Pathophysiology of Obesity Hypoventilation Syndrome

Mechanical Respiratory Dysfunction

The American Thoracic Society notes that increased work of breathing and oxygen cost result from the physical weight on the chest wall and thoracic cage, which mechanically interferes with lung expansion 1, 2, 3
Low lung volumes and decreased respiratory system compliance develop as abdominal fat mass restricts diaphragmatic excursion, particularly when lying supine 1, 2, 3
Increased small airway resistance occurs due to poor expansion of lung bases and altered chest wall mechanics 1, 2
Reduced respiratory capacity when supine worsens as abdominal pressure pushes up the diaphragm, increasing intrathoracic pressure 3, 4

The American College of Chest Physicians states that decreased ventilatory responsiveness to CO2 represents impaired central respiratory drive, preventing appropriate compensatory hyperventilation despite rising PaCO2 1, 2
Shallow and inefficient breathing patterns develop as the respiratory center adapts maladaptively to chronic hypercapnia 3, 4

The American Academy of Sleep Medicine reports that obstructive sleep apnea coexists in approximately 90% of OHS patients, with nearly 70% having severe OSA (AHI >30 events/h) 5, 6
Nocturnal alveolar hypoventilation occurs even during periods without discrete apneas or hypopneas 1, 2
Inadequate respiratory muscle strength fails to meet the increased ventilatory demand during sleep, when respiratory drive is naturally reduced 3, 4

Diurnal hypercapnia and hypoxia induce pathologic cardiovascular effects including pulmonary hypertension, right ventricular hypertrophy, and cor pulmonale 1, 2
Resting daytime hypoxemia and hypercarbia (PaCO2 >45 mm Hg) develop as the final common pathway of these multiple mechanisms 1, 2, 5, 6

Pulmonary hypertension and cor pulmonale develop from chronic hypoxemia and hypercapnia 1, 2
Increased risk of acute-on-chronic hypercapnic respiratory failure requiring hospitalization 5, 6
Higher mortality rates compared to eucapnic obese patients with sleep-disordered breathing 5, 6
Impaired exercise tolerance, disability, and reduced quality of life result from the combined cardiopulmonary dysfunction 1, 2