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