Learning objectives

• Describe the differences in anatomy and physiology between pediatric patients and adults
• Understand how these differences impact anesthesia practice in pediatric patients

Background

• Pediatric patients include the following groups
  • Neonates: A baby within 44 weeks of age from the date of conception
  • Infants: Up to 12 months of age
  • Children: 1-12 years
  • Adolescents: 13-16 years
• The differences between pediatric and adult anesthetic practice are reduced as patients become older

Anatomy and physiology

Airway and respiratory system

• Large head, short neck, and prominent occiput
• The tongue is relatively large
• The larynx is high and anterior (C3-C4 level)
• Epiglottis is long, stiff, and U-shaped → flops anteriorly → head needs to be in the neutral position to visualize it
• Neonates breathe through their nose → narrow nasal passages are easily blocked by secretions and may be damaged by a nasogastric tube or nasally placed endotracheal tube (ETT) 
• 50% of airway resistance comes from the nasal passages
• The airway is funnel-shaped and narrowest at the level of the cricoid cartilage
  • Trauma to the airway results in edema
  • 1 mm of edema can narrow a baby’s airway by 60% 
  • Presence of a leak around the ETT to prevent trauma resulting in subglottic edema and subsequent post-extubation stridor
• ETT must be inserted to the correct length to sit at least 1 cm above the tracheal carina and taped securely to prevent tube dislodgement with head movement
• Neonates and infants have a limited respiratory reserve
• Horizontal ribs prevent the “bucket handle” action seen in adult breathing and limit an increase in tidal volume (TV)
  • Ventilation is primarily diaphragmatic
  • Bulky abdominal organs or a stomach filled with gases from poor bag-mask ventilation can impinge on the contents of the chest and splint the diaphragm, reducing the ability to ventilate adequately
• The chest wall is more compliant → functional residual capacity (FRC) is relatively low
  • FRC decreases with apnea and anesthesia, causing lung collapse
• Minute ventilation is rate-dependent as there are little means to increase TV
• Closing volume is larger than FRC until 6-8 years → increased tendency for airway closure at end-expiration → neonates and infants need intermittent positive-pressure ventilation (IPPV) during anesthesia and benefit from a higher respiratory rate (RR) and use of PEEP
• Continuous positive airway pressure (CPAP) during spontaneous ventilation improves oxygenation and decreases the work of breathing
• Work of respiration may be 15% of oxygen consumption
• Muscles of ventilation are easily subject to fatigue due to the low percentage of type I muscle fibers in the diaphragm → number increases to the adult level over the first year of life
• Alveoli are thick-walled at birth → only 10% of the total number of alveoli found in adults → alveoli clusters develop over the first 8 years of life
• Apnea is common postoperatively in premature infants → associated with desaturation and bradycardia
• RR = 24 – age/2
• Spontaneous ventilation TV = 6-8 mL/kg; IPPV TV = 7-10 mL/kg
• Physiological dead space = 30% and increased by anesthetic equipment

Cardiovascular system

• The myocardium is less contractile in neonates, causing the ventricles to be less compliant and less able to generate tension during contraction → limits the size of the stroke volume → cardiac output is rate-dependent
• The infant behaves with a fixed cardiac output state
• The vagal parasympathetic tone is predominant, making neonates and infants more prone to bradycardia
• Bradycardia is associated with reduced cardiac output
• Treat bradycardia associated with hypoxia with oxygen and ventilation
• External cardiac compression is required in neonates with a heart rate of ≤60 bpm or 60-80 bpm with adequate ventilation
• Cardiac output = 300-400 mL/kg/min at birth; 200 mL/kg/min within a few months
• Sinus arrhythmia is common in children, all other irregular rhythms are abnormal
• The patent ductus contracts in the first few days of life and will fibrose within 2-4 weeks
• Closure of the foramen ovale is pressure-dependent and closes in the first day of life but it may reopen within the next 5 years

Normal heart rates (beats/min) and systolic blood pressure (mmHg)

Normal blood volumes

Renal system

• Renal blood flow and glomerular filtration are low in the first 2 years of life due to high renal vascular resistance
• The tubular function is immature until 8 months → infants are unable to excrete a large sodium load
• Dehydration is poorly tolerated
• Urine output = 1-2 mL/kg/h

