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.

Continuous Mnadatory Ventilation (CMV)

• The main feature is that it ignores attempts at breathing and all components of the respiratory phase delivered by the ventilator are mandatory (i.e. controlled by time)
  • This means it is time triggered and typically time cycled
• Control variable: Typically volume controlled but can be pressure
• Time triggered
• Volume, pressure or flow limited
• Time-cycled but can be volume cycled to avoid an inspiratory pause
• Advantages:
  • Gives optimal control over minute volume; therefore useful when maintaining a narrow $\mathrm{CO}{}_2$ range (e.g. in a paralysed sedated head injury patient)
• Disadvantages
  • Uncomfortable; requires decent sedation

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 but can be volume cycled to avoid inspiratory pauses

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
  • Uncomfortable
• Indications
  • Critically ill patients requiring full ventilatory support and in whom fluctuations in $V_{\text{T}}$ is undesirable

Intermittent Mandatory Ventilation (IMV)

• Not continuously mandatory, the patient can decide their own respiratory rate and take a number of pressure supported breaths; the machine also gives a set number of mandatory breaths
• Advantages:
  • Guarantees a minimum minute ventilation
• Mandatory breaths are volume controlled
• Mandatory breaths are time triggered and the pressure supported breaths are flow triggered
• Time cycled but can be volume cycled to avoid inspiratory pauses

Synchronised Intermittent Mandatory Ventilation (SIMV)

• Main difference with IMV is that it is synchronised (as in the name); the patient can decide their own respiratory rate and they can take a number of pressure-supported breaths
• Among these breaths, the machine also gives a set number of mandatory breaths and if the patient makes an effort around the same time the next mandatory breath is due, the ventilator delivers to them an assist-control breath, with volume control
• Mix of mandatory breaths (some of which are synchronised with spontaneous breaths), and assisted breaths
• Mandatory (non-synchronised breaths):
  • Trigger: time
  • Control: volume
  • Cycling: time or volume
• Synchronised breaths:
  • Trigger: pressure or flow
  • Control: volume
  • Cycling: time or volume
• 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
  • Comfortable for the patient
• Disadvantages
  • Increased work of breathing for patient
  • ? Lower cardiac output in patients with LV dysfunction
  • Limited control over the minute ventilation
• 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)

• The main feature is that pressure is the control variable (CMV, IMV, SIMV, ACV can all be pressure controlled)
• Trigger: time (but typically flow triggered; the patient will become tachypnoeic if the $V_{\text{T}}$ is inadequate compensating for the low compliance)
• 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 (i.e. those with low compliance)
• Little control over the minute volume

Pressure Support Ventilation (PSV)

• The ventilator only controls the pressure support level and the rate, volume, cycling time etc are controlled by the patient
  • You can only control the the pressure support value (indirectly controlling the tidal volume) and the flow cycling trigger (decreasing it will make the breath last longer)
• There are no mandatory breaths; every breath must be triggered by the patient
  • This mode is typically combined with SIMV as the non-mandatory component
• 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

  • First breath tests compliance and is typically a low volume low pressure calibration breath
  • The next breath is delivered at a calculated pressure depending on the compliance from the previous breath; if the controlled target volume is exceeded, the ventilator adjusts the flow rate so that the next breath the pressure is slightly lower

• Using this data, it then determines the appropriate pressure level required to achieve the prescribed tidal volume

• The main feature is that the lowest possible pressure to achieve the volume control target is applied

• Advantages and disadvantages are similar to SIMV with the added benefits from PCV

• Advantages:

  • Mean airway pressure is as high as with PCV
  • Pressure is minimised for any given prescribed tidal volume
  • A minimum minute volume is guaranteed, preserving a degree of control over PaCO2