Wijesinghe et al randomized and crossed-over 24 people who were recently diagnosed with obesity hypoventilation syndrome to breathe either 100% oxygen or room air for 20 minutes on 2 separate days, while measuring their minute ventilation, expired tidal pCO2, and dead space-to-tidal volume ratio. In 44% of patients, pCO2 increased by at least 4 mm Hg on 100% O2; in 3 patients, pCO2 increased by 10 mm Hg within 15 minutes. Only 5 experienced no change or a slight decline in pCO2. The worsened hypercarbia was accompanied by a 13% decrease in minute ventilation while breathing oxygen. Those more hypoxic at baseline exhibited the largest increases in hypercarbia.(n=24) CHEST 2011;139:1018-1024.