Respiratory · Year 1 · from Respiratory
Case 3: Obesity Hypoventilation Syndrome
Case Presentation
A 52-year-old woman with BMI 48 kg/m2 presents with progressive daytime somnolence, morning headaches, and lower extremity edema. She has a history of obstructive sleep apnea diagnosed 5 years ago but has been non-adherent with CPAP therapy. On examination, she is obese and plethoric with elevated jugular venous pressure, lower extremity edema, and a loud P2. Vital signs show respiratory rate 16/min, heart rate 92 bpm, blood pressure 155/95 mmHg, and oxygen saturation 84% on room air.
Arterial blood gas on room air shows pH 7.35, PaCO2 58 mmHg, PaO2 52 mmHg, HCO3 32 mEq/L. Serum bicarbonate is elevated at 34 mEq/L. The elevated bicarbonate with near-normal pH indicates chronic respiratory acidosis with metabolic compensation. Echocardiogram shows right ventricular hypertrophy and elevated pulmonary artery pressures consistent with cor pulmonale.
This patient has obesity hypoventilation syndrome (OHS), characterized by obesity (BMI greater than 30), chronic daytime hypercapnia (PaCO2 greater than 45 mmHg), and sleep-disordered breathing, after excluding other causes of hypoventilation. The pathophysiology involves multiple factors: increased work of breathing from chest wall loading and decreased compliance; reduced central chemosensitivity to CO2 (possibly from leptin resistance); and upper airway obstruction during sleep.
The chronic hypercapnia leads to renal bicarbonate retention to maintain pH near normal - this is the expected compensation for chronic respiratory acidosis (3.5 mEq/L increase in HCO3 for each 10 mmHg rise in PCO2). The chronic hypoxemia causes pulmonary vasoconstriction and eventually pulmonary hypertension with cor pulmonale.
Treatment includes positive airway pressure therapy (CPAP or BiPAP depending on severity), weight loss, and management of comorbidities. The patient is started on nocturnal BiPAP and improves significantly over several months with resolution of morning headaches and improved daytime alertness.
Key Learning Points
- Obesity hypoventilation syndrome requires obesity, chronic daytime hypercapnia, and sleep-disordered breathing
- Chronic hypercapnia leads to renal bicarbonate retention with near-normal pH (compensated respiratory acidosis)
- Elevated bicarbonate further blunts the central chemoreceptor response to CO2
- Chronic hypoxemia causes pulmonary hypertension and cor pulmonale
- Positive airway pressure therapy is the cornerstone of treatment