SECTION XI. Pediatrics
A Anatomy and Physiology
1. Cardiovascular physiology
a) During fetal development, oxygenation and carbon dioxide (CO 2) elimination are accomplished through the placenta. Oxygenated blood to the fetus travels from the placenta through the umbilical vein through the ductus venosus near the liver, to the inferior vena cava. The foramen ovale, the opening between the right and left atria, allows the oxygenated blood direct access to the left circulation. From the left atrium, the blood is transferred to the left ventricle and then to the body. The blood returns to the placenta through the umbilical arteries. Deoxygenated blood from the superior vena cava is deposited into the right atrium. It is then ejected into the pulmonary artery. Because of high pulmonary vasculature pressure, the blood bypasses the lungs and is instead transferred through the ductus arteriosus to the aorta. The blood travels to the placenta through the umbilical arteries.
b) Clamping of the umbilical cord increases systemic vascular resistance, increasing aortic and left-sided pressures and allowing the foramen ovale to close and the lungs to assume their role in oxygenation. Pulmonary vascular resistance decreases, and the ductus arteriosus closes as arterial oxygen pressure (P o2) levels increase.
c) Hypoxia, hypercarbia, and acidosis lead to persistent pulmonary hypertension and maintenance of fetal circulation. The diagnosis is made when right radial (preductal) and umbilical line (postductal) samples reveal a P o2 difference of 20 mm Hg. Shunting continues across a patent ductus arteriosus, resulting in increased hypoxemia and acidosis.
d) Treatment of persistent pulmonary circulation includes hyperventilation, maintenance of adequate oxygenation, and alkalosis.
e) Neonatal cardiac output is heart rate dependent because of a noncompliant left ventricle and fixed stroke volume.
f) The pediatric basal heart rate is higher than that of adults, although parasympathetic stimulation, hypoxia, or deep anesthesia can cause profound bradycardia and decreased cardiac output.
g) Sympathetic nervous system and baroreceptor reflexes are immature. Infants have low catecholamine stores and decreased responsiveness to exogenous catecholamines. Infants cannot respond to hypovolemia with vasoconstriction. Therefore, hypovolemia is suspected when there is hypotension in the absence of an increased heart rate.
h) Normal parameters are given in the following table.
i) Physiologic anemia of the newborn: Hematocrit at birth is 50%, 80% of which is fetal hemoglobin. Fetal hemoglobin binds more strongly to O 2 than adult hemoglobin. This facilitates O 2 uptake in utero. After birth, the presence of fetal hemoglobin causes a shift in the oxyhemoglobin curve to the left and a decrease in O 2 delivery to the tissues. At age 1 to 3 months, hemoglobin levels decrease and levels of 2,3-diphosphoglycerate increase. This causes a shift of the oxyhemoglobin curve to the right and increased O 2 delivery to tissues.
2. Respiratory physiology
a) Metabolic rate, CO 2 production, and O 2 consumption are increased.
b) Functional residual capacity and O 2 reserves are decreased.
c) Infants have a paradoxical response to hypoxia—initial hyperpnea followed by respiratory depression and depressed response to hypercarbia.
d) The larynx is at C2 to C4 in children and at C3 to C6 in the adult. This results in increased difficulty in alignment of the pharyngeal and laryngeal axes. A straight blade is useful for laryngoscopy in children.
e) Children have a stiff, omega-shaped epiglottis. The vocal cords slant up and back.
f) The narrowest part of the pediatric airway is the cricoid cartilage, as opposed to the adult glottis. The cricoid cartilage can form a seal around the endotracheal tube, eliminating the need for a cuffed tube. The cartilage is funnel shaped. Do not force-fit the endotracheal tube. Properly fitted tubes allow a leak at 15 to 25 cm H 2O.
g) Children have large occiputs that flex the head onto the chest, large tongues, and small chins. Tonsils and adenoids grow rapidly from ages 4 to 7 and may obstruct breathing.
h) Infants are obligatory nasal breathers. The position of the epiglottis in relation to the soft palate allows simultaneous breathing and sucking/drinking.
