Abstract
Proportional assist ventilation (PAV) is a patient-triggered mode in which the elastic and resistive work of breathing (WOB) can be unloaded. The unloading level is set with the aim of achieving an optimal balance of the WOB between the ventilator and the patient. In vitro studies, however, have shown that during PAV, there are trigger delays and the pressure wave forms of the ventilator and lung models differ which may influence the relationship between the level of unloading and the WOB of the patient and ventilator.
Aim: To assess the WOB of the ventilator and a lung model with increasing amounts of elastance unloading.
Methods: A dynamic lung model, with a compliance of 0.4ml/cmH2O, was connected to a Stephanie ventilator in PAV mode. Elastance unloading was applied in stepwise increments and pressure, flow and tidal volume were recorded. Pressure/volume loops were constructed, from which the WOB was calculated for the ventilator and the lung model.
Results: The median tidal volume of breaths analysed was 3.75 (range 3.1 to 4.4) ml. As the elastance unloading was increased, the lung model showed a greater reduction in the elastic WOB than the increase of WOB of the ventilator.
The WOB in the lung model, however, was underestimated by the distortion of the inflation pressure waveform due to the trigger delay.
Conclusion: Although elastance unloading during PAV enables transfer of the WOB from the patient to ventilator, the infant has no support during the trigger delay, thus the benefit is over-estimated.
- © 2012 ERS
