Anesthesia for Pediatric Surgery
Susan A. Vassallo
Michael P. Puglia
I. ANATOMY AND PHYSIOLOGY
A. Upper Airway
1. Neonates are obligate nose breathers due to weak oropharyngeal muscles and increased compliance of the pharynx, larynx, and bronchial tree. Their nares are relatively narrow, and a significant fraction of the work of breathing is needed to overcome nasal resistance. Occlusion of the nares by bilateral choanal atresia or tenacious secretions can cause complete airway obstruction; however, some infants will convert to mouth breathing. Placement of an oral airway, a laryngeal mask airway, or an endotracheal tube may be necessary to reestablish airway patency during sedation or anesthesia.
2. Infants have relatively large tongues, which can make mask ventilation and laryngoscopy challenging. A recent study called tongue size into question and found the tongue to be proportional in children aged 1 to 12. Clinically, the tongue can easily obstruct the airway if excessive submandibular pressure is applied during mask ventilation.
3. Infants and children have a more cephalad glottis (C3 vertebral level in premature infants, C4 in infants, and C5 in adults) and a narrow, long, angulated epiglottis, which can make laryngoscopy difficult.
4. In infants and young children, the narrowest part of the airway is at the cricoid cartilage (recent studies have questioned this; see suggested readings), rather than at the glottis (as in adults). An endotracheal tube that passes through the cords may still be too large distally.
5. Deciduous teeth erupt within the first year and are shed between ages 6 and 13 years. To avoid dislodging a loose tooth, it is safest to open the mandible directly, without introducing a finger or appliance into the oral cavity. Loose teeth should be documented on the preoperative evaluation. In some instances, unstable teeth should be removed before laryngoscopy. Parents and patients should be informed of this possibility in advance.
6. Airway resistance in infants and children can be increased dramatically by subtle changes in an already small-caliber system. Even a small amount of edema can significantly increase airway resistance and cause airway compromise.
B. Pulmonary System
1. Neonates have higher metabolic rates, resulting in an elevated oxygen consumption (6 mL/kg/min) when compared with adults (3 mL/kg/min).
2. Neonatal lungs have high closing volumes, which fall within the lower range of their normal tidal volume. Below closing volume, alveolar collapse and shunting occur.
3. To meet the higher oxygen demand, infants have a higher respiratory rate and minute ventilation. An infant’s functional residual capacity (FRC) is nearly equivalent to that of an adult (FRC of an infant, 25 mL/kg; adult, 35 mL/kg). Their higher minute ventilation to FRC ratio results in rapid
inhalational induction. The tidal volume for infants and adults is equivalent (6 to 7 mL/kg).
inhalational induction. The tidal volume for infants and adults is equivalent (6 to 7 mL/kg).
4. Anatomic shunts including patent ductus arteriosus and patent foramen ovale may develop significant right-to-left flow with increases in pulmonary artery pressure (e.g., hypoxia, acidosis, or high positive airway pressure). This may predispose to air emboli if care is not taken to remove it from the IV tubing.
5. The characteristics of the infant’s pulmonary system contribute to rapid desaturation during apnea. Profound desaturation can occur when an infant coughs or strains and alveoli collapse. Treatment may require deepening anesthesia, using neuromuscular relaxants, as well as alveolar recruitment.
6. The diaphragm is the infant’s major muscle of ventilation. Compared with the adult diaphragm, the newborn has only half the number of Type I, slow-twitch, high-oxidative muscle fibers essential for sustained increased respiratory effort. Thus, the infant’s diaphragm fatigues earlier than the adult’s. By 2 years of age, the child’s diaphragm would have attained mature levels of Type I fibers.
7. The pliable rib cage (compliant chest wall) of an infant cannot maintain negative intrathoracic pressure easily. This diminishes the efficacy of the infant’s attempts to increase ventilation.
8. An infant’s dead space is 2 to 2.5 mL/kg, equivalent to an adult’s.
9. Infants’ high baseline minute ventilation limits their ability to increase their ventilatory effort further. End-tidal CO2 concentrations should be followed if spontaneous ventilation is permitted under anesthesia; assisted or controlled ventilation may be necessary.
