Pediatric Anesthetic Considerations

Overview

Neonate 0-1 month; infants 1-12 months; toddlers 1-3  yrs; small children 4-12 yrs

Weight estimation for 50^th percentile: (age x 2) + 9

Larger volume distribution due to higher total water content

Immature hepatic biotransformation pathways (longer elimination)

Increased organ blood flow

Decreased protein binding due to lower levels at birth

Higher metabolic rate

Lower GFR normalizing 12-24 months

Tube diameter: 4 + age/4           

Endotracheal length: 3 x diameter

Uncuffed vs. cuffed tubes - check for leaks in uncuffed using the APL valve

Predictors of mortality: ASA 3-5 and emergency surgery

Respiratory

Neonates and infants

Weak intercostal and diaphragmatic musculature due to few type 1 fibers (fatigue-resistant, slow-twitching, highly oxidative) result in Increased work of breathing

• Adults have 55%, full-term 25%, and preterm infants 10%

Horizontal and more pliable ribs promote chest wall collapse during inspiration and relatively low residual lung volumes at expiration

• May see paradoxical chest wall motion

Decreased FRC, which limits O₂ reserve (intubation) and predisposes to atelectasis and hypoxemia

Hypoxia and hypercapnia depress respiration in these patients

Larger head and tongue, narrow nasal passage, cricoid at C4 vs C6 in adults, short trachea and neck

The epiglottis is large but narrow and longer, so it is okay to use either blade type

Mouth breathers until 5 months

Vocal cords are slanted, posterior more cephalad than anterior commissure

Cricoid narrowest point < 5 yr, glottic rim in adults is the narrowest point

<10 kg consider Mapleson D or Bain System (less dead space)

• Always monitor airway pressure

Cardiovascular

Fixed stroke volume due to noncompliant and poorly developed left ventricle

CO dependent on HR

Limited ability to handle fluid overload

High risk for bradycardia leading to hypotension, asystole, and intraoperative death

Lower catecholamine stores, minimum response to exogenous catecholamines

Intravascular fluid depletion characterized by hypotension without tachycardia

Decreased ability to respond to hypovolemia with vasoconstriction

Metabolic

The goal is the production of water-soluble compounds that are readily excreted. Phase 1 (oxidation, reduction, and hydrolysis) and phase 2 (conjugation, which takes place in the liver, immature at birth) - the latter causes more prolonged elimination half-lives

Temperature Regulation

Significant heat loss due to the larger surface area/kg, thin skin, and low-fat content - worsened by cold OR, IV fluids, and dry anesthetic gases

Hypothermia causes delayed awakening, cardiac irritability, respiratory depression, increased PVR, and altered drug responses (prolonged)

Heat production

Nonshivering thermogenesis by the metabolism of brown fat. Exposure to cold leads to sympathetic stimulation of brown adipocytes via norepinephrine binding to beta-adrenergic receptors.

• As in white fat, sympathetic stimulation promotes triglyceride hydrolysis, releasing fatty acids and glycerol. However, within brown adipocytes, most fatty acids are immediately oxidized in mitochondria, and because of the uncoupling protein, a large amount of heat is produced. This process is part of what is called non-shivering thermogenesis

Metabolism of brown fat is severely limited in premature infants and sick neonates

Volatile anesthetics inhibit thermogenesis in brown adipocytes

Renal and GI functions

Urine production

• 20 wks ~ 5 mL/hr
• 30 wks ~ 18 mL/hr 
• 40 wks ~ 50 mL/hr 

An immature liver causes impaired hepatic conjugation early in life

• This is phase 2 of degradation pathways
• May influence the excretion of waste products

Neonates have low glycogen stores

High incidence of GI reflux in neonates

Anesthetic Considerations

Use precordial stethoscope

Pulse ox probe on the right hand or right ear to measure preductal oxygen saturation

Neonates: A-line preferably in the right radial artery

• Preductal O₂ content in carotid and retinal arteries

Inhalational Agents

Rapid induction with risk of myocardial depression due to rapid rise in FA/FI ratio (influenced by blood gas solubility, CO, minute ventilation), increased blood flow to organs, and higher anesthetic concentrations

Neonates, infants, and young children have higher alveolar ventilation and lower FRC compared to older children and adults = faster induction