i) The neonatal trachea is 4 cm. Flexion of the head onto the chest forces the endotracheal tube to extend deeper into the right mainstem. Extension of the head may dislodge the tube.
j) The number of alveoli increases until age 6 years. Mature levels of surfactant are reached at 35 weeks of gestation. Decreased amounts of alveoli and surfactant in the neonatal period increase the risk of infant respiratory distress syndrome.
k) Increased work of breathing in the infant results from a decreased amount of type I muscle fibers in the diaphragm; this causes predisposition to fatigue. Poor chest wall mechanics, lack of rib cage rigidity, horizontal orientation of the ribs, weak intercostal muscles, and increased fatigue result in paradoxical chest movements in the newborn.
3. Nervous system
a) Cranial sutures are not fused in the infant; the cranium is pliable. Fluid status is indicated by fullness of the fontanelles.
b) Myelination of the nervous system continues until age 3. The spinal cord ends at L1 in adults and at L3 in pediatric patients. This is important to consider when using regional anesthesia techniques in the pediatric population.
c) Preterm and low-birth-weight infants are at risk for intracranial hemorrhage resulting from fragile cerebral vessels. Intracranial bleeding may result from hypoxia, hypercarbia, hyperglycemia or hypoglycemia, hypernatremia, or wide variations in blood pressure.
4. Renal system
a) The total body water in proportion to body weight is higher in neonates than in adults. Kidneys function in utero to eliminate urine into the amniotic fluid, whereas the placenta eliminates waste.
b) Neonates have the complete number of nephrons at birth. Nephrons are immature in function until age 6 to 12 months.
c) The glomerular filtration rate (GFR) is decreased by renal vasoconstriction, low plasma flow in the renal system, and low blood pressure. GFR increases until age 1 year.
d) Infants are obligate sodium excretors because of their inability to conserve sodium. Renal tubules are not responsive to the renin-angiotensin-aldosterone system. The infant kidney cannot concentrate urine, leading to an increased risk of dehydration. The ability to reabsorb glucose is also impaired. If excessive glucose is given intravenously, the result is osmotic diuresis.
e) Pediatric patients have a tendency toward acidosis, because the metabolic rate and CO 2 production are double those of the adult. There is a decreased ability to conserve bicarbonate and to excrete acids.
5. Hepatic system
a) Near birth, the fetal liver increases glycogen stores. Preterm infants are at increased risk for hypoglycemia because of a lack of glycogen stores.
b) Hepatic metabolism of drugs is decreased in the early weeks of life. The liver functions at the adult level by age 2.
6. Impaired thermogenesis
a) Infants are at risk for hypothermia from the following:
(1) Increased ratio of surface area to body weight
(2) Ineffective shivering mechanism
(3) Decreased amounts of subcutaneous fat present in preterm infants
b) Heat loss results from the following:
(1) Radiation: This is the transfer of heat between two objects of different temperatures not in direct contact. Reduce radiant loss by decreasing the temperature gradient (raise the room temperature closer to patient temperature).
(2) Convection: This is the transfer of heat to moving molecules such as air or liquid. Cover exposed skin.
(3) Evaporation: This occurs through the skin and respiratory systems, including sweat, insensible water loss through skin, wounds, respiratory tract, and evaporation of liquids applied to the skin.
(4) Conduction: This is the transfer of heat from the warm infant to a cool object in direct contact.
c) Patients assume room temperature under anesthesia, a condition termed poikilothermia.
d) Nonshivering thermogenesis: Infants have impaired shivering capabilities. Autonomic nervous system activation during periods of cold results in metabolism of brown fat stores. Brown fat is located around the neck, kidneys, axilla, and adrenals, in addition to spaces between shoulders, under the sternum, and along the spine. Fatty acids in the brown fat stores are oxidated in an exothermic reaction to produce heat. Nonshivering thermogenesis is used until age 1 to 2 years.
e) Hypothermia in the neonate results in the release of norepinephrine, peripheral and pulmonary vasoconstriction, increasing acidosis, increased pulmonary pressures and right-to-left shunting, and eventually hypoxia, further perpetuating the cycle.
f) Avoid hypothermia by instituting the following: Increase room temperature, cover the patient’s head and exposed extremities, and use overhead warming light. Beware of burns. Use recommended distances for safe use. Heat and humidify delivered gases.
