Ventilator Modes: APRV, PRVC, and HFOV
High-Frequency Oscillatory Ventilation (HFOV)
Do not use HFOV routinely in adult patients with moderate or severe ARDS—this mode is associated with potential harm and no mortality benefit. 1
Strong Recommendation Against Routine Use in Adults
The American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine strongly recommends against routine use of HFOV in adult patients with moderate or severe ARDS (strong recommendation, high confidence in effect estimates). 1
Two large multicenter trials demonstrated either significant harm (RR 1.41 for mortality) or no benefit (adjusted OR 1.03) when HFOV was compared to lung-protective conventional ventilation. 1
Meta-analysis of three studies with 1,371 patients showed no mortality difference (RR 1.14; 95% CI 0.88-1.48), and when all six RCTs were pooled (1,705 patients), there remained no significant mortality benefit (RR 0.94; 95% CI 0.71-1.24). 1
Limited Role as Rescue Therapy
HFOV may be considered only as rescue therapy in patients with severe ARDS and refractory hypoxemia when conventional lung-protective ventilation has failed, though definitive evidence for this indication remains lacking. 1, 2
In pediatric populations, HFOV should be considered as rescue therapy when conventional mechanical ventilation fails in severe respiratory failure, using an open lung strategy. 3
Pediatric-Specific Indications
- The American Thoracic Society recommends HFOV for pediatric patients with acute hypoxemic respiratory failure, restrictive lung disease, mixed disease, obstructive airway disease (with caution), cardiac children with severe respiratory failure, and chronically ventilated children experiencing acute exacerbations. 3
Initial HFOV Settings (When Used as Rescue)
Mean airway pressure: Start 3-5 cmH₂O above the mean airway pressure used during conventional ventilation to optimize lung recruitment. 4
Frequency: 3-6 Hz in adults (lower frequencies for obstructive disease, higher for restrictive). 4
Amplitude (ΔP): Adjust to achieve visible chest wall vibration, typically starting at 90 cmH₂O and titrating based on CO₂ elimination. 4
FiO₂: Start at 1.0 and wean based on oxygenation response. 4
Inspiratory time: 33% (1:2 ratio). 4
Critical Monitoring and Limitations
Plateau pressure should be limited to ≤28 cmH₂O in the absence of transpulmonary pressure measurements, or ≤29-32 cmH₂O if chest wall elastance is increased. 3
HFOV requires heavy sedation and often paralysis, which limits mobilization and increases associated risks. 1, 5
No significant differences were found in oxygenation at 24 hours, CO₂ tension, or barotrauma rates compared to conventional ventilation. 1
Airway Pressure Release Ventilation (APRV)
APRV may be used as a rescue or alternative mode for patients with ARDS and refractory hypoxemia when conventional lung-protective ventilation is inadequate, though high-quality evidence supporting mortality benefit is lacking. 6
Indications for APRV
APRV is typically utilized as a rescue or alternative mode for patients with ARDS and hypoxemia refractory to conventional mechanical ventilation. 6
APRV may safely enhance hemodynamics in patients with ALI/ARDS compared to pressure control ventilation, with decreased need for paralysis, sedation, and vasopressor support. 7
All patients with ventilator-induced lung injury (VILI) or at risk of developing VILI or ARDS who have failed conventional mechanical ventilation are suitable candidates. 2
Initial APRV Settings
P-high (high pressure): Set to plateau pressure from conventional ventilation or 25-30 cmH₂O, maintaining lung recruitment. 4
T-high (time at high pressure): 4-6 seconds, allowing for spontaneous breathing during this phase. 4
P-low (low pressure): Typically 0-5 cmH₂O to allow brief lung deflation. 4
T-low (release time): 0.4-0.8 seconds, adjusted to achieve 50-75% of peak expiratory flow (termination of release when flow reaches 50-75% of peak). 4
Physiologic Advantages
APRV increases cardiac index from 3.2 to 4.6 L/min/m² BSA and oxygen delivery from 997 to 1409 mL/min compared to pressure control ventilation with inverse ratio. 7
Peak airway pressures decrease from 38 to 25 cmH₂O and mean pressures from 18 to 12 cmH₂O when switching from PCV to APRV. 7
Central venous pressure declines from 18 to 12 cmH₂O, and urine output increases from 0.83 to 0.96 mL/kg/hour. 7
APRV allows spontaneous breathing, which may reduce sedation requirements and preserve respiratory muscle function. 6
Evidence Limitations
Despite theoretical benefits, APRV's indication and efficacy remain unclear due to lack of consensus amongst practitioners, inconsistent methodology, and scarcity of convincing evidence from randomized controlled trials. 6
APRV's use may begin to outpace HFOV for management of refractory hypoxemia given benefits of spontaneous breathing and minimizing sedation. 6
Pressure Regulated Volume Control (PRVC)
PRVC is a dual-control mode that combines volume-targeted ventilation with pressure-limited breaths, but it is not specifically addressed in major ARDS guidelines and should not replace standard lung-protective ventilation strategies.
General Principles
PRVC automatically adjusts inspiratory pressure breath-to-breath to deliver a set tidal volume, theoretically combining the benefits of pressure control (lower peak pressures) with volume control (guaranteed minute ventilation).
This mode is not mentioned in the 2017 American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine ARDS guidelines, which focus on conventional volume-cycled assist-control as the standard. 1, 8
Standard Lung-Protective Ventilation Remains Primary
For all patients with ARDS, use mechanical ventilation with lower tidal volumes (4-8 mL/kg predicted body weight) and lower inspiratory pressures (plateau pressure <30 cmH₂O) regardless of mode. 1
The American Thoracic Society recommends starting with volume-cycled assist-control ventilation when initiating mechanical ventilation, as this provides complete ventilatory support and ensures a backup respiratory rate. 8
Target 6 mL/kg predicted body weight to reduce mortality in ARDS and sepsis-induced respiratory failure (strong recommendation with high-quality evidence). 8
Key Pitfall to Avoid
- Do not use actual body weight for tidal volume calculations—always use predicted body weight calculated as: Men = 50 + 2.3 × (height in inches - 60); Women = 45.5 + 2.3 × (height in inches - 60). 8