10. Alveolar maturation occurs by 8 to 10 years of age when alveoli number and size reach adult ranges.
11. Retinopathy of prematurity (see Chapter 30).
12. Apnea and bradycardia after general anesthesia occur with increased frequency in infants who are premature and in infants who have anemia, sepsis, hypothermia, central nervous system disease, hypoglycemia, hypothermia, or other metabolic derangements. These patients should have cardiorespiratory monitoring for a minimum of 24 hours postoperatively. Such infants are not candidates for ambulatory day surgery. The guidelines for discharge vary among institutions. Most hospitals agree that infants who are less than 45 to 60 weeks of postconceptual age are monitored postoperatively. Any full-term infant who displays apnea after general anesthesia should also be monitored.
C. Cardiovascular System
1. Heart rate and blood pressure vary with age and should be maintained at age-appropriate levels perioperatively (Tables 31.1 and 31.2).
2. Cardiac output is 180 to 240 mL/kg/min in newborns, which is two to three times that of adults. This higher cardiac output is necessary to meet the higher metabolic oxygen consumption demands.
3. The ventricles are less compliant and have a relatively smaller contractile muscle mass in newborns and infants. The ability to increase contractility is limited; increases in cardiac output occur by increasing heart rate rather than stroke volume. Bradycardia is the most deleterious dysrhythmia in infants, and hypoxemia is a frequent cause of bradycardia in infants and children.
4. Neonates have immature calcium signaling and handling in the scarcoplasmic reticulum and myocardium and are more dependent on ionized calcium concentrations for myocardial function.
TABLE 31.1 Age Dependence of Typical Respiratory Parameters | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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D. Fluid and Electrolyte Balance
1. The glomerular filtration rate at birth is 15% to 30% of the normal adult value. Adult value is reached by 1 year of age. Renal clearance of drugs and their metabolites is diminished during the first year of life.
2. Neonates have an intact renin-angiotensin-aldosterone pathway, but the distal tubules resorb less sodium in response to aldosterone. Thus, newborns are “obligate sodium losers,” and IV fluids should contain sodium.
3. The total body water in the preterm infant is 90% of body weight. In term infants, it is 80%; at 6 to 12 months, it is 60%. This increased percentage of total body water affects drug volumes of distribution. The dosages of some drugs (e.g., propofol, succinylcholine, pancuronium, and rocuronium) are 20% to 30% greater than are the equally effective dose for adults.
E. Hematologic System
1. Normal values for hemoglobin and hematocrit are listed in Table 31.1. The nadir of physiologic anemia is at 3 months of age, and the hemoglobin may reach 10 to 11 g/dL in an otherwise healthy infant. Premature infants may demonstrate a decrease in hemoglobin concentration as early as 4 to 6 weeks of age.
2. At birth, fetal hemoglobin (HbF) predominates, but β-chain synthesis shifts to the adult type (HbA) by 3 to 4 months of age. HbF has a higher
affinity for oxygen; that is, the oxyhemoglobin dissociation curve is shifted to the left, but debate exists regarding the clinical relevance.
affinity for oxygen; that is, the oxyhemoglobin dissociation curve is shifted to the left, but debate exists regarding the clinical relevance.
TABLE 31.2 Cardiovascular Variables | ||||||||||||||||||||||||||||
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3. See section IX.B for calculations of blood volume and red cell mass.
F. Hepatobiliary System
1. Liver enzyme systems, particularly those involved in phase-II (conjugation) reactions, are immature in the infant. Drugs metabolized by the P-450 system may have prolonged elimination times.
2. Jaundice is common in neonates and can be physiologic or have pathologic causes.
3. Hyperbilirubinemia and displacement of bilirubin from albumin by drugs can result in kernicterus. Premature infants develop kernicterus at lower levels of bilirubin than do term infants (see Chapter 30).
4. Plasma levels of albumin are lower at birth and as a result, drugs that are protein bound may have a higher free fraction and higher effective concentration.
G. Endocrine System
1. Newborns, particularly premature babies and those small for gestational age, have decreased glycogen stores and are more susceptible to hypoglycemia. Infants of diabetic mothers have high insulin levels because of prolonged exposure to elevated maternal serum glucose levels and are prone to hypoglycemia. Infants who fall into these groups may have dextrose requirements as high as 5 to 15 mg/kg/min. Normal glucose concentrations in the full-term infant are ≥45 mg/dL (2.5 mmol/L).
2. Hypocalcemia is common in infants who are premature, are small for gestational age, have a history of asphyxia, are offspring of diabetic mothers, or have received transfusions with citrated blood or freshfrozen plasma. Serum calcium concentration should be monitored in these patients and calcium administered if the ionized calcium is less than 4.0 mg/dL (1.0 mmol/L).
