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Background  Proportional assist ventilation+ (PAV+) delivers airway pressure ( P  aw ) in proportion to patient effort ( P  mus ) by using the equation of motion of the respiratory system. PAV+ calculates automatically respiratory mechanics (elastance and resistance); the work of breathing (WOB) is estimated by the ventilator. The accuracy of  P  mus  estimation and hence accuracy of the delivered  P  aw  and WOB calculation have not been assessed. This study aimed at assessing the accuracy of delivered  P  aw  and calculated WOB by PAV+ and examining the factors influencing this accuracy.    Methods  Using an active lung model with different respiratory mechanics, we compared (1) the actual delivered  P  aw  by the ventilator to the theoretical  P  aw  as defined by the equation of motion and (2) the WOB value displayed by the ventilator to the WOB measured from a Campbell diagram.    Results  Irrespective of respiratory mechanics and gain, the ventilator provided a  P  aw  approximately 25 % lower than expected. This underassistance was greatest at the beginning of the inspiration. Intrinsic PEEP (PEEPi), associated with an increase in trigger delay, was a major factor affecting PAV+ accuracy. The absolute value of total WOB displayed by the ventilator was underestimated, but the changes in WOB were accurately detected by the ventilator.    Conclusion  The assistance provided by PAV+ well follows  P  mus  but with a constant underassistance. This is associated with an underestimation by the ventilator of the WOB. PEEPi can be a major factor contributing to PAV+ inaccuracy. Clinical recommendations should include using a high trigger sensitivity and a careful PEEP titration.    Electronic supplementary material  The online version of this article (doi:10.1186/s13613-016-0131-y) contains supplementary material, which is available to authorized users.

Accuracy of delivered airway pressure and work of breathing estimation during proportional assist ventilation: a bench study