Modes of ventilation

Intensive care ventilators use positive pressure generated by either a turbine or valves within the ventilator that control the amount of gas flow (O2, Air) from wall connections to generate changes in lung volume. 

The classification of modes has changed over time and is different from manufacturer to manufacturer.

Let us give you way to understand the different modes commonly used. 

Different physical parameters can control a breath

Volume controlled ventilation is ventilation where both volume and flow are controlled by the ventilator. Normally, flow is set to a fixed amount, meaning volume increases linearly over time.

Any mode that relies on flow to control inspiration falls under the VC- category.

 

Pressure controlled ventilation is where pressure as a function of time is controlled by the ventilator. Normally, pressure is set to a specific amount for a specific breath duration, letting volume and flow vary according to patient demands.

Any mode that relies on pressure to deliver a breath falls under the PC- category.

Volume Control
  • Set TV and Ti

  • Ventilator applies constant flow until TV reached

  • PIP and Pplat depend on the compliance and resistance of the respiratory system

  • Maintains MV and therefore quasi-constant CO2 level

Pressure Control
  • Set PIP and Ti

  • Ventilator applies decelerating flow pattern to reach PIP fast and then maintain it

  • TV and therefore MV depend on respiratory system compliance and resistance

  • Varying MV and therefore CO2 level

  • More comfortable for patient

  • Better equilibration of air throughout the lung as time constants vary between different lung areas

 
  • A breath is a cycle of positive flow (inspiration) and negative flow (expiration)

  • Inspiratory time (Ti) is defined as the period from the start of positive flow to the start of negative flow

  • Expiratory time is defined as the period from the start of expiratory flow to the start of inspiratory flow

  • A breath is assisted if the ventilator does work on the patient

  • An unassisted breath is one for which the ventilator simply provides the inspiratory flow demanded by the patient

  • A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system

  • Volume control (VC) means that both volume and flow are preset prior to inspiration. In other words, the right hand side of the equation of motion remains constant while pressure changes with changes in elastance and resistance

  • Pressure control (PC) means that inspiratory pressure is preset. In other words, the left-hand side of the equation of motion remains constant while volume and flow change with changes in elastance and resistance

  • Breaths are classified by the criteria that trigger (start) and cycle (stop) inspiration. The start of inspiration is called the trigger event. The end of inspiration is called the cycle event

  • Patient triggering means starting inspiration based on a patient signal independent of a machine trigger signal

  • Machine triggering means starting inspiration based on a signal from the ventilator, independent of a patient trigger signal

  • Patient cycling means ending inspiratory time based on signals representing the patient's efforts

  • Flow cycling is a form of patient cycling used in pressure support ventilation

  • Machine cycling means ending inspiratory time independent of signals from the patient 

  • Breaths are classified as spontaneous or mandatory

  • A spontaneous breath is a breath which the patient triggers

  • A spontaneous breath may be assisted or unsupported

  • A mandatory breath is a breath for which the machine triggers and/or cycles the breath. It is, by definition, assisted. It can be triggered by the patient

  • There are 3 breath sequences (patterns of spontaneous and/or mandatory breaths

    • Continuous mandatory ventilation (CMV) - Spontaneous breaths are not allowed. All breaths are mandatory

    • Intermittent Mandatory Ventilation (IMV) - Spontaneous breaths may occur between mandatory breaths

    • Continuous Spontaneous Ventilation (CSV) - All breaths are spontaneous

Key aspects

There are 5 basic ventilatory patterns

VC-CMV 

Replaces: Assist/Control, A/C, CMV, Volume Assist / Control, Volume Control

VC-IMV 

Replaces: Synchronized Intermittent Mandatory Ventilation, SIMV

PC-CMV

Replaces: Assist/Control, A/C, ​CMV, Pressure Assist / Control, Pressure Control

PC-IMV

Replaces: Synchronized Intermittent Mandatory Ventilation, SIMV

PC-CSV

Replaces: Spont, Spontaneous

  • Within each ventilatory pattern there are several variations that can be distinguished by their targeting scheme(s).

  • A targeting scheme is a description of how the ventilator achieves preset targets

    • Examples of within-breath targets include inspiratory flow, pressure and rise time, tidal volume and constant of proportionality between inspiratory pressure and patient effort (servo targeting)

    • Examples of between-breath targets and targeting schemes include average tidal volume (for adaptive targeting), percent minute ventilation (for optimal targeting) and combined pCO2, volume, and frequency values describing a “zone of comfort” (for intelligent targeting, e.g., SmartCarePS or IntelliVent-ASV).