H. Temperature Regulation
1. Compared with adults, infants and children have a greater surface area to body weight ratio, which increases loss of body heat.
2. Infants have significantly less muscle mass and cannot compensate for cold by shivering or adjusting their behavior to avoid the cold.
3. Infants respond to cold stress by increasing norepinephrine production, which enhances metabolism in brown fat. Norepinephrine also produces pulmonary and peripheral vasoconstriction, which can lead to right-to-left shunting, hypoxemia, and metabolic acidosis. Sick and preterm infants have limited stores of brown fat and therefore are more susceptible to cold. Strategies to prevent cold stress are discussed in section IV.C.
II. THE PREANESTHETIC VISIT
General principles of the preanesthetic visit are discussed in Chapter 1. The preoperative visit is an excellent opportunity to address the concerns of the child and parents.
A. History should include the following:
1. Maternal health during gestation, including alcohol or drug use, smoking, diabetes, and viral infections
2. Prenatal tests (e.g., ultrasound and amniocentesis)
3. Gestational age and weight
4. Events during labor and delivery, including Apgar scores and length of hospital stay
5. Hospitalizations/emergency room visits
6. Congenital, chromosomal, metabolic anomalies or syndromes
7. Recent upper respiratory infections, tracheobronchitis, “croup,” reactive airway disease (asthma), exposure to communicable diseases, cyanotic episodes, or history of snoring
8. Sleeping position (prone, side, or supine)
9. Respiratory quality and pattern (i.e., noisy breathing that increase with sleep; periods of apnea with sleep)
10. Growth history
11. Vomiting and gastroesophageal reflux
12. Siblings’ health
13. Parents who smoke
14. Past surgical and anesthetic history
15. Allergies (environmental, drugs, food, and latex)
16. Bleeding tendencies
B. Physical examination should include the following:
1. General appearance, alertness, color, tone, congenital anomalies, head size and shape, activity level, and social interaction
2. Vital signs, height, and weight
3. Loose teeth, craniofacial anomalies, or large tonsils that could complicate airway management
4. Respiratory pattern and quality. Signs of upper respiratory infection and/or reactive airways disease (excessive secretions may predispose patients to laryngospasm and bronchospasm during induction and emergence of anesthesia).
5. Cardiac exam including age-appropriate heart rate, rhythm, and heart murmurs (which may indicate flow through anatomic shunts)
6. Potential vascular access sites
7. Strength, developmental milestones, activity level, and motor and verbal skills
8. Additional exam(s) pertinent to specific anesthetic/surgical condition
C. Laboratory data appropriate for the child’s illness and proposed surgery should be obtained. Most centers agree that a “routine hemoglobin” is unnecessary for healthy children. If indicated, laboratory tests can often be obtained after induction of general anesthesia (e.g., blood bank sample).
III. PERIPROCEDURAL CONSIDERATIONS AND FASTING GUIDELINES
A. Periprocedural Considerations
1. Children face a multiple stressors in the perioperative period. They may not understand the specific disease; the concept of anesthesia, or the procedure itself. Honesty about the procedure(s), associated pain, and the sequence of events is essential in maintaining the trust of children regardless of their level of development.
a. Approaches to optimize the periprocedural experience include the use of nonpharmacologic techniques. Examples of these strategies include distraction, humor, sharing control (e.g., would you like to sit up or lie down for induction?), and medical reinterpretation of equipment. The use of a child life specialist, tablet, and/or a teaching module may decrease the need for preprocedure pharmacologic therapy and improve a child’s experience.
2. Infants less than 8 months old generally tolerate short periods of separation from parents and usually do not require premedication.
3. Children 8 months to 5 years of age likely have developed separation anxiety and may require sedation before the induction of anesthesia (see section V.B).
TABLE 31.3 Fasting Guidelines | ||||||||||
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4. Older children generally respond well to information and reassurance. Parental and patient anxiety may be reduced by having parents accompany their children to the operating room. An especially anxious child may benefit from premedication.
5. Premedication with intramuscular (IM) anticholinergics generally is not recommended. If vagolytic drugs are indicated, they are usually administered intravenously (IV) at the time of induction of anesthesia.