B Pediatric Pharmacologic Considerations
1. Introduction
a) Immature organ systems are responsible for existing pharmacologic differences between the infant and child.
b) Physiologic characteristics that modify the pharmacokinetic (what the body does to the drug) and pharmacodynamic (what the drug does to the body) activity include differences in total body water (TBW) composition, immaturity of metabolic degradation pathways, reduced protein binding, immaturity of the blood-brain barrier, greater proportion of blood flow to the vessel-rich organs (brain, heart, liver, and lungs), reductions in glomerular filtration, a smaller functional residual capacity, and increased minute ventilation.
c) TBW, expressed in liters, is determined as a percentage of total body weight (1 L of water weighs 1 kg). The changes in TBW, intracellular fluid (ICF), and extracellular fluid (ECF) during maturation are listed in the following table.
2. Volume of drug distribution
a) The infant has a larger extracellular fluid compartment and greater TBW content.
b) There is a greater adipose content and a higher ratio of water to lipid. Fat content is approximately 12% at birth, doubling by 6 months of age, and reaching 30% at 12 months of age.
c) These factors lower plasma drug concentrations when water-soluble drugs are administered according to weight.
d) A larger drug loading dose is required to achieve the desired plasma concentration. The effect of immaturity upon the volume of distribution is not as evident for lipophilic drugs that are transported across cell membranes.
3 Protein binding
a) Total plasma protein is decreased in the infant reaching equivalent adult concentrations by childhood.
b) Both albumin and alpha 1-acid glycoprotein (AAG) concentrations are diminished at birth but reach the adult equivalency by infancy (age 4 weeks).
4. Metabolism
a) Phase II reactions, which are immature at birth, consist of conjugation or synthesis. Conjugation couples the drug with an endogenous substrate (glucuronidation, methylation, acetylation, and sulfation) to facilitate excretion.
b) The newborn lacks the capacity to efficiently conjugate bilirubin (decreased glucuronyl transferase activity), and metabolize acetaminophen, chloramphenicol, and sulfonamides.
c) Although the necessary enzyme systems are present at birth, enzyme activity is reduced, increasing drug elimination half-lives.
5. Rectal and oral drug administration
a) Drugs are usually formulated as liquids for oral administration in children.
b) Midazolam may be administered orally for premedication, and the rectal route may be selected for the administration of acetaminophen, opioids, barbiturates (thiopental and methohexital), and benzodiazepines.
c) Both routes rely on passive diffusion for drug absorption. The resulting plasma drug concentration is dependent on the molecular weight, degree of drug ionization, and lipid solubility.
d) Orally administered drugs are generally reserved for older children, as gastric pH is elevated in the neonate at birth (pH 6 to 8), and although decreased to a pH level of 1 to 3 within 24 hours, adult gastric pH values are not consistent until age 2.
e) Gastric absorption is reduced following oral administration of acidic drugs in infants. Gastric emptying time reaches adult values by 6 months of age. Although gastric emptying time does not affect drug absorption, it may alter peak drug concentration.
f) Acetaminophen, a metabolite of phenacetin, is a popular and safe analgesic and antipyretic commonly administered to children during the perioperative period.
g) The analgesic and antipyretic effects of acetaminophen are equivalent to those of aspirin when the drugs are administered in equipotent dosages.
h) Suppositories should not be divided in an attempt to provide the exact calculated dose, because the suspended acetaminophen is distributed unevenly within the suppository. Recommended acetaminophen doses have been based on the age of the child, weight, body surface area calculations, and fractions of adult dosages.
i) Currently recommended oral and rectal doses of acetaminophen range from 10 to 15 mg/kg every 4 hours. Because of the variable absorption of acetaminophen suppositories, some practitioners have advocated the administration of larger initial rectal dosages. It should be emphasized that subsequent rectal doses should be decreased (20 mg/kg) and the dosing interval should be extended to every 6 to 8 hours.