Sevoflurane causes the least respiratory depression

May see agitation on emergence with sevoflurane and desflurane

• May switch to isoflurane after induction)

MAC values are higher in infants than in neonates and adults. Sevoflurane has almost the same MAC in neonates and infants

Nonvolatile Anesthetics

Infants and young children require higher doses of Propofol due to the larger volume of distribution. May need up to 250 mcg/kg/min for maintenance

Morphine to be used with caution in neonates (decreased hepatic conjugation and renal clearance)

Fentanyl and midazolam can cause profound hypotension

Midazolam has the fastest clearance, less in neonates than in older children

70% nitrous oxide/30% O₂, increase sevoflurane concentration every three to five breaths

Steady application of 10 cm positive end-expiratory pressure may overcome laryngospasm

Muscle Relaxants

Shorter onset due to shorter circulation time

Variable response in neonates due to immaturity of the neuromuscular junction (increased sensitivity), the dilutional effect from the sizeable extracellular compartment, and the duration depends on hepatic maturity

Atracurium and cisatracurium are reliable due to Hoffman's elimination

Avoid succinylcholine if possible - if given pre-treat with atropine 0.1 mg minimum to prevent profound bradycardia and sinus node arrest

Rocuronium can be given IM (1-1.5 mg/kg) but requires 3-4 min for onset

Caudal Block

Procedures below the diaphragm; circumcision, inguinal herniorrhaphy, hypospadias and clubfoot repair, anal surgery

Sacral hiatus above the coccyx, between the sacral cornua - note dural sac extends to the third sacral vertebra in children (1^st in adults), higher risk for intrathecal injection

Usually combined with GA and placed post-induction. Lateral position or prone - think baricity

Use 22g needle, loss of resistance with SALINE to prevent air embolism (patent foramen ovale)

"Pop" signals penetration of sacrococcygeal membrane, lower needle, advance only a few mm. Test dose with 2 mL of LA with epinephrine (1/200,000 = 5 mcg) to exclude vascular placement. May use 1% lidocaine up to 7 mg/kg or bupivacaine 0.125-0.25% up to 2.5 mg/kg

Pain Management

Fentanyl 1-2 mcg/kg

Morphine 0.05-0.1 mg/kg

Ketorolac 0.5-0.75 mg/kg

PONV Management

Zofran 0.1 mg/kg

Reglan 0.15 mg/kg

Laryngospasm

Laryngospasm is an sustained, uncontrolled/involuntary muscular contraction (spasm) of the laryngeal cords. Forceful, involuntary spasm of the laryngeal musculature due to stimulation of the superior laryngeal nerve

• The reflex is mediated by the branches of the superior laryngeal nerve influencing the tone of the striated muscle of the upper airway causing contraction of the adductor muscles
• The pattern of incidence in indirectly proportional to age, most prevalent in young pediatrics 1-3 months old
  • Neonates 3%
  • Children 2%
  • Older <1%

The condition can cause complete or partial obstruction of airflow through the vocal cords. It typically lasts less than 30 or 60 seconds. Stridors and/or retractions characterize it

Usually, it is a postoperative event, but it can occur at any time. Place pt in lateral position post-surgery to prevent secretions from hitting the vocal cords

Treatment

Positive pressure ventilation with 100% oxygen

Forward jaw-trust

• Larson Maneuver where bilateral digital pressure is applied to the area between the mastoid process and the ear lobe
  • The intense pain caused by the maneuver is thought to cause vocal cord relaxation via an autonomic reflex

IV lidocaine 1-1.5 mg/kg

IV succinylcholine 0.5-1.0 mg/kg or IM succinylcholine 4-6 mg/kg if no IV access

• Evidence suggest that doses less than that used for intubation of IV succinylcholene have simalr efficacy in terminating the laryngospams as those doses used to intubate
• In contrast, doses of syccinylcholne higher than those routinely used in intubation are regularly employed when using IM succinylcholene

Croup

Due to glottic or tracheal edema, it usually appears within 3 hrs. of extubation

Usually associated with 1-4 years of age, repeated intubation attempts, large ETT, prolonged surgery, head and neck procedures, or excessive movement of the tube due to coughing and/or movement of the head

Treat with Decadron (0.25-0.5 mg/kg) and inhalation of nebulized racemic epinephrine (0.25-0.5 mL of a 2.25% solution in 2.5mL of saline)