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

Po-Lan Su, Pei-Shan Kao, Wei-Chieh Lin, Pei-Fang Su, Chang-Wen Chen, Po-Lan Su, Pei-Shan Kao, Wei-Chieh Lin, Pei-Fang Su, Chang-Wen Chen

Abstract

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 (Pmus, oes) and from ΔP (Pmus, aw) and inspiratory pressure time product derived from oesophageal pressure (PTPoes) and from ΔP (PTPaw) were determined from the last minute of each level. Pmus, oes and PTPoes with consideration of PEEPi were expressed as Pmus, oes, PEEPi and PTPoes, PEEPi, respectively. Pressure time product was expressed as per minute (PTPoes, PTPoes, PEEPi, PTPaw) and per breath (PTPoes, br, PTPoes, PEEPi, br, PTPaw, br).

Results: PAV significantly reduced the breathing effort of patients with increasing PAV gain (PTPoes 214.3 ± 80.0 at PAV20 vs. 83.7 ± 49.3 cmH2O•s/min at PAV80, PTPoes, PEEPi 277.3 ± 96.4 at PAV20 vs. 121.4 ± 71.6 cmH2O•s/min at PAV80, p < 0.0001). Pmus, aw overestimates Pmus, oes for low-gain PAV and underestimates Pmus, oes for moderate-gain to high-gain PAV. An optimal Pmus, aw could be achieved in 91% of cases with PAV60. When the PAV gain was adjusted to Pmus, aw of 5-10 cmH2O, there was a 93% probability of PTPoes <224 cmH2O•s/min and 88% probability of PTPoes, PEEPi < 255 cmH2O•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 Pmus, aw of 5-10 cmH2O, there was a 90% probability of PTPoes and PTPoes, PEEPi within acceptable ranges. This information should be considered when applying ΔP to set PAV under various gains.

Keywords: Airway pressure; Pressure time product; Proportional assist ventilation.

Figures

Fig. 1
Fig. 1
Graphic illustration of flow, airway pressure (Paw), and oesophageal pressure tracing (Poes) during proportional assist ventilation. Chest wall recoil pressure (Pcw) was calculated from the product of tidal volume and dynamic chest wall elastance. Upper bound oesophageal pressure time product (PTPoes, PEEPi) was calculated as the integration of the difference between Pcw, PEEPi and Poes. Lower bound oesophageal pressure time product (PTPoes) was calculated as the integration of the difference between Pcw and Poes. Pmus, oes and Pmus, oes, PEEPi represent the maximal difference between passive and active Poes
Fig. 2
Fig. 2
Tidal volume, peak airway pressure (Paw, peak) and respiratory mechanics during proportional assist ventilation (PAV) under different gains. PAV20 indicates a mean gain level of 20%. Significant differences in tidal volume were found between PAV60 vs. PAV20, PAV70 vs. PAV20, PAV70 vs. PAV30, and PAV70 vs. PAV40. Significant differences in Paw, peak were found among individual Paw, peak levels under different gains, except the Paw, peak of PAV20 vs. Paw, peak of PAV30 and Paw, peak of PAV70 vs. Paw, peak of PAV80. For PAV-based patient elastance (Epav), significant differences were found between PAV20 vs. PAV50, PAV60, PAV70, and PAV80; PAV30 vs. PAV50, PAV60, PAV70, and PAV80; PAV40 vs. PAV50, PAV60, PAV70, and PAV80; PAV50 vs. PAV70 and PAV80; PAV60 vs. PAV80; and PAV70 vs. PAV80. No significant difference was found in PAV-based patient resistance (Rpav) among various gains. For the Epav and Rpav comparison, one patient was not included because of insufficient numbers of Epav and Rpav in PAV20 and PAV30
Fig. 3
Fig. 3
Inspiratory pressure time product (PTP) under different gain levels. PTP calculated from the difference between the oesophageal pressure and the relaxed chest wall elastance curve (PTPoes) decreased progressively with increasing gain with or without intrinsic positive end-expiratory pressure (PEEPi). For PTPoes, a significant difference was found between proportional assist ventilation 20% gain (PAV20) vs. PAV40, PAV50, PAV60, PAV70, and PAV80; PAV30 vs. PAV50, PAV60, PAV70, and PAV80; PAV40 vs. PAV60, PAV70, and PAV80; PAV50 vs. PAV70 and PAV80; PAV60 vs. PAV80; and PAV70 vs. PAV80. Similar patterns were found with PTPoes, PEEPi. Values in parentheses are the number of breaths analysed in each gain level
Fig. 4
Fig. 4
a Maximum muscular pressure (Pmus) determined using either oesophageal pressure tracing or airway pressure under different proportional assist ventilation (PAV) gains. Significant differences (p < 0.05) were observed for all gain levels. b Bland-Altman analysis plot showing bias and agreement between maximal muscular pressure calculated from ΔP and PAV gain (Pmus, aw) and maximal muscular pressure calculated from maximum difference between passive and active Poes without consideration of PEEPi (Pmus, oes). The middle dashed line is the mean difference (bias). The outer dashed line is the 95% confidence interval of the difference between Pmus, aw and Pmus, oes (±1.96 SD)

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