Anatomical and Physiological Differences in Children

Pediatric Airway & Respiration - Tiny Passages, Big Impact

• Anatomical Differences (vs. Adult):
  • Larynx: Higher (C3-C4), anterior, funnel-shaped.
  • Epiglottis: Long, U/Ω-shaped, floppy.
  • Tongue: Relatively larger, prone to obstruction.
  • Cricoid Ring: Narrowest point < 5-8 yrs.

    ⭐ Cricoid cartilage is the narrowest part of the airway in children under 5-8 years.

  • Trachea: Shorter (4-5 cm neonates), narrower, compliant.
  • Nares: Small, easily obstructed.
• Physiological Differences:
  • Obligate nose breathers (infants < 3-6 months). 📌 Mnemonic: "Infants Inhale Nasally."
  • $VO_2$: ↑↑ ($6-8 \text{ mL/kg/min}$ vs $3-4$ adult).
  • FRC: ↓ relative to $VO_2$; rapid desaturation.
  • $V_T$: $6-8 \text{ mL/kg}$ (similar/kg to adults).
  • RR: ↑ (Neonate: 30-60/min).
  • Chest Wall: Compliant, ribs horizontal; diaphragmatic breathing, less efficient.

Pediatric Cardiovascular System - Little Hearts, Fast Rhythms

• Heart Rate (HR): Higher (Neonate: 120-160 bpm, Infant: 100-150 bpm), ↓ with age.
• Stroke Volume (SV): Relatively fixed in neonates; limited capacity to increase.

  ⭐ Neonatal cardiac output is primarily heart rate-dependent due to a fixed stroke volume.

• Cardiac Output (CO): Higher per kg body weight; $CO = HR \times SV$.
• Blood Pressure (BP): Lower (Neonate Systolic: 60-80 mmHg, Infant Systolic: 70-90 mmHg), ↑ with age.
• Myocardium:
  • Less compliant (stiffer ventricles).
  • Fewer contractile elements, more non-contractile tissue.
• Autonomic Control:
  • Parasympathetic system (vagal tone) dominant at birth.
  • Sympathetic innervation immature; blunted response to catecholamines.
• Blood Volume: Relatively larger (Neonate: 80-90 ml/kg; Infant: 75-80 ml/kg).
• Response to Hypoxia: Bradycardia (unlike adult tachycardia). 📌 Peds Brady Hypoxia.

Pediatric Pharmacology - Small Bodies, Dose Adjustments

• Unique Pharmacokinetics (PK) & Pharmacodynamics (PD) in children.
• PK Differences:
  • Body Composition: ↑ Total Body Water (TBW), ↓ fat → ↑ Volume of Distribution (Vd) for water-soluble drugs; ↓Vd for lipid-soluble drugs initially.
  • Protein Binding: ↓ (e.g., albumin, alpha-1-acid glycoprotein) → ↑ free drug fraction.
  • Hepatic Metabolism: Immature; enzyme activity (Phase I & II) varies with age. Glucuronidation ↓ in neonates.
  • Renal Excretion: Immature (↓ Glomerular Filtration Rate (GFR), ↓ tubular function) → prolonged drug $t_{1/2}$ in neonates/infants.
• PD Differences:
  • Altered receptor sensitivity and density (e.g., opioid receptors, neuromuscular junction).

  ⭐ Minimum Alveolar Concentration (MAC) for volatile anesthetics is highest in infants aged 1-6 months.

• Dosing Strategies:
  • Primarily weight-based (mg/kg).
  • Titrate to clinical effect; requires close monitoring.
  • Age-specific adjustments are crucial due to organ maturation.

Key Systemic Differences - Warmth, Fluids & Nerves

• Thermoregulation (Warmth):
  • Prone to rapid hypothermia: ↑ surface area/volume ratio, ↓ fat, ↓ shivering.
  • Reliant on Non-Shivering Thermogenesis (NST) via brown fat.
  • Hypothermia risks: ↑O2 consumption, acidosis, hypoglycemia.

  ⭐ Infants and neonates are prone to rapid hypothermia due to a large surface area-to-volume ratio and rely on non-shivering thermogenesis.

• Fluid & Renal Balance:
  • ↑ Total Body Water (TBW) (70-80% neonates).
  • Immature kidneys: GFR ~25% adult, matures by 1-2 yrs.
  • Limited urine concentration/dilution; risk of dehydration/overload.
  • Blood volume: Neonate 80-90 mL/kg.
• Nervous System:
  • Immature CNS, permeable Blood-Brain Barrier (BBB).
  • Incomplete myelination (until ~2 yrs).
  • MAC peaks 1-6 months, then ↓.
  • Spinal cord ends at L3 (neonates) vs L1 (adults).

High‑Yield Points - ⚡ Biggest Takeaways

• Airway: Larger tongue, anterior/cephalad larynx, omega-shaped epiglottis, cricoid narrowest point (subglottis).
• Respiratory: Higher O2 consumption, lower FRC, leading to rapid desaturation.
• Cardiovascular: HR-dependent cardiac output; bradycardia poorly tolerated; sensitive to vagal stimuli.
• Pharmacology: Larger Vd for water-soluble drugs; immature hepatic/renal function alters drug metabolism/excretion.
• Thermoregulation: Prone to hypothermia due to high surface area to volume ratio.
• Infants: Obligate nose breathers; higher closing volumes.