Comparison of airway opening pressure measurements: low-constant-flow vs. constant low-slope pressure ramp methods

The airway closure phenomenon occurs when the airway collapses, isolating the proximal airway from the distal alveoli. Airway opening pressure (AOP) is required to reopen closed airways. Two methods are available to measure AOP: the low-constant flow (AOP_flow) and constant low-slope pressure ramp methods (AOP_pres). The discrepancies between these two methods remain unclear. We investigated whether there is a difference between AOP_flow and AOP_pres when used for mechanically ventilated patients.

In this single-center retrospective observational study, we included 42 patients who were mechanically ventilated owing to respiratory failure in the intensive care unit of a university hospital between January 2023 and October 2024. AOP was measured using two methods: AOP_flow (5 L/min) and AOP_pres (2 cmH₂O/s). Agreement and correlation between the two methods were evaluated using Bland-Altman plots, Passing-Bablok regression, and Spearman's rank correlation.

AOP measured using AOP_flow (median: 4.7 cmH₂O) was higher than that measured using AOP_pres (median: 1.9 cmH₂O, P < 0.001). Nevertheless, the two were strongly correlated (ρ = 0.86, P < 0.001) in all patients. The regression equation was y = 1.39x + 0.90 (95% confidence interval [CI] for slope b: 1.20-1.65, 95% CI for intercept a: 0.48-1.64). In patients with AOP_flow ≥5cmH₂O, AOP_flow was moderately correlated with AOP_pres (ρ = 0.77, P < 0.001). The regression equation was y = 0.82x + 4.51 (95% CI for slope b: 0.50-1.17, 95% CI for intercept a: 2.57-6.72). The rate of pressure increase from the pressure at the beginning (0 cmH₂O) of inflation up to AOP_flow was 26.4 cmH₂O/s (10.5-30.0) in all patients and was moderately correlated with the difference between AOP_flow and AOP_pres (ρ = 0.61, P < 0.001).

Systematic biases were observed between AOP_flow and AOP_pres, with AOP_flow tending to yield higher values. However, the physiological significance of the AOP values obtained from each method remains unclear, and caution is needed for clinical application.