% water, whereas muscle contains 75% water. Total body water (TBW) is composed of intracellular fluid (ICF) (two-thirds) and extracellular fluid (ECF) (one-third). There is a higher TBW content in a full-term neonate (75%) and a 1-year-old infant (65%) as compared to adults (60%). ICF represents about two-thirds of the TBW, which is equivalent to 30–40% of total body weight. The ECF decreases with age, mainly as a result of loss of water in ECF. Thus the proportion of ECF is much greater than that of ICF in preterm infants and reaches 60% of TBW at term.

  
  
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  Glomerular filtration rate (GFR) : At birth, renal function is not completely mature, although all nephrons of the mature kidneys are formed by 36 weeks’ gestation during healthy intrauterine life. The renal size increases by cellular hyperplasia followed by hypertrophy during the postnatal period. The GFR of a full-term newborn infant averages 40.6 ± 14.8 mL/min per 1.73 m ² and increases to 65.8 ± 24.8 mL/min per 1.73 m ² by the end of the second postnatal week. GFR reaches adult levels after 2 years of age. Premature newborns have a lower GFR that increases more slowly than that in full-term infants. The low GFR at birth is attributed to the low systemic arterial blood pressure, high renal vascular resistance, and low ultrafiltration pressure, together with decreased capillary surface area for filtration.

  
  
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  Tendency to hyponatremia : Neonates and infants are obligate sodium losers due to their impaired renal-concentrating ability and are more prone to hyponatremia than adults. This tendency to hyponatremia is seen more in preterm than in full-term infants. Preterm infants have a prolonged glomerulotubular imbalance, so that GFR is high relative to tubular capacity to reabsorb sodium. In addition, there is a structural immaturity of the proximal convoluted tubule and incomplete development of the transport system responsible for conserving Na, all aggravating the loss of sodium.

  
  
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  The tubular mechanisms involved in the excretion of organic acids are poorly developed in neonates. The reasons are possibly low GFR, immaturity of the systems providing energy for transport, and a low number of transporter molecules. Additionally, premature infants have a lower renal threshold for bicarbonate. Hence, they are more prone to acidosis and may need base supplementation.

  
  
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  Development of dehydration : The kidneys’ ability to concentrate urine is lower at birth, especially in premature infants. After water deprivation in the full-term newborn, urine concentrates to only 600–700 mOsm/kg, or 50–60% of maximum adult levels. The major cause for the reduced concentration of urine in the neonate is the hypotonicity of the renal medulla in addition to lower sensitivity of the collecting duct to antidiuretic hormone (ADH). Thus, pediatric patients cannot tolerate dehydration.

  
  
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  In addition, neonates, infants, and children have a smaller blood volume and due to a higher resting sympathetic tone, may not manifest signs and symptoms of hemodynamic instability until a significant percentage of their blood volume is lost.

Fluid and Electrolyte Choices

The relative relaxation of “nil per os” guidelines for children presenting for elective surgeries has, in part, eliminated the dehydration encountered preoperatively. Clear liquids are now allowed up to 2 h prior to elective surgeries. In general, the perioperative fluid management for an otherwise healthy child older than 1 year coming in for an elective intracranial or spine surgery would follow the 4-2-1 rule per body weight per hour, which is intended to cover the deficits and maintenance. The 4-2-1 rule means 4 mL/kg for the first 10 kg, 2 mL/kg for the next 10 kg, and 1 mL/kg after that.

In 2007, Holliday pointed out several problems with applying the original 4-2-1 rule to acutely ill children and revised the approach to fluid therapy in children. They point out that there is dysregulation of ADH, which is a hallmark of critical illness because ADH secretion is affected by a variety of nonosmotic factors such as pain, stress, mechanical ventilation, and many medications. As a result, the choice of intravenous fluid and the rapidity of deficit replacement must be approached with care, and isotonic fluids are preferred to hypotonic fluids. Additionally, most ongoing volume losses in the perioperative period are isotonic, consisting of shed blood and interstitial fluids. They now recommend that when a child (who is without significant heart or kidney disease) presents with marginal to moderate hypovolemia (e.g., after fasting for surgery), 20–40 mL/kg of isotonic fluids should be given during surgery and the postanesthesia care unit stay (as rapidly as 10–20 mL/kg/h). Clinical judgment must always allow for modification of these recommendations if indicated for an individual child.

