What is the recommended pressure limit in volume support (VS) mode on a ventilator?

Pressure Limit in Volume Support Mode

In volume support (VS) mode, maintain plateau pressure ≤30 cmH₂O, with the ventilator automatically adjusting pressure support breath-by-breath to deliver the preset tidal volume while respecting this pressure ceiling. 1, 2

Understanding Volume Support Mode

Volume support ventilation is a pressure-limited mode that uses tidal volume as feedback control—the ventilator continuously adjusts inspiratory pressure level breath-by-breath to deliver a preset tidal volume target. 3 This differs from traditional pressure support where tidal volume varies with patient effort and lung mechanics. 3

Recommended Pressure Limits

The plateau pressure ceiling should be set at ≤30 cmH₂O for patients with ARDS and acute respiratory failure. 1, 2 This represents the most widely validated pressure threshold associated with reduced mortality and ventilator-induced lung injury. 1

• For patients with normal lungs, consider a slightly lower threshold of ≤28 cmH₂O 2
• In cases of increased chest wall elastance (obesity, ascites, pleural effusions), plateau pressures up to 29-32 cmH₂O may be acceptable since the elevated pressure reflects chest wall stiffness rather than excessive lung stretch 2, 4

Target Tidal Volume Settings

Set target tidal volume at 6 mL/kg predicted body weight (PBW) as your starting point. 1 The ventilator will adjust pressure support to achieve this volume while respecting the pressure limit.

• Calculate PBW using: Male = 50 + 2.3 [height (inches) - 60]; Female = 45.5 + 2.3 [height (inches) - 60] 1
• If plateau pressure exceeds 30 cmH₂O despite 6 mL/kg, reduce tidal volume target progressively down to 4 mL/kg PBW 1, 4
• Accept permissive hypercapnia as long as pH remains >7.15-7.20 unless contraindicated by elevated intracranial pressure 1, 4

Maximum Pressure Support Settings

The maximum pressure support (IPAP-EPAP difference) should not exceed 20 cmH₂O. 1 For patients ≥12 years, maximum IPAP should be ≤30 cmH₂O; for patients <12 years, limit IPAP to ≤20 cmH₂O. 1

Critical Monitoring Parameters

Monitor these parameters continuously to ensure safe ventilation:

• Plateau pressure: Must remain ≤30 cmH₂O 1, 2
• Driving pressure (plateau pressure minus PEEP): Values >15 cmH₂O predict worse outcomes and warrant intervention 4
• Delivered tidal volume: VS mode may "sacrifice" tidal volume to maintain pressure limits, particularly in patients with poor compliance 5
• Minute ventilation adequacy: Ensure the automatically adjusted tidal volumes maintain adequate minute ventilation 5

Common Pitfalls and Management

Volume support may fail to deliver adequate tidal volume when compliance is severely reduced. 5 In simulation studies, VS mode sacrificed tidal volume and minute ventilation in 17-37% of scenarios to maintain plateau pressure limits. 5

When this occurs:

• First verify the pressure limit is appropriately set at 30 cmH₂O (not lower) 1, 2
• Optimize PEEP to improve compliance while monitoring that total pressure (PEEP + driving pressure) remains acceptable 1, 4
• Consider switching to pressure support with guaranteed volume mode, which has demonstrated reduced risk of excessive tidal volumes while maintaining volume targets 6
• Avoid traditional pressure support without volume guarantee, as this carries 19-fold increased odds of delivering tidal volumes >6 mL/kg 6

Adjusting for Patient Effort and Asynchrony

Increase pressure support by 1-2 cmH₂O increments every 5 minutes if tidal volume remains inadequate (below 6-8 mL/kg for most patients), but never exceed the 30 cmH₂O plateau pressure ceiling. 1, 2

• Optimize trigger sensitivity and inspiratory flow settings to minimize work of breathing and prevent double-triggering 7
• If patient-ventilator asynchrony persists despite optimization, increase sedation rather than accepting elevated plateau pressures 7
• Patient effort generates higher transalveolar pressures for a given plateau pressure, so actively breathing patients may require more conservative pressure limits 1

Mode Selection Considerations

No single ventilation mode (volume control, pressure control, or volume support) has proven superior when lung-protective principles are maintained. 1, 4 However, volume support offers theoretical advantages by automatically adjusting to changes in compliance and patient effort while maintaining pressure limits. 3, 5

Consider alternative modes if volume support fails to maintain adequate ventilation within pressure limits:

• Airway pressure release ventilation (APRV) demonstrated significantly reduced risk of excessive tidal volumes (OR 0.44) and may be considered for refractory cases 6
• Traditional volume control with 6 mL/kg may result in plateau pressures exceeding 30 cmH₂O in 35-39% of scenarios with varying lung mechanics 5