Physiological changes of pediatrics related to anesthesia

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Physiological differences in pediatric patients relevant to anesthesia

Children, especially neonates and infants, are not small adults. The differences are most pronounced in premature neonates, then lessen progressively with age.

System	Pediatric physiological feature	Anesthetic implication
Airway anatomy	Large occiput and tongue, relatively small mandible and nasal passages; larynx is more cephalad and anterior; epiglottis is long and floppy; short neck and trachea. Infants are predominantly nasal breathers in early life.	Airway obstruction occurs easily during sedation or induction. Position with a shoulder roll rather than a large pillow under the head. Intubation can be more difficult and accidental endobronchial intubation or extubation can occur because the trachea is short.
Airway caliber	Small airway diameter, with a highly clinically important small reduction in radius from edema, secretions, or laryngospasm.	Markedly increased resistance and work of breathing. Use appropriate-sized equipment, gentle instrumentation, humidification, and prompt management of croup, laryngospasm, or bronchospasm.
Chest wall and lungs	Chest wall is compliant because ribs are cartilaginous; lungs have fewer/smaller alveoli in neonates; respiratory muscles have less fatigue-resistant type I fibers.	Increased work of breathing, chest-wall retraction, early fatigue, atelectasis, and respiratory failure during anesthesia or postoperative opioid/sedative use.
FRC and oxygen reserve	Functional residual capacity is small, while oxygen consumption per kg is high.	Apnea produces rapid desaturation, particularly in neonates and infants. Effective preoxygenation, continuous airway support, and brief intubation attempts are important. Morgan and Mikhail notes that low FRC predisposes infants to atelectasis and hypoxemia during apnea (Morgan and Mikhail’s Clinical Anesthesiology, 7e, Pediatric Anesthesia chapter).
Ventilation and inhalational induction	Alveolar ventilation relative to FRC is high, and cerebral blood flow is relatively high.	Inhaled anesthetic concentration rises quickly, producing rapid induction and potentially rapid deepening of anesthesia. Ventilation also changes PaCO₂ quickly.
Control of breathing	Immature respiratory control, especially in preterm infants; greater susceptibility to apnea after anesthetics, opioids, hypoxemia, and hypothermia.	Preterm or former-preterm infants need careful postoperative monitoring for apnea. Avoid excessive respiratory depression and maintain normothermia.
Cardiovascular system	Neonatal myocardium is less compliant and has limited ability to increase stroke volume. Cardiac output is largely heart-rate dependent.	Bradycardia can cause a major fall in cardiac output. Treat causes promptly, especially hypoxemia, vagal stimulation, anesthetic overdose, and hypothermia. Avoid abrupt myocardial depression.
Blood pressure and volume	Blood volume is small in absolute terms, though higher per kg than in adults. Even modest surgical blood loss may represent a large percentage of total circulating volume.	Estimate allowable blood loss carefully, measure blood loss accurately, use appropriately sized IV access, and replace volume/blood promptly when indicated. Hypotension may be a late sign of shock.
Fetal-to-neonatal circulation	The neonatal circulation is transitional. Pulmonary vascular resistance may increase with hypoxia, hypercarbia, acidosis, hypothermia, or high airway pressure.	These factors can worsen pulmonary hypertension or promote right-to-left shunting in susceptible neonates, especially with congenital heart disease. Maintain oxygenation, normocarbia, normothermia, and acid-base balance.
Thermoregulation	Large surface-area-to-body-weight ratio, thin skin, little subcutaneous fat, immature temperature regulation, and exposure of large body areas during surgery.	Rapid heat loss is common. Hypothermia delays drug metabolism and awakening, causes respiratory depression/arrhythmias, increases pulmonary vascular resistance, and impairs coagulation. Warm the room, fluids, gases when applicable, and patient.
Glucose metabolism	Limited glycogen and fat stores, relatively high cerebral glucose use, and fasting intolerance, particularly in neonates/infants.	Risk of perioperative hypoglycemia. Avoid unnecessarily long fasting, check glucose in high-risk children, and use glucose-containing fluid selectively when clinically indicated.
Fluid/electrolyte balance	Total body water and extracellular fluid fraction are relatively high. Renal concentrating ability is immature in neonates.	Both dehydration and fluid overload can occur quickly. Use weight-based, carefully measured fluid therapy and monitor urine output, electrolytes, glucose, and acid-base status for major procedures.
Renal function	Glomerular filtration, tubular secretion, and concentrating ability are immature at birth and mature over the first months to years of life.	Renally cleared drugs and active metabolites may have prolonged effects in neonates. Dose intervals and fluid/electrolyte treatment require age-specific adjustment.
Hepatic metabolism and plasma proteins	Hepatic enzyme activity and hepatic blood flow are immature in neonates. Albumin and alpha-1-acid glycoprotein concentrations are lower.	Reduced clearance may prolong effects of some drugs; a greater free fraction of highly protein-bound drugs may increase toxicity risk. Titrate drugs to response rather than relying only on adult-style dosing assumptions.
Blood and coagulation	Lower absolute circulating volume; neonatal hemoglobin differs from adult hemoglobin and coagulation factors are developmentally lower.	Anemia develops quickly with blood loss or repeated phlebotomy. Interpret coagulation tests using pediatric/neonatal reference ranges and consider developmental hemostasis rather than assuming adult norms.
Neuromuscular junction	Immature neuromuscular junction and smaller muscle mass.	Responses to neuromuscular blockers can differ by age. Use quantitative monitoring when feasible and ensure complete reversal before extubation.
Pharmacodynamics	Greater sensitivity to myocardial depression, respiratory depression, and temperature-related changes in drug effect. Rapid changes in anesthetic depth may occur.	Dose all anesthetics by weight and developmental age, administer incrementally, monitor closely, and anticipate rapid physiologic deterioration.

High-yield anesthesia points

The major immediate danger is hypoxemia. Small FRC plus high oxygen consumption means infants desaturate much faster than adults during apnea.
Bradycardia is dangerous. In neonates and infants it usually means reduced cardiac output until proven otherwise, often secondary to hypoxia.
Avoid hypothermia. It is common, preventable, and worsens respiratory, cardiovascular, coagulation, and drug-recovery problems.
Drug handling is age-dependent. Neonates have immature liver and kidney function, lower protein binding, and altered drug sensitivity.
Minor fluid or blood losses are proportionately major. Calculate all volumes in mL/kg and reassess frequently.
Prematurity increases risk. Former preterm infants are particularly susceptible to postoperative apnea and require appropriate monitoring.

A useful developmental respiratory review explains why airway obstruction and hypoxemia are disproportionately common during pediatric anesthesia, especially in infants: developmental respiratory physiology.

Recent systematic reviews retrieved on pediatric anesthesia mainly concern premedication, radiotherapy anesthesia, and neurobehavioral outcomes rather than changing these foundational physiologic principles.

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