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# Mechanical ventilation modes: volume, pressure and support

- Concept

The ways a ventilator can deliver a breath: by pushing a set volume, by holding a set pressure, or by helping the patient's own breaths.

Updated Nov 13, 2026

## Key points

- Every breath has a trigger (time or patient), a limit (volume/flow or pressure) and a cycle (time, volume or falling flow).
- Volume control guarantees tidal volume but lets pressure vary; pressure control caps pressure but lets tidal volume vary.
- Assist-control delivers a full breath for every trigger; rapid breathing on assist-control can cause breath stacking and auto-PEEP.
- Pressure support is a spontaneous mode: the patient sets rate and timing, so it needs an intact respiratory drive and an apnoea backup.
- Minute ventilation (rate times tidal volume) mainly controls CO2; inspired oxygen and PEEP mainly control oxygenation.

On this page

- The basics Key points
- What it is
- Key points
- Components
- How it works
- The main modes compared
- In practice
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

Invasive mechanical ventilation moves gas into the lungs through a tracheal tube or tracheostomy by applying positive pressure, while expiration stays passive. Every breath is described by three variables: what starts it (the trigger: a timer, or the patient's own effort sensed as a fall in pressure or flow), what limits it during inspiration (a target volume or flow, or a target pressure) and what ends it (the cycle: a set time, a delivered volume, or a fall in inspiratory flow). A mode is simply a rule for combining these variables. In volume-controlled modes the clinician sets the tidal volume, so ventilation is guaranteed but airway pressure varies with the lung's compliance and resistance. In pressure-controlled modes the clinician sets the inspiratory pressure and time, so pressure is capped but the tidal volume rises and falls with the lung and the patient's effort. Assist-control gives a full mandatory breath for every trigger, synchronised intermittent mandatory ventilation adds spontaneous breaths between mandatory ones, and pressure support only boosts breaths the patient starts, ending each one when inspiratory flow falls. Hybrid modes adjust the pressure breath by breath to reach a target volume. No mode is best for every patient: the choice follows the goal (full rest, lung protection or weaning), and what matters most is the tidal volume, pressures, oxygen and PEEP chosen within it, checked against blood gases and the patient's comfort.

## Key points

- Ventilation removes CO2 and depends on minute ventilation; oxygenation depends on inspired oxygen fraction, PEEP and mean airway pressure.
- Indications: failure to oxygenate, failure to ventilate, inability to protect the airway, and high work of breathing in shock or exhaustion.
- Mandatory breaths are fully delivered by the machine; spontaneous breaths are started and ended by the patient, with or without support.
- Patient-ventilator asynchrony (missed triggers, double triggering, flow starvation) raises work of breathing, discomfort and sedation needs.

## Components

- Volume assist-control: set tidal volume, rate and flow; pressure is the dependent variable to watch.
- Pressure assist-control: set inspiratory pressure, inspiratory time and rate; tidal volume is the dependent variable to watch.
- Pressure-regulated volume control and similar hybrids: the machine adjusts pressure breath by breath toward a target volume.
- Synchronised intermittent mandatory ventilation: a set number of mandatory breaths, with spontaneous (often supported) breaths in between.
- Pressure support and CPAP: spontaneous breathing with an inspiratory boost and/or a constant positive pressure; used for weaning.
- Airway pressure release ventilation: a long high-pressure phase with brief releases; a rescue mode in specialised hands.

## How it works

- Positive pressure opens airways and alveoli, increases lung volume and lowers the work of the breathing muscles.
- The same pressure raises intrathoracic pressure, reducing venous return and preload, so hypotension is common after starting ventilation.
- Too much volume or pressure overstretches alveoli; repeated opening and closing injures them: hence lung-protective settings.
- In obstructive disease, too little expiratory time traps gas and builds intrinsic PEEP.

