How does pressure support ventilation (PSV) work?

How Pressure Support Ventilation Works

Pressure support ventilation (PSV) is a patient-triggered, pressure-limited, and flow-cycled mode of ventilation that augments the patient's spontaneous inspiratory efforts with a clinician-selected level of positive airway pressure to reduce work of breathing. 1, 2

Basic Mechanism of Action

• In PSV, the patient's respiratory effort triggers the ventilator to deliver a preset pressure during inspiration, while the patient controls both respiratory rate and timing of each breath 1
• The ventilator delivers a constant pressure throughout inspiration until the flow decreases to a certain threshold (typically 25% of peak flow), at which point the ventilator cycles to expiration 2
• If the patient fails to make respiratory efforts, no respiratory assistance will occur, unless the ventilator has a backup rate feature (many modern ventilators incorporate a backup rate of 6-8 breaths per minute) 1

Key Components and Settings

• Trigger: Patient's inspiratory effort initiates breath delivery (flow or pressure triggered) 2
• Pressure Support Level: Clinician-selected pressure that determines the amount of assistance provided during inspiration 3
• Rise Time (Pressurization Rate): Speed at which the ventilator reaches the set pressure support level after triggering 2
• Flow Cycling Criteria: The percentage of peak inspiratory flow at which the ventilator cycles from inspiration to expiration (typically 25%, but adjustable on newer ventilators) 2

Physiological Effects

• PSV reduces respiratory muscle workload by decreasing the work of breathing required by the patient 3
• It improves patient comfort compared to volume-controlled ventilation modes 4
• PSV allows for a more balanced pressure and volume change form of muscle work for the patient 3
• The delivered tidal volume depends on the pressure support level, patient effort, and respiratory system mechanics (resistance and compliance) 2

Clinical Applications

Advantages

• PSV is more comfortable for patients compared to volume-controlled continuous mandatory ventilation (VC-CMV), with studies showing significantly higher comfort scores 4
• It can be used during weaning from mechanical ventilation, as it allows patients to gradually resume respiratory muscle work 1
• PSV can be used in spontaneous breathing trials (SBTs) with modest pressure support (5-8 cmH2O) to overcome the resistance of the endotracheal tube 1

Potential Issues

• High levels of PSV may cause sleep disruption from periodic breathing due to hyperventilation and hypocapnia 1
• If pressure support delivers higher-than-needed alveolar minute ventilation, it can lead to central apneas during sleep 1
• In patients with increased ventilatory demand, fixed PSV may not adequately respond to changing patient needs 5

Advanced PSV Variations

• Volume-Assured Pressure Support (VAPS): Automatically adjusts inspiratory pressure between set minimum and maximum limits to deliver a targeted tidal volume 6

  • Particularly useful in preventing hypoventilation in central sleep apnea 6
  • Should be used with a backup rate (ST mode) for patients with central apneas 6

• Volume Support Ventilation (VSV): Continuously adjusts pressure support level to deliver a preset tidal volume 5

  • Caution: When ventilatory demand increases, VSV may decrease the pressure support provided, potentially causing respiratory distress 5

Practical Considerations

• Triggering difficulties during PSV are usually due to intrinsic positive end-expiratory pressure (auto-PEEP) 2
• Patient-ventilator dyssynchrony may occur if the flow at which the ventilator cycles to exhalation does not coincide with the termination of neural inspiration 2
• Adjusting the rise time and flow-termination criteria can improve patient-ventilator synchrony 2
• Ventilator waveforms are useful for appropriately adjusting PSV settings 2

Common Pitfalls

• Setting excessive pressure support levels can cause hyperventilation, leading to central apneas during sleep 1
• Inadequate pressure support may result in increased work of breathing and patient discomfort 3
• Failure to set a backup rate in patients with central hypoventilation or apneas can lead to inadequate ventilation 6
• Patient-ventilator asynchrony can occur if flow cycling criteria are not properly adjusted 2