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To compare short-term administration of noninvasive proportional assist ventilation (NIV-PAV) and pressure support ventilation (NIV-PSV).Prospective, crossover, randomized study.Medicosurgical intensive care unit in a nonteaching hospital.Twelve chronic obstructive pulmonary disease patients admitted for hypercapnic acute respiratory failure.NIV-PSV and NIV-PAV given in a randomized order after baseline evaluation in continuous positive airway pressure. Using a flow-triggering ventilator, NIV-PAV was adjusted using the runaway method and compared with NIV-PSV at similar peak inspiratory airway pressure.Flow, airway pressure, and changes in esophageal pressure were measured and the tidal volume, the patient’s inspiratory work of breathing, and the esophageal pressure–time product were calculated. Arterial pH and Paco2 were measured and breathing comfort was assessed using a visual analogic scale. Peak inspiratory airway pressure (17 ± 3 cm H2O) and tidal volume were similarly increased with the two modalities with no change in respiratory rate. The change in esophageal pressure was similarly decreased (from 20 ± 8 cm H2O in continuous positive airway pressure to 12 ± 7 in NIV-PSV and 10 ± 5 cm H2O in NIV-PAV) as well as inspiratory muscle effort indexes. Arterial pH and Paco2 were similarly improved. Breathing comfort was significantly improved in NIV-PAV (+38 ± 38%) but not in NIV-PSV (+11 ± 23%). The tidal volume was more variable in NIV-PAV (89 ± 18%) than in NIV-PSV (15 ± 8%) and changes in tidal volume variability were significantly correlated (p = .02) with changes in breathing comfort.In chronic obstructive pulmonary disease patients with hypercapnic acute respiratory failure, NIV-PAV was able to unload inspiratory muscles similarly to NIV-PSV but may be more comfortable than NIV-PSV.