## The main modes compared

| Mode | Clinician sets | Varies with the lung | Typical use |
| --- | --- | --- | --- |
| Volume assist-control | Tidal volume, rate, flow | Airway pressures | Initial support; lung-protective ventilation |
| Pressure assist-control | Inspiratory pressure, time, rate | Tidal volume | Initial support; poor compliance, leaks |
| SIMV | Mandatory breaths plus support | Spontaneous breath size | Partial support; less used now |
| Pressure support | Inspiratory pressure, PEEP | Tidal volume, rate | Spontaneous breathing, weaning, trials |
| CPAP | Constant pressure only | Everything else | Spontaneous breathing trial, oxygenation |

## In practice

- Start most patients on an assist-control mode with a lung-protective tidal volume based on predicted body weight, not actual weight.
- On volume control, a rising airway pressure is the warning; on pressure control, a falling tidal volume is the warning.
- Match the mode to the goal: control modes to rest or protect the lung, support modes as the patient recovers.
- Agitation or asynchrony is first a ventilator and comfort problem: adjust flow, trigger and timing, treat pain, then consider sedation.
- Check an arterial blood gas after starting or changing ventilation, and review settings at least daily.

## Next step

- After starting ventilation, check an arterial blood gas, chest X-ray (tube position) and blood pressure; treat hypotension promptly.
- Measure plateau pressure and look at the waveforms on volume control; record tidal volume per predicted body weight.
- Review the mode every day: move to a support mode and screen for a spontaneous breathing trial as the cause improves.
- Call a senior for rising pressures, falling tidal volumes, worsening oxygenation or persistent asynchrony.

## Memory aid

Trigger, limit, cycle: what starts the breath, what caps it, what ends it.

## Test yourself

Think of the answer, then tap the question to see it.

## In volume assist-control, which variable does the clinician set and which one must be watched?

The tidal volume is set; the airway pressures (peak and plateau) vary with compliance and resistance and must be watched.

## What ends an inspiration in pressure support ventilation?

Inspiratory flow falling to a set fraction of its peak: the breath is flow-cycled, so the patient controls its timing.

## Why is pressure support unsafe in a patient with no respiratory drive?

It only supports breaths the patient starts; without effort there are no breaths, so it relies on an apnoea backup.

## Why does blood pressure often fall after starting positive pressure ventilation?

Raised intrathoracic pressure reduces venous return and preload, added to the loss of sympathetic drive from sedation.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anaesthesia/mechanical-ventilation-modes-volume-pressure-and-support/)

## Elsewhere in DurgiPedia

- [Mechanics of breathing: pressures and muscles](https://durgical.com/en/physiology/mechanics-of-breathing-pressures-and-muscles/)
- [Lung and chest wall compliance](https://durgical.com/en/physiology/lung-and-chest-wall-compliance/)
- [The work of breathing](https://durgical.com/en/physiology/the-work-of-breathing/)
- [Hypoxemic and hypercapnic respiratory failure](https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/)

## Connections

## Related 7

- [PEEP, plateau pressure and lung protection](https://durgical.com/en/anaesthesia/peep-plateau-pressure-and-lung-protection/)
- [Ventilating ARDS: low volumes, prone position and more](https://durgical.com/en/anaesthesia/ventilating-ards-low-volumes-prone-position-and-more/)
- [Ventilator alarms, auto-PEEP and the DOPE approach](https://durgical.com/en/anaesthesia/ventilator-alarms-auto-peep-and-the-dope-approach/)
- [Weaning and the spontaneous breathing trial](https://durgical.com/en/anaesthesia/weaning-and-the-spontaneous-breathing-trial/)
- [NIV and high-flow nasal oxygen in the ICU](https://durgical.com/en/anaesthesia/niv-and-high-flow-nasal-oxygen-in-the-icu/)
- [Tracheal intubation](https://durgical.com/en/anaesthesia/tracheal-intubation/)
- [Analgesia and sedation in the ICU: targets and scales](https://durgical.com/en/anaesthesia/analgesia-and-sedation-in-the-icu-targets-and-scales/)

## Related conditions 3

- [Acute respiratory failure (hypoxaemic and hypercapnic)](https://durgical.com/en/conditions/acute-respiratory-failure/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)

## In Pharmacology 1

- [Oxygen](https://durgical.com/en/pharmacology/oxygen/)

[← All of anaesthesia](https://durgical.com/en/anaesthesia/)

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: [Invasive Mechanical Ventilation](https://www.ncbi.nlm.nih.gov/books/NBK539742/) (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.

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