Ventilator Modes

Part of: Mechanical Ventilation

• Ventilator modes are just pre-set combinations of ventilator settings
• Generally classified according to their major characteristics:
  • The control variable: pressure vs. volume
  • Breath sequence: spontaneous vs. mandatory
  • Targeting scheme: set point, dual targeting, servo control, adaptive etc.

Assist Control (AC)

• Mix of mandatory and assisted breaths with all breaths, once triggered, are treated the same and have a consistent tidal volume
• Trigger: time, pressure or flow
• Control: flow (volume)
• Cycling: time

Figure 1. Assist control. Note that the 1st, 3rd and 4th breaths are patient triggered with the second breath being time triggered.

• Advantages
  • Guarantees a minimum minute ventilation
  • Low work of breathing
• Disadvantages
  • Can lead to respiratory alkalosis, auto-PEEP and hypotension in hyperventilating patients as every breath leads to a fully supported breath
• Indications
  • Critically ill patients requiring full ventilatory support and in whom fluctuations in $V_{\text{T}}$ is undesirable

Synchronised Intermittent Mandatory Ventilation (SIMV)

• Mix of mandatory breaths (some of which are synchronised with spontaneous breaths), and assisted breaths
• Mandatory (non-synchronised breaths):
  • Trigger: time
  • Control: flow (volume)
  • Cycling: time
• Synchronised breaths:
  • Trigger: pressure or flow
  • Control: flow (volume)
  • Cycling: time
• Non-synchronised breaths
  • Trigger: pressure or flow
  • Control: pressure
  • Cycling: flow

Figure 2. SIMV (with pressure support). Note that the first and fourth breaths are synchronised pressure triggered fully supported breath with tidal volume set for ~600 mL. The second, third and fifth breaths are spontaneous non-synchronised breaths assisted with pressure support with 10 cm of water.

• User sets a rate and volume
• There is a window of opportunity around each timed breath, during which a patient effort will be converted into a mandatory breath
• Advantage
  • Guarantees a minimum minute ventilation
  • Lower mean airway pressure when compared with AC
  • Can provide a wide range of respiratory support
• Disadvantages
  • Increased work of breathing for patient
  • ? Lower cardiac output in patients with LV dysfunction
• Indications
  • Critically ill patients who are hyperventilating or otherwise prone to auto-PEEP or high airway resistance

NOTE

The main difference between SIMV and AC is that spontaneous breaths in excess of the set respiratory rate:

• In AC receive full support
• In SIMV receive partial support Therefore in a patient with no spontaneous breaths AC = SIMV

Pressure Control Ventilation (PCV)

• Mandatory breaths only. The patient is unable to trigger the ventilator
• Trigger: time
• Control: pressure
• Cycling: time

Figure 3. Pressure Control Ventilation. Inspiratory pressure is set at 25cm of water. Flow starts high with each breath and rapidly declines (known as a decelerating flow contour).

• Indications
  • Patients who are at a particularly high risk of barotrauma

Pressure Support Ventilation (PSV)

• There are no mandatory breaths; every breath must be triggered by the patient
• Trigger: Pressure or flow
• Control: Pressure
• Cycling: Flow

Figure 4. Pressure support ventilation. The pressure support is set at ~15cm of water. Inspiration is terminated by the removal of pressure support once the flow drops to 25% of its maximum

• Advantages:
  • Probably the most comfortable mode for the awake, conscious patient
• Disadvantages:
  • Patient must trigger each breath
  • A minimum minute ventilation cannot be guaranteed
  • Associated with poorer quality sleep
  • Generally incapable of providing full ventilatory support
• Indications
  • Conscious patient
  • As a stepping stone immediately prior to extubation

Dual Control Modes

• Use instantaneous feedback to control aspects of lung volume and airway pressure simultaneously
• Examples include
  • Pressure-regulated volume control
  • Volume support
  • Volume assured pressure support

Pressure Regulated Volume Control (PRVC)

• The user sets a target tidal volume (as in volume control modes)
• The ventilator performs an assessment of the dynamic lung compliance over the course of three initial calibrating breaths
• Using this data, it then determines the appropriate pressure level required to achieve the prescribed tidal volume
• Advantages:
  • Square pressure waveform favours early and sustained lung unit recruitment
  • Mean airway pressure is as high as with PCV
  • Pressure is minimised for any given prescribed tidal volume
  • A minute volume is guaranteed, preserving a degree of control over PaCO2