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

Abstract

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.

Fig. 1

Percentage of difference between measured airway pressure ($P_{{\text{aw}}_{\text{meas}}}$) and theoretical airway pressure ($P_{{\text{aw}}_{\text{th}}}$) (%ΔP _aw) at 25, 50, 75 and 100 % of inspiration with different lung mechanics with gain 30 % (a) and 60 % (b). %ΔP _aw is expressed in percentage of $P_{{\text{aw}}_{\text{th}}}$ (%ΔP _aw = ($P_{{\text{aw}}_{\text{meas}}}$ − $P_{{\text{aw}}_{\text{th}}}$)/$P_{{\text{aw}}_{\text{th}}}$ × 100). Representative tracing of $P_{{\text{aw}}_{\text{meas}}}$ and $P_{{\text{aw}}_{\text{th}}}$ in 4 respiratory mechanics with gain 30 % (c) and 60 % (d). Black lines $P_{{\text{aw}}_{\text{th}}}$ waveforms; blue lines $P_{{\text{aw}}_{\text{meas}}}$ waveforms. Inspiratory trigger = 5 L/min; muscular pressure = 10 cmH₂O; PEEP = 5 cmH₂O; respiratory rate = 20/min. Respiratory mechanics; normal: resistance (R) = 10 cmH₂O/L/s and compliance (C) = 60 mL/cmH₂O; obstructive: R = 20 cmH₂O/L/s and C = 60 mL/cmH₂O; restrictive: R = 10 cmH₂O/L/s and C = 30 mL/cmH₂O; and mixed: R = 20 cmH₂O/L/s and C = 30 mL/cmH₂O

Fig. 2

Percentage of difference between measured airway pressure and theoretical airway pressure (%ΔP _aw) at 25, 50, 75 and 100 % of inspiration with different inspiratory trigger (IT) (a), muscular pressure (P _mus) (b) and positive end-expiratory pressure (PEEP) (c) under normal respiratory mechanics. Difference between $P_{{\text{aw}}_{\text{meas}}}$ and $P_{{\text{aw}}_{\text{th}}}$ is expressed in percentage of $P_{{\text{aw}}_{\text{th}}}$ (%ΔP _aw = ($P_{{\text{aw}}_{\text{meas}}}$ − $P_{{\text{aw}}_{\text{th}}}$)/$P_{{\text{aw}}_{\text{th}}}$ × 100). Resistance = 10 cmH₂O/L/s; compliance = 60 mL/cmH₂O; gain = 60 %; and respiratory rate = 20/min; a different IT at 0.8, 5 and 15 L/min; P _mus = 10 cmH₂O; PEEP = 5 cmH₂O. b Different P _mus at 10 and 15 cmH₂O; IT 5 L/min; PEEP = 5 cmH₂O; c different PEEP at 0 and 5 cmH₂O; IT 5 L/min; P _mus 10 cmH₂O

Fig. 3

Percentage of difference between measured airway pressure ($P_{{\text{aw}}_{\text{meas}}}$) and theoretical airway pressure ($P_{{\text{aw}}_{\text{th}}}$) (%ΔP _aw) at 25, 50, 75 and 100 % of inspiration with different respiratory rates in obstructive respiratory mechanics (a). Difference between $P_{{\text{aw}}_{\text{meas}}}$ and $P_{{\text{aw}}_{\text{th}}}$ is expressed in percentage of $P_{{\text{aw}}_{\text{th}}}$(% ΔP _aw = ($P_{{\text{aw}}_{\text{meas}}}$ − $P_{{\text{aw}}_{\text{th}}}$)/$P_{{\text{aw}}_{\text{th}}}$ × 100). Representative tracing of $P_{{\text{aw}}_{\text{th}}}$ and $P_{{\text{aw}}_{\text{meas}}}$ (b). Black lines $P_{{\text{aw}}_{\text{th}}}$ waveforms. Blue lines $P_{{\text{aw}}_{\text{meas}}}$ waveforms. Resistance = 20 cmH₂O/L/s; compliance = 60 mL/cmH₂O; gain = 60 %; inspiratory trigger = 5 L/min; PEEP = 0 cmH₂O; and P _mus = 10 cmH₂O

Fig. 4

Correlation between the total work of breathing calculated by the ventilator (${\text{WOB}}_{{\text{tot}}_{\text{displayed}}}$) and the corresponding calculated by Campbell (${\text{WOB}}_{{\text{tot}}_{\text{real}}}$)

Fig. 5

Bland–Altman plot of total inspiratory work of breathing measurements, expressed in J/L, between the two methods compared (Campbell and Ventilator). ${\text{WOB}}_{{\text{tot}}_{\text{displayed}}}$, inspiratory work of breathing calculated by the ventilator; ${\text{WOB}}_{{\text{tot}}_{\text{real}}}$, inspiratory work of breathing calculated by the Campbell diagram