What is APRV Mode Ventilator?
Airway Pressure Release Ventilation (APRV) is a pressure-limited, time-cycled mode of mechanical ventilation that maintains prolonged high airway pressure (Phigh) for most of the respiratory cycle with brief intermittent releases to a lower pressure, while allowing unrestricted spontaneous breathing throughout the entire cycle. 1
Core Mechanism and Design
APRV utilizes inverse ratio ventilation where inspiratory time (Thigh) significantly exceeds expiratory time (Tlow), fundamentally distinguishing it from conventional volume or pressure control modes 1
The mode maintains a continuous positive airway pressure (CPAP) with brief releases, originally conceptualized as CPAP with intermittent pressure release 2, 3
The defining technical feature separating APRV from standard bilevel modes is the intentionally brief Tlow setting, which is personalized based on the slope of the expiratory flow curve to prevent alveolar de-recruitment 1
Patients can breathe spontaneously at any phase of the ventilatory cycle without restriction, including during both the high-pressure and release phases 2, 4
The Four Key Time and Pressure Settings
High Pressure Phase (Phigh and Thigh)
Phigh should be set at 20-30 cmH₂O to promote alveolar recruitment and improve oxygenation, though exceeding 30 cmH₂O increases risk of barotrauma and hemodynamic compromise 5
Thigh represents the prolonged inspiratory time that maintains alveolar recruitment and increases mean airway pressure 5
Release Phase (Plow and Tlow)
Tlow should be set to 0.5-0.8 seconds to allow partial exhalation while generating tidal volumes for CO₂ clearance 5
The release phase should terminate when expiratory flow reaches approximately 50-75% of peak expiratory flow on the flow-time waveform, which typically occurs at 0.5-0.8 seconds 5
This brief release creates auto-PEEP that maintains alveolar recruitment between release phases, acting as a "brake" to prevent newly recruited lung units from collapsing 5, 6
Physiological Rationale and Lung Protection Strategy
APRV follows the "open the lung and keep it open" strategy, with prolonged CPAP duration recruiting collapsed alveoli and minimal release duration preventing lung collapse 2
The mode achieves higher mean airway pressures compared to conventional modes, which increases alveolar recruitment and improves oxygenation in ARDS patients 1
The time-controlled adaptive ventilation (TCAV) method uses the brief expiratory release to progressively "ratchet" open small volumes of collapsed tissue with each breath over hours to days, gradually achieving lung homogeneity 6
Animal studies demonstrate that personalized APRV (P-APRV) stabilizes alveoli and reduces the incidence of ARDS by reversing the main drivers of ventilator-induced lung injury 2
Clinical Indications
The American Thoracic Society recommends considering APRV as the primary alternative mode for patients with ARDS who have ventilator asynchrony or refractory hypoxemia 1
The Society of Critical Care Medicine suggests APRV specifically for ARDS patients with ventilator asynchrony or when intracranial pressure management is required 1
In animal models, APRV improved P/F ratios with higher mean airway pressures while maintaining stable intracranial pressure (ICP) and cerebral perfusion pressure (CPP) 1
APRV reduced cerebral lactate levels in animal models, suggesting potential neuroprotective effects 1
Critical Distinction: Fixed vs. Personalized APRV
Fixed-setting APRV (F-APRV) uses a constant release phase that is set and left unchanged, while personalized-APRV (P-APRV) adjusts the release phase based on changes in lung mechanics using the slope of the expiratory flow curve 2
Over 30 years since inception, there have been no strict criteria defining APRV, with tremendous variation in settings used in experimental studies and clinical practice 2
P-APRV has shown greater promise as a lung-protective ventilation strategy compared to F-APRV in clinically relevant animal models and trauma patients 2
Clinical Decision Algorithm
When first-line conventional AC or SIMV with lung-protective ventilation fails, consider APRV for refractory hypoxemia with ARDS and ventilator asynchrony 1
Monitor for negligible ICP increases when transitioning to APRV in patients with intracranial pathology, though expect higher mean airway pressures 1
Ensure plateau pressures remain <30 cmH₂O to minimize ventilator-induced lung injury risk, regardless of the high Phigh settings 1
Common Pitfalls and Caveats
The performance of APRV is highly dependent on operator-selected settings and ventilator performance—small variations in the very short expiratory time can lead to undesired outcomes including de-recruitment or excessive tidal volumes 7
Ventilator performance is not uniform across manufacturers, and outcomes depend on high-precision settings where minor deviations can compromise lung protection 7
Avoid excessive Phigh (>30 cmH₂O) as this increases risk of barotrauma and hemodynamic compromise 5
In no published study has APRV shown statistically significant worse outcomes regardless of settings (F-APRV or P-APRV), though definitive evidence of improved clinical outcomes in ARDS remains lacking 2, 7
Comparison to Other Modes
Unlike pressure support ventilation where the patient triggers both on and off, APRV provides time-cycled mandatory pressure levels with unrestricted spontaneous breathing superimposed 8, 4
Compared to SIMV with pressure support, APRV as a primary ventilatory mode in ARDS showed similar ventilator-free days and mortality, though APRV required significantly lower inspiratory pressures (25.9 vs. 28.6 cmH₂O) 4
APRV differs from standard bilevel pressure support by the intentionally brief Tlow designed to prevent alveolar collapse rather than simply providing two pressure levels 1