Obstructive Sleep Apnea (osa) Is a Common Disorder Characterized by Recurrent Col- Lapse of the Upper Airway Leading to Repeti-

429 OBSTRUCTIVE SLEEP APNEA (OSA) IS A COMMON DISORDER CHARACTERIZED BY RECURRENT COLLAPSE OF THE UPPER AIRWAY LEADING TO REPETITIVE episodes of hypoxemia, hypercapnia, and arousal from sleep. Sleep-induced decrements in lung volume are believed to result in increased upper airway collapsibility and contribute to inspiratory flow limitation,1 although the exact mechanisms have not been clearly delineated. Our group has recently demonstrated that, in OSA patients, increased end expiratory lung volume (EELV) decreases the number of respiratory events during sleep.2,3 These data suggest that increments in lung volume have a stabilizing effect on the upper airway during sleep in OSA patients. In these studies, lung volume was increased using an iron lung with a constant negative extrathoracic pressure, which would be impractical to use at home. We therefore sought to determine the effect of expiratory positive airway pressure (EPAP) on EELV and sleep disordered breathing in OSA patients, under the assumption that such an approach may be easier to achieve in the home than an iron lung apparatus. METHODS