What is the preferred ventilation mode, pressure control mode or volume control mode, for a patient requiring mechanical ventilation, particularly those with acute respiratory distress syndrome (ARDS) or chronic obstructive pulmonary disease (COPD)?

Ventilation Mode Selection: Volume Control vs Pressure Control

For patients with ARDS requiring passive mechanical ventilation, volume control ventilation (VCV) is the preferred initial mode because it ensures delivery of lung-protective tidal volumes (4-8 mL/kg predicted body weight), facilitates measurement of plateau pressure and driving pressure, and allows precise titration of PEEP—all critical for reducing mortality. 1, 2

Initial Ventilation Strategy for ARDS

Begin with volume control ventilation during the early passive ventilation phase of ARDS. 2 The rationale is straightforward:

• VCV guarantees fixed tidal volume delivery (4-8 mL/kg predicted body weight), which is essential for implementing the lung-protective ventilation strategy that has demonstrated mortality reduction in ARDS patients. 1

• VCV allows automatic collection of plateau pressure and driving pressure during inspiratory pause, which are critical measurements for assessing lung stress and titrating PEEP appropriately. 2

• VCV is essential when reducing tidal volume from 6 mL/kg to 4 mL/kg predicted body weight for plateau pressures >30 cm H₂O, ensuring consistent delivery despite changing lung mechanics. 2

• Target plateau pressure <30 cm H₂O and driving pressure <15 cm H₂O regardless of mode, as these parameters predict outcomes independent of ventilation mode. 2

When to Transition to Pressure Control Ventilation

Transition from VCV to pressure control ventilation (PCV) when switching from controlled to assisted invasive mechanical ventilation as patient recovery allows. 2 This transition should occur when:

• Patient comfort and synchrony become priorities, particularly during assisted or spontaneous breathing phases. 2

• The patient demonstrates intact respiratory drive and begins triggering breaths, as PCV offers superior respiratory comfort by not limiting inspiratory flow and allowing the ventilator to match variable patient demand. 2

• Patient work of breathing becomes a concern, as PCV significantly reduces work of breathing compared to VCV (0.59 vs 0.70 J/L) due to higher peak inspiratory flow rates (103.2 vs 43.8 L/min). 3

Critical Nuances for COPD Patients

For patients with COPD exacerbations and acute hypercapnic respiratory failure, the approach differs:

• Non-invasive ventilation using bi-level pressure support (CPAP 4-8 cmH₂O plus pressure support 10-15 cmH₂O) is the most effective first-line approach. 1

• If invasive mechanical ventilation is required, VCV using assist-control mode is appropriate initially to provide complete ventilatory support. 1

• Monitor for auto-PEEP and gas trapping, which are aggravated by hyperventilation and can compromise cardiac output—this risk exists with both modes but requires particular attention in COPD. 1

Common Pitfalls and How to Avoid Them

Pitfall #1: Using PCV during early ARDS without adequate monitoring

• PCV may fail to deliver programmed tidal volumes in high-resistance conditions, potentially leading to hypoventilation. 4
• Solution: Use VCV during passive ventilation to ensure consistent tidal volume delivery, then transition to PCV only when patients begin assisted breathing. 2

Pitfall #2: Assuming PCV automatically provides lung protection

• For the same tidal volume, there is no outcome advantage between PCV and VCV in terms of stress and strain generated in the lung. 2
• Solution: Both modes can achieve lung protection when tidal volume, plateau pressure, and driving pressure are appropriately managed—the mode itself is less important than these parameters. 2

Pitfall #3: Failing to monitor delivered volumes when using PCV

• In severely obstructed patients, PCV delivered only 120 mL when 200 mL was programmed (Evita XL) and 104 mL (Servo-i). 4
• Solution: Closely monitor exhaled tidal volumes when using PCV, especially in patients with high airway resistance or changing compliance. 4, 5

Pitfall #4: Using low tidal volumes in VCV without addressing patient-ventilator dyssynchrony

• Low tidal volumes (6 mL/kg) during VCV result in dyssynchrony index of 100% in many patients, compared to 25% at 9 mL/kg. 5
• Solution: When dyssynchrony occurs with lung-protective VCV, transition to adaptive pressure control (APC) mode, which reduces dyssynchrony while maintaining target volumes—but monitor carefully to avoid larger-than-target delivered volumes. 5

Specific Clinical Scenarios

For obesity hypoventilation syndrome (OHS) with acute hypercapnic respiratory failure:

• Pressure-controlled mechanical ventilation is recommended initially, with high PEEP settings (10-15 cm H₂O range) often required to recruit collapsed lung units. 1
• If tidal volume delivery is inadequate despite high inspiratory pressures (>30 cm H₂O), consider volume-controlled ventilation or volume-assured modes. 1
• Forced diuresis is often indicated as fluid overload commonly contributes to ventilatory failure and its degree is easily underestimated (may exceed 20 L). 1

For post-cardiac arrest patients:

• Maintain normocapnia (PaCO₂ 40-45 mm Hg or ETCO₂ 35-40 mm Hg) as routine hyperventilation with hypocapnia worsens global brain ischemia. 1
• Use low tidal volume ventilation (6-8 mL/kg predicted body weight) to reduce ventilator-associated lung injury, as these patients are at risk for ARDS. 1
• Either VCV or PCV is acceptable, but avoid hyperventilation and excessive tidal volumes that increase intrathoracic pressure and compromise hemodynamics. 1

Algorithm for Mode Selection

1. Initial assessment: Is the patient with ARDS requiring passive ventilation?

   • Yes → Start with VCV in assist-control mode 1, 2
   • No → Proceed to step 2

2. Is the patient beginning assisted breathing with intact respiratory drive?

   • Yes → Transition to PCV for improved comfort and synchrony 2
   • No → Continue VCV

3. Is severe patient-ventilator dyssynchrony present despite appropriate settings?

   • Yes → Consider adaptive pressure control or dual-control modes 5
   • No → Continue current mode

4. Monitor continuously: Plateau pressure, driving pressure, delivered tidal volumes, and patient-ventilator synchrony regardless of mode selected. 1, 2