Choosing Between Pressure Support and PRVC Ventilation Modes
For awake, spontaneously breathing critically ill patients, pressure support (PS) ventilation is the preferred mode due to superior patient comfort and synchrony, while pressure-regulated volume control (PRVC) or volume control modes should be used during passive ventilation in early acute respiratory failure to ensure lung-protective ventilation and facilitate measurement of critical respiratory mechanics. 1, 2
Initial Ventilator Mode Selection Algorithm
Start with Volume Control in These Situations:
Early ARDS or severe acute respiratory failure requiring passive ventilation: Volume control (VC) or PRVC in assist-control mode is essential because it facilitates automatic measurement of plateau pressure and driving pressure during inspiratory pause, which are critical for titrating PEEP and implementing lung-protective ventilation strategies. 1, 3
When implementing strict tidal volume targets: VC guarantees fixed tidal volume delivery of 6 mL/kg predicted body weight, crucial for lung protection when plateau pressures exceed 30 cm H₂O. 1, 3
Patients with unstable respiratory mechanics: VC better ensures consistent alveolar ventilation when compliance or airway resistance changes acutely, and it better compensates for air leaks (though tidal volumes must be increased). 1
When permissive hypercapnia is employed: VC ensures consistent minute ventilation delivery. 1
Transition to Pressure Support When:
Patient becomes awake and initiates spontaneous breathing efforts: PS offers superior respiratory comfort because it does not limit inspiratory flow, allowing the ventilator to match variable patient demand. 1, 2
Weaning from mechanical ventilation: PS allows patient-controlled respiratory frequency and timing of each breath, promoting respiratory muscle activity. 4
Patient comfort becomes a priority: In a randomized trial of intubated patients, PS achieved significantly higher comfort scores (83 ± 11) compared to volume control (70 ± 18, p=0.02), with 11 of 14 patients preferring PS. 2
Critical Physiologic Differences
Pressure Support Characteristics:
Patient-triggered on and off: The patient's respiratory effort triggers both inspiration and expiration, with respiratory frequency and timing determined entirely by the patient. 4
Flow delivery: Does not limit inspiratory flow, automatically adjusting to meet variable patient demand, which explains superior comfort during assisted breathing. 1, 5
Backup rate: Many modern ventilators incorporate a backup rate of 6-8 breaths per minute if the patient fails to make respiratory effort. 4
Tidal volume variability: Delivered tidal volume varies based on set pressure, patient effort, and respiratory mechanics—requires close monitoring. 6, 5
PRVC/Volume Control Characteristics:
Guaranteed tidal volume: Ensures preset tidal volume delivery regardless of changes in compliance or resistance. 1, 5
Respiratory mechanics measurement: Allows automatic collection of plateau pressure and driving pressure during inspiratory pause, essential for lung-protective ventilation. 1
Fixed flow patterns: Inspiratory flow and flow waveform are preset (though decelerating flow is available on many modern ventilators). 5
Outcome Data: No Mortality Difference When Properly Applied
For the same tidal volume, there is no outcome advantage between pressure control and volume control modes in terms of stress, strain, or clinical outcomes when both are properly managed. 1, 7
A systematic review of 34 studies found no differences between pressure control and volume control modes for compliance, gas exchange, hemodynamics, work of breathing, or clinical outcomes. 7
Both modes can achieve lung protection when tidal volume, plateau pressure (<30 cm H₂O), and driving pressure (<15 cm H₂O) are appropriately managed. 1
Any perceived benefit of pressure control likely results from the associated decelerating flow waveform, which is now available during volume control on most modern ventilators. 5
Common Pitfalls and How to Avoid Them
When Using Pressure Support:
Rebreathing risk in tachypneic patients: Significant rebreathing potential exists with bi-level pressure support systems, especially at high respiratory rates (>20/min), which can paradoxically worsen hypercapnia in anxious, tachypneic patients. 8, 3
Monitor arterial blood gases closely after transitioning to PS, particularly if respiratory rate increases, to detect unexpected hypercapnia from rebreathing. 1
Ensure exhalation ports function properly: Occlusion by sputum can exacerbate hypercapnia through rebreathing. 1, 3
Patient-ventilator asynchrony: May result from undetected inspiratory effort, delayed response to start of inspiration, or excessive air leakage—when asynchrony cannot be resolved, switch back to assist-control mode. 8
When Using Volume Control/PRVC:
Always use predicted body weight for tidal volume calculations, never actual body weight (Men = 50 + 2.3 × [height in inches - 60]; Women = 45.5 + 2.3 × [height in inches - 60]). 3
Monitor plateau pressure continuously: Do not assume pressure-limited modes automatically protect against ventilator-induced lung injury—plateau pressure monitoring is essential regardless of mode. 3
Avoid hyperventilation: This causes cerebral vasoconstriction, hemodynamic instability, and increased mortality. 3
Practical Transition Strategy
Begin with volume control assist-control during early passive ARDS ventilation (6 mL/kg PBW, plateau pressure ≤30 cm H₂O, driving pressure <15 cm H₂O), then transition to pressure support when weaning sedation and allowing spontaneous breathing efforts. 1, 3
During volume control: Continuously monitor plateau pressure, driving pressure, and tidal volume delivery. 1
During transition: Gradually reduce sedation while monitoring for patient-ventilator synchrony. 1
After transition to PS: Monitor respiratory rate, tidal volume consistency, and arterial blood gases for signs of rebreathing or inadequate ventilation. 1, 8
Special Consideration for Sleep Quality
For critically ill patients requiring nighttime ventilation, assist-control mode (rather than pressure support) improves sleep quality. 9
Assist-control ventilation was associated with increased sleep efficiency (18.33% improvement, 95% CI 7.89-28.76) and more time spent in REM sleep (2.79% increase, 95% CI 0.53-5.05) compared to pressure support. 9
For patients who remain dyssynchronous despite optimizing assist-control settings, clinicians must make case-by-case decisions whether to return to pressure support, considering the deleterious effects of sedation on sleep quality. 9