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) (ΔP) during PAV. We conjectured that deducing muscle pressure from ΔP 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 ΔP (P(mus, aw)) and inspiratory pressure time product derived from oesophageal pressure (PTP(oes)) and from ΔP (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 cmH(2)O•s/min at PAV80, PTP(oes, PEEPi) 277.3 ± 96.4 at PAV20 vs. 121.4 ± 71.6 cmH(2)O•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 cmH(2)O, there was a 93% probability of PTP(oes) <224 cmH(2)O•s/min and 88% probability of PTP(oes, PEEPi) < 255 cmH(2)O•s/min. CONCLUSION: Deducing maximal muscular pressure from ΔP during PAV has limited accuracy. The extrapolated pressure time product from ΔP 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 cmH(2)O, there was a 90% probability of PTP(oes) and PTP(oes, PEEPi) within acceptable ranges. This information should be considered when applying ΔP to set PAV under various gains.