What ventilation strategy is recommended for a patient with Acute Respiratory Distress Syndrome (ARDS) following aspiration, considering the use of inverse ratio ventilation or Airway Pressure Release Ventilation (APRV)?

Ventilation Strategy for ARDS Following Aspiration

For ARDS following aspiration requiring mechanical ventilation, use conventional low tidal volume ventilation (6 mL/kg predicted body weight) with moderate PEEP as first-line therapy, targeting driving pressure ≤15 cmH₂O and plateau pressure <30 cmH₂O; reserve APRV for refractory hypoxemia or severe ventilator asynchrony after conventional strategies have failed. 1, 2, 3

First-Line Ventilation Strategy

Start with volume-controlled ventilation using lung-protective parameters:

• Set tidal volume at 6 mL/kg predicted body weight 1
• Maintain plateau pressure <30 cmH₂O as an absolute ceiling 1, 3
• Target driving pressure (plateau pressure minus PEEP) ≤15 cmH₂O, as this predicts mortality better than tidal volume or plateau pressure alone 3
• Apply moderate PEEP guided by severity: for moderate-severe ARDS (PaO₂/FiO₂ <200), higher PEEP strategies reduce mortality 3

Volume-controlled ventilation is recommended in the early stage because it facilitates measurement of respiratory mechanics and driving pressure, which are critical for lung protection 1.

When to Consider APRV

APRV should be considered as a rescue therapy only when:

• Refractory hypoxemia persists despite optimized conventional ventilation 2, 4
• Severe ventilator asynchrony occurs that cannot be managed with sedation adjustments 2
• First-line lung-protective ventilation with AC or SIMV has failed 2

The American Thoracic Society recommends APRV as the primary alternative mode specifically for ARDS patients with ventilator asynchrony or refractory hypoxemia 2. Recent evidence suggests early application of APRV may reduce ventilator duration (median 19 vs 2 ventilator-free days) and ICU stay compared to conventional low tidal volume ventilation 5.

Understanding APRV Settings

If APRV is initiated, use these parameter guidelines:

• Set P_high at the last plateau pressure (not exceeding 30 cmH₂O) 5
• Set P_low at 5 cmH₂O 5
• Adjust T_low (release phase) to terminate peak expiratory flow at ≥50% - this brief release time prevents alveolar de-recruitment and is the defining feature separating APRV from standard bilevel modes 2, 5
• Target release frequency of 10-14 cycles/min 5

APRV utilizes inverse ratio ventilation where inspiratory time exceeds expiratory time, increasing mean airway pressure and alveolar recruitment while remaining pressure-limited and time-cycled 2, 6.

Critical Monitoring Parameters

Monitor these parameters closely regardless of mode:

• Calculate driving pressure at bedside: ΔP = plateau pressure - PEEP 3
• If ΔP >15 cmH₂O, decrease tidal volume below 6 mL/kg if necessary or increase PEEP to recruit collapsed alveoli 3
• Driving pressure ≥18 cmH₂O specifically increases right ventricular failure risk in ARDS 3
• Expect higher mean airway pressures with APRV compared to conventional modes 2, 6

Additional Rescue Strategies Before or Alongside APRV

For severe ARDS (PaO₂/FiO₂ <150 mmHg), implement prone positioning:

• Apply for 16-20 hours per day in the early phase 1
• Prone positioning reduces 28-day mortality (16% vs 32%) in severe ARDS 1
• This strategy redistributes lung densities, recruits dorsal regions, and reduces ventilator-induced lung injury risk 1

Consider neuromuscular blockade in the first 48 hours:

• Reserve for most severe ARDS to prevent excessive transpulmonary pressure generation 1
• Reduces patient-ventilator dyssynchrony and prevents expiratory derecruitment 1
• Requires sustained deep sedation and should be limited to acute phase only 1

Common Pitfalls to Avoid

Do not confuse APRV with standard bilevel pressure support - the critical difference is the intentionally brief T_low setting in APRV designed to prevent alveolar collapse, whereas bilevel modes use longer expiratory times 2. The European Respiratory Society recommends EPAP of 3-5 cmH₂O in bilevel support to offset intrinsic PEEP 2.

Avoid excessive tidal volumes during spontaneous breathing - in ARDS, large transpulmonary pressures and resulting large tidal volumes may exacerbate lung injury through patient self-inflicted lung injury (P-SILI), whether on NIV or during spontaneous efforts on APRV 1. This is particularly concerning as spontaneous ventilation can induce harm similar to ventilator-induced lung injury in severe lung injury 1.

Do not use high-frequency oscillatory ventilation (HFOV) with high baseline pressures - prospective trials have shown harm when used early in ARDS, though it may have limited rescue utility in very severe cases (PaO₂/FiO₂ <70 mmHg) 1.