When is volume‑assured pressure therapy indicated for adults with obstructive sleep apnea who have persistent apnea‑hypopnea index >5 despite maximal CPAP (≥15 cm H₂O), nocturnal hypoventilation/hypercapnia (e.g., obesity‑hypoventilation, COPD‑OSA overlap, neuromuscular disease), emergent central or mixed apneas on high CPAP, or intolerance to high inspiratory pressures?

When Volume-Assured Pressure Therapy is Indicated for Obstructive Sleep Apnea

Volume-assured pressure support (AVAPS or volume-targeted BiPAP) should be initiated when CPAP fails to control OSA at maximum tolerable pressures (≥15 cm H₂O), when treatment-emergent central apneas develop on high CPAP, or when concurrent hypoventilation/hypercapnia exists (obesity-hypoventilation syndrome, COPD-OSA overlap, neuromuscular disease). 1, 2

Primary Indications for Volume-Assured Pressure Therapy

Failed CPAP Titration Despite Maximal Pressure

• Switch to volume-targeted BiPAP when obstructive events (AHI >5) persist despite CPAP pressures reaching 15-20 cm H₂O during attended polysomnography titration 3, 2
• In a study of 45 patients who failed CPAP titration, AVAPS reduced AHI from 65.3±29.3 to 22.3±16.1 events/hour (p<0.001), with 71% achieving AHI <30 2
• The American Academy of Sleep Medicine supports BiPAP as the next therapeutic step when CPAP intolerance occurs despite mask adjustments or when obstructive events persist at 15 cm H₂O 3

Treatment-Emergent Central Sleep Apnea

• Volume-assured pressure support is indicated when central apneas or mixed apneas emerge during CPAP titration at high pressures 2
• In one series, 55.6% of patients requiring AVAPS had treatment-emergent central sleep apnea as the primary reason for CPAP failure 2
• The automatic adjustment of inspiratory pressure in volume-targeted modes helps stabilize ventilation and reduce central events 1

Concurrent Hypoventilation Syndromes

Obesity-Hypoventilation Syndrome (OHS)

• Attended polysomnography with BiPAP titration is the standard method for patients with OHS, as many require high CPAP levels and may have residual desaturation or persistent hypoventilation 1, 4
• Start with IPAP 8 cm H₂O and EPAP 4 cm H₂O, maintaining minimum pressure support of 4 cm H₂O 4
• Never use CPAP alone as initial therapy for documented OHS with hypoventilation, as CPAP only addresses upper airway obstruction without providing ventilatory support 4
• Increase pressure support if tidal volume remains below 6-8 mL/kg ideal body weight 4

COPD-OSA Overlap Syndrome

• BiPAP or volume-targeted modes are preferred when OSA coexists with chronic hypercapnia or significant obstructive lung disease 5, 6
• Volume-assured pressure support automatically adjusts inspiratory pressure to maintain adequate tidal volumes as respiratory muscle strength varies 1

Neuromuscular Disease

• Attended NPPV titration with polysomnography allows definitive identification of adequate ventilatory support for patients with neuromuscular disorders 1
• Volume-targeted BiPAP has the advantage of automatically varying pressure support to deliver targeted tidal volume when respiratory muscle strength declines 1

Initial Settings for Volume-Targeted BiPAP

Standard Starting Parameters

• EPAP: 4 cm H₂O 1, 4
• IPAP minimum: EPAP + 4 cm H₂O (typically 8 cm H₂O) 1, 4
• IPAP maximum: 25-30 cm H₂O 1, 4
• Target tidal volume: approximately 8 mL/kg ideal body weight 1
• Maximum pressure support should not exceed 20 cm H₂O 4

Titration Algorithm

• Adjust EPAP first to eliminate obstructive apneas, hypopneas, RERAs, and snoring following standard OSA titration protocols 4
• Increase pressures in 1-2 cm H₂O increments every 5 minutes minimum 4
• Increase pressure support (IPAP-EPAP difference) if tidal volume remains inadequate despite elimination of obstructive events 4
• Monitor for persistent oxygen desaturation despite elimination of obstructive events, indicating hypoventilation 4

Monitoring Requirements During Titration

• Attended polysomnography is the standard method for determining effective pressure settings when OSA coexists with chronic alveolar hypoventilation 1
• Monitor tidal volume, respiratory rate, and respiratory muscle EMG to document impact of NPPV 1
• Continuous pulse oximetry monitoring is essential 1, 4
• Transcutaneous or end-tidal PCO₂ monitoring allows precise documentation of adequate ventilatory support 1, 4
• Evidence of reduced work of breathing includes increased tidal volume, reduced respiratory rate, and reduced inspiratory EMG activity 1

Critical Pitfalls to Avoid

• Do not add supplemental oxygen without addressing the underlying ventilatory problem, as this may worsen hypercapnia 4
• Never decrease CPAP below 12 cm H₂O in severe OSA when switching to BiPAP, as this would result in inadequate treatment 3
• Do not increase pressure support beyond what is needed for adequate ventilation, as excessive pressure worsens tolerance 4
• Avoid prematurely abandoning PAP therapy without attempting volume-assured pressure support, as this leaves severe OSA with dangerous hypoxemia untreated 3

Predictors of Response

• Improvement in AHI with AVAPS correlates inversely with BMI: higher BMI patients show less AHI reduction (∆AHI = 12.2 - 1.4 × BMI; p=0.05) 2
• Response is not related to gender, age, or opioid use 2
• AVAPS improves sleep architecture with median N3 sleep increasing from 1.4% to 19.6% of total sleep time (p<0.001) and REM sleep increasing from 6.4% to 13.6% (p<0.01) 2

Follow-Up Requirements

• Close follow-up after BiPAP initiation is crucial, with objective adherence monitoring and side effect management 3
• Initial follow-up during the first few weeks establishes utilization patterns and provides remediation if needed 6
• Longer-term follow-up is recommended yearly or as needed to address mask, machine, or usage problems 6