The cause of arrhythmias during PDP is uncertain. Huseby et investigated oxygenation during PDP after noting arrhythmias during PDP, apparently sus pecting hypoxemia. We have previously demonstrated that some acutely ill patients experience hypoxemia during PDP.2 However, even though 18 patients had arterial blood gases drawn during PDP, none had a major fall in PaO2. There was a tendency for NA and MNA patients to have improved oxygenation following PDP, whereas the MJA group had a decline in PaO2. One of the MJA patients had development of hypoxe mia, hypercarbia, and acidosis in positions 2 and 3. Arterial oxygen decreased from 67 to 60 and 61 mm Hg, arterial carbon dioxide increased from 43 to 55 and 51 mm Hg, while the pH decreased from 7.34 to 7.29 and 7.31 . Therefore, it is possible that hypoxemia with acidosis may have been a contributing factor in this patient's arrhythmias.