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.
Design: Prospective cohort study.
Setting: Polyvalent intensive care unit of a teaching hospital of Jaen, Spain.
Patients and methods: Twelve long-term mechanical ventilated patients who met criteria to initiate weaning from the ventilator.
Interventions: 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.
Main variables of interest: 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.