Obesity Hypoventilation Syndrome: Unloading the Respiratory System with Expiratory Pressure.

Obesity hypoventilation syndrome (OHS) is traditionally interpreted through the lens of coexistent obstructive sleep apnea (OSA), yet the mechanisms by which continuous positive airway pressure (CPAP) improves daytime hypercapnia without providing ventilatory support remain incompletely explained. This Perspective proposes that abdominal obesity-induced depletion of expiratory reserve volume (ERV) and functional residual capacity (FRC), with consequent reduction in end-expiratory lung volume (EELV), may represent upstream mechanical abnormalities linking OSA, increased work of breathing, sleep hypoventilation, and daytime hypercapnia. Reduced EELV worsens pharyngeal collapsibility by decreasing caudal traction, while simultaneously shifting tidal volume toward a mechanically inefficient portion of the respiratory system curve, characterized by impaired compliance, and increased elastic, resistive respiratory work and possibly threshold loading. Within this framework, OHS emerges when respiratory burden (the mechanical load on the pulmonary apparatus due to excess weight) exceeds ventilatory reserve (an ensemble of respiratory drive and its modifiers such as sleep-state control, leptin signaling, and genetic factors). CPAP and expiratory PAP may provide, in addition to upper-airway splinting, a mechanical unloading by raising EELV, improving respiratory mechanics, and reducing the pressure cost of ventilation. In this context, weight loss represents the ultimate structural unloading therapy, whereas pressure support may be required when unloading alone is insufficient to restore adequate ventilation. Future studies should measure ERV, FRC/EELV, respiratory effort, and mechanics to better phenotype OHS, and understand whether optimizing expiratory pressure before adding ventilatory support actually improves ventilation.