Pressure vs Volume Controlled Ventilation

Figuring out a mechanical ventilator, and understanding the basic modes of ventilation can be a difficult task.

At the end of this brief summary, you should have a better understanding of what is meant by the some of the different terms used when ventilating a patient mechanically, and you should leave with a basic understanding of how pressure and volume controlled ventilation work.

For the purposes of this post, we will not be going into the different modes of ventilation, these will be covered in more depth in a different post, however, these definitions will set you up with a better understanding for all the ventilation chapters to follow.

First some definitions

Volume control

In this mode of ventilation, the parameter that is controlled is the volume administered to the patient (tidal volume)
Safe tidal volumes of around 4-8ml/kg are recommended for most patients, with somewhere near the 6ml/kg mark being a safe starting point

Pressure control

In this mode of ventilation, the controlled parameter is the pressure
The peak airway pressure is constant, and is used to achieve a tidal volume which may vary depending on many factors

Peak Inspiratory Pressure

This is the highest level of pressure applied to the lungs during inspiration
This is a factor of airway resistance, PEEP and compliance of the lungs

Pressure that is generally applied to the larger airways (those that are not involved in gaseous exchange) and the airways not capable of distending (conducting airways)

Ideally this pressure should not exceed 35cmH20 in lungs that are not obstructed. The ventilation of the patient with obstructive lung disease is discussed in a different post)

Mean Airway Pressure

This is the average pressure applied to the lungs throughout the respiratory cycle (on inspiration and expiration) – see the graph below

The mean airway pressure can be increased by allowing more time to be spent under higher pressures; increasing PEEP; increasing inspiratory time and increasing inspiratory pressure

PEEP – Positive END-Expiratory pressure

This is the baseline pressure left over in the lungs once expiration is complete. It can be set by the ventilator, or may be a reading of the intrinsic pressure that is left over due to air trapping in the asthma/COPD patient for instance
This is the pressure that allows the alveoli to stay partially open throughout the respiratory cycle (too much PEEP = too little tidal volume, too little PEEP – increased effort to open the alveoli and oxygenate.

Plateau Pressure

This is the pressure applied to the alveoli and is measured at the end of inspiration (or during an inspiratory hold). It is the pressure that is exerted on the actual tissues which can distend in the lungs.

Ideally this pressure should not exceed 30cmH20

Driving Pressure

This is a measurement of the difference between the plateau pressure and the PEEP in the system. The higher the driving pressure, the more the lung is placed under stress. A driving pressure of around 14cmH20 – 18cmH20 is recommended when following the “open lung” or lung protective ventilation strategy. The lower, the better (provided that the appropriate tidal volume is produced for the patient).

I:E Ratio

This is the time in the respiratory cycle that is allocated to the inspiration of air, and expiration of air, normally a person breathes at a ratio of 1:2 (inspiration is usually 1/2 as long as expiration). This is usually as a result of the active nature of inspiration (it is driven by actively moving muscle), whilst expiration is about relaxing muscle and so usually takes a bit longer.

In advanced ventilation, the inspiratory time can be made equal to or more than the expiratory time to maximize mean airway pressure and increase oxygenation, this is called inverse ratio ventilation and is NOT recommended in normal ventilation in the EM setting.

Tidal Volume

This is the volume of air required to distend the lungs in one single breath (normal breath without any active work to get more air in) and is normally around 7ml/kg when a person is at rest.

Pressure Support

This is a preset pressure value that is delivered when the patient triggers a breath. It is a pressure mode of ventilation, that assists an existing drive to breathe. It makes the work of breathing easier by decreasing the effort the patient has to put in to achieve an appropriate tidal volume.

Respiratory Cycle

The respiratory cycle includes two phases: inspiration of environmental air and the expiration of gases from inside the lung including carbon dioxide.


This refers to the method used by the ventilator to decide when to give the next breath.

Time trigger:

uses time alone to determine when the next breath should be given. It does not really take the patient into account at all, and uses the respiratory rate to determine the time between breaths (example: rate set at 12b/min, a breath will be given every 5 seconds regardless of the patient’s desire to breath at a different time)

Pressure trigger:

this trigger is patient driven and uses the fact that spontaneous breaths create negative pressure. When a certain amount of negative pressure is generated in the ventilator circuit by the patient, then the ventilator will deliver the breath. Because the patient needs to generate a change in pressure, it requires that the patient work relatively hard to get a breath.

Flow trigger:

When the patient makes an inspiratory effort, some of the gas that was previously flowing continuously through the circuit is diverted to the patient. The ventilator senses the decrease in flow returning through the circuit, and a breath is triggered, this requires much less work than the pressure trigger but also requires a flow sensor to be calibrated and in place on the ventilator with constant monitoring

Pressure Modes

In a pressure controlled mode of ventilation, the inspiratory pressure is the control variable (this is the one you can set), and is maintained during the inspiratory phase. The volume will be variable depending on a number of different things and can vary on a breath to breath basis.

On all ventilators there are three specific parameters/graphs that indicate how a breath has been delivered, based on the pressure, flow and volumes.

The graphs below are representative of a pressure-controlled mode of ventilation for the following reasons:
The pressure graph is the same for each breath. There is a pressure that has been set and this is achieved for each breath

The pressure waveform is square

The flow graph shows what is called a “decelerating waveform” with the highest flow present at the start of inspiration, which then tapers off at the end of the inspiratory time. This mode most closely resembles the flow pattern of “normal” breathing.

The tidal volume varies with each breath as this is dependent on the pressure reached and the pressure set
You cannot see the plateau pressure, this is really only visible in the volume control mode of ventilation

Why is pressure controlled ventilation good?

Pressure controlled ventilation means that:

BUT because it is not “normal” for the time of inspiration to be longer than the expiratory time, this benefit is mostly achieved through the use of PEEP (we tend not to ventilate a patient with I:E ratios that favor prolonged inspiratory time unless we run out of options)

The pressure is exerted evenly across the alveoli and not directed down the path of least resistance (as sometimes happens with volume-controlled ventilation), so de-recruited alveoli are more likely to distend with this mode of ventilation.

Why is pressure controlled ventilation not so good?

If the patient is ventilated for a longer time, there needs to be a close watch on the tidal volume and minute volume achieved to ensure adequate control of the carbon dioxide levels. As compliance decreases, so the pressure increases and the tidal volume decreases.

Examples of pressure controlled modes:

These and more will be discussed in detail in a different post.

Volume Modes

In volume-controlled modes of ventilation, the tidal volume is the control variable (this is the one you can set), depending on a whole lot of different parameters, the pressure will be variable from breath to breath.

On all ventilators there are three specific parameters/graphs that indicate how a breath has been delivered, based on the pressure, flow and volumes.

The graphs below are representative of a volume-controlled mode of ventilation for the following reasons:

If lung compliance increases, the pressure applied to the lungs will decrease, and if lung compliance increases, the pressure required to achieve an appropriate tidal volume (distend the elastic tissues) will decrease.

In theory, or at least long ago when ventilators only had the ability to control one or the other parameter, this was dangerous as high levels of pressure could be achieved.

Most ventilators now will allow you to set a volume or pressure as their static parameter, but also set limits on the other parameter to allow for safer ventilation, this will be discussed in more depth in further posts (Ventilation modes and what they mean).

Why is volume controlled ventilation good?

Why is volume controlled ventilation good?

Examples of pressure controlled modes:

These and more will be discussed in detail in a different post.