Describe the respiratory changes that occur in morbid obesity.
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
| Obesity-related changes | Effect of these changes |
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Airway function and structure |
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Structural properties of the chest wall and lung volumes |
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| Airway resistance |
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Function of respiratory muscles |
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Gas exchange |
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Control of ventilation |
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Demands on the respiratory system |
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