In deciding the volume and composition of intraoperative fluids, one should take into consideration that both premature and term neonates have a limited capacity to excrete K possibly because of distal tubular insensitivity to aldosterone, that hyponatremia may develop after administration of large volumes of hypotonic fluids, and that fluid excretion may be limited mainly because of low GFR. Stress may cause profound reduction in GFR in premature and term neonates through release of various extrarenal vasoactive and hormonal substances, further disturbing fluid and electrolyte homeostasis. The higher body content of water and the higher metabolic rate, as well as a propensity to metabolic acidosis and hypocalcemia, in premature newborns are other important factors in deciding the volume and composition of intraoperative fluids. Dextrose-containing solutions must be added for additional maintenance in neonates and premature infants due to the risk of hypoglycemia during prolonged surgeries, due to impaired body glucose homeostasis.

Type of Fluids for Perioperative Administration in Pediatric Patients

The compositions of commonly used fluids are given in Table 37.1 .

Assuming normal plasma osmolality of 275–290 mOsm/L, it is worthy to note that 0.9% NaCl (normal saline, NS) is slightly hypertonic to plasma and that lactated Ringer solution is isotonic (273 mOsm/L), although slightly hyponatremic (130 mEq/L). For dextrose-containing solutions, added osmolality is rapidly dissipated as sugar is metabolized, resulting in increased volumes of free water. Therefore, administration of 5% dextrose in water is ultimately equivalent to administration of free water and could potentially contribute to cerebral edema in a situation of raised intracranial pressure (ICP).

The controversy regarding the perioperative use of colloid versus crystalloid fluid replacement remains unresolved. Colloids like 5% albumin and synthetic colloids like the hydroxyethyl starches are gaining popularity in pediatrics. Human albumin is available for infusion in 5% and 25% solutions. Albumin has a molecular weight of approximately 69,000 and constitutes 50% of the total plasma proteins by weight and is responsible for 80% of the colloid osmotic pressure of plasma. Albumin is considered as an ideal volume expander in neurosurgical cases due to the aforementioned reasons and due to its lack of side effects on coagulation. However, it is expensive. In the very large randomized SAFE (saline versus albumin fluid evaluation) trial, it was found that there was no difference in the 28-day survival between albumin and saline resuscitation for patients in the intensive care. However, the subset of patients with TBI (traumatic brain injury) who received fluid resuscitation with albumin had higher mortality rates than those who received fluids with crystalloids ^. Thus the role of albumin is very controversial. The various starch-containing solutions should be used cautiously in neurosurgery because, in addition to a dilutional reduction of coagulation factors, they interfere directly with both platelets and the factor VIII complex. Synthetic colloids such as these should be considered in surgical patients who demonstrate the need for aggressive intraoperative fluid resuscitation, such as children with large-volume blood loss or excessive insensible losses.

The routine intraoperative use of glucose-containing solutions has also been a subject of debate. As a rule, operative stress evokes physiologic responses that increase serum glucose. In practice, therefore, hypoglycemia is seldom a problem in healthy children when glucose is omitted from perioperative IV fluids. Indeed, the risk should be particularly small if the period of fasting is limited to less than 10 h. At the same time, rapid administration of dextrose solutions may certainly produce acute hyperglycemia and hyperosmolality. Therefore, glucose-containing solutions should not be used to replace fluid deficits, third space losses, or blood losses. The potential association of larger cerebral infarct size with hyperglycemia (i.e., blood glucose values in excess of 250 mg/dL) during ischemia is of particular concern.

Fluid Management in Pediatric Neurosurgery

The blood–brain barrier (BBB) is a highly selective permeability barrier that separates the circulating blood from the brain ECF in the central nervous system. The BBB is formed by brain endothelial cells, which are connected by tight junctions with an extremely high electrical resistivity of at least 0.1 Ωm. Under normal conditions, the BBB allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids, which are crucial to neural function.

Disruption of the BBB by underlying pathologic processes, trauma, or surgery predisposes neurosurgical patients to cerebral edema, which may be exacerbated by excessive administration of intravenous fluids. Hence, meticulous management of fluids and blood products to minimize cerebral edema, ensure cerebral perfusion, and maintain water and sodium homeostasis and serum glucose concentration is the cornerstone of pediatric neuroanesthesia. All children should have secure, large-bore intravenous access, and blood products should be available along with the means for warming the blood.

Crystalloid solutions are commonly administered in the perioperative period. Lactated Ringer solution tends to be slightly hypotonic because its osmolality is 273 mOsm/L (normal: 285–290 mOsm/L). Normal saline, which is slightly hypertonic (308 mOsm/L), is the fluid of choice because reduction of serum osmolality is not desirable. However, rapid infusion of large volumes of normal saline has been associated with a hyperchloremic nonanion gap metabolic acidosis. If there are large fluid requirements during surgery, alternating bags of lactated Ringer solution with normal saline can minimize the risk of hypernatremia and acidosis and avoid hypo-osmolality.