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

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.