j) Following acetaminophen administered during the perioperative period, the parents should be informed as to the time of administration and be advised of appropriate acetaminophen dosages (60 to 65 mg/kg/day).
k) The daily acetaminophen dosage administered either rectally or orally should be limited to 100 mg/kg/day for children and 75 mg/kg/day for infants.
l) Sedation with nasally administered midazolam (0.2 mg/kg) may be achieved in as little as 10 to 20 minutes and is explained in part through drug absorption via the olfactory mucosa. Nasal administration is unpleasant as the midazolam produces a burning of the nasal mucosa.
m) Oral fentanyl, although effective in producing significant sedation, has been plagued by significant side effects, including facial pruritus (up to 80%) and postoperative nausea and vomiting, seven times greater than when a child receives an oral meperidine, midazolam, or atropine premedicant.
n) Water-soluble drugs (atropine, fentanyl, lidocaine, morphine) may be administered via inhalation; however, only 5% to 10% of the administered dose will reach the systemic circulation.
INHALATION AGENTS
1. Introduction
a) Although tidal volume is similar between children and adults (5 to 7 mL/kg), children have greater minute ventilation and a higher ratio of tidal volume to functional residual capacity (5:1) compared to the adult (1.5:1).
b) The greater minute ventilation and higher cardiac output in infants and children are responsible for rapid inhalation anesthetic uptake and rapidly increasing alveolar anesthetic concentration. In addition, their decreased distribution of adipose tissue and decreased muscle mass affect the rate of equilibration among the alveoli, blood, and brain.
c) The percentage of blood flow to the vessel-rich organs is greater than in the adult, and the blood-gas partition coefficients are lower in infants and children.
d) Anesthetic requirements are known to change with age. Neonates have a somewhat lower minimum alveolar concentration (MAC) than infants, which peaks at around 30 days of age.
e) MAC is higher in infants from age 1 to 6 months of age; thereafter, MAC values are known to decrease with increasing age.
f) Myocardial depression may be exaggerated when inhalation anesthetics are administered to pediatric patients. A more rapid rise F A/F I ratio, and the greater percentage of blood flow to the vessel-rich organs, and higher administered anesthetic concentrations are central to the cause of myocardial depression.
g) Inhalation induction is more rapid in pediatric patients and is accompanied by a higher incidence of myocardial depression than in adults.
2. Isoflurane
a) The MAC of isoflurane in oxygen is 1.6% in infants and children.
b) Inhalation induction with isoflurane produces more adverse respiratory events (breath-holding, coughing, and laryngospasm with copious secretions) than either halothane or sevoflurane.
c) Administration of isoflurane to adults produces dose-dependent decreases in peripheral vascular resistance, while increases in heart rate maintain blood pressure. This touted advantage (e.g., increase in heart rate to maintain blood pressure) does not occur in infants.
d) Anesthetic induction in infants with isoflurane produces significant decreases in heart rate, blood pressure, and mean arterial pressure that are not corrected with prior atropine administration.
3. Desflurane
a) The MAC of desflurane in oxygen is 9% for infants, and 6% to 10% for children.
b) Desflurane has the lowest blood-gas partition coefficient of all the inhalation anesthetics (0.42), which facilitates a rapid induction, rapid alterations in anesthetic depth, and emergence.
c) Like isoflurane, desflurane is pungent and is associated with more adverse respiratory events during inhalation induction, including breath holding, laryngospasm, coughing, and increased secretions with accompanying hypoxia.
d) Following inhalation induction with sevoflurane, desflurane is appropriate for the maintenance of general anesthesia with facemask, endotracheal tube, or laryngeal mask airway (LMA).
e) As in the adult population, dramatic increases in desflurane concentrations may induce sympathetic stimulation evidenced by tachycardia and hypertension.
