When to transition from volume control to pressure control setting and its impact on arterial blood gases (ABGs)?

Transitioning from Volume Control to Pressure Control Ventilation

Direct Answer

You should transition from volume control (VC) to pressure control (PC) ventilation when moving from passive/controlled ventilation to assisted spontaneous breathing phases, prioritizing patient comfort and synchrony during weaning, or when managing patients requiring high inflation pressures (such as obesity hypoventilation syndrome). 1, 2

Primary Indications for Mode Transition

Weaning and Assisted Breathing Phase

• Pressure control is preferred during assisted or spontaneous breathing phases when patient comfort and synchrony become priorities, after the acute passive ventilation phase has stabilized 1, 3
• PC ventilation offers superior respiratory comfort during assisted breathing because it does not limit inspiratory flow, allowing the ventilator to match variable patient demand 1
• The transition should occur when switching from controlled to assisted invasive mechanical ventilation as patient recovery allows 2
• PC significantly reduces patient work of breathing compared to VC (0.59 vs 0.70 J/L, p<0.05) due to higher peak inspiratory flow rates 4

Obesity Hypoventilation Syndrome (OHS)

• Volume control or volume-assured modes are more effective when high inflation pressures are required in OHS patients (e.g., IPAP >30, EPAP >8) 2
• In OHS requiring invasive mechanical ventilation, pressure-controlled MV is recommended initially with high PEEP settings to recruit collapsed lung units 2

Difficult-to-Ventilate Patients

• Some patients who fail pressure support can be successfully managed with volume control, particularly when ensuring consistent alveolar ventilation despite changing compliance or resistance is crucial 2, 1
• Volume control better ensures alveolar ventilation when compliance or airway resistance changes acutely 2

Impact on Arterial Blood Gases

Oxygenation Effects

• PC ventilation may promote more homogeneous ventilation distribution and improve oxygenation, particularly in obese patients 1
• The decelerating flow profile of PC may result in better distribution of ventilation 2
• However, one study showed arterial oxygenation slightly deteriorated with PC inverse ratio ventilation despite higher mean airway pressure 5

CO₂ Elimination

• For the same tidal volume, there is no outcome advantage between PC and VC in terms of stress and strain generated in the lung 1
• VC ensures consistent minute ventilation when permissive hypercapnia is employed, which is critical for maintaining predictable CO₂ elimination 1
• PC with inverse ratios may slightly improve alveolar ventilation (lower PaCO₂ trend) but this is not clinically significant 5

Critical Caveat on Rebreathing

• PC bi-level systems have significant rebreathing potential that can worsen hypercapnia, especially in tachypneic patients with respiratory rates >20/min 2
• Normally used EPAP levels (3-5 cmH₂O) do not completely eliminate rebreathing during bi-level pressure support 2
• This must be considered if a patient fails to improve or develops worsening hypercapnia after transitioning to PC 2

When Volume Control Should Be Maintained

Early ARDS Management

• During the early passive ventilation phase of ARDS, VC is recommended because it facilitates measurement of respiratory mechanics and driving pressure, which are critical for lung-protective ventilation 1
• VC allows automatic collection of plateau pressure and driving pressure during inspiratory pause, essential for titrating PEEP 1
• VC guarantees fixed tidal volume delivery at 6 mL/kg predicted body weight, crucial for lung protection 1

Plateau Pressure Management

• VC is essential when reducing tidal volume from 6 to 4 mL/kg predicted body weight for plateau pressures >30 cmH₂O 1
• The overdistension-collapse method for PEEP titration requires VC with inspiratory pause >0.5 seconds to measure driving pressure 1

Leak Compensation

• VC better compensates for air leaks (from mask or mouth in NIV, or around endotracheal tubes), though tidal volumes must be arbitrarily increased 2

Algorithmic Approach to Mode Selection

Initial Phase (First 24-48 hours):

• Start with volume assist-control at 6 mL/kg PBW 6, 3
• Maintain plateau pressure ≤30 cmH₂O 6, 1
• Use this phase to measure respiratory mechanics and optimize PEEP 1

Transition Phase (After stabilization):

• When reducing sedation and allowing spontaneous breathing efforts, transition to PC 1
• Ensure adequate monitoring for delivered tidal volumes, as PC can deliver excessive volumes 7
• Monitor for worsening hypercapnia due to rebreathing 2

Special Populations:

• OHS patients: Consider volume-assured modes for high pressure requirements 2
• ARDS patients: Begin with VC, transition to PC only during weaning 1
• Obstructive disease: Either mode acceptable, but ensure adequate expiratory time (I:E 1:2 or 1:3) 6, 3

Common Pitfalls to Avoid

• Do not assume PC automatically provides lung protection—delivered tidal volumes with pressure-regulated volume control (PRVC) are often significantly higher than set volumes, frequently exceeding 6 mL/kg ideal body weight 7
• Monitor ABGs closely after transitioning to PC for unexpected hypercapnia from rebreathing, especially if respiratory rate increases 2
• Ensure exhalation ports are functioning properly—occlusion by sputum can exacerbate hypercapnia through rebreathing 2
• Do not transition to PC too early in ARDS—wait until the passive ventilation phase is complete and respiratory mechanics are optimized 1