When should Airway Pressure Release Ventilation (APRV) be used instead of Pressure Regulated Volume Control (PRVC), and what are the recommended ventilator settings for each mode?

APRV Should Be Considered as First-Line Lung-Protective Ventilation in Moderate-to-Severe ARDS, While PRVC Serves as a Standard Volume-Control Mode for Less Severe Cases

For patients with moderate-to-severe ARDS (PaO2/FiO2 < 200), initiate APRV using the Time-Controlled Adaptive Ventilation (TCAV) method early rather than as rescue therapy, as it provides superior lung recruitment while maintaining lung-protective principles. For patients with less severe respiratory failure or those requiring predictable tidal volumes, PRVC offers a reasonable alternative that adapts pressures to deliver set volumes 1, 2, 3.

Understanding the Fundamental Difference

These modes operate on completely different physiological principles:

• APRV functions as extended continuous positive airway pressure (CPAP) with brief pressure releases, maintaining high mean airway pressure to recruit and stabilize collapsed lung tissue 2, 4
• PRVC is an adaptive volume-control mode that adjusts inspiratory pressure breath-by-breath to achieve a target tidal volume at the lowest possible airway pressure 5

The 2017 ATS/ERS/SCCM guidelines strongly recommend lung-protective ventilation with tidal volumes of 4-8 ml/kg predicted body weight and plateau pressures <30 cmH2O for all ARDS patients 1. Both modes can theoretically achieve this, but through different mechanisms.

When to Choose APRV

Initiate APRV early in moderate-to-severe ARDS (PaO2/FiO2 ≤ 200) rather than waiting for conventional ventilation to fail 2, 3. The paradigm has shifted from using APRV as salvage therapy to implementing it as primary lung-protective ventilation.

APRV Settings Using the TCAV Method:

Initial Settings:

• P-High: 20-30 cmH2O (start at 25 cmH2O for most patients; may match plateau pressure from conventional ventilation or use mean airway pressure + 2-5 cmH2O) 6
• P-Low: 0 cmH2O 6
• T-High: 4-6 seconds 6
• T-Low: Set to terminate expiratory flow at 75% of peak expiratory flow (typically 0.4-0.8 seconds) - this is the critical personalized setting 2

Key Principle: The T-Low setting acts as a "brake" preventing newly recruited alveoli from collapsing, while the extended T-High progressively "ratchets" open collapsed tissue over hours to days 2. This timing must be individualized based on the patient's expiratory flow curve, which reflects their changing lung elastance 2.

Physiological Advantages:

• Maintains alveolar recruitment through sustained high pressure
• Minimizes atelectrauma from repetitive opening/closing
• Allows spontaneous breathing at any point in the cycle (reduces sedation needs)
• Improves hemodynamics compared to conventional modes 3
• May improve secretion clearance through expiratory-dominant flow profiles 7

Evidence for Mortality Benefit:

Recent data suggest APRV may reduce mortality in severe PARDS (79% vs 91% mortality, HR 0.27, P=0.001) and prolong survival times (7.5 vs 4.3 days median time to death) 8. While this is pediatric data, the physiological principles apply to adults.

When to Choose PRVC

Use PRVC for:

• Mild ARDS or less severe respiratory failure where aggressive recruitment is not needed
• Patients requiring guaranteed minute ventilation with predictable tidal volumes
• Early stabilization when you need straightforward volume delivery with pressure limitation 9

PRVC Settings:

• Tidal Volume: 4-8 ml/kg predicted body weight (target 6 ml/kg) 1
• PEEP: Titrate based on ARDS severity (higher PEEP for moderate-severe ARDS with PaO2/FiO2 ≤ 200) 1, 9
• Target Plateau Pressure: <30 cmH2O 1
• Driving Pressure: Minimize (ΔP = Plateau - PEEP); lower driving pressures associated with better outcomes 9

The ventilator will automatically adjust inspiratory pressure to maintain your set tidal volume while keeping pressures as low as possible.

Critical Caveats and Pitfalls

APRV Pitfalls:

1. Excessive Release Volumes: Survey data show many practitioners accept release volumes that exceed lung-protective limits 6. Monitor that release tidal volumes stay within 4-8 ml/kg PBW.
2. Incorrect T-Low Setting: Only 47% of practitioners correctly use expiratory flow termination to set T-Low 6. This is the most critical setting - if T-Low is too long, you lose recruitment; too short, and you don't adequately clear CO2.
3. Late Implementation: Historical use as rescue therapy means many clinicians wait too long 3. Consider APRV early when PaO2/FiO2 drops below 200.
4. Accepting High P-High: While APRV uses sustained high pressures, ensure P-High doesn't create excessive transpulmonary pressure (consider esophageal manometry in obese patients or those with high chest wall elastance) 9.

PRVC Pitfalls:

1. Maladaptive Behavior: PRVC can become maladaptive, leading to patient-ventilator dyssynchrony when the algorithm misinterprets patient effort 5
2. False Security: "Set it and forget it" mentality is dangerous - you must still monitor plateau pressures, driving pressure, and patient-ventilator synchrony 5
3. Variable Pressures: Unlike volume control, pressures will vary breath-to-breath, which can mask changes in compliance

Adjustments for Gas Exchange

For Hypoxemia (both modes):

• APRV: Increase P-High (in 2-3 cmH2O increments) or increase T-High
• PRVC: Increase PEEP (particularly in moderate-severe ARDS) 1, 9
• Consider prone positioning for >16 hours/day if PaO2/FiO2 <150 1, 9

For Hypercarbia:

• APRV: Increase release frequency (decrease T-High) or increase P-High-to-P-Low differential 6
• PRVC: Increase tidal volume (if <8 ml/kg PBW and plateau <30 cmH2O) or increase respiratory rate

For Hypocarbia:

• APRV: Decrease release frequency (increase T-High)
• PRVC: Decrease respiratory rate or tidal volume

The Bottom Line

Volume-controlled ventilation (including PRVC) facilitates measurement of respiratory mechanics and driving pressure, making it recommended for early ARDS management by some experts 9. However, emerging evidence suggests APRV using TCAV methodology may provide superior lung protection through sustained recruitment and prevention of atelectrauma 2, 3. The choice depends on ARDS severity: PRVC for mild cases requiring straightforward lung-protective ventilation, APRV for moderate-to-severe cases where aggressive recruitment and lung homogeneity are priorities. Both must adhere to fundamental lung-protective principles of low tidal volumes and limited plateau pressures 1.