What are alternative modes of mechanical ventilation, such as Airway Pressure Release Ventilation (APRV) or Neurally Adjusted Ventilatory Assist (NAVA), for patients not responding to conventional ventilation?

Alternative and New Modes of Mechanical Ventilation

For patients failing conventional ventilation, consider Airway Pressure Release Ventilation (APRV) as the primary alternative mode, particularly when ventilator asynchrony or refractory hypoxemia complicates ARDS, while Neurally Adjusted Ventilatory Assist (NAVA) should be reserved for patients with intact respiratory drive who demonstrate significant patient-ventilator asynchrony. 1

Airway Pressure Release Ventilation (APRV)

Mechanism and Rationale

• APRV functions as continuous positive airway pressure (CPAP) with intermittent pressure releases, maintaining high airway pressures to recruit and sustain lung volumes near functional residual capacity while allowing spontaneous breathing throughout the respiratory cycle 2
• The mode utilizes inverse ratio ventilation where inspiratory time exceeds expiratory time, increasing alveolar recruitment and improving oxygenation 1
• APRV is pressure-limited and time-cycled, distinguishing it from conventional volume or pressure control modes 1

Clinical Application and Evidence

• Consider APRV specifically for ARDS patients with ventilator asynchrony or when intracranial pressure management is required, as animal studies demonstrate improved P/F ratios with higher mean airway pressures while maintaining stable ICP and CPP 1
• In controlled swine models mimicking ARDS with elevated ICP (30-40 mmHg), APRV showed statistically improved oxygenation compared to ARDSNet protocols without worsening intracranial dynamics 1
• APRV reduced cerebral lactate levels in animal models, suggesting potential neuroprotective effects, though lactate-pyruvate ratios remained similar 1

Critical Settings and Monitoring

• Expect higher mean airway pressures with APRV compared to conventional modes—this is the intended mechanism for recruitment 1
• Monitor for negligible ICP increases (typically <5 mmHg elevation) when transitioning to APRV in patients with intracranial pathology 1
• As patients recruit lung volume and approach functional residual capacity, their spontaneous breathing improves and release breaths can be gradually reduced until patients support themselves on CPAP alone 2

Limitations and Caveats

• Evidence remains limited to case reports, animal studies, and small case series—no large randomized controlled trials demonstrate mortality benefit 1, 3, 4
• APRV methodology varies widely between centers with no standardized approach, making comparison of outcomes difficult 4
• The mode requires careful titration and understanding of its mechanics; inappropriate settings can worsen outcomes 3

Neurally Adjusted Ventilatory Assist (NAVA)

Mechanism and Patient Selection

• NAVA delivers ventilatory support proportional to the electrical activity of the diaphragm (EAdi), requiring placement of a specialized esophageal catheter to measure diaphragmatic electrical activity 1, 5
• The mode integrates the ventilator into neuro-ventilatory coupling at a higher level than conventional modes, using the patient's neural respiratory drive to trigger and cycle breaths 5
• NAVA requires intact ventilatory drive and breathing reflexes—do not use in patients with absent respiratory effort or severe neurological impairment 5

Evidence for Improved Synchrony

• NAVA demonstrates superior patient-ventilator synchrony compared to pressure support ventilation, reducing asynchrony index from 24.3% to 11.5% during spontaneous breathing trials 6
• Patient-ventilator asynchrony affects 25% of mechanically ventilated ICU patients and contributes to sleep fragmentation, increased sedation requirements, delirium, prolonged ventilation, and mortality 5
• Studies show NAVA improves synchrony and may reduce asynchrony-related complications, though mixed results exist regarding sleep quality improvement 1

Clinical Application

• Target patient populations include COPD patients and those with major patient-ventilator asynchrony who have failed conventional pressure support ventilation 5
• NAVA can be used during spontaneous breathing trials with similar respiratory patterns to PSV but improved synchrony 6
• The EAdi signal provides valuable monitoring information to guide ventilator management and weaning decisions 5

Limitations

• No evidence demonstrates superiority on mortality or other hard clinical endpoints—benefits remain primarily physiologic 5
• Requires specialized equipment (EAdi catheter) and training not universally available 5
• Three of 20 patients failed spontaneous breathing trials in NAVA versus zero failures in PSV in one crossover study, raising questions about appropriate use during weaning 6

Proportional Assist Ventilation (PAV)

• PAV delivers pressure proportional to inspiratory muscle pressure calculated from respiratory mechanics, adjusting support breath-by-breath 1
• Studies show PAV improves patient-ventilator synchrony and may increase slow-wave and REM sleep compared to PSV, though individual sleep components did not reach statistical significance 1
• PAV with load-adjustable gain factor (PAV+) improves synchrony but does not consistently improve sleep quality despite better patient-ventilator interaction 1

Bi-Level Pressure Support and CPAP

• Bi-level pressure support combines inspiratory positive airway pressure (IPAP) for ventilation with expiratory positive airway pressure (EPAP) to recruit underventilated lung and offset intrinsic PEEP 1
• For COPD exacerbations with acute hypercapnic respiratory failure, use CPAP 4-8 cmH₂O plus PSV 10-15 cmH₂O as the most effective non-invasive approach 1
• CPAP alone corrects hypoxemia by maintaining constant positive pressure throughout the respiratory cycle, recruiting collapsed alveoli similar to PEEP in invasive ventilation 1

Critical Decision Algorithm

1. First-line failure of conventional AC or SIMV with lung-protective ventilation:

   • If refractory hypoxemia with ARDS and ventilator asynchrony → Consider APRV 1
   • If significant patient-ventilator asynchrony with intact respiratory drive → Consider NAVA 5, 6
   • If concern for ICP management with ARDS → APRV preferred over conventional modes 1

2. Monitor for mode-specific complications:

   • APRV: Expect mean airway pressure elevation; monitor ICP if applicable 1
   • NAVA: Ensure adequate EAdi signal; monitor for weaning readiness 5, 6
   • Avoid excessive pressure support levels that cause hyperventilation and central apneas during sleep 1

3. Recognize when alternative modes are inappropriate:

   • Do not use NAVA without intact respiratory drive 5
   • Do not expect mortality benefit from APRV based on current evidence 3, 4
   • Recognize that high-frequency oscillatory ventilation (HFOV) does not improve outcomes and should not be used routinely 7