4. Sevoflurane
a) The MAC of sevoflurane in oxygen is 3% for the infant up to 6 months of age, decreasing to 2.5% to 2.8% up to 1 year of age. The MAC of sevoflurane in oxygen is 2% to 3%.
b) Sevoflurane produces a more rapid induction and emergence than halothane because of its low blood-gas partition coefficient.
c) Sevoflurane is readily accepted for mask induction, and its safe cardiovascular profile (when compared to halothane) is responsible for the increasing popularity of sevoflurane in pediatric anesthesia.
d) Both minute ventilation and respiratory rate are significantly lower after sevoflurane administration. With the introduction of high inspired concentrations, apnea is likely to occur.
e) Sevoflurane metabolism may produce concentration-dependent elevations in serum fluoride levels that decline when sevoflurane is discontinued.
f) Some clinicians, when performing longer procedures, use sevoflurane for anesthetic induction and subsequently introduce either desflurane or isoflurane for anesthetic maintenance. This clinical decision reduces patient cost and limits sevoflurane exposure.
g) Sevoflurane does not sensitize the myocardium to the effects of endogenous and exogenous catecholamines, but concentration-dependent myocardial depression may occur.
5. Emergence delirium
a) A variety of terms are used interchangeably when referring to postoperative agitation. These include emergence delirium, emergence agitation, and post-anesthetic excitement. These terms describe altered behavior in the immediate postoperative period manifesting as non-purposeful restlessness, crying, moaning, incoherence, and disorientation (known here as emergency delirium, or ED).
b) Case reports also suggest that ED occurs more frequently in preschool-aged children (< age 6).
c) The reported incidence of ED is between 25% and 80%, although the incidence has been difficult to pinpoint as previous studies are confounded by the previously mentioned varying definitions.
d) The Pediatric Anesthesia Emergence Delirium (PAED) scale for the assessment of ED is listed in the following box.
The Pediatric Anesthesia Emergence Delirium (PAED) Scale
1. The child makes eye contact with caregiver.
2. The child’s actions are purposeful.
3. The child is aware of his or her surroundings.
4. The child is restless.
5. The child is inconsolable.
Items 1, 2, and 3 are reverse scored as follows: 4 = not at all, 3 = just a little, 2 = quite a bit, 1 = very much, 0 = extremely. Items 4 and 5 are scored as follows: 0 = not at all, 1 = just a little, 2 = quite a bit, 3 = very much, 4 = extremely. The scores are summed to obtain a total. The degree of emergence delirium increases directly with total score.
e) ED is fortunately self-limiting but may manifest for as long as 45 minutes.
f) In a search for the causation of ED, several emerging themes have been examined. Proposed etiologies include rapid emergence in a strange environment, pain upon awakening, and preoperative behavior.
g) Several strategies have been advocated for the prevention of ED, although a scientific, clinically tested strategy for prevention has yet to be advanced. Following inhalation induction with sevoflurane, propofol infusion for maintenance has been demonstrated to reduce ED.
h) Some anesthesia providers advocate the substitution of sevoflurane with isoflurane, yet there are no studies that detail the effectiveness of this strategy.
i) The phenomenon of ED is clearly increased following the administration of sevoflurane (and likely desflurane).
INTRAVENOUS ANESTHETIC
1. Introduction
a) Infants and children have a higher proportion of cardiac output delivered to vascular-rich tissues (i.e., heart, brain, kidneys, and liver).
b) Intravenously administered drugs are readily taken up by these tissues and are subsequently redistributed to muscle and fat—tissues that are less well perfused.
c) Intravenously administered drugs may have a prolonged duration of action in infants and children because of decreased percentages of muscle and fat.
d) The central nervous system effects of opioids and barbiturates may also be prolonged because of the immaturity of the blood-brain barrier.
e) Although this evidence suggests that intravenously administered anesthetic doses should be reduced, one must also recall the effect of increased body water. Increased doses of thiopental, propofol, and ketamine are required, presumably because of a greater volume of distribution.
2. Thiopental
a) Intravenous induction dosages range from 4 to 6 mg/kg in children without significant cardiovascular disease.
b) Neonates have limited fat stores and may have an extended duration of action after thiopental administration.
c) Thiopental, 25 to 30 mg/kg, may be administered rectally for anesthetic induction.
