Pathophysiology
Clinical meaning
Hypoventilation syndromes are characterized by inadequate alveolar ventilation resulting in chronic CO2 retention (hypercapnia, PaCO2 >45 mmHg) and secondary hypoxemia. Alveolar ventilation = (tidal volume - dead space) × respiratory rate. Anything reducing tidal volume, increasing dead space, or decreasing respiratory rate causes hypoventilation. In chronic hypoventilation, medullary chemoreceptors become desensitized to elevated CO2 (CO2 narcosis), shifting the primary breathing stimulus to hypoxemia detected by peripheral chemoreceptors. This has critical implications: high-flow oxygen can remove the hypoxic drive, causing respiratory depression and potentially fatal arrest. Obesity hypoventilation syndrome (OHS/Pickwickian syndrome) is the most common form: BMI >30 with daytime hypercapnia (PaCO2 >45) not explained by other conditions. Excess adipose tissue restricts diaphragmatic excursion, reduces chest wall compliance, and increases work of breathing. ~90% have concurrent OSA. Other causes include neuromuscular disease (ALS, myasthenia), severe kyphoscoliosis, chronic opioid use, and advanced COPD. Complications include cor pulmonale (right heart failure from chronic hypoxemia-induced pulmonary vasoconstriction and pulmonary hypertension), polycythemia (erythropoietin response to chronic hypoxemia), and CO2 narcosis. Treatment centers on BiPAP (bilevel positive airway pressure), which provides IPAP to augment tidal volume...