Hepatic system

• Immature liver function with decreased function of hepatic enzymes
• E.g., barbiturates and opioids have a longer duration of action due to the slower metabolism

Glucose metabolism

• Hypoglycemia is common in the stressed neonate
  • Monitor glucose levels regularly
  • Neurological damage may result from hypoglycemia → infusion of 10% glucose to prevent this
  • Infants and older children maintain blood glucose better and rarely need glucose infusions
• Hyperglycemia is usually iatrogenic

Hematology

• 70-90% of hemoglobin molecules are fetal hemoglobin (HbF)
• Within 3 months, the levels of HbF drop to 5%, and adult hemoglobin (HbA) predominates
• Vitamin K-dependent clotting factors (II, VII, IX, X) and platelet function are deficient in the first months → administer vitamin K at birth to prevent hemorrhagic disease
• Transfusion is recommended when 15% of the circulating blood volume has been lost

Temperature control

• Babies and infants have a large surface area to weight ratio with minimal subcutaneous fat → poorly developed shivering, sweating, and vasoconstriction mechanisms
• Brown fat metabolism is required for non-shivering thermogenesis 
• The optimal ambient temperature to prevent heat loss is 34°C for the premature infant, 32°C for neonates, and 28°C for adolescents and adults
• Hypothermia causes respiratory depression, acidosis, decreased cardiac output, increased duration of action of drugs, decreased platelet function, and increased risk of infection

Central nervous system

• Pain is associated with increased heart rate, blood pressure, and neuroendocrine response
• Narcotics depress the ventilation response to a rise in PaCO2
• The blood-brain barrier (BBB) is poorly formed → barbiturates, opioids, antibiotics, and bilirubin cross the BBB easily, causing a prolonged duration of action
• Cerebral vessels in the preterm infant are thin-walled and fragile → prone to intraventricular hemorrhages → risk is increased with hypoxia, hypercarbia, hypernatremia, low hematocrit, awake airway manipulations, rapid bicarbonate administration, and fluctuations in blood pressure and cerebral blood flow
• Cerebral autoregulation is present and functional from birth

Psychology

• Infants <6 months are usually not upset by separation from their parents and will accept a stranger
• Children up to 4 years are upset by separation from their parents, and unfamiliar people and surroundings 
• School-age children are more upset by the surgical procedure and the possibility of pain
• Adolescents fear narcosis and pain, loss of control, and the possibility of not being able to cope with the illness 
• Parental anxiety is readily perceived and reacted to by the child

Anesthetic considerations

• Preoperative fasting
  • Solids and milk >12 months: 6 hours
  • Breast milk and formula feed <12 months: 4 hours
  • Unlimited clear fluids: 2 hours
  • Increased incidence of nausea and vomiting with long fasting periods
• Preoperative medical and anesthetic history
• • Previous problems with anesthetics, including family history
  • Allergies
  • Previous medical problems, including congenital anomalies
  • Recent respiratory illness
  • Current medications
  • Recent immunizations
  • Fasting times
  • Presence of loose teeth
• Weigh the child → all drug doses are related to body weight
• Physical examination of the airway and cardiorespiratory systems
• Further investigations that may be necessary
  • Hemoglobin: Large expected blood loss, premature infants, systemic disease, congenital heart disease
  • Electrolytes: Renal or metabolic disease, intravenous fluids, dehydration
  • Chest radiograph: Active respiratory disease, scoliosis, congenital heart disease
• Uncooperative patient
• Altered airway anatomy
• Increased risk of laryngospasm
• Inhalational induction: Halothane and sevoflurane
• Intravenous induction: Propofol, thiopentone, or ketamine
• Rapid desaturation on induction
• Increased vagal tone and potential for bradycardia
• Rate-dependent cardiac output
• Altered pharmacokinetics and -dynamics
  • Increased minimum alveolar concentration (MAC)
  • Immature liver and kidney function
  • Increased total body water

Suggested reading

• Macfarlane F. Paediatric anatomy and physiology and the basics of paediatric anaesthesia. December 16, 2005. Accessed February 2, 2023. https://resources.wfsahq.org/atotw/paediatric-anatomy-and-physiology-and-the-basics-of-paediatric-anaesthesia/ 

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