3. Propofol
a) Propofol has a rapid onset and a short duration of action and has been established as a sole agent for induction and maintenance of general anesthesia, or may be combined with an opioid and nitrous oxide to provide total intravenous anesthesia.
b) Propofol may be delivered as a continuous infusion for short diagnostic and radiological procedures, and is used as a primary sedative in chronically ventilated intensive care patients.
c) Its antiemetic properties may reduce the incidence of postoperative nausea and vomiting in children undergoing strabismus correction.
d) Infants require larger induction doses (2.5 to 3 mg/kg) than children (2 to 2.5 mg/kg). These induction doses produce moderate decreases in systolic blood pressure.
e) The pain that accompanies intravenous administration may be reduced with the addition of as little as 0.2 mg/kg of lidocaine.
f) Additional strategies promulgated for decreasing the pain of injection include a slower injection of propofol into a rapid-running intravenous line, or the injection into larger intravenous catheters placed in the antecubital space.
Induction agents and analgesics are listed in the following tables.
| Narcotics | |||
|---|---|---|---|
| Drug | Dose: Analgesic | Dose: Induction | Comments |
| Morphine | 0.05-0.2 mg/kg | ||
| Meperidine (Demerol) | 1-2 mg/kg | Causes less respiratory depression in neonates than morphine | |
| Fentanyl (Sublimaze) | 1-3 mcg/kg | 10-100 mcg/kg | ∗ Can cause bradycardia and muscle rigidity |
| Sufentanil (Sufenta) | 0.5-2 mcg/kg | ∗ Can cause bradycardia | |
| Remifentanil (Ultiva) | Loading dose: 0.5-1 mcg/kg Continuous infusion: 0.25-0.5 mcg/kg/min | ∗ Bolus can cause bradycardia | |
| Naloxone (Narcan) | 0.01 mg/kg | ||
| Nonnarcotic Analgesics | |||
| Drug | Dose | ||
| Ketorolac (Toradol) | 0.4-1 mg/kg IV | ||
| Acetaminophen (Tylenol) | 10-20 mg/kg orally 30-40 mg/kg rectally | ||
| ∗The use of pancuronium with fentanyl, sufentanil, and remifentanil can help to decrease the incidence of bradycardia. | |||
NEUROMUSCULAR RELAXANTS
1. Introduction
a) Increases in ECF volume and the ongoing maturation of neonatal skeletal muscle and acetylcholine receptors affect the pharmacokinetics and pharmacodynamics of neuromuscular relaxants.
b) The effective doses of clinical neuromuscular blocking drugs in various age groups are listed in the following table.
| Neonate | Infant | Child | Adult | |
|---|---|---|---|---|
| Succinylcholine∗ | 620 | 729 | 423 | 290 |
| Atracurium | 120 | 156-175 | 170-350 | 110-280 |
| Vecuronium | 47 | 42-47 | 56-80 | 27-56 |
| Rocuronium | 600 | 600 | 600 | 300 |
| Pancuronium | – | 55 | 55-81 | 50-70 |
| ∗Should be used for emergency airway stabilization in children younger than 12 years. Not for routine intubation. | ||||
c) The neuromuscular junction is incompletely developed at birth, maturing after 2 months of age. Skeletal muscle, acetylcholine receptors, and the accompanying biochemical processes essential in neuromuscular transmission mature during infancy into childhood.
d) The presynaptic release of acetylcholine is slowed compared with the adult, which explains the decreased margin of safety for neuromuscular transmission in the neonate. The acetylcholine receptors of the newborn are anatomically different from the adult receptors, which may explain the sensitivity of the neonate to the nondepolarizing class of neuromuscular relaxants.
e) This neuromuscular immaturity may be demonstrated with the appearance of fade after tetanic stimulation in the absence of neuromuscular blocking drugs.
