• Francisco Ruiz-Ferrón, Juan Machado, Antonia Morante, Silvia Galindo, Ana Castillo, and Luis Rucabado.
• Servicio de Cuidados Críticos y Urgencias, Unidad de Medicina Intensiva, Hospital Médico-Quirúrgico, Complejo Hospitalario de Jaén, Jaén, España. frferron@terra.es
• Med Intensiva. 2009 Aug 1;33(6):269-75.

ObjectiveTo 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.DesignProspective cohort study.SettingPolyvalent intensive care unit of a teaching hospital of Jaen, Spain.Patients And MethodsTwelve long-term mechanical ventilated patients who met criteria to initiate weaning from the ventilator.InterventionsWe 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 InterestBefore PAV, we measured the respiratory mechanics and the breathing pattern and work of breathing during this mode.ResultsThe 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.ConclusionsIn 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.

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