PRVC vs Volume Control
A clinical comparison of Pressure Regulated Volume Control and traditional Volume Control ventilation — when to use each and what the evidence says.
Written for respiratory therapists, students, and clinicians.
Understanding the Fundamentals
The debate between pressure-regulated and volume-targeted ventilation sits at the heart of modern mechanical ventilation management. Both Volume Control (VC) and Pressure Regulated Volume Control (PRVC) aim to deliver safe tidal volumes while protecting the lungs — they differ in how they achieve this goal.
Volume Control ventilation has been the workhorse of mechanical ventilation for decades. It delivers a precise, pre-set tidal volume with each breath, adjusting inspiratory flow and pressure as needed to achieve that volume. The trade-off is that airway pressures can vary widely based on lung compliance and resistance.
PRVC (also called Adaptive Pressure Ventilation or Auto-Flow depending on the ventilator manufacturer) is a dual-control mode that combines the predictability of volume control with the comfort of pressure control. It automatically adjusts the inspiratory pressure breath-by-breath to deliver the target tidal volume using the lowest pressure necessary.
Volume Control (VC) Ventilation
Mechanics
In Volume Control, the clinician sets the tidal volume (Vt), respiratory rate (RR), inspiratory flow rate or inspiratory time (I:E ratio), PEEP, and FiO₂. The ventilator delivers a constant flow to achieve the set tidal volume. Because flow is constant, airway pressure will rise or fall depending on lung compliance — if the lungs become stiffer, peak airway pressure increases.
Advantages of Volume Control
- • Guaranteed tidal volume delivery — exactly what is set is delivered
- • Predictable minute ventilation, making CO₂ management straightforward
- • Changing compliance is immediately reflected in peak airway pressure — early warning of airway or lung changes
- • Widely understood and applied across institutions
- • Preferred by many clinicians during initial ventilator management when lung mechanics are unknown
Disadvantages of Volume Control
- • Constant inspiratory flow can cause patient-ventilator dyssynchrony (flow starvation)
- • Peak pressures can become dangerously high if compliance worsens acutely
- • Less adaptive to changes in patient effort or lung mechanics
Tidal Volume
6–8 mL/kg IBW
Lung-protective target
Inspiratory Flow
40–60 L/min
Typical starting point
Plateau Pressure
< 30 cmH₂O
Safety target
PRVC: Pressure Regulated Volume Control
How PRVC Works
PRVC uses a test breath to measure the patient's respiratory system compliance, then calculates the pressure needed to deliver the target tidal volume. On each subsequent breath, the ventilator adjusts its pressure delivery — up or down by no more than 3 cmH₂O per breath — to consistently deliver the set tidal volume using the least pressure necessary. This adaptive, breath-by-breath adjustment is the defining feature of PRVC.
If the patient generates more spontaneous effort (reducing the needed driving pressure), PRVC backs off on pressure. If lung compliance worsens, PRVC increases pressure up to a set maximum (typically set 5–10 cmH₂O above the working pressure). If the target volume cannot be delivered below the pressure limit, an alarm is triggered.
Advantages of PRVC
- • Decelerating flow pattern (like pressure control) improves gas distribution and patient comfort
- • Volume is still guaranteed despite pressure variations
- • Automatically adapts to changes in lung mechanics without manual adjustments
- • Lower mean airway pressures compared to fixed-volume VC in some situations
- • Improved patient-ventilator synchrony due to decelerating flow
Disadvantages and Risks of PRVC
- • Volume delivery can be unpredictable when patient effort is high and highly variable
- • In highly active patients, PRVC may reduce support excessively (patient pulls too much volume, triggering PRVC to lower pressure further)
- • Risk of patient self-inflicted lung injury (P-SILI) if strong patient effort is not recognized
- • Adapts well to improving lungs — but this means you may not notice worsening as quickly as in pure VC
Direct Clinical Comparison
| Feature | Volume Control | PRVC |
|---|---|---|
| Volume guarantee | Yes | Yes |
| Flow pattern | Constant (square) | Decelerating |
| Pressure variation | Yes — follows compliance | Minimal — adapts automatically |
| Patient synchrony | Can cause dyssynchrony | Generally improved |
| Adapts to mechanics | No (alarm-driven) | Yes (breath-by-breath) |
| Clinical complexity | Simple, predictable | More nuanced monitoring needed |
| Best for | Initial vent setup, ARDS | Spontaneously breathing, post-op |
Clinical Decision Guide: Which Mode to Use
Neither mode is universally superior. The choice depends on patient condition, goals of ventilation, and institutional practice.
Consider Volume Control when:
- Initiating ventilation and lung mechanics are unknown or highly variable
- Managing ARDS with strict lung-protective ventilation targets
- Patient is deeply sedated with minimal spontaneous effort
- You need precise, predictable minute ventilation
Consider PRVC when:
- Patient is spontaneously breathing and making some respiratory effort
- Improving synchrony is a priority (post-operative, moderate sedation)
- Lung mechanics are improving and you want the ventilator to automatically reduce support
- Weaning from ventilation and transitioning toward lower pressure support
Frequently Asked Questions
Is PRVC the same on every ventilator?
No. PRVC is a concept, but manufacturers implement it differently and use different names. Puritan Bennett calls it "VC+" (Volume Control Plus). Maquet uses "AutoFlow." Dräger uses "VC-CMV with AutoFlow." Hamilton uses "APVcmv." The underlying principle is similar — adaptive pressure to deliver a target volume — but the algorithms differ.
Can PRVC cause volutrauma?
In patients with strong spontaneous inspiratory effort, PRVC may drop pressure too low, causing the patient to generate very large tidal volumes on their own (P-SILI — patient self-inflicted lung injury). Monitor exhaled tidal volumes closely in spontaneously breathing patients on PRVC, especially when effort is vigorous.
What is the pressure limit setting in PRVC?
Most ventilators allow setting a maximum pressure limit in PRVC mode (often called "Pmax" or "Pressure Limit"). If PRVC cannot deliver the target volume below this limit, an alarm fires. This limit should typically be set 10–15 cmH₂O above the working inspiratory pressure as a safety ceiling.
Does the ARDSNet protocol specify Volume Control?
The original ARDSNet ARMA trial used volume-assist control ventilation with a set 6 mL/kg IBW tidal volume and plateau pressure target of ≤30 cmH₂O. PRVC can achieve the same volume targets but requires careful monitoring of plateau pressure and spontaneous effort in ARDS patients.
Summary
- Volume Control delivers a fixed tidal volume with variable pressure — reliable but less adaptive
- PRVC adapts pressure breath-by-breath to deliver volume at the lowest needed pressure
- PRVC's decelerating flow improves synchrony in spontaneously breathing patients
- Monitor PRVC carefully in highly active patients — risk of P-SILI if effort is underestimated
- Know your ventilator's PRVC algorithm by its manufacturer name
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