How should airway opening pressure (AOP) be interpreted in mechanically ventilated patients?

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Last updated: September 16, 2026 • View editorial policy

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Airway Opening Pressure Interpretation

Airway opening pressure (AOP) represents the minimum pressure required to overcome airway collapse and initiate inspiratory flow in mechanically ventilated patients, and should be measured using low-flow insufflation maneuvers with values typically ranging from 2-12 cmH₂O, though global measurements often underestimate regional AOP by 2-4 cmH₂O.

What AOP Represents Physiologically

AOP indicates the pressure threshold needed to reopen collapsed airways during inspiration 1. When present, it reflects:

  • Elevated pleural pressures - AOP correlates strongly with baseline esophageal pressure (R² = 0.57, P = 0.005), with patients having AOP showing mean Pes of 11.3 ± 4.3 cmH₂O versus 7.0 ± 5.5 cmH₂O in those without 1
  • Auto-PEEP presence - Auto-PEEP correlates with both elevated Pes (R² = 0.51) and AOP (R² = 0.35) 1
  • Airway collapse - 67% of patients with AOP also demonstrate expiratory flow limitation, suggesting bidirectional airway collapse 1

How to Measure AOP

Standard Low-Flow Method

Perform a slow-flow insufflation maneuver (< 10 L/min) starting from PEEP 0-5 cmH₂O. AOP appears as an abrupt change in the pressure-volume curve slope 2, 3.

Alternative Constant-Flow Method (Pcond)

A newer validated approach uses standard 60 L/min constant flow during volume assist-control ventilation 4:

  • Identify the abrupt slope change on the airway pressure waveform at inspiration onset
  • Calculate Pcond as the difference between this pressure level and PEEP
  • Advantages: 93% sensitivity, 91% specificity, better hemodynamic tolerance, and less oxygen desaturation compared to low-flow method 4

Critical Interpretation Pitfalls

Regional Heterogeneity

Global AOP measurements significantly underestimate the highest regional AOP 2, 3:

  • Only 25% of patients show global AOP ≥ 5 cmH₂O, but 53% have regional AOP ≥ 5 cmH₂O 2
  • Global AOP averages 3.0 cmH₂O while highest regional AOP averages 5.0 cmH₂O (difference up to 8 cmH₂O) 2
  • Dependent lung regions have higher AOP than non-dependent regions (4.0 vs 3.0 cmH₂O) 2

Pressure-Time Curve Patterns

The shape of the low-flow pressure-time curve indicates regional AOP variation 2:

  • Progressive slope change (47% of patients): Indicates multiple regional AOPs with large differences between global and highest regional AOP (3.0 cmH₂O difference, maximum 8 cmH₂O)
  • Abrupt change: Smaller regional variation (1.0 cmH₂O difference)
  • No change: Minimal regional variation (1.0 cmH₂O difference)

Body Position Effects

AOP increases in supine versus semi-recumbent position 3:

  • The highest regional AOP rises when moving from 30° semi-recumbent to flat supine
  • This correlates with increased end-expiratory esophageal pressure in supine positioning
  • Consider position-dependent AOP changes when adjusting PEEP

Clinical Application for PEEP Titration

Minimum PEEP Requirements

Set PEEP to exceed the highest regional AOP, not just global AOP, to prevent tidal recruitment/derecruitment 2, 3:

  1. If global AOP detected: Set PEEP at minimum to global AOP value
  2. If progressive slope change pattern: Suspect higher regional AOP; consider PEEP 2-4 cmH₂O above global AOP
  3. If dependent atelectasis suspected: Add 1-2 cmH₂O to account for gravitational gradient

Integration with ARDS Guidelines

For moderate-to-severe ARDS, combine AOP assessment with guideline recommendations 5:

  • Use low tidal volumes (4-8 mL/kg PBW) and plateau pressure ≤ 30 cmH₂O (strong recommendation)
  • Apply higher PEEP strategies when AOP detected (conditional recommendation)
  • Consider recruitment maneuvers if AOP > 5 cmH₂O (conditional recommendation)

Monitoring Caveat

Do not calibrate esophageal balloons below AOP 6:

  • When total PEEP < AOP, the ΔPaw/ΔPes ratio is erroneously low (0.42 vs 0.95)
  • Always perform positive pressure occlusion tests during end-inspiratory occlusion when AOP suspected
  • This prevents unreliable transpulmonary pressure calculations

Special Populations

In obesity, expect higher AOP due to 7:

  • Increased intra-abdominal pressure
  • Elevated pleural pressures
  • Greater prevalence of airway closure
  • Consider electrical impedance tomography for regional assessment in this population

References

7
Research

Obesity: biomechanical implications for mechanical ventilation.

Critical care (London, England), 2026

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