  • The targeting schema is what distinguishes one ventilatory pattern from another. There are a number of basic targeting schemes that comprise the wide variety seen in different modes of ventilation

 

   - Set-point: A targeting scheme for which the operator sets all the parameters of the pressure waveform (pressure
     control modes) or volume and flow waveforms (volume control modes)
   - Dual: A targeting scheme that allows the ventilator to switch between volume control and pressure control during a
     single inspiration
   - Bio-variable: A targeting scheme that allows the ventilator to automatically set the inspiratory pressure or tidal
     volume randomly to mimic the variability observed during normal breathing
   - Servo: A targeting scheme for which inspiratory pressure is proportional to inspiratory effort
   - Adaptive: A targeting scheme that allows the ventilator to automatically set one target (eg, pressure within a breath)
     to achieve another target (eg, average tidal volume over several breaths)
   - Optimal: A targeting scheme that automatically adjusts the targets of the ventilatory pattern to either minimize or
     maximize some overall performance characteristic (eg, minimize the work rate done by the ventilatory pattern)
   - Intelligent: A targeting scheme that uses artificial intelligence programs such as fuzzy logic, rule based expert
     systems, and artificial neural networks

- A mode of ventilation is classified according to its control variable, breath sequence, and targeting scheme(s). The
  taxonomy is based has 4 hierarchical levels:

  • Control Variable (Pressure or Volume for the primary breath)

  • Breath Sequence (CMV, IMV, or CSV)

  • Primary Breath Targeting Scheme (for CMV or CSV)

  • Secondary Breath Targeting Scheme (for IMV)

The targeting schemes can be represented by single, lower case letters: set-point = s, dual = d, servo = r, bio-variable = b, adaptive = a, optimal = o, intelligent = i. A tag is an abbreviation for a mode classification, such as PC-IMVs,s. Compound tags are possible, eg, PC-IMVoi,oi.

 

Control Variable

- Pressure

- Volume

Breath Sequence 

- CMV

- IMV

- CSV

I° Breath Targeting Scheme

- Set-Point (s)

- Dual (d)

- Bio-variable (b)

-Servo (r)

- Adaptive (a)

- Optimal (o)

- Intelligent (i)

II° Breath Targeting Scheme

- Set-Point (s)

- Dual (d)

- Bio-variable (b)

-Servo (r)

- Adaptive (a)

- Optimal (o)

- Intelligent (i)

Modes of ventilation

Initiation

What triggers the initiation of a breath?

- Time

- Flow

- Pressure

- Edi

Limitation

What limits the breath?

- Volume

- Pressure

Cycling

What cycles from in- to expiration?

- Volume

- Time

- Flow

Support

Does the ventilator allow for supported breaths on top of the mandatory breaths?

 

- Yes

- No

Criteria within modes
 
Spontaneous ventilation modes
CPAP/ Continuous positive airway pressure

Positive airway pressure is maintained during the entire respiratory cycle at a set level called PEEP (Positive end-expiratory pressure)

PEEP maintains airway patency and lung volume (FRC) and is therefore an anti-derecruitment force. This is particularly effective in neonates and children as their FRC is close to the closing capacity.

There are fluctuations of airway pressure during in- and expiration. 

This mode is only suitable for patients that have adequate respiratory drive as there is NO back-up ventilation

 

PC-CSVs

Chatburn taxonomy
CPAP/ Pressure Support (PS)

Positive airway pressure is maintained at a set level called PEEP (Positive end-expiratory pressure)

The patient's own efforts trigger breaths that are supported with a given amount of pressure (= pressure support).

Inspiration is cycled off as the inspiratory flow decreases to a certain percentage of the peak inspiratory flow.  

There are no mandatory breaths. 

Back-Up ventilation can be chosen depending on the ventilator / manufacturer. 

 
Chatburn taxonomy

PC-CSVs

Volume controlled modes
 

Uses a constant flow pattern to deliver a set TV.

PEEP, TV, RR, Ti and a pressure limit are set by the operator.

The PIP reflects the pressure needed to overcome the resistance of the respiratory system during inspiration. Pplat ensues after the flow stops and pressure equilibrates throughout the respiratory system. It is then an estimate of alveolar pressure. --> Physics

Inspiration is cycled off after Ti.

The patient can trigger additional breaths which will be delivered with the same settings as the mandatory breaths. 

