Pressure Support Ventilation (PSV)

see also:

• assisted ventilation
• chronic obstructive pulmonary disease (COPD)
• Assist-Control Ventilation (ACV)
• Intermittent Mandatory Ventilation (IMV)

• PSV is a flow-limited mode of ventilation that delivers inspiratory pressure until the inspiratory flow decreases to a predetermined percentage of its peak value. This is usually 25%.
• During PSV, each breath is augmented by inspiratory pressure.
• The clinician sets the pressure support level (inspiratory pressure level), applied PEEP, and FiO2
• There is no set respiratory rate, thus, the patient triggers each inspiration, which continues until the inspiratory flow decreases to a system-specific minimal level.
• Exhalation follows.
• The tidal volume is determined by the pressure support level, effort, and mechanics.
• Minute volume is variable and dependent upon tidal volume and patient initiated respiratory rate, and thus does not guarantee an adequate minute volume.
• increasing the level of pressure support decreases the work of breathing provided that inspiratory flow is sufficient to meet patient demand.
• PSV seems particularly well suited for weaning from mechanical ventilation because it tends to be a comfortable mode, giving the patient greater control over the inspiratory flow rate and respiratory rate.
• PSV is frequently combined with SIMV. The ventilator delivers the set respiratory rate using SIMV, but patient-initiated breaths beyond the set respiratory rate are delivered using PSV. The purpose of adding PSV for patient-initiated breaths is to overcome the resistance of the endotracheal tube and ventilator circuit.
• in general, PSV is poorly suited to provide full or nearly full ventilatory support, it is a particularly poor choice for patients who also have increased airway resistance (eg, chronic obstructive pulmonary disease (COPD) or asthma exacerbation).

• machine breathes are pressure limited and either time triggered or patient triggered
• ventilator delivers flow to achieve a preset level of pressure early in inspiration
• decelerating flow pattern

• frequency
• upper and lower level of pressure
• inspiratory time
• thus flow adjusts automatically
• tidal volume is determined by:
  • pressure gradient - the difference between the set PIP and PEEP, and also patient effort
  • inspiratory time
  • patient and machine characteristics including lung and chest wall compliance, airway resistance, and circuit characteristics

• improved oxygenation via:
  • higher mean airway pressure
  • better gas flow distribution
  • spontaneous breathing at upper level (if allowed)
• able to meet high inspiratory flow demands
• better patient and ventilator synchrony in some modes
  • less sedation needs
  • active exhalation valve
  • can breathe at any stage during the cycle

• applies to spontaneous breathing only
• maintains and supports the patient's inspiratory effort
• used as a mode on its own or often combined with SIMV
• flow cycled
• once synchronisation is detected, a constant flow is delivered at the preset pressure
• expiration is triggered by a reduction in the inspiratory flow to a given level

• improved patient and ventilator synchrony
• increased patient comfort
• minimal or no sedation may be required
• minimal cardiovascular effects
• patient retains control over frequency, inspiratory flow and time

• minute volume is not guaranteed
• large changes in patients breathing may occur without changing the actual minute volume
• periods of hypoventilation may occur
• increased work of breathing if inappropriate rise time or trigger settings
• patient may fail to cycle to expiration if leak
• failure to detect apnoea if use of an inline nebuliser

• used to overcome ETT and circuit resistance encountered during spontaneous breathing
• used to augment tidal volume in spontaneous breathes
• reduces work of breathing
• to assess if extubation is likely to be successful
• weaning from intubation