OBJECTIVE To study the minimum assistance level during proportional assist ventilation (PAV) to decrease the work of breathing to physiological limits (0.6 j/l) and the relationship between breathing pattern changes and respiratory effort at different PAV levels. METHODS Prospective cohort study. METHODS Polyvalent intensive care unit of a teaching hospital of Jaen, Spain. METHODS Twelve long-term mechanical ventilated patients who met criteria to initiate weaning from the ventilator. METHODS We used the Puritan-Bennett 840 ventilator in proportional assist ventilation. The percentage of support was randomly modified between 5% and 80%, in intervals of 10%. Prior to the change in the PAV level, the patients were ventilated in assist-volume control followed by pressure support ventilation. METHODS Before PAV, we measured the respiratory mechanics and the breathing pattern and work of breathing during this mode. RESULTS The decrease in respiratory assist in PAV was related to significantly higher work of breathing, this going from 0.2+/-0,07 (0.1-0.3) j/l with PAV80 to 0.9+/-0.2 (0.4-1.5) j/l with PAV5 (p=0.002). The coefficient correlation between PAV level and work of breathing (measured as j/l and j/min) was r=-0.8 and -0.6, respectively. Minimum PAV level related with physiological work of breathing was 30% (0.63+/-0.13 j/l). Except for the tidal volume that increased significantly (PAV80 vs PAV5=0.4+/-0.1 vs 0.3+/-0.1; p=0.02), the remaining variables defining the breathing pattern did not changed with the increase in PAV. CONCLUSIONS In the group of patients studied, the increase in the PAV levels decreases work of breathing, without significantly changing the breathing pattern. Levels lower than 30% of PAV are associated to excessive work of breathing.