6. In the presence of gastroesophageal reflux, ranitidine (2 to 4 mg/kg PO, 2 mg/kg IV) along with metoclopramide (0.1 mg/kg) can be administered 2 hours before surgery to increase gastric pH and reduce gastric volume.
7. Children receiving medications for medical problems such as reactive airways disease, seizures, or hypertension should continue to take these medications preoperatively.
B. Fasting Guidelines
1. Milk, breast milk, formula, and solid foods should be restricted as outlined in Table 31.3.
2. The last feeding should consist of clear fluids or sugar water. Studies document that there is no increased risk of aspiration if clear fluids are offered up to 2 hours preoperatively. This policy minimizes preoperative dehydration and hypoglycemia and contributes to a smooth induction and stable operative course. We recommend that patients receive clear fluids until 2 hours before surgery is scheduled. Oral intake is then restricted (see Table 31.3).
3. If schedule delays occur, clear fluids may be given. Some patients may require IV placement for hydration and/or glucose administration.
IV. PREPARATION OF THE OPERATING ROOM
A. Perioperative/procedure huddle is a useful meeting to confirm procedure details, anesthetic requirements, special concerns, patient-specific factors, and nonroutine steps.
B. Anesthetic Circuit
1. The semiclosed circuit normally used in adults has some disadvantages if used in very small infants:
a. The inspiratory and expiratory valves increase resistance during spontaneous ventilation: This can be overcome by closing the APL valve to 3 to 5 cm of water and providing adequate gas flow rates to maintain pressure, or the use of pressure support).
b. The large volume of the absorber system acts as a reservoir for anesthetic agents.
c. The breathing circuit tubing has a large compression volume; however, neonatal and pediatric tubing are available that help compensate for this, as well as decrease the circuit dead space volume.
2. The nonrebreathing, open circuit (Mapleson D) solves these problems (see Chapter 9). Rebreathing is prevented by using fresh gas flows 2.0 to 2.5 times the minute ventilation to wash out carbon dioxide. Capnography is essential in recognizing rebreathing (inspired CO2 >0) and avoiding excessive hyperventilation. This circuit is useful for very small infants who are allowed to breathe spontaneously and during transport.
3. A passive heat and moisture exchanger may be used with either circuit.
4. The reservoir bag volume should be at least as large as the child’s vital capacity but small enough so that a comfortable squeeze does not overinflate the chest. General guidelines for bag volumes are as follows: newborns, 500-mL bag; 1 to 3 years, 1,000-mL bag; and more than 3 years, 2,000-mL bag.
5. For most infants and children, the semiclosed-circuit-absorber system can be used with a smaller reservoir bag and a pediatric breathing circuit with small-caliber tubing (i.e., circle system).
C. Airway Equipment
1. A mask with minimal dead space should be chosen. A clear plastic type is preferred because the lips (for color) and mouth (for secretions and vomitus) can be visualized.
2. The appropriate size of oral airway can be estimated by holding the airway in position next to the child’s face. The tip of the oral airway should reach to the angle of the mandible with the base at the lips.
3. Laryngoscopy
a. A narrow handle is often preferred because it has a more natural feel when using a smaller blade.
b. Straight blade (Miller or Wis-Hipple) is recommended for children less than 2 years old. The smaller flange and long tapered tip of the straight blade provide better visualization of the larynx and manipulation of the epiglottis in the confined spaces of a small oral cavity(see section Endotracheal Intubation, Oral Approach)
c. Curved blades are generally used for patients more than 5 years old.
d. Guidelines for laryngoscope blade sizes (Table 31.4)
4. Endotracheal tubes. Traditionally, uncuffed tubes were used for children under 6 to 7 years of age (5.5-mm inner diameter endotracheal tube or smaller). The risk of tracheal stenosis is minimal with modern
low-pressure cuffs, and cuffed tubes may be used when indicated (e.g., tonsillectomy or proximal bowel obstruction) and may even provide additional benefit against aspiration. Concern for tracheal injury (including postintubation stridor) in extremely small infants with microcuff endotracheal tubes has been reported.
low-pressure cuffs, and cuffed tubes may be used when indicated (e.g., tonsillectomy or proximal bowel obstruction) and may even provide additional benefit against aspiration. Concern for tracheal injury (including postintubation stridor) in extremely small infants with microcuff endotracheal tubes has been reported.
TABLE 31.4 Guidelines for Choice of Laryngoscope Blades | ||||||||||||||
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