What are the optimal settings for Average Volume Assured Pressure Support (AVAPS) mode in mechanical ventilation?

Optimal Settings for Average Volume Assured Pressure Support (AVAPS)

For AVAPS mode in noninvasive ventilation, start with EPAP = 4 cm H₂O, IPAP minimum = EPAP + 4 cm H₂O (8 cm H₂O), IPAP maximum = 25-30 cm H₂O, and target tidal volume = 8 mL/kg ideal body weight. 1

Initial Parameter Settings

Pressure Parameters

• EPAP (Expiratory Positive Airway Pressure): Set at 4 cm H₂O initially 1

  • Adjust EPAP upward using standard CPAP titration protocols if obstructive events (apneas, hypopneas, RERAs, snoring) are present 1

• IPAP Minimum: Set at EPAP + 4 cm H₂O (typically 8 cm H₂O at initiation) 1

  • This provides modest initial pressure support to allow patient adaptation 1

• IPAP Maximum: Set at 25-30 cm H₂O 1

  • Use 20 cm H₂O maximum for patients < 12 years of age 1
  • The device will automatically adjust IPAP between these limits to achieve target tidal volume 1

Volume Target

• Target Tidal Volume: 8 mL/kg ideal body weight 1
  • Acceptable range is 6-8 mL/kg ideal body weight 1, 2
  • Calculate ideal body weight: Men = 50 + 2.3 × (height in inches - 60); Women = 45.5 + 2.3 × (height in inches - 60) 3

Mode Selection

Ventilatory Modes Available

AVAPS can be used in three modes 1:

• Spontaneous mode: Patient triggers all breaths
• Spontaneous-Timed (ST) mode: Backup rate ensures minimum respiratory rate if patient effort is inadequate
• Timed mode: Rarely used; all breaths are machine-triggered at set rate and inspiratory time 1

Backup Rate and Inspiratory Time (for ST or Timed modes)

• Backup respiratory rate: Set 2-4 breaths/minute below the patient's spontaneous rate 1
• Inspiratory time:
  • Target 30-40% of total breath cycle time (%IPAP time) 1
  • At respiratory rate of 15 breaths/minute, this corresponds to 1.2-1.6 seconds 1
  • Lower %IPAP time (shorter inspiratory time) for obstructive airway disease to allow adequate expiratory time 1
  • Higher %IPAP time (longer inspiratory time) for restrictive lung disease 1

Titration Goals and Adjustments

Primary Objectives

The goals of AVAPS titration are 1:

1. Select effective EPAP to eliminate obstructive respiratory events
2. Document adequate pressure support delivery to achieve target tidal volume
3. Correct hypoventilation and hypoxemia

When to Increase Pressure Support

Increase IPAP maximum if 1:

• Tidal volume remains below goal after 5 minutes of observation
• SpO₂ remains < 90% for 5 minutes despite adequate tidal volume
• Respiratory muscle rest not achieved after 10 minutes (evidenced by persistent tachypnea or excessive inspiratory effort)
• Transcutaneous CO₂ remains elevated despite adequate settings

Incremental Changes

• Minimum increment: 1 cm H₂O 1
• Maximum increment: 2 cm H₂O to avoid over-titration 1
• Maximum pressure support: 20 cm H₂O (difference between IPAP and EPAP) 1

Clinical Advantages and Evidence

Mechanism of Action

AVAPS automatically varies pressure support between the set IPAP minimum and maximum to deliver the targeted tidal volume 1. If respiratory muscle strength declines and tidal volume decreases, the device increases pressure support to maintain the target volume 1.

Comparative Effectiveness

• AVAPS demonstrated similar efficacy to conventional bilevel pressure support in treating acute hypercarbic respiratory failure, with identical ICU length of stay, hospital length of stay, and mortality rates 2
• AVAPS produced better perceived sleep efficiency compared to conventional pressure support in stable hypercarbic COPD patients (sleep efficiency score improved from 5.1 to 4.1, P = 0.001) 4
• In pediatric nocturnal hypoventilation, AVAPS showed significant improvement in peak and mean transcutaneous CO₂ compared to conventional bilevel (P = 0.009 and P = 0.001 respectively) 5
• AVAPS delivered higher tidal volumes using similar pressures compared to conventional modes 5

Common Pitfalls and Caveats

Pressure Delivery Considerations

• AVAPS uses higher maximum IPAP levels than conventional bilevel to achieve target volumes 2
• Monitor for patient discomfort with higher pressures; if patient awakens complaining pressures are too high, decrease to a comfortable level that allows return to sleep 1
• Leak management is critical: Reduction in mask leak may improve effectiveness of current pressure support before increasing IPAP 1

Volume Targets in Special Populations

• Slightly lower tidal volumes with higher respiratory rates may be better tolerated in restrictive thoracic cage disorders 1
• In patients with lung disease, higher minute ventilation is needed due to increased physiological dead space (normal dead space ≈ 2 mL/kg) 1

Oxygenation Management

• Target SpO₂ 90-94% in most patients 1
• Add supplemental oxygen via T-connector at NPPV device outlet if needed 1
• Note that effective FiO₂ decreases as IPAP or EPAP increases due to higher intentional leak 1

Monitoring Requirements

• Continuous monitoring of delivered tidal volume, leak, and patient-ventilator synchrony is essential 1
• Download and review adherence data and delivered parameters from device memory regularly 5
• Perform arterial or transcutaneous blood gas monitoring to confirm adequate ventilation 2, 5