Diagnosis and Management of Obesity Hypoventilation Syndrome (OHS) and Obstructive Sleep Apnea (OSA)
Defining Characteristics
- The American Thoracic Society defines Obesity Hypoventilation Syndrome (OHS) as requiring three components: obesity (BMI > 30 kg/m²), sleep-disordered breathing, and awake daytime hypercapnia (PaCO₂ > 45 mm Hg), after excluding other causes of hypoventilation 1, 2
- OHS represents the most severe form of obesity-induced respiratory compromise with significantly higher morbidity and mortality than OSA alone 1
- Patients with OHS develop chronic hypercapnic respiratory failure that persists during wakefulness 2, 3
- The American Academy of Sleep Medicine characterizes Obstructive Sleep Apnea (OSA) by recurrent upper airway obstruction during sleep causing apneas and hypopneas 3
- Approximately 90% of patients with OHS also have coexistent OSA (AHI > 5 events/h), with nearly 70% having severe OSA (AHI > 30 events/h) 1, 2, 3
Pathophysiologic Distinctions
- OHS requires multiple failing mechanisms that OSA patients compensate for, including impaired central respiratory drive, severe mechanical respiratory dysfunction, inadequate respiratory muscle strength, and chronic nocturnal hypoventilation 5
Clinical Consequences
- OHS carries substantially worse prognosis, including increased mortality rates compared to eucapnic obese patients with OSA 1, 2
- Pulmonary hypertension develops in 30-88% of OHS patients versus lower rates in OSA alone 4, 5
- Chronic heart failure and cor pulmonale from chronic hypoxemia and hypercapnia are more common in OHS patients 1, 5
Diagnostic Algorithm
- For obese patients with known or suspected sleep-disordered breathing, the American College of Chest Physicians recommends measuring arterial PaCO₂ directly if there is a high pretest probability, or using serum bicarbonate as an initial screen if the probability is low to moderate 1, 2
- PaCO₂ > 45 mm Hg at sea level during wakefulness confirms OHS, if other causes are excluded 1, 2, 6