Question 5(p.2)

Describe the respiratory changes that occur in morbid obesity.

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College Answer

Obesity is an increasing problem in the broader community and in Intensive Care practice. Hence it is important candidates understand the physiological and pharmacological consequences of obesity. This question confined its scope towards obesity and the respiratory system. Major area of weakness of candidates was a lack of depth and breadth in knowledge of this topic and in applying basic physiology. A good answer required the following points - 

Definition of morbid obesity (>200% ideal body weight or body mass index > 35) 

Upper airway effects: fat infiltration of pharyngeal soft tissues  difficult airway, prone to
airway obstruction eg OSA
 O2 consumption and CO2 production: due to  total body fat, requires  cardiac output and
 alveolar ventilation
 FRC mainly via  ERV: due to mass loading and splinting of diaphragm, upright obese,
closing capacity > FRC  small airway closure,  V/Q mismatch,  venous admixture and
arterial hypoxaemia,  O2 stores
 total respiratory system compliance: chest wall compliance , subcutaneous and intra
abdominal fat excess,  lung compliance,  airways resistance,  work of breathing,  resp
muscle efficiency
Altered ventilatory control: Obstructive sleep apnoea, Obesity hypoventilation syndrome
Syllabus: B1k B1d2k
Reference Text: Nunn’s Applied Respiratory Physiology / A B Lumb & J F Lunn - 6th ed


Respiratory Changes that Occur in Morbid Obesity

Obesity-related changes Effect of these changes

Airway function and structure

  • Decreased pharyngeal diameter
  • Increased tendency to collapse during sleep and sedation

Structural properties of the chest wall and lung volumes

  • Decreased chest wall compliance
  • Decreased FRC (mainly due to decreased ERV)
  • Decreased lung compliance due to decreased lung volume (FRC)
  • Slightly decreased total lung capacity (TLC)
Airway resistance
  • Increased airway resistance due to decreased lung volumes
  • Specific airway conductance remains the same.

Function of respiratory muscles

  • Increased total respiratory muscle mass
  • Increased respiratory effort and increased oxygen use by respiratory muscles

Gas exchange

  • Lower PaO2 chronically (in some studies)
  • Increased V/Q mismatch due to decreased FRC

Control of ventilation

  • Obesity hypoventilation syndrome:
    • Resting increased PaCO2 even when awake
  • Decreased reactivity of respiratory control reflexes

Demands on the respiratory system

  • Increased body mass = increased total body oxygen demand and increased ventilatory requirements for the clearance of the excess CO2


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