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Obesity Hypoventilation Syndrome (OHS)

  • OHS characterised by
    • Obesity
    • Daytime hypercapnia (>45mmHg PaCO2)
  • Difference between OHS and obstructive sleep anpea is that OHS has
    • Longer and more continuous episodes of hypoventilation overnight (± upper airway obstruction)
    • Daytime hypercapnia

Pathophysiology

  • ↑work of breathing as fat restricts movement of chest muscles
    • Makes chest wall less compliant – so diaphragm moves less effectively
    • Therefore, patients need to expend more energy to breathe efficiently
  • Leads to sleep disordered breathing
  • Inadequate removal of CO2 → hypercapnia → acidosis
  • Under normal circumstances, brainstem chemoreceptors detect acidity and respond by increasing respiration rate
  • In OHS this ventilator response is blunted due to ↑leptin levels in obese people
  • ↓O2 levels → pulmonary vasoconstriction → pulmonary hypertension → RV strain → RV hypertrophy and remodelling → peripheral edema (pitting edema)
  • Chronic hypoxemia → ↑erythropoietin → polycythemia

Clinical features

  • Features of sleep apnea
  • Chronic mouth breathing
  • Daytime sleepiness
  • Morning headaches
  • Typical patient is obese, with fat deposition around chin and abdomen
  • Diagnostic criteria
    • BMI >30
    • Daytime PaCO2 >45mmHg
    • Associated sleep-related breathing disorder e.g. OSA
    • Absence of other causes of hypoventilation

Diagnosis

  • ABG
  • Nocturnal oximetry
  • CXR – to see signs of heart failure
  • Echo – RVH
  • FBC – for polycythemia

Treatment

  • Advise pt to lose weight
  • CPAP/assisted ventilation
  • Treat concomitant OSA/asthma/COPD
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