What are the differences between VC+ (Volume Control) mode and regular VC (Volume Control) mode in terms of flow delivery and patient suitability?

VC+ vs Regular VC: Flow Delivery Differences and Clinical Implications

The statement is correct: VC+ (Volume Control Plus or Pressure-Regulated Volume Control) delivers variable decelerating flow that automatically adjusts breath-to-breath to maintain target tidal volume at the lowest possible pressure, while regular VC delivers fixed constant flow throughout inspiration. 1, 2

Flow Pattern Mechanics

Regular Volume Control (VC):

• Delivers a pre-set constant inspiratory flow that remains fixed regardless of patient demand or changing lung mechanics 3
• The clinician must manually set inspiratory flow rate, flow waveform, and inspiratory time 3
• Airway pressure increases passively in response to reduced compliance, increased resistance, or active exhalation 3
• Peak inspiratory pressure can rise dangerously high when lung mechanics deteriorate 4

VC+ Mode (Pressure-Regulated Volume Control):

• Delivers variable decelerating flow with high initial flow that tapers throughout inspiration 3, 5
• The ventilator automatically adjusts inspiratory pressure breath-to-breath to deliver the target tidal volume at the lowest possible peak pressure 4
• Peak inspiratory pressure is significantly lower (approximately 20 cmH₂O vs 24 cmH₂O in VC) while maintaining identical tidal volumes 4
• Combines the safety of guaranteed tidal volume delivery with pressure-limited characteristics 3

Patient Suitability and Clinical Selection

When Regular VC is Superior:

• Early passive ARDS ventilation when measuring respiratory mechanics and driving pressure is critical for lung-protective ventilation 2
• Reducing tidal volume from 6 to 4 mL/kg predicted body weight when plateau pressures exceed 30 cmH₂O 2
• Ensuring consistent minute ventilation during permissive hypercapnia strategies 2
• Initial stabilization phase of acute respiratory failure when guaranteeing alveolar ventilation despite changing compliance is essential 1, 6
• Air leak compensation where tidal volumes must be arbitrarily increased 2

When VC+ is Superior:

• Assisted or spontaneous breathing phases when patient comfort and synchrony become priorities 2
• Patients with variable respiratory demand who require breath-to-breath flow adjustments 3, 5
• Acute lung injury/ARDS during weaning when transitioning from controlled to assisted ventilation 2
• Reducing work of breathing: VC+ significantly decreases patient work of breathing (0.59 ± 0.42 J/L vs 0.70 ± 0.58 J/L with regular VC) due to higher peak inspiratory flow (103.2 ± 22.8 L/min vs 43.8 L/min) 5
• Preventing ventilator-induced lung injury by automatically limiting peak pressures while maintaining target volumes 4

Critical Nuances and Common Pitfalls

Important Caveats:

• No outcome differences exist between modes when tidal volume, plateau pressure, and driving pressure are appropriately managed 2, 7
• For the same tidal volume delivery, there is no advantage in terms of lung stress and strain generated between modes 2
• Meta-analysis of 34 studies showed no differences in compliance, gas exchange, hemodynamics, work of breathing, or clinical outcomes when comparing pressure-control variants to volume control 7
• The beneficial effects attributed to VC+ primarily result from the decelerating flow waveform, which is now available during regular VC on many modern ventilators 3

Clinical Algorithm for Mode Selection:

1. Start with regular VC during acute respiratory failure requiring passive ventilation 1, 6
2. Maintain regular VC when titrating PEEP using the overdistension-collapse method (requires inspiratory pause >0.5 seconds) 2
3. Transition to VC+ when reducing sedation and allowing spontaneous breathing efforts 2
4. Consider VC+ when peak pressures exceed 35-40 cmH₂O despite appropriate tidal volume settings 4
5. Return to regular VC if tidal volume delivery becomes inconsistent or if precise respiratory mechanics measurements are needed 2

Avoid These Mistakes:

• Do not assume VC+ provides superior lung protection—both modes can cause ventilator-induced lung injury if tidal volume and plateau pressure limits are exceeded 2, 7
• Do not use VC+ as initial mode in early ARDS when accurate measurement of driving pressure is essential for PEEP titration 2
• Do not expect improved oxygenation with VC+—studies show no gas exchange advantage and some suggest poorer oxygenation index with pressure-control variants 7
• Monitor delivered tidal volume continuously with VC+ as variable compliance can result in inconsistent volume delivery 3