Ventilator Modes: Explained! PEEP, CPAP, Pressure vs. Volume

Are you ready to learn about the ventilator modes of mechanical ventilation? If so, you’re in the right place because that is what this study guide is all about.

As you will see below, we have listed out pretty much everything you need to know about Ventilator Modes as a Respiratory Therapy Student. Not to mention, you’ll get access to some helpful practice questions as well. 

So if you’re ready to get started, let’s go ahead and dive right in.


Ventilator Modes

What is a Ventilator Mode?

A ventilator mode is a way of describing how the mechanical ventilator assists the patient with inspiration. The characteristics of a particular mode controls how the ventilator functions. 

Understanding the different ventilator modes is one of the most important aspects of mechanical ventilation.

Primary Control Variables:

In mechanical ventilation, there are two primary control variables:

  1. Volume Control
  2. Pressure Control

Volume Control

Volume Control means that you can set (or control) the patient’s tidal volume.

So with a set tidal volume and a set respiratory rate, this means that there is a known minute ventilation. This is good when it comes to making adjustments to achieve a desired PaCO2.

One of the negative aspects of using Volume Control is that, since the tidal volume is preset, if the patients lung compliance were to decrease, this could result in high peak pressures.

Another drawback of Volume Control is patient-ventilator dyssynchrony.

Pressure Control

Pressure Control means that you can set (or control) the patient’s pressure in order to achieve a desired tidal volume.

As with Volume Control, a Pressure-Controlled tidal volume and set rate can help you reach a desired PaCO2.

The main disadvantage of using Pressure Control is the patient’s tidal volume can potentially be unstable if there are changes in the patient’s lung compliance or airway resistance.

So again, Volume Control and Pressure Control — those are the two control variables. When initiating mechanical ventilation on a patient, once you select the control variable, now you can choose the actual operational mode that determines the pattern of breathing for the patient.

What are the Primary Ventilator Modes?

In mechanical ventilation, there are two primary ventilator modes:

  1. Assist/Control (A/C) Mode
  2. Synchronous Intermittent Mandatory Ventilation (SIMV) Mode

Assist/Control (A/C) Mode

In this mode, a minimum number of preset mandatory breaths are delivered by the ventilator but the patient can also trigger assisted breaths. The patient makes an effort to breathe and the ventilator assists in delivering the breath.

With that said, this mode of ventilation does not allow the patient to take spontaneous breaths. In this mode, the operator can set either a controlled pressure or a controlled volume.

The sensitivity control can be adjusted to make it easier or harder for the patient to initiate a breath.

When to Use Assist/Control?

This mode is most often used when mechanical ventilation is first initiated for a patient because this mode provides full ventilatory support.

That is also one of the advantages of using Assist/Control because it keeps the patient’s work of breathing requirement very low.

One of the major complications of Assist/Control is hyperventilation, which results in respiratory alkalosis. This is the result of too many breaths given to the patient, whether patient-triggered or machine-triggered.

Synchronous Intermittent Mandatory Ventilation (SIMV) Mode

In this mode, the ventilator delivers a preset minimum number of mandatory breaths. However, it also allows the patient to initiate spontaneous breaths in between the mandatory breaths.

This mode also allows the operator to set either a controlled pressure or a controlled volume. 

When to Use SIMV? 

The primary indication for SIMV is when a patient needs partial ventilatory support. That is because, since the patient can takes spontaneous breaths, that means they can contribute to some of their minute ventilation.

SIMV is a mode that is used for weaning as well.

Advantages of Using SIMV:

  • Since the patient is able to take spontaneous breaths, it helps to maintain their respiratory muscle strength and avoid muscular atrophy.
  • It distributes tidal volumes evenly throughout the lung fields, which reduces V/Q mismatching.
  • It helps to decrease the patient’s mean airway pressure.

As a Respiratory Therapist (or student), SIMV and Assist/Control are the two ventilator modes that you should be most familiar with.

However, it’s also important to develop an understanding of some of the secondary modes of mechanical ventilation as well.

Ventilator Modes for Spontaneous Breathing:

Keep in mind that, in order to use any of the following modes, the patient must be breathing spontaneously.
  1. Continuous Positive Airway Pressure (CPAP)
  2. Pressure Support Ventilation (PSV)
  3. Volume Support (VS)

Continuous Positive Airway Pressure (CPAP)

In CPAP, or continuous positive airway pressure, a continuous pressure that is above atmospheric pressure is maintained throughout the breathing cycle.

The patient must be breathing spontaneously to be in this mode because no mandatory breaths are given. This is a useful mode for weaning patients off of the ventilator. 

Pressure Support Ventilation (PSV)

A mode of mechanical ventilation in which the patient’s spontaneous breaths are supported by the ventilator during the inspiratory phase of breathing. As the patient triggers a breath, the ventilator assists by adding pressure to make breathing easier.

The level of pressure is preset by the operator, so you have control over how much support you give the patient. For example, the higher the level of pressure support that is set, the easier it will be for the patient to take a breath.

In PSV, the breaths are time-cycled and pressure-limited.

PSV is often used to help the patient overcome the airway resistance that is caused by the endotracheal tube.

For example, let’s say there is a patient who needs to be weaned from the ventilator that is in the SIMV mode. If their endotracheal tube size is too small, the airway resistance would be increased which would make weaning difficult.

That’s when PSV would come in handy to help the patient overcome the airway resistance so that they can be extubated.

Volume Support (VS)

A mode of mechanical ventilation in which the ventilator delivers a supported breath to help the patient reach a set tidal volume. This mode is totally dependent on the patient’s effort, meaning that, the machine varies the inspiratory pressure support level with each breath in order to achieve the target volume.

This mode is not quite as common as some of the others, but it’s often used to wean patients from anesthesia.

Other Modes of Mechanical Ventilation:

Now that we’ve covered the primary and spontaneous modes of ventilation, now let’s go through the unconventional ventilator modes. These are sometimes referred to as the secondary modes of ventilation.
  1. Control Mode Ventilation (CMV)
  2. Airway Pressure Release Ventilation (APRV)
  3. Mandatory Minute Ventilation (MMV)
  4. Inverse Ratio Ventilation (IRV)
  5. Pressure Regulated Volume Control (PRVC)
  6. Proportional Assist Ventilation (PAV)
  7. Adaptive Support Ventilation (ASV)
  8. Adaptive Pressure Control (APC)
  9. Volume-Assured Pressure Support (VAPS)
  10. Neurally Adjusted Ventilatory Assist (NAVA)
  11. Automatic Tube Compensation (ATC)
  12. High-Frequency Oscillatory Ventilation (HFOV)

Control Mode Ventilation (CMV)

It’s a mode where the ventilