Perioperative monitoring

In the 1940s and 1950s, the techniques of pediatric anesthesia, as well as the skills of those using and teaching them, evolved more as an art than as a science, as ^†Dr. Robert Smith vividly and eloquently recollects through his firsthand experiences in his chapter on the history of pediatric anesthesia (see Chapter 41, History of Pediatric Anesthesia, as updated by Mark Rockoff). The anesthetic agents and methods available were limited, as was the scientific knowledge of developmental differences in organ-system function and anesthetic effect in infants and children. Monitoring pediatric patients was limited to inspection of chest movement and occasional palpation of the pulse until the late 1940s, when Smith introduced the first physiologic monitoring to pediatric anesthesia by using the precordial stethoscope for continuous auscultation of heartbeat and breath sounds (Smith, 1953; 1968.). Until the mid-1960s, many anesthesiologists monitored only the heart rate in infants and small children during anesthesia and surgery. Electrocardiographic and blood-pressure measurements were either too difficult or too extravagant and were thought to provide little or no useful information. Measurements of central venous pressure were thought to be inaccurate and too invasive even in major surgical procedures. The insertion of an indwelling urinary (Foley) catheter in infants was considered invasive and was resisted by surgeons.

Smith also added an additional physiologic monitoring: soft, latex blood-pressure cuffs suitable for newborn and older infants, which encouraged the use of blood pressure monitoring in children (Smith, 1968). The “Smith cuff ” (see Chapter 41, History of Pediatric Anesthesia, Fig. 41-4) remained the standard monitoring device in infants and children until the late 1970s, when it began to be replaced by automated blood pressure devices.

The introduction of pulse oximetry for routine clinical use in the early 1990s has been the single most important development in monitoring and patient safety, especially related to pediatric anesthesia, since the advent of the precordial stethoscope in the 1950s (see Chapters 10, Equipment; 11, Monitoring; and 40, Safety and Outcome) (Smith, 1956). Pulse oximetry is superior to clinical observation and other means of monitoring, such as capnography, for the detection of intraoperative hypoxemia (Coté et al., 1988, 1991). In addition, Spears and colleagues (1991) have indicated that experienced pediatric anesthesiologists may not have an “educated hand” or a “feel” adequate to detect changes in pulmonary compliance in infants. Pulse oximetry has revealed that postoperative hypoxemia occurs commonly among otherwise healthy infants and children undergoing simple surgical procedures, presumably as a result of significant reductions in functional residual capacity (FRC) and resultant airway closure and atelectasis (Motoyama and Glazener, 1986). Consequently, the use of supplemental oxygen in the postanesthesia care unit (PACU) has become a part of routine postanesthetic care (see Chapter 3, Respiratory Physiology).

Although pulse oximetry greatly improved patient monitoring, there were some limitations, namely motion artifact and inaccuracy in low-flow states, and in children with levels of low oxygen saturation (e.g., cyanotic congenital heart disease). Advances have been made in the new generation of pulse oximetry, most notably through the use of Masimo Signal Extraction Technology (SET). This device minimizes the effect of motion artifact, improves accuracy, and has been shown to have advantages over the existing system in low-flow states, mild hypothermia, and moving patients (Malviya et al., 2000; Hay et al., 2002; Irita et al., 2003).

Monitoring of cerebral function and blood flow, as well as infrared brain oximetry have advanced the anesthetic care and perioperative management of infants and children with congenital heart disease and traumatic brain injuries. Depth of anesthesia can be difficult to assess in children, and anesthetic overdose was a major cause of anesthesia-associated cardiac arrest and mortality. Depth-of-anesthesia monitors (bisectral index monitor [BIS], Patient State Index, Narcotrend) have been used in children and have been associated with the administration of less anesthetic agent and faster recovery from anesthesia. However, because these monitors use electroencephalography and a sophisticated algorithm to predict consciousness, the reliability of these monitors in children younger than 1 year old is limited.

In addition to advances in monitors for individual patients, hospital, patient, and outside-agency initiatives have focused on more global issues. Issues of patient safety, side-site markings, time outs, and proper patient identification together with the appropriate administration of prophylactic antibiotics have now become major priorities for health care systems. The World Health Organization (WHO) checklists have been positive initiatives that have ensured that the correct procedure is performed on the correct patient, as well as fostered better communication among health care workers. In anesthesia, patient safety continues to be a mantra for the specialty. Improved monitoring, better use of anesthetic agents, and the development of improved airway devices coupled with advancements in minimally invasive surgery, continue to advance the frontiers of pediatric anesthesia as a specialty medicine, as well as improve patient outcome and patient safety.

Anesthetic agents

More than a decade after the release of isoflurane for clinical use, two volatile anesthetics, desflurane and sevoflurane, became available in the 1990s in most industrialized countries. Although these two agents are dissimilar in many ways, they share common physiochemical and pharmacologic characteristics: very low blood-gas partition coefficients (0.4 and 0.6, respectively), which are close to those of nitrous oxide and are only fractions of those of halothane and isoflurane; rapid induction of and emergence from surgical anesthesia; and hemodynamic stability (see Chapters 7, Pharmacology; 13, Induction Maintenance and Recovery; and 34, Same-Day Surgical Procedures). In animal models, the use of inhaled anesthetic agents has been shown to attenuate the adverse effects of ischemia in the brain, heart, and kidneys.

Although these newer, less-soluble, inhaled agents allow for faster emergence from anesthesia, emergence excitation or delirium associated with their use has become a major concern to pediatric anesthesiologists (Davis et al., 1994; Sarner et al., 1995; Lerman et al., 1996; Welborn et al., 1996; Cravero et al., 2000; Kuratani and Oi, 2008). Adjuncts, such as opioids, analgesics, serotonin antagonists, and α₁-adrenergic agonists, have been found to decrease the incidence of emergence agitation (Aono et al., 1999; Davis et al., 1999; Galinkin et al., 2000; Cohen et al., 2001; Ko et al., 2001; Kulka et al., 2001; Voepel-Lewis et al., 2003; Lankinen et al., 2006; Aouad et al., 2007; Tazeroualti et al., 2007; Erdil et al., 2009; Bryan et al., 2009; Kim et al., 2009).

Propofol has increasingly been used in pediatric anesthesia as an induction agent, for intravenous sedation, or as the primary agent of a total intravenous technique (Martin et al., 1992). Propofol has the advantage of aiding rapid emergence and causes less nausea and vomiting during the postoperative period, particularly in children with a high risk of vomiting. When administered as a single dose (1 mg/kg) at the end of surgery, propofol has also been shown to decrease the incidence of sevoflurane-associated emergence agitation (Aouad et al., 2007).

Remifentanil, a μ-receptor agonist, is metabolized by nonspecific plasma and tissue esterases. The organ-independent elimination of remifentanil, coupled with its clearance rate (highest in neonates and infants compared with older children), makes its kinetic profile different from that of any other opioids (Davis et al., 1999; Ross et al., 2001). In addition, its ability to provide hemodynamic stability, coupled with its kinetic profile of rapid elimination and nonaccumulation, makes it an attractive anesthetic option for infants and children. Numerous clinical studies have described its use for pediatric anesthesia (Wee et al., 1999; Chiaretti et al., 2000; Davis et al., 2000, 2001; German et al., 2000; Dönmez et al., 2001; Galinkin et al., 2001; Keidan et al., 2001; Chambers et al., 2002; Friesen et al., 2003). When combined, intravenous hypnotic agents (remifentanil and propofol) have been shown to be as effective and of similar duration as propofol and succinylcholine for tracheal intubation.

The development of more predictable, shorter-acting anesthetic agents (see Chapter 7, Pharmacology) has increased the opportunities for pediatric anesthesiologists to provide safe and stable anesthesia with less dependence on the use of neuromuscular blocking agents.

Airway devices and adjuncts

Significant changes in pediatric airway management that have patient-safety implications have emerged over the past few years. The laryngeal mask airway (LMA), in addition to other supraglottic airway devices (e.g., the King LT-D, the Cobra pharyngeal airway), has become an integral part of pediatric airway management. Although the LMA is not a substitute for the endotracheal tube, LMAs can be safely used for routine anesthesia in both spontaneously ventilated patients and patients requiring pressure-controlled support. The LMA can also be used in the patient with a difficult airway to aid in ventilation and to act as a conduit to endotracheal intubation both with and without a fiber optic bronchoscope.

In addition to supraglottic devices, advances in technology for visualizing the airway have also improved patient safety. Since the larynx could be visualized, at least 50 devices intended for laryngoscopy have been invented. The newer airway-visualization devices have combined better visualizations, video capabilities, and high resolution.

The development and refinement of airway visualization equipment such as the Glidescope, Shikani Seeing Stylet, and the Bullard laryngoscope have added more options to the management of the pediatric airway and literally give the laryngoscopist the ability to see around corners (see Chapters 10, Equipment; and 12, Airway Management).

The variety of pediatric endotracheal tubes (ETTs) has focused on improved materials and designs. ETTs are sized according to the internal diameter; however, the outer diameter (the parameter most likely involved with airway complications) varies according to the manufacturer (Table 1-1). Tube tips are both flat and beveled, and a Murphy eye may or may not be present. The position of the cuff varies with the manufacturer. The use of cuffed endotracheal tubes in pediatrics continues to be controversial. In a multicenter, randomized prospective study of 2246 children from birth to 5 years of age undergoing general anesthesia, Weiss and colleagues (2009) noted that cuffed ETTs compared with uncuffed ETTs did not increase the risk of postextubation stridor (4.4% vs. 4.7%) but did reduce the need for ETT exchanges (2.1% vs. 30.8%). However, the role of cuffed ETTs in neonates and infants who require prolonged ventilation has yet to be determined.

Intraoperative and postoperative analgesia in neonates

It has long been thought that newborn infants do not feel pain the way older children and adults do and therefore do not require anesthetic or analgesic agents (Lippman et al., 1976). Thus, in the past, neonates undergoing surgery were often not afforded the benefits of anesthesia. Later studies, however, indicated that pain experienced by neonates can affect behavioral development (Dixon et al., 1984; Taddio et al., 1995, 2005). Rats exposed to chronic pain without the benefit of anesthesia or analgesia showed varying degrees of neuroapoptosis (Anand et al., 2007). However, to add further controversy to the issue of adequate anesthesia for infants, concerns regarding the neurotoxic effects of both intravenous and inhalational anesthetic agents (GABAminergic and NMDA antagonists) have been raised. Postoperative cognitive dysfunction (POCD) has been noted in adult surgical patients (Johnson et al., 2002; Monk et al., 2008). In adults, POCD may also be a marker for 1-year survival after surgery. Although POCD is an adult phenomenon, animal studies by multiple investigators have raised concerns about anesthetic agents being toxic to the developing brains of infants and small children (Jevtovic-Todorovic et al., 2003, 2008; Mellon et al., 2007; Wang and Slikker, 2008). Early work by Uemura and others (1985) noted that synaptic density was decreased in rats exposed to halothane in utero. Further work with rodents, by multiple investigators, has shown evidence of apoptosis in multiple areas of the central nervous system during the rapid synaptogenesis period. This window of vulnerability appears to be a function of time, dose, and duration of anesthetic exposure. In addition to the histochemical changes of apoptosis, the exposed animals also demonstrated learning and behavioral deficits later in life.

In addition to apoptotic changes that occurred in rodents, Slikker and colleagues have demonstrated neuroapoptotic changes in nonhuman primates (rhesus monkeys) exposed to ketamine (an NMDA antagonist). As with the rodents, ketamine exposure in monkeys resulted in long-lasting deficits in brain function (Dr. Merle Poule, personal communication on the Safety of Key Inhaled and Intravenous Drugs in Pediatric Anesthesia [SAFEKIDS] Scientific Workshop, November 2009, White Oaks Campus Symposium). How these animal studies relate to human findings is unclear to date. However, three clinical studies have been reported, and all three studies are retrospective. Wilder et al. (2009) studied a cohort group of children from Rochester, Minnesota, and noted that children exposed to two or more anesthetics in the first 4 years of life were more likely to have learning disabilities, compared with children exposed to one anesthetic or none at all. Kalkman and others (2009) studied a group of children undergoing urologic surgery before age 6 years and reported that there was a tendency for parents to report more behavioral disturbances than those operated on at a later age. However, in a twin cohort study from the Netherlands, Bartels and coworkers (2009) reported no causal relationship between anesthesia and learning deficits in 1,143 monozygotic twin pairs.

In an effort to determine the impact of anesthetic agents or neurocognitive development, a collaborative partnership between the U.S. Food and Drug Administration (FDA) and the International Anesthesia Research Society has formed Safety of Key Inhaled and Intravenous Drugs in Pediatric Anesthesia (SAFEKIDS), a program designed to fund and promote research in this area.

Regional analgesia in infants and children

Although conduction analgesia has been used in infants and children since the beginning of the twentieth century, the controversy about whether anesthetic agents can be neurotoxic has caused a resurgence of interest in regional anesthesia (Abajian et al., 1984; Williams et al., 2006).

As newer local anesthetic agents with less systemic toxicity become available, their role in the anesthetic/analgesic management of children is increasing. Studies of levobupivacaine and ropivacaine have demonstrated safety and efficacy in children that is greater than that of bupivacaine, the standard regional anesthetic used in the 1990s (Ivani et al., 1998, 2002, 2003; Hansen et al., 2000, 2001; Lönnqvist et al., 2000; McCann et al., 2001; Karmakar et al., 2002). A single dose of local anesthetics through the caudal and epidural spaces is most often used for a variety of surgical procedures as part of general anesthesia and for postoperative analgesia. Insertion of an epidural catheter for continuous or repeated bolus injections of local anesthetics (often with opioids and other adjunct drugs) for postoperative analgesia has become a common practice in pediatric anesthesia. The addition of adjunct drugs, such as midazolam, neostigmine, tramadol, ketamine, and clonidine, to prolong the neuroaxial blockade from local anesthetic agents has become more popular, even though the safety of these agents on the neuroaxis has not been determined (see Chapters 15, Pain Management; and 16, Regional Anesthesia) (Ansermino et al., 2003; de Beer and Thomas, 2003).

In addition to neuroaxial blockade, specific nerve blocks that are performed with or without ultrasound guidance have become an integral part of pediatric anesthesia (see Chapter 16, Regional Anesthesia). The use of ultrasound has allowed for the administration of smaller volumes of local anesthetic and for more accurate placement of the local anesthetic (Ganesh et al., 2009; Gurnaney et al., 2007; Willschke et al., 2006). The use of catheters in peripheral nerve blocks has also changed the perioperative management for a number of pediatric surgical patients. Continuous peripheral nerve catheters with infusions are being used by pediatric patients at home after they have been discharged from the hospital (Ganesh et al., 2007). The use of these at-home catheters has allowed for shorter hospital stays. In addition, the use of regional techniques with ultrasound guidance, coupled with the natural interest in pain management, has allowed for pediatric anesthesiologists to spearhead pediatric acute and chronic pain management programs.

In addition to advances in anesthetic pharmacology and equipment, advances in the area of pediatric minimal invasive surgery have improved patient morbidity, shortened the length of hospital stays, and improved surgical outcomes (Fujimoto et al., 1999).

Although minimally invasive surgery (MIS) imposes physiologic challenges in the neonate and small infant, numerous neonatal surgical procedures can nevertheless be successfully approached with such methods, even in infants with single ventricle physiology (Georgeson, 2003; Ponsky and Rothenberg, 2008). The success of MIS has allowed for the evolution of robotic techniques, stealth surgery (scarless surgery), and Natural Orifice Transluminal Endoscopic Surgery (NOTES) (Dutta and Albanese, 2008; Dutta et al., 2008; Isaza et al., 2008).

Fundamental differences in infants and children

Regardless of all the advances in equipment, monitoring, and patient safety initiatives, pediatric anesthesia still requires a special understanding of anatomic, psychological, and physiologic development. The reason for undertaking a special study of pediatric anesthesia is that children, especially infants younger than a few months, differ markedly from adolescents and adults. Many of the important differences, however are not the most obvious. Although the most apparent difference is size, it is the physiologic differences related to general metabolism and immature function of the various organ systems (including the heart, lungs, kidneys, liver, blood, muscles, and central nervous system) that are of major importance to the anesthesiologist.

Anatomic and Physiologic Differences

Body Size

As stated, the most striking difference between children and adults is size, but the degree of difference and the variation even within the pediatric age group are hard to appreciate. The contrast between an infant weighing 1 kg and an overgrown and obese adolescent weighing more than 100 kg who appear in succession in the same operating room is overwhelming. It makes considerable difference whether body weight, height, or body surface area is used as the basis for size comparison. As pointed out by Harris (1957), a normal newborn infant who weighs 3 kg is one third the size of an adult in length but the adult size in body surface area and of adult size in weight (Fig. 1-1). Of these body measurements, body surface area (BSA) is probably the most important, because it closely parallels variations in basal metabolic rate measured in kilocalories per hour per square meter. For this reason, BSA is believed to be a better criterion than age or weight in judging basal fluid and nutritional requirements. For clinical use, however, BSA proves somewhat difficult to determine, although a nomogram such as that of Talbot and associates (1952) facilitates the procedure considerably (Fig. 1-2). For the anesthesiologist who carries a pocket calculator, the following formulas may be useful to calculate BSA:

FIGURE 1-1 Proportions of newborn to adult with respect to weight, surface area, and length.

(From Crawford JD, Terry ME, Rourke GM: Simplification of drug dosage calculation by application of the surface area principle, Pediatrics 5:785, 1950.)

FIGURE 1-2 Body surface area nomogram for infants and young children.

(From Talbot NB, Sobel FH, McArthur JW, et al.: Functional endocrinology from birth through adolescence, Cambridge, 1952, Harvard University Press.)

Formula of DuBois and DuBois (1916)

Formula of Gehan and George (1970)

At full-term birth, BSA averages 0.2 m², whereas in the adult it averages 1.75 m². A table of average height, weight, and BSA is given for reference in Table 1-3. A simpler, crude estimate of BSA for children of average height and weight is given in Table 1-4. The formula:

TABLE 1-3 Relation of Age, Height, and Weight to Body Surface Area (BSA)*

TABLE 1-4 Approximation of Body Surface Area (BSA) Based on Weight

Weight (kg)	Approximate BSA (m2)
1-5	0.05 × kg + 0.05
6-10	0.04 × kg + 0.10
11-20	0.03 × kg + 0.20
21-40	0.02 × kg + 0.40

Modified from Vaughan VC III, Litt IF: Assessment of growth and development. In Behrman RE, Vaughn VC III, editors: Nelson’s textbook of pediatrics, ed 13, Philadelphia, 1987, WB Saunders.

is also reasonably accurate in children of normal physique weighing 21 to 40 kg (Vaughan and Litt, 1987).

The caloric need in relation to BSA of a full-term infant is about 30 kcal/m² per hour. It increases to about 50 kcal/m² per hour by 2 years of age and then decreases gradually to the adult level of 35 to 40 kcal/m² per hour.

Relative Size or Proportion

Less obvious than the difference in overall size is the difference in relative size of body structure in infants and children. This is particularly true with the head, which is large at birth (35 cm in circumference)—in fact, larger than chest circumference. Head circumference increases by 10 cm during the first year and an additional 2 to 3 cm during the second year, when it reaches three-fourths of the adult size (Box 1-1).

Box 1-1 Typical Patterns of Physical Growth

Weight

Birth weight (BW) is regained by the tenth to fourteenth day.

Average weight gain per day: 0-6 mo = 20 g; 6-12 mo = 15 g.

BW doubles at ≈︀4 mo, triples at ≈︀12 mo, quadruples at ≈︀24 mo.

During second year, average weight gain per month: ≈︀0.25 kg.

After age 2 years, average annual gain until adolescence: ≈︀2.3 kg.

Length/Height

By end of first year, birth length increases by 50%.

Birth length doubles by age 4 years, triples by 13 years.

Average height gain during second year: ≈︀12 cm.

After age 2 years, average annual growth until adolescence: ≈︀5 cm.

Head Circumference

Average head growth per week: 0-2 mo = ≈︀0.5 cm; 2-6 mo = ≈︀0.25 cm.

Average total head growth: 0-3 mo = ≈︀5 cm; 3-6 mo = ≈︀4 cm; 6-9 mo = ≈︀2 cm; 9-12 mo = ≈︀1 cm.

At full-term birth, the infant has a short neck and a chin that often meets the chest at the level of the second rib; these infants are prone to upper airway obstruction during sleep. In infants with tracheostomy, the orifice is often buried under the chin unless the head is extended with a roll under the neck. The chest is relatively small in relation to the abdomen, which is protuberant with weak abdominal muscles (Fig. 1-3). Furthermore, the rib cage is cartilaginous and the thorax is too compliant to resist inward recoil of the lungs. In the awake state, the chest wall is maintained relatively rigid with sustained inspiratory muscle tension, which maintains the end-expiratory lung volume functional residual capacity (FRC). Under general anesthesia, however, the muscle tension is abolished and FRC collapses, resulting in airway closure, atelectasis, and venous admixture unless continuous positive airway pressure (CPAP) or positive end-expiratory pressure (PEEP) is maintained.

FIGURE 1-3 A normal infant has a large head, narrow shoulders and chest, and a large abdomen.

Central and Autonomic Nervous Systems

The brain of the neonate is relatively large, weighing about 1/10 of body weight compared with about 1/50 of body weight in the adult. The brain grows rapidly; its weight doubles by 6 months of age and triples by 1 year. By the third week of gestation, the neural plate appears, and by 5 weeks the three main subdivisions of the forebrain, midbrain, and hindbrain are evident. By the eighth week of gestation, neurons migrate to form the cortical layers, and migration is complete by the sixth month. Cell differentiation continues as neurons, astrocytes, aligodendrocytes, and glial cells form. Axons and synaptic connections continually form and remodel. At birth, about one fourth of the neuronal cells are present. The development of cells in the cortex and brain stem is nearly complete by 1 year of age. Myelinization and elaboration of dendritic processes continue well into the third year. Incomplete myelinization is associated with primitive reflexes, such as the Moro and grasp reflexes in the neonate; these are valuable in the assessment of neural development.

At birth the spinal cord extends to the third lumbar vertebra. By the time the infant is 1 year old, the cord has assumed its permanent position, ending at the first lumbar vertebra (Gray, 1973).

In contrast to the central nervous system, the autonomic nervous system is relatively well developed in the newborn. The parasympathetic components of the cardiovascular system are fully functional at birth. The sympathetic components, however, are not fully developed until 4 to 6 months of age (Friedman, 1973). Baroreflexes to maintain blood pressure and heart rate, which involve medullary vasomotor centers (pressor and depressor areas), are functional at birth in awake newborn infants (Moss et al., 1968; Gootman, 1983). In anesthetized newborn animals, however, both pressor and depressor reflexes are diminished (Wear et al., 1982; Gallagher et al., 1987).

The laryngeal reflex is activated by the stimulation of receptors on the face, nose, and upper airways of the newborn. Reflex apnea, bradycardia, or laryngospasm may occur. Various mechanical and chemical stimuli, including water, foreign bodies, and noxious gases, can trigger this response. This protective response is so potent that it can cause death in the newborn (see Chapters 3, Respiratory Physiology; and 4, Cardiovascular Physiology).

Respiratory System

At full-term birth, the lungs are still in the stage of active development. The formation of adult-type alveoli begins at 36 weeks post conception but represents only a fraction of the terminal air sacs with thick septa at full-term birth. It takes more than several years for functional and morphologic development to be completed, with a 10-fold increase in the number of terminal air sacs to 400 to 500 million by 18 months of age, along with the development of rich capillary networks surrounding the alveoli. Similarly, control of breathing during the first several weeks of extrauterine life differs notably from control in older children and adults. Of particular importance is the fact that hypoxemia depresses, rather than stimulates, respiration. Anatomic differences in the airway occur with growth and development. Recently, the concept of the child having a funnel-shaped airway with the cricoid as the narrowest portion of the airway has been challenged. Based on bronchoscopic images, Dalal and colleagues (2009) suggest for infants and children the glottis, not the cricoid, may be the narrowest portion. The development of the respiratory system and its physiology are detailed in Chapter 3, Respiratory Physiology.

Cardiovascular System

During the first minutes after birth, the newborn infant must change his or her circulatory pattern dramatically from fetal to adult types of circulation to survive in the extrauterine environment. Even for several months after initial adaptation, the pulmonary vascular bed remains exceptionally reactive to hypoxia and acidosis. The heart remains extremely sensitive to volatile anesthetics during early infancy, whereas the central nervous system is relatively insensitive to these anesthetics. Cardiovascular physiology in infants and children is discussed in Chapter 4.

Fluid and Electrolyte Metabolism

Like the lungs, the kidneys are not fully mature at birth, although the formation of nephrons is complete by 36 weeks’ gestation. Maturation continues for about 6 months after full-term birth. The glomerular filtration rate (GFR) is lower in the neonate because of the high renal vascular resistance associated with the relatively small surface area for filtration. Despite a low GFR and limited tubular function, the full-term newborn can conserve sodium. Premature infants, however, experience prolonged glomerulotubular imbalance, resulting in sodium wastage and hyponatremia (Spitzer, 1982). On the other hand, both full-term and premature infants are limited in their ability to handle excessive sodium loads. Even after water deprivation, concentrating ability is limited at birth, especially in premature infants. After several days, neonates can produce diluted urine; however, diluting capacity does not mature fully until after 3 to 5 weeks of life (Spitzer, 1978). The premature infant is prone to hyponatremia when sodium supplementation is inadequate or with overhydration. Furthermore, dehydration is detrimental to the neonate regardless of gestational age. The physiology of fluid and electrolyte balance is detailed in Chapter 5, Regulation of Fluids and Electrolytes.

Temperature Regulation

Temperature regulation is of particular interest and importance in pediatric anesthesia. There is a better understanding of the physiology of temperature regulation and the effect of anesthesia on the control mechanisms. General anesthesia is associated with mild to moderate hypothermia, resulting from environmental exposure, anesthesia-induced central thermoregulatory inhibition, redistribution of body heat, and up to 30% reduction in metabolic heat production (Bissonette, 1991). Small infants have disproportionately large BSAs, and heat loss is exaggerated during anesthesia, particularly during the induction of anesthesia, unless the heat loss is actively prevented. General anesthesia decreases but does not completely abolish thermoregulatory threshold temperature to hypothermia. Mild hypothermia can sometimes be beneficial intraoperatively, and profound hypothermia is effectively used during open heart surgery in infants to reduce oxygen consumption. Postoperative hypothermia, however, is detrimental because of marked increases in oxygen consumption, oxygen debt (dysoxia), and resultant metabolic acidosis. Regulation of body temperature is discussed in detail in Chapter 6, Thermoregulation.

Summary

Pediatric anesthesia as a subspecialty has evolved, because the needs of infants and young children are fundamentally different from those of adults. The pediatric anesthesiologist should be aware of the child’s cardiovascular, respiratory, renal, neuromuscular, and central nervous system responses to various drugs, as well as to physical and chemical stimuli, such as changes in blood oxygen and carbon dioxide tensions, pH, and body temperature. Their responses are different both qualitatively and quantitatively from those of adults and among different pediatric age groups. More importantly, the pediatric anesthesiologist should always consider the child’s emotional needs and create an environment that minimizes or abolishes fear and distress.

There have been many advances in the practice of anesthesia to improve the comfort of young patients since the seventh edition of this book was published in 2006. These advances include a relaxation of preoperative fluid restriction, more focused attention to the child’s psychological needs with more extensive use of preoperative sedation via the transmucosal route, the wide use of topical analgesia with a eutectic mixture of local anesthetic cream before intravenous catheterization, expanded use of regional anesthesia with improved accuracy and safety by means of ultrasound devices, and more generalized acceptance of parental presence during anesthetic induction and in the recovery room. Furthermore, a more diverse anesthetic approach has evolved through the combined use of regional analgesia, together with the advent of newer and less soluble volatile anesthetics, intravenous anesthetics, and shorter-acting synthetic opioids and muscle relaxants. Finally, the scope of pediatric anesthesia has significantly expanded with the recent development of organized pain services in most pediatric institutions. As a result, pediatric anesthesiologists have assumed the leading role as pain management specialists, thus further extending anesthesia services and influence beyond the boundary of the operating room.