0989. Accuracy of delivered airway pressure during proportional assist ventilation +. A bench study

Proportional assist ventilation (PAV+) is a partial ventilatory support mode delivering airway pressure (Paw) in proportion to patient effort, enhancing patient-ventilator interactions. The ventilator estimates muscular pressure by using the respiratory system equation of motion with the instantaneous volume (V) and flow (V´) and the automatically calculated compliance and resistance. The mode gains in popularity but the accuracy of the delivered Paw by PAV+ is unknown.


Introduction
Proportional assist ventilation (PAV+) is a partial ventilatory support mode delivering airway pressure (Paw) in proportion to patient effort, enhancing patient-ventilator interactions. The ventilator estimates muscular pressure by using the respiratory system equation of motion with the instantaneous volume (V) and flow (V´) and the automatically calculated compliance and resistance. The mode gains in popularity but the accuracy of the delivered Paw by PAV+ is unknown.

Objectives
To assess the accuracy of PAV+ by comparing the delivered Paw by the ventilator (Paw meas ) to the theoretical Paw as defined by the equation of motion (Paw Th ) and to examine the factors influencing this accuracy.

Results
Irrespective of respiratory mechanics and gain, mean Paw meas was lower than mean Paw Th , Table1.
This underassistance by the ventilator was greatest at the beginning (25%) of the cycle and decreased later (75%) in inspiration. These findings were replicated under different IT, Pmus or PEEP settings. A high IT led to greater underassistance at the end of inspiration versus a low IT. A high Pmus was associated with a greater underassistance during the entire inspiration versus a low Pmus. A decrease in PEEP was associated with a major underassistance at the start of the inspiration. A higher RR resulted in a higher %Δ, showing that PEEPi increases total trigger delay and affects PAV+ accuracy, fig. 1. Combining the data from all conditions, PEEPi was correlated with the mean %Δ (R 2 = 0.61, p < 0.001).
Conclusions PAV+ assistance is globally accurate compared to Paw Th even if underassistance is often observed, especially at the start of inspiration. PEEPi leading to increased trigger delay is a major factor contributing to PAV+ inaccuracy. Clinical recommandations should include using a high trigger sensitivity and a careful PEEP titration when PEEPi is suspected.  All conditions 8.5 11.4 -2.9 -25.4 Figure 1 Representative tracings of measured airway pressure (Paw meas ) and theoretical airway pressure (Paw Th ) with progressive increases in rate (RR) and PEEPi. Black lines: Paw Th wareforms. Blue lines: Paw meas waveforms. Inspiratory trigger = 5 L.min; PEEP = 0 cmH 2 O; gain = 60%; Pmus = 10 cmH 2 O; Resistance = 20 cmH 2 O/L/s and compliance = 60 mL/cmH 2 O.