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.


  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.
Last updated 2017-09-21

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