Is average volume pressure support (AVPS) suitable for 24-hour use?

Average Volume Assured Pressure Support (AVAPS) for 24-Hour Use

AVAPS is appropriate for 24-hour use in patients with chronic respiratory failure requiring continuous noninvasive ventilation, particularly when transitioning from nocturnal-only support to around-the-clock ventilation. 1

Clinical Context for 24-Hour AVAPS Use

The primary indication for 24-hour noninvasive ventilation occurs when patients progress from nocturnal hypoventilation to constant daytime hypoventilation, requiring continuous ventilatory support. 1 This transition is indicated when:

• Waking PaCO₂ exceeds 50 mm Hg 1
• Hemoglobin saturation remains ≤92% while awake 1
• Progressive respiratory muscle weakness prevents adequate spontaneous ventilation throughout the day 1

AVAPS vs. Standard BiPAP for Extended Use

Efficacy Evidence

AVAPS demonstrates comparable or superior efficacy to standard BiPAP S/T mode for continuous use:

• More rapid CO₂ clearance: AVAPS achieves significantly better improvement in pH at 6 hours (p=0.027) and 24 hours (p=0.032), and better pCO₂ reduction at 6 hours (p=0.012) and 24 hours (p=0.013) compared to BiPAP S/T 2
• Faster neurological recovery: In patients with hypercapnic encephalopathy, AVAPS produces statistically significant improvements in Glasgow Coma Scale scores (p=0.00001) and pCO₂ (p=0.03) compared to standard BiPAP 3
• Shorter hospital stays: AVAPS is associated with reduced duration of hospitalization (p=0.003) 2

Mechanism of Advantage

The key advantage of AVAPS for 24-hour use is its automatic adjustment of inspiratory pressure to maintain target tidal volume (typically 6-8 mL/kg) despite changing pulmonary mechanics throughout day and night. 4, 5 This addresses the limitation of fixed-pressure BiPAP, which cannot adapt to variations in:

• Patient position changes (sitting vs. lying)
• Sleep-wake transitions
• Fluctuating respiratory muscle strength
• Changes in lung compliance 4

Practical Implementation for 24-Hour Support

Initial Settings

When initiating 24-hour AVAPS:

• Maximum IPAP: Start at 20 cm H₂O, titrate up to 30 cm H₂O as needed 5
• Minimum IPAP: Set at 5 cm H₂O higher than EPAP 5
• Target tidal volume: 6-8 mL/kg ideal body weight 1, 5
• EPAP: Typically 4-8 cm H₂O, adjusted to eliminate upper airway obstruction 1
• Backup rate: Set equal to or slightly less than spontaneous sleeping respiratory rate (minimum 10 breaths/minute) 6

Daytime Delivery Methods

For daytime continuous support, multiple interfaces can be used:

• Mouthpiece intermittent positive pressure ventilation: Most commonly used technique, allowing eating and speaking without interruption; successfully used in patients with mean FVC of 0.6 L (5% predicted) for >8 years 1
• Nasal mask: Can be alternated with full-face mask to reduce skin breakdown 1
• Full-face mask: For patients unable to use mouthpiece or nasal interface 1

Monitoring Requirements

Because most bilevel machines lack built-in alarms, additional monitoring is essential for 24-hour use:

• Pulse oximetry: Continuous monitoring recommended, especially during initial 24 hours 1
• Target SpO₂: Maintain between 85-90% 1
• Arterial blood gas analysis: Measure at baseline, 1 hour, 4-6 hours, then as clinically indicated 1
• Clinical assessment: Regular evaluation of respiratory rate, accessory muscle use, patient comfort, and coordination with ventilator 1

Limitations and Caveats

Evidence Quality

While AVAPS shows clinical benefits, important limitations exist:

• No mortality benefit demonstrated: Studies show no significant difference in ICU mortality, hospital mortality, 6-month, or 1-year mortality between AVAPS and BiPAP S/T 5
• No difference in intubation rates: Need for invasive mechanical ventilation is similar between modes (p=0.338) 2
• Small sample sizes: Most chronic respiratory failure studies involve limited patient numbers 4, 7
• Equivalent long-term outcomes: No significant differences in PaCO₂, PaO₂, sleep efficiency, or compliance when comparing AVAPS to PS-NIV for chronic respiratory failure 7

Clinical Pitfalls

Mask displacement risk: In fragile patients requiring 24-hour support, mask displacement can rapidly lead to severe hypoxemia and hypercapnia; continuous monitoring is critical 1

Higher pressure requirements: AVAPS typically requires significantly higher maximum IPAP levels compared to fixed BiPAP (p<0.001 across all time points), which may increase risk of air leaks and patient discomfort 5

Not a substitute for intubation: When patients meet criteria for invasive ventilation (progressive deterioration despite optimized NIV, inability to protect airway, hemodynamic instability), AVAPS should not delay definitive airway management 1

Algorithm for 24-Hour AVAPS Implementation

1. Confirm indication: Waking PaCO₂ >50 mm Hg or SpO₂ ≤92% while awake despite nocturnal NIV 1

2. Assess contraindications: Rule out inability to protect airway, hemodynamic instability, uncontrolled arrhythmias, or facial trauma 1

3. Select interface: Start with mouthpiece for daytime if patient can coordinate; use nasal or full-face mask as backup 1

4. Initiate AVAPS settings: Maximum IPAP 20 cm H₂O, minimum IPAP 5 cm H₂O above EPAP, target TV 6-8 mL/kg, backup rate ≥10/min 1, 6, 5

5. Monitor response: ABG at 1 hour and 4-6 hours; continuous pulse oximetry for 24 hours 1

6. Titrate pressures: Increase maximum IPAP by 1-2 cm H₂O increments every 5 minutes if TV remains below target or hypoventilation persists 1

7. Schedule reassessment: Periodic follow-up appropriate to disease stage, monitoring for progression requiring tracheostomy 1