2. Succinylcholine
a) Because succinylcholine contains acetylcholine moieties, its intravenous administration will reproduce the effects of acetylcholine when it interacts with nicotinic and muscarinic receptors, provoking both sympathetic and parasympathetic cardiovascular responses.
b) Stimulation of the parasympathetic ganglia or direct stimulation of cardiac muscarinic receptors will produce sinus bradycardia, junctional rhythms, unifocal premature ventricular contractions, and ventricular fibrillation.
c) The prior administration of atropine 0.02 mg/kg will block cardiac muscarinic receptors and minimize the decreases in heart rate.
d) Dysrhythmia is more common in children, particularly after repeated doses in the presence of hypoxia or concurrent electrolyte imbalance.
e) Myoglobinemia may occur in up to 20% of children who receive intravenous succinylcholine and in 40% of children who receive succinylcholine and halothane.
f) The prior administration of a small dose of a nondepolarizing neuromuscular blocking drug will modify the degree of myoglobinuria.
g) Myalgia is common following succinylcholine administration.
h) Succinylcholine is a known triggering agent for the development of malignant hyperthermia (MH).
i) Neonates are more resistant to the effects of succinylcholine than children and adults. This sensitivity is illustrated by the ED 95 for neonates (620 mcg/kg), infants (729 mcg/kg), children (423 mcg/kg), and adults (290 mcg/kg).
j) The increase in dose requirement is in part a result of the increased volume of distribution within the large extracellular compartment.
k) Plasma cholinesterase activity is reduced in neonates; however, the duration of action after a single dose is of expected duration (6 to 10 minutes). A longer duration of action after a single bolus dose suggests the presence of an inherited deficiency of plasma cholinesterase activity.
l) Intramuscular succinylcholine may facilitate endotracheal intubation in children without suitable intravenous access. Because of the increased volume of distribution, a larger dose is required to achieve satisfactory relaxation. Although a dose of 3 mg/kg will produce satisfactory relaxation in 85% of patients, an intramuscular dose of 4 mg/kg in the deltoid muscle will provide skeletal muscle relaxation in all, with a duration of action of up to 21 minutes.
m) To attenuate the effects of succinylcholine at both the nicotinic and muscarinic receptors, atropine at a dose of 0.02 mg/kg may be combined in the same syringe with the calculated dose of succinylcholine or in an additional syringe, which is administered in a selected muscle group before succinylcholine administration.
n) Unexpected cardiac arrest has been reported after the routine administration of succinylcholine, with less than 40% of patients successfully resuscitated.
o) Succinylcholine should not be routinely employed for airway management in children younger than 8 years of age.
NONDEPOLARIZING NEUROMUSCULAR BLOCKING AGENTS
1. Introduction
a) Infants and children are more sensitive than adults to the effects of nondepolarizing neuromuscular blocking drugs.
b) A lower plasma concentration of the selected neuromuscular relaxant is required to achieve the desired clinical level of neuromuscular blockade.
c) This does not imply that the selected dosage should be decreased as infants have a greater volume of distribution.
d) The larger volume of distribution and slower drug clearance result in longer half-life elimination, decreasing the need for repeated drug dosing (longer dosing intervals).
e) Neuromuscular function monitoring must be employed to guide repeated administration of these drugs in all pediatric patients.
f) The selection of a nondepolarizing neuromuscular relaxant should take into consideration the desired degree and duration of skeletal muscle paralysis, the immaturity of organ systems, and the associated side effects of the selected relaxant.
2. Atracurium
a) An intermediate-acting neuromuscular relaxant that is metabolized by nonspecific esterases and spontaneous breakdown of the parent compound by Hofmann elimination.
b) cis-Atracurium also uses Hofmann elimination and nonspecific ester hydrolysis for the metabolism of the parent compound.
c) The duration of action of atracurium is relatively the same as in the adult.
d) The volume of distribution is greater in infants, yet the clearance is more rapid. Accordingly, an intubating dose (0.5 mg/kg) may be administered in infants and children with the same expected duration of action.
e) Atracurium (intubating dose 0.5 mg/kg, maintenance dose 0.2 to 0.3 mg/kg) and cis-atracurium (intubating dose 0.1 mg/kg, maintenance dose 0.08 to 0.1 mg/kg) may be the drugs of choice for the infant, as these drugs are independent of mature organ function for elimination.
