Anatomical and Physiological Differences in Children

Anatomical and Physiological Differences in Children

Anatomical and Physiological Differences in Children

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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.

Adult vs Pediatric Airway Anatomy

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. Pediatric Pharmacokinetics by Age and Critical Illness

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. Neonate in radiant warmer with temperature probe

  • 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.

Practice Questions: Anatomical and Physiological Differences in Children

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A _____mm endotracheal tube and laryngoscopy blade of size _____ are used to intubate a preterm infant

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