For a patient on Airway Pressure Release Ventilation (APRV) with settings of Phigh 30 cmH2O, Plow 0 cmH2O, Thigh 5 seconds, Tlow 0 seconds, and FiO2 55%, are Thigh and Tlow settings of 5 and 0 or 5 and 1 second more common?

APRV Tlow Settings: 0.5-0.8 Seconds is Standard, Not 0 or 1 Second

Your current settings with Tlow of 0 seconds are incorrect and potentially harmful—Tlow should typically be set to 0.5-0.8 seconds to allow adequate but incomplete exhalation, achieving approximately 50-75% of peak expiratory flow, which is essential for the auto-PEEP mechanism that maintains alveolar recruitment in APRV. 1, 2

Understanding APRV Time Settings

The Critical Role of Tlow (Release Time)

• Tlow is NOT set to 0 or 1 second in standard APRV practice—the release time is typically set to 0.5-0.8 seconds to allow partial exhalation 1, 2

• 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 2, 3

• This incomplete exhalation is intentional and creates auto-PEEP that maintains alveolar recruitment between release phases 2

• Setting Tlow to 0 seconds (as in your current settings) would not allow any release phase at all, defeating the entire purpose of APRV 1, 2

Standard APRV Initial Settings

• Phigh: Set to plateau pressure from previous conventional ventilation (typically 20-30 cmH2O) 1, 2

• Plow: Usually set to 0-5 cmH2O 1, 2, 3

• Thigh: Set to 4-6 seconds initially (your setting of 5 seconds is appropriate) 2, 3

• Tlow: Set to 0.5-0.8 seconds (NOT 0 or 1 second)—adjusted based on expiratory flow termination 1, 2

Why Your Current Settings Are Problematic

Tlow = 0 Seconds Creates Multiple Issues

• No pressure release occurs, eliminating the ventilation component of APRV 1, 2

• No tidal volume generation from the release phase, which is the primary mechanism of CO2 clearance in APRV 2, 3

• Converts APRV into continuous positive airway pressure (CPAP) with spontaneous breathing only, which is not APRV 2

• Inadequate minute ventilation will result, leading to hypercapnia 2, 4

The Correct Tlow Setting Strategy

• Start with 0.5-0.8 seconds and observe the expiratory flow waveform 1, 2

• Adjust Tlow so that expiratory flow terminates at 50-75% of peak expiratory flow 2

• Shorter Tlow (closer to 0.5 seconds) in patients with obstructive disease to prevent air trapping 2

• Longer Tlow (closer to 0.8 seconds) may be needed in patients with very stiff lungs, but should still terminate before complete exhalation 2

APRV Mechanism and Benefits

How APRV Works

• APRV maintains prolonged high pressure (Phigh) for most of the respiratory cycle (Thigh), promoting alveolar recruitment and improving oxygenation 5, 6, 2

• Brief pressure releases (Tlow) allow partial exhalation, generating tidal volumes for CO2 clearance 2, 3

• Spontaneous breathing is permitted throughout the cycle, reducing sedation needs and improving hemodynamics 2, 3, 4

• The inverse ratio ventilation pattern (inspiratory time > expiratory time) increases mean airway pressure and alveolar recruitment 5, 6

Clinical Benefits in ARDS

• Improved oxygenation through sustained alveolar recruitment with lower peak airway pressures (typically 30% reduction) 1, 2, 3

• Enhanced cardiac performance with increased cardiac index from 3.2 to 4.6 L/min/m² BSA compared to pressure control ventilation 3

• Reduced sedation and paralytic requirements due to allowance of spontaneous breathing 3, 4

• Decreased pressor requirements and improved hemodynamics 3

Critical Pitfalls to Avoid

• Never set Tlow to 0 seconds—this eliminates the ventilation component entirely 1, 2

• Do not allow complete exhalation during Tlow—terminating at 50-75% of peak expiratory flow maintains auto-PEEP for recruitment 2

• Monitor for excessive auto-PEEP if Tlow is too short, which can impair venous return and worsen hemodynamics 3

• Avoid excessive Phigh (>30 cmH2O) as this increases risk of barotrauma and hemodynamic compromise 7, 1

• Do not use APRV without monitoring expiratory flow waveforms—proper Tlow adjustment requires visual confirmation of flow termination point 2

Recommended Correction for Your Settings

Immediate adjustment needed: Change Tlow from 0 to 0.6 seconds as a starting point, then titrate based on expiratory flow waveform to achieve termination at 50-75% of peak flow 1, 2

Your other settings (Phigh 30, Plow 0, Thigh 5, FiO2 55%) are within acceptable ranges, though Phigh of 30 cmH2O is at the upper safety limit 7, 1