What is the difference between volume-controlled and pressure-controlled ventilation?

In volume-controlled ventilation the tidal volume is fixed and the airway pressure varies with the patient's lung mechanics, whereas in pressure-controlled ventilation the inspiratory pressure is fixed and the delivered volume varies; each makes a different variable the dependent one.

Is one ventilator mode clearly better than the others?

For most patients, comparative trials have not shown a clear survival advantage of one conventional mode over another; the evidence instead emphasises the overall protective strategy, such as limiting tidal volume and plateau pressure, more than the specific mode.

Mechanical Ventilation Modes and Strategies

A ventilator mode is the set of rules by which a mechanical ventilator delivers and ends each breath: what triggers a breath, how the breath is delivered (by a set volume or a set pressure), and what ends inspiration. Modes and the broader strategies that govern settings such as tidal volume and positive end-expiratory pressure determine how well ventilation supports gas exchange while limiting harm to the lung.

Definition

A mode of mechanical ventilation is defined by the breath trigger (patient effort or machine timer), the breath control variable (volume-targeted or pressure-targeted delivery), and the cycle criterion that ends inspiration; a ventilation strategy is the overall plan combining mode, tidal volume, respiratory rate, positive end-expiratory pressure, and inspired oxygen to meet a patient's gas-exchange needs.

Scope

This entry describes the common categories of invasive ventilator modes and the principles behind protective ventilation strategies. It is conceptual and educational, summarising how modes differ and why settings are chosen; it does not prescribe ventilator settings for any individual patient.

Core questions

What three variables define any ventilator mode?
How do volume-controlled and pressure-controlled breaths differ?
How do assist-control, synchronised intermittent mandatory ventilation, and pressure support relate to one another?
What makes a ventilation strategy lung-protective?

Key concepts

Breath trigger, control variable, and cycle
Volume-controlled ventilation
Pressure-controlled ventilation
Assist-control ventilation
Synchronised intermittent mandatory ventilation (SIMV)
Pressure support ventilation
Positive end-expiratory pressure (PEEP)
Lung-protective ventilation

Mechanisms

Every mechanical breath can be described by how it is triggered, how the delivered flow is shaped, and how it is cycled off. In volume-targeted modes the clinician sets a tidal volume and the resulting airway pressure depends on respiratory mechanics; in pressure-targeted modes the clinician sets an inspiratory pressure and the delivered volume varies with compliance and resistance. Mandatory modes such as assist-control deliver a full breath for every trigger, intermittent mandatory modes mix mandatory and spontaneous breaths, and support modes such as pressure support augment the patient's own efforts. Across modes, positive end-expiratory pressure maintains alveolar recruitment at end-expiration, and protective strategies constrain tidal volume and plateau pressure to limit overdistension (Tobin-2013-textbook; ARDSnet-2000).

Clinical relevance

The choice of mode and strategy frames much of day-to-day critical-care practice and underlies the appraisal of ventilation trials. This entry explains how modes are categorised and why protective strategies are favoured; it describes evidence and concepts and is not a basis for setting an individual patient's ventilator.

Epidemiology

International observational studies of mechanically ventilated adults show that assist-control and synchronised intermittent mandatory ventilation, often combined with pressure support, are among the most frequently used modes, with practice patterns varying across regions and over time (Esteban-2002).

Evidence & guidelines

Although head-to-head trials have generally not shown one conventional mode to be clearly superior to another for survival, trial and guideline evidence strongly supports protective strategy elements, particularly limiting tidal volume in ARDS, independent of the named mode (ARDSnet-2000; Fan-2017-guideline). This entry conveys that emphasis without specifying numeric targets.

History

Early intensive care relied on volume-controlled ventilation delivered by mechanical ventilators developed after the mid-twentieth-century polio era. Microprocessor-controlled ventilators later enabled patient-triggered and pressure-targeted modes, synchronised intermittent mandatory ventilation, and pressure support, expanding the repertoire of partial support. Over time the field's emphasis shifted from the named mode toward the strategy, especially protective limits on tidal volume and pressure (Tobin-2013-textbook; ARDSnet-2000).

Debates

Does the named ventilator mode matter for outcomes?: Comparative trials have rarely shown a clear survival advantage for one conventional mode over another, leading many to argue that the ventilation strategy (protective tidal volume and pressure limits) matters more than the mode label itself.

Key figures

Martin J. Tobin
Robert M. Kacmarek
Andres Esteban
Laurent Brochard

Seminal works

esteban-2002
ardsnet-2000
tobin-2013-textbook

Frequently asked questions

What is the difference between volume-controlled and pressure-controlled ventilation?: In volume-controlled ventilation the tidal volume is fixed and the airway pressure varies with the patient's lung mechanics, whereas in pressure-controlled ventilation the inspiratory pressure is fixed and the delivered volume varies; each makes a different variable the dependent one.
Is one ventilator mode clearly better than the others?: For most patients, comparative trials have not shown a clear survival advantage of one conventional mode over another; the evidence instead emphasises the overall protective strategy, such as limiting tidal volume and plateau pressure, more than the specific mode.