Introduction Definition supplemental oxygen given to a patient with chronic hypercapnia worsens CO2 retention chronic hypercapnia seen in longstanding COPD as a result of V/Q mismatching (increased dead space) higher minute ventilation required to maintain normal PaCO2 Pathophysiology blood pH, determined by changes in PaCO2, is main driver of ventilation peripheral and central chemoreceptors elevated PaCO2 normally triggers hyperventilation chronic elevations alter sensitivity of respiratory center to CO2 and new threshold is set this makes O2 receptors in the carotid bodies, and hypoxemia an important driver for respiratory activity in patients with COPD Changes with oxygen supplementation that result in increased PaCO2 Decreased minute ventilation hypoxic drive for ventilation may be reduced, but this is likely not major factor in worsening hypercapnia decrese in minute ventilation is insufficient to fully explain rise in CO2 Increased dead space to tidal volume ratio worsens V/Q matching because of loss of hypoxic vasoconstriction most important contributing factor to worsening hypercapnia Haldane effect oxyhemoglobin binds CO2 less avidly than deoxyhemoglobin increased dissolved CO2 in blood = increased PaCO2 Presentation altered mental status after supplemental oxygen as a result of acute on chronic respiratory acidosis and elevated PaCO2