The passive pharyngeal critical closing pressure (Pcrit) is measured using a series of pressure drops. However, pressure drops also lower end-expiratory lung volume (EELV), which independently affects Pcrit. We describe a technique to measure Pcrit at a constant EELV. Continuous positive airway pressure (CPAP)-treated obstructive sleep apnea (OSA) patients and controls were instrumented with an epiglottic catheter, magnetometers (to measure change in EELV), and nasal mask/pneumotachograph and slept supine on nasal CPAP. Pcrit was measured in standard fashion and using our novel "biphasic technique" in which expiratory pressure only was lowered for 1 min before the inspiratory pressure was dropped; this allowed EELV to decrease to the drop level before performing the pressure drop. Seven OSA and three controls were studied. The biphasic technique successfully lowered EELV before the inspiratory pressure drop. Pcrit was similar between the standard and biphasic techniques (-0.4 ± 2.6 vs. -0.6 ± 2.3 cmH(2)O, respectively, P = 0.84). Interestingly, we noted three different patterns of flow limitation: 1) classic Starling resistor type: flow fixed and independent of downstream pressure; 2) negative effort dependence within breaths: substantial decrease in flow, sometimes with complete collapse, as downstream pressure decreased; and 3) and negative effort dependence across breaths: progressive reductions in peak flow as respiratory effort on successive breaths increased. Overall, EELV changes do not influence standard passive Pcrit measurements if breaths 3-5 of pressure drops are used. These results also highlight the importance of inspiratory collapse in OSA pathogenesis. The cause of negative effort dependence within and across breaths is not known and requires further study.