Respiratory Changes with Obesity

Discuss the effect of morbid obesity on ventilation

Obesity is a multisystem disorder defined by an elevated body mass index (BMI):

• Normal: BMI < 25
• Overweight: BMI 25 - 30
• Obese: BMI > 30
• Morbidly Obese:
  • Obesity related disease and a BMI > 35
  • BMI > 40

Characteristics of obesity include:

• Complex genetic and environmental causes
• Increased caloric intake
• Increased metabolic rate (normal for BSA)

Morbid obesity causes several changes to the respiratory system:

• Airway
  • Increased risk of OSA
  • Increased risk of GORD and aspiration
  • Increased risk of difficult bag-mask ventilation
  • Increased risk of difficulty laryngoscopy

• Changes to respiratory pattern
  • Increased minute ventilation
    • Secondary to increased VO₂ and VCO₂
      Due to the increase in LBW and adiposity.
    • Increased airway reactivity
      Central adiposity increases circulating cytokines, including TNF-α, IL-6, leptin.

• Changes to volumes and capacities
  • Reduced respiratory system compliance
    • Decreased chest wall compliance
      Due to abdominal and chest wall fat.
      • Fat distribution may be more important than absolute BMI
    • Decreased lung compliance
      Basal atelectasis due to abdominal compression and reduced respiratory compliance.
  • Decreased ERV and FRC
    • Note that RV is generally relatively unchanged
  • Increased airway resistance
    Due to decreased airway radius at lower lung volumes.
  • Increased work of breathing
    Due to reduced respiratory compliance and increased airway resistance.
  • Closing capacity encroaches on FRC
    As FRC falls, closing capacity becomes closer to FRC.
    • If closing volume exceeds expiratory reserve volume, then small airways will collapse during normal tidal breathing, causing shunt

• Changes to blood gases
  • Increased A-a gradient
    Occurs when closing capacity exceeds FRC.

• Changes to respiratory circulation
  • PVR increases due to reduced FRC causing increased HPV
    May lead to secondary PHTN and right heart dysfunction.

References

1. Alvarez A, Brodsky J, Lemmens H, Morton J. Morbid Obesity: Peri-operative Management. Cambridge: Cambridge University Press. 2010.
2. Lotia S, Bellamy MC. Anaesthesia and morbid obesity. Contin Educ Anaesth Crit Care Pain 2008; 8 (5): 151-156.