What is Airway Pressure Release Ventilation (APRV) mode in mechanical ventilation for critically ill patients, particularly those with Acute Respiratory Distress Syndrome (ARDS)?

What is Airway Pressure Release Ventilation (APRV)?

APRV is a pressure-controlled ventilation mode that maintains a continuous high airway pressure (P high) for most of the respiratory cycle with brief intermittent releases to a lower pressure (P low), while allowing unrestricted spontaneous breathing at any point—however, it is not recommended as a primary mode for ARDS because it lacks evidence for improved mortality or outcomes compared to standard lung-protective ventilation. 1, 2

Mechanism and Design

APRV operates through four key parameters that define its function 3:

• P high: The elevated airway pressure maintained for most of the cycle (typically 20-30 cmH₂O), which serves as the primary mechanism for oxygenation and alveolar recruitment 3
• T high: The duration of P high (typically 4-6 seconds), representing the majority of the respiratory cycle 3
• P low: The lower pressure during brief release phases (typically 0-5 cmH₂O) 3
• T low: The duration of pressure release (typically 0.4-0.8 seconds), designed to be very short to prevent alveolar collapse 3

The mode functions as inverse ratio, pressure-controlled, intermittent mandatory ventilation with the distinguishing feature of unrestricted spontaneous breathing throughout all phases 3. This represents a fundamental difference from conventional modes where spontaneous efforts are restricted or synchronized 4.

Theoretical Advantages (Unproven Clinically)

APRV proponents cite several physiological benefits, though these have not translated to improved clinical outcomes 2, 4:

• Alveolar recruitment: The sustained high pressure theoretically maintains open lung units and prevents cyclic collapse 3
• Spontaneous breathing preservation: Allows patients to breathe without restriction, potentially reducing sedation requirements 2, 4
• Hemodynamic effects: May improve cardiac output compared to conventional ventilation in some studies 5
• Lower peak pressures: The brief release mechanism may reduce peak airway pressures compared to volume-controlled modes 6

Critical Evidence Gap and Guideline Position

The American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine 2017 guidelines make no recommendation for APRV use in ARDS, as it was not included in their evidence-based recommendations. 1 The guideline strongly recommends low tidal volume ventilation (4-8 ml/kg predicted body weight) with plateau pressure ≤30 cmH₂O as the standard approach 1.

A 2021 meta-analysis of 6 RCTs with 360 ARDS patients showed 6:

• Reduced 28-day mortality (RR 0.66,95% CI: 0.47-0.94)
• Lower peak pressures (MD -2.04 cmH₂O)
• Higher mean arterial pressure (MD 2.35 mmHg)
• No improvement in oxygenation (PaO₂/FiO₂ MD 26.24,95% CI: -26.50 to 78.97, P=0.33)

However, all included studies had unclear risk of bias, and the evidence quality remains insufficient to change practice 6.

Major Limitations and Risks

APRV has greater potential for harm than benefit in ARDS unless definitive outcome data emerge. 2 Critical concerns include:

• Volutrauma risk: The very short T low settings required for effectiveness can lead to inadequate exhalation and large tidal volumes if not precisely calibrated 2, 3
• Ventilator-dependent performance: Different ventilators deliver APRV inconsistently, and small variations in T low can cause either de-recruitment or excessive tidal volumes 2
• Lack of standardized protocols: No consensus exists on optimal settings, making reproducible application impossible 4, 3
• Increased work of breathing: Unrestricted spontaneous breathing may increase energy expenditure in critically ill patients 3
• Operator expertise required: Effective use demands high precision in setting adjustments that most clinicians lack experience with 2

Clinical Context and Current Use

APRV is primarily used as a rescue therapy for refractory hypoxemia in ARDS when conventional lung-protective ventilation fails 4, 3. A 2004 RCT comparing APRV to SIMV with pressure support as a primary mode found no difference in ventilator-free days (13.4 vs 12.2 days) or mortality (17% vs 18%) 5.

Before considering APRV, ensure implementation of evidence-based ARDS interventions 1, 7:

• Low tidal volume ventilation (4-8 ml/kg PBW) with plateau pressure ≤30 cmH₂O 1
• Higher PEEP (10-15 cmH₂O) for moderate-severe ARDS 1
• Prone positioning >12 hours daily for severe ARDS (PaO₂/FiO₂ <150) 1
• Recruitment maneuvers with hemodynamic monitoring 8

Common Pitfalls

• Using APRV as first-line therapy: No evidence supports this approach over standard lung-protective ventilation 2, 5
• Inadequate monitoring of tidal volumes: The spontaneous breathing component can generate large, injurious tidal volumes that go unrecognized 2
• Inconsistent terminology: APRV is often confused with BiPAP (bilevel positive airway pressure), though they have different applications and settings 3
• Assuming oxygenation improvement equals outcome benefit: While APRV may improve oxygenation in some cases, this has not translated to mortality reduction in high-quality studies 6, 5