ventpsv

Pressure Support Ventilation (PSV)

introduction

  • 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).

pressure control ventilation (PCV)

  • 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

ventilator settings

  • 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

advantages

  • 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

PSV

  • 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

advantages

  • 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

disadvantages

  • 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

uses

  • 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
ventpsv.txt · Last modified: 2011/04/12 16:17 by gary