We conducted a study to assess the validity of the occlusion pressure (P0.1) measured during activation of the trigger mechanism (P0.1(aw)trig) in patients showing variable levels of PEEPi during pressure-support ventilation. We first compared P0.1(aw)trig and P0.1 measured with the conventional method (i.e., the airway pressure drop after the first 100 ms of an occluded inspiration) in 16 patients with chronic obstructive pulmonary disease (COPD). We observed good agreement and a highly significant correlation (r = 0.99; bias = 0.3 +/- 0.5 cm H20) between the two methods. In a second part of the study, we compared, in 17 patients, P0.1(aw)trig with (P0.1(es)), measured as the depression generated on the esophageal pressure tracing in the first 100 ms of the inspiratory negative swing, and with P0.1 measured on the P(es) tracing simultaneously with P(aw)trig (P0.1(es-synchro)). Our results showed a good correlation and good agreement between P(aw)trig and P0.1(es) (r = 0.92; bias = 0.3 +/- 0.5 cm H20); P(aw)trig and P0.1(es-synchro) (r = 0.97; bias = 0.1 +/- 0.2 cm H20); and P0.1(es) and P0.1(es-synchro) (r = 0.95, bias = 0.2 +/- 0.4 cm H20), respectively. This study suggests that reliable measurements of inspiratory drive can be obtained easily, on a breath-by-breath basis, from airway pressure tracings during pressure-support ventilation in patients with variable levels of PEEPi.