Volume control - Continuous mandatory ventilation
 
Chatburn taxonomy

VC-CMVs

This is the same as: 

Assist/Control, A/C, ACV, CMV, Volume Assist/Control, Volume Control, Volume Limited Ventilation, Volume Controlled Ventilation, Controlled Ventilation, Volume Targeted Ventilation

 
Volume Control intermittent mandatory ventilation +/- PS

Uses a constant flow pattern to deliver a set TV.

PEEP, TV, RR, Ti, a pressure limit +/- PS are set by the operator. 

Pmax limits the amount of pressure during volume delivery to minimise the risk of barotrauma. 

The ventilator allows for synchronisation of mandatory breaths within a trigger-window. 

 

Mandatory breaths are initiated by either time or patient effort within the trigger window. Inspiration is cycled off after Ti.

Spontaneous breaths in addition to the mandatory rate can either be unsupported (left side in between two mandatory breaths) OR pressure supported (right side). 

Supported breaths are cycled off after the inspiratory flow reaches a percentage of peak inspiratory flow. 

Chatburn taxonomy

VC-IMVs

This is the same as: 

Synchronized Intermittent Mandatory Ventilation +/- PS, SIMV +/- PS

Pressure controlled modes
 

This is the same as: Assist/Control, A/C, ACV, CMV, Pressure Assist/Control, Pressure Control, Pressure Limited Ventilation, Pressure Controlled Ventilation, Controlled Ventilation, Pressure Targeted Ventilation

PC-CMVs

Chatburn taxonomy
Pressure control - Continuous mandatory ventilation

Uses a set decelerating flow pattern to deliver a set pressure above PEEP.

PEEP, PIP, RR, and Ti  are set by the operator.

The PIP reflects the pressure needed to overcome the resistance of the respiratory system during inspiration. There is no Pplat as the PIP is maintained through the decelerating flow pattern. --> Physics

The TV and therefore MV changes with the patient's lung compliance and airway resistance over time. 

Inspiration is cycled off after Ti.

The patient can trigger additional breaths which are delivered with the same settings as the mandatory breaths. 

 

Uses a decelerating flow pattern to deliver a set pressure.

PEEP, PIP, RR,  and Ti +/- PS are set by the operator. 

The ventilator allows for synchronisation of mandatory breaths within a trigger-window. 

 

Mandatory breaths are initiated by either time or patient effort within the trigger window. Inspiration is cycled off after Ti.

Spontaneous breaths in addition to the mandatory rate can either be unsupported (left side in between two mandatory breaths) OR pressure supported (right side). 

Supported breaths are cycled off after the inspiratory flow reaches a percentage of peak inspiratory flow

Pressure control - Intermittent mandatory ventilation +/- PS
Chatburn taxonomy

PC-IMVs

This is the same as: Synchronized Intermittent Mandatory Ventilation +/- PS, SIMV +/- PS

 

PC-IMVs,s

Chatburn taxonomy
BiLevel +/- PS

Uses a decelerating flow pattern to deliver a set pressure.

PEEP, PIP (=Pduo), RR, Ti +/- PS are set by the operator. 

The ventilator allows for spontaneous breaths on both pressure levels. These can be pressure supported or not. 

Mandatory breaths are initiated by time and inspiration is cycled off after Ti.

Supported breaths are cycled off after the inspiratory flow reaches a percentage of peak inspiratory flow.

The primary set-point is PIP / Duo for the mandatory breaths and the secondary set-point is the amount of PS for the spontaneous / supported breaths. 

 
 

These are non-invasive modes of ventilation.

In spontaneous mode (S) the device triggers IPAP when flow sensors detect spontaneous inspiratory effort and then cycles back to EPAP

In timed mode (T) the IPAP/EPAP cycling is purely machine-triggered, at a set rate (breaths per minute)

S/T (Spontaneous/Timed): The device triggers to IPAP on patient inspiratory effort but features a "backup" rate to ensure that patients still receive a minimum number of breaths per minute if they fail to breathe.

 

Uses a decelerating flow pattern to deliver a set pressure.

The inspiratory time is variable for patient-triggered breaths and cycled by a decrease in flow. For the back-up rate (=machine triggered breaths), the inspiratory time is fixed.

BiLevel S, T, S/T (Spontaneous / Timed)
Chatburn taxonomy

PC-IMVs,s

BiLevel S/T
Contact/Suggestions

Juerg Burren, MBBS MD FMH (CH) EPIC FCICM

Paediatric Intensivist and Ventilation Enthusiast

​​

Tel: +61 (0) 481 751 073

info@pivares.com

© 2019-2020 by Juerg Burren for PIVARES

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