Limited predictability of maximal muscular pressure using the difference between peak airway pressure and positive end-expiratory pressure during proportional assist ventilation (PAV)

Background: If the proportional assist ventilation (PAV) level is known, muscular effort can be estimated from the difference between peak airway pressure and positive end-expiratory pressure (PEEP) (∆P) during PAV. We conjectured that deducing muscle pressure from ∆P may be an interesting method to set PAV, and tested this hypothesis using the oesophageal pressure time product calculation. Methods: Eleven mechanically ventilated patients with oesophageal pressure monitoring under PAV were enrolled. Patients were randomly assigned to seven assist levels (20-80%, PAV20 means 20% PAV gain) for 15 min. Maximal muscular pressure calculated from oesophageal pressure (P_{mus, oes}) and from ∆P (P_{mus, aw}) and inspiratory pressure time product derived from oesophageal pressure (PTP_oes) and from ∆P (PTP_aw) were determined from the last minute of each level. P_{mus, oes} and PTP_oes with consideration of PEEPi were expressed as P_{mus, oes, PEEPi} and PTP_{oes, PEEPi}, respectively. Pressure time product was expressed as per minute (PTP_oes, PTP_{oes, PEEPi}, PTP_aw) and per breath (PTP_{oes, br}, PTP_{oes, PEEPi, br}, PTP_{aw, br}). Results: PAV significantly reduced the breathing effort of patients with increasing PAV gain (PTP_oes 214.3 ± 80.0 at PAV20 vs. 83.7 ± 49.3 cmH₂O•s/min at PAV80, PTP_{oes, PEEPi} 277.3 ± 96.4 at PAV20 vs. 121.4 ± 71.6 cmH₂O•s/min at PAV80, p < 0.0001). P_{mus, aw} overestimates P_{mus, oes} for low-gain PAV and underestimates P_{mus, oes} for moderate-gain to high-gain PAV. An optimal P_{mus, aw} could be achieved in 91% of cases with PAV60. When the PAV gain was adjusted to P_{mus, aw} of 5-10 cmH₂O, there was a 93% probability of PTP_oes <224 cmH₂O•s/min and 88% probability of PTP_{oes, PEEPi} < 255 cmH₂O•s/min. Conclusion: Deducing maximal muscular pressure from ∆P during PAV has limited accuracy. The extrapolated pressure time product from ∆P is usually less than the pressure time product calculated from oesophageal pressure tracing. However, when the PAV gain was adjusted to P_{mus, aw} of 5-10 cmH₂O, there was a 90% probability of PTP_oes and PTP_{oes, PEEPi} within acceptable ranges. This information should be considered when applying ∆P to set PAV under various gains.