Distribution of airway pressure opening in the lungs measured with electrical impedance tomography (POET): a prospective physiological study

Background

In patients with acute hypoxemic respiratory failure (AHRF) under mechanical ventilation, the change in pressure slope during a low-flow insufflation indicates a global airway opening pressure (AOP) needed to reopen closed airways and may be used for titration of positive end-expiratory pressure.

Objectives

To understand 1) if airways open homogeneously inside the lungs or significant regional AOP variations exist; 2) whether the pattern of the pressure slope change during low-flow insufflation can indicate the presence of regional AOP variations.

Methods

Using electrical impedance tomography, we recorded low-flow insufflation maneuvers (< 10 L/min) starting from end-expiratory positive pressure 0–5 cmH₂O. We measured global (AOP_global) and regional AOPs from pressure-impedance curves in the four different lung quadrants, and compared AOP_global with the highest quadrantal AOP (AOP_highest). We categorized the slope change of the low-flow inflation pressure–time curve into three patterns: no change, progressive change, abrupt change.

Results

Among the 36 patients analyzed, 9 (25%) had AOP_global ≥ 5 cmH₂O whereas 19 (53%) exhibited regional AOP_highest ≥ 5 cmH₂O. AOP_global was on average similar to AOP of the upper right quadrant (P = 0.182) but was lower than AOPs of the other three quadrants (P < 0.01 of each). AOP_global was significantly lower than AOP_highest: 3.0 [2.0–4.3] vs. 5.0 [2.8–8.3] cmH₂O, P < 0.001. AOP was higher in the dependent than the non-dependent ventilated lung (4.0 [2.0–6.3] vs. 3.0 [2.0–5.0] cmH₂O, P < 0.001). Seventeen (47%) patients exhibited a ‘progressive change’ pattern in the pressure–time curve. These patients had a larger difference between AOP_highest and AOP_global (3.0 [2.0–4.0] cmH₂O with a maximum of 8 cmH₂O) compared to the other two patterns: 1.0 [0–1.0] cmH₂O in ‘no change’ , P < 0.001 and 1.0 [0–2.0] cmH₂O in ‘abrupt change’ , P = 0.003.

Conclusion

AOP_global mostly reflects the lowest opening pressure in the lung and frequently underestimates the highest regional AOP in mechanically ventilated patients with AHRF. A progressive slope change during the low-flow pressure–time curve indicates the presence of several and higher regional AOPs.

Key Points

1. Global vs. Regional AOP: The study demonstrates that global AOP often underestimates the highest regional AOP in the lungs, with significant variations between dependent and non-dependent lung regions, emphasizing the need for regional assessment.
2. Heterogeneous Airway Closure: Regional airway closure is more prevalent than global airway closure, with over 50% of patients showing regional AOP ≥ 5 cmH₂O compared to 25% with complete airway closure, highlighting the inhomogeneity of lung mechanics in AHRF.
3. Dependent Lung Regions: AOP is consistently higher in dependent lung regions, influenced by gravitational forces, lung weight, and diaphragm displacement, requiring tailored ventilation strategies to prevent overdistension and collapse.
4. EIT as a Diagnostic Tool: EIT-derived quasi-static pressure-impedance curves provide real-time insights into regional AOP variations, offering a non-invasive approach to guide mechanical ventilation settings and optimize PEEP.
5. Patterns of Pressure-Time Curve: Three distinct slope change patterns were identified in the low-flow pressure-time curve (no change, progressive change, abrupt change), with the progressive pattern linked to the highest regional AOP differences and increased lung injury risk.
6. Implications for ARDS: In patients with ARDS, regional AOP heterogeneity was particularly pronounced, underlining the importance of region-specific assessments to refine lung-protective ventilation in this population.
7. Impact of Obesity: Obese patients exhibited higher AOPs in both global and regional measurements, further supporting personalized approaches to mechanical ventilation based on patient-specific factors.
8. Clinical Applications: The study underscores the potential of regional AOP assessments to inform PEEP titration and reduce ventilator-induced lung injury, with progressive slope patterns serving as a marker for uneven lung mechanics.
9. Limitations in Current Practice: Conventional methods relying solely on global AOP measurements may misguide ventilatory strategies, highlighting the need for advanced diagnostic techniques like EIT to capture regional lung dynamics.
10. Future Directions: Further research is necessary to validate regional AOP measurements in diverse patient populations and clinical scenarios, as well as to explore their integration into routine critical care practice for individualized ventilation strategies.

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