3. Vecuronium
a) Vecuronium produces minimal alterations in cardiovascular function and stimulates the release of histamine.
b) Infants are more sensitive to the effects of vecuronium than children (ED 95 0.047 versus 0.081 mg/kg).
c) Vecuronium may be administered as a continuous infusion at a rate of 0.8 to 1 mcg/kg/min.
4. Rocuronium
a) An intermediate-acting neuromuscular blocker with a rapid-to-intermediate onset of 60 to 90 seconds after an intubating dose of 0.6 mg/kg.
b) The potency of rocuronium is greater in infants than children; however its onset is faster in children.
c) Unlike vecuronium, rocuronium in intubating doses may produce transient increases in heart rate.
d) Skeletal muscle relaxation can be maintained with repeat doses of 0.075 to 0.125 mg/kg.
e) In clinical situations in which intravenous access is not available, rocuronium may be administered intramuscularly. Acceptable intubating conditions in lightly anesthetized infants occurs 2.5 to 3 minutes after a deltoid intramuscular dose of 1000 mcg/kg and within 3 minutes after 1800 mcg/kg in children.
f) The onset of action approximates the onset of succinylcholine after intramuscular injection.
g) Rocuronium injection into the deltoid provides a faster onset of twitch and ventilatory depression than does injection into the quadriceps muscle group.
h) A disadvantage of this route of administration is the accompanying prolonged duration of relaxation—in excess of 60 minutes.
i) Rocuronium may also be administered by continuous infusion at doses of 0.004 to 0.016 mg/kg/min.
5. Antagonism of neuromuscular blockade
a) Residual neuromuscular blockade places the infant and child at risk of hypoventilation and the inability to independently and continuously maintain a patent airway.
b) Because of increased basal oxygen consumption, impaired respiratory function will lead to arterial oxygen desaturation and CO 2 retention. The resulting acidosis will potentiate residual neuromuscular blockade.
c) Accordingly, the infant and child must have neuromuscular function restored at the conclusion of the surgical procedure. The detection of residual neuromuscular blockade requires the integration of clinical criteria and the assessment of neuromuscular blockade via a peripheral nerve stimulator.
d) Conventional doses of the anticholinesterase inhibitors (50 to 60 mcg/kg of neostigmine, or 500 to 1000 mcg/kg of edrophonium) combined with appropriate doses of atropine or glycopyrrolate are acceptable for antagonism of nondepolarizing neuromuscular blockade.
e) Useful clinical signs of successful antagonism of neuromuscular blockade include the ability to flex the arms, lift of the legs, and flex the thighs upon the abdomen, providing evidence of the return of abdominal muscle tone, in addition to the return of a normal train-of-four response as assessed by the peripheral nerve stimulator.
f) Neonates are capable of generating a negative inspiratory force (NIF) of −70 cm H 2O with the first few breaths after birth.
g) An negative inspiratory force of at least −32 cm H 2O has been found to correspond with leg lift, which is indicative of the adequacy of ventilatory reserve required before tracheal extubation.
h) Clinical investigation is ongoing in examining a novel antagonist of neuromuscular blockade. Sugammadex, a water-soluble, modified γ-cyclodextrin, is being investigated as a reversal of steroidal neuromuscular blocking agents.
i) The drug does not affect acetylcholinesterase, eliminating the need for the co-administration of an anticholinergic.
j) The application of Sugammadex for antagonism of neuromuscular blockade in the pediatric population is being studied.
C Pediatric Anesthesia Equipment
1. Introduction
The child’s age, weight, and proposed surgical procedure guide the selection of essential pediatric anesthesia equipment. The anesthesia workroom should be appropriately stocked with a variety of sizes of masks, airways, LMAs, laryngoscope blades, endotracheal tubes, endotracheal tube stylets, blood pressure cuffs, pulse oximeter probes, calibrated pediatric fluid sets, syringe pumps for the delivery of both fluids and drugs, an assortment of intravenous catheters, tape, and arm boards.
