AVAPS: Understanding Fixed vs. Variable Parameters
In AVAPS (Average Volume-Assured Pressure Support), the tidal volume is fixed (targeted) while the pressure automatically varies within set limits—neither parameter is truly "fixed" in the traditional sense, but the tidal volume is the controlled target and pressure is the variable means to achieve it. 1
How AVAPS Actually Works
AVAPS is a volume-targeted mode that automatically adjusts IPAP between preset minimum and maximum limits (IPAPmin to IPAPmax) to deliver a targeted tidal volume. 1 This represents a fundamental departure from traditional bilevel ventilation where you set a fixed pressure and accept whatever tidal volume results.
The Core Mechanism
- The tidal volume target is set by the clinician (typically ~8 mL/kg ideal body weight initially) and remains the therapeutic goal throughout ventilation 1
- The inspiratory pressure varies automatically within the range you define (IPAPmin to IPAPmax, typically EPAP + 4 cm H₂O to 25-30 cm H₂O) to achieve that target volume 1
- The device continuously monitors delivered tidal volume and adjusts pressure support breath-by-breath to maintain the target 2, 3
Clinical Example of Dynamic Adjustment
If a patient's respiratory muscle strength declines during sleep and tidal volume begins to fall, AVAPS automatically increases the pressure support to return the delivered tidal volume to the targeted level 1. Conversely, if the patient's effort increases and tidal volume rises above target, the device reduces pressure support accordingly 3, 4.
Initial Settings Framework
The American Academy of Sleep Medicine consensus recommendations specify: 1
- EPAP = 4 cm H₂O (adjusted upward to eliminate obstructive events)
- IPAPmin = EPAP + 4 cm H₂O
- IPAPmax = 25 to 30 cm H₂O
- Target tidal volume = approximately 8 mL/kg ideal body weight
Key Clinical Advantages
AVAPS provides automatic compensation for changing respiratory mechanics that would otherwise require manual titration adjustments throughout the night 3, 4. This is particularly valuable in:
- Patients with progressive neuromuscular weakness where respiratory muscle strength varies 2, 4
- COPD patients with hypercapnic encephalopathy where AVAPS demonstrated significantly faster improvement in Glasgow Coma Scale scores (P = 0.00001) and pCO₂ reduction (P = 0.03) compared to fixed bilevel pressure 3
- Pediatric nocturnal hypoventilation where AVAPS showed significant improvement in peak (P = 0.009) and mean (P = 0.001) transcutaneous CO₂ compared to conventional bilevel 4
Critical Distinction from Traditional Modes
Unlike volume control ventilation where tidal volume is fixed and pressure is whatever results from circuit compliance and thoracic mechanics 5, and unlike pressure control where inspiratory pressure is fixed and tidal volume varies with resistance and compliance 5, AVAPS actively modulates pressure to maintain volume consistency 2, 3, 4.
Common Pitfall to Avoid
Do not confuse AVAPS with pressure support ventilation (PSV), where pressure is fixed and tidal volume can vary dangerously—studies show PSV has significantly increased risk (OR 19.36,95% CI 12.37-30.65) of delivering excessive tidal volumes >6 mL/kg in certain conditions 6. AVAPS maintains tighter control over delivered volume 4.