Obesity Definition

• Excess body fat
• BMI > 30 kg/m²

Lung Volumes

• ↓FRC, ↓ERV, ↓TLC
• Due to mass loading & diaphragm splinting
• FRC ↓ exponentially with ↑BMI
• FRC < CC → small airway closure, V/Q mismatch, R → L shunt & arterial hypoxaemia

Compliance/Resistance

• ↑BMI = exponential ↓compliance
  • ↓lung C > chest wall C
• ↓lung compliance due to ↑pulm. blood volume
• ↓chest wall C due to ↑ weight on thorax/abdomen
• ↓compliance assoc. with ↓FRC → FRC < CC → impaired gas exchange
• ↑resistance
  • Upper airway
    • ↑fat deposition oropharynx
    • ↑turbulent airflow = ↑airway resistance
  • Lower airway
    • ↓FRC = ↑resistance
    • Polycythaemia = ↑resistance

Gas Exchange

Modest ↓PaO₂ → small ↑A – a gradient

O2 Consumption / CO2 Production

• XS fats → ↑metabolic activity
• ↑O₂ consumption & ↑CO₂ production
• Normocapnia maintained by ↑MV (O₂)

WoB

• ↑↑↑
  • ↓Compliance
  • ↑Resistance
  • ↓Metabolic demand of resp muscles

Control of Respiration

OSA

• ↑fat deposition pharyngeal wall
• Airway collapses in sleep
• Frequent episodes of apnoea
• Daytime somnolence, memory impairment
• Morning headaches 2° ↑PCO₂ & cerebral VD
• Hypoxaemia → 2° polycythaemia
• Hypercapnia
• ↑ pulmonary vessel VC
• RV failure

Obesity Hypoventilation Syndrome

• OSA → ↑PCO₂ & ↓PO₂ at night
• Return to baseline in day
• But chronically → desensitises resp. centre
• Eventual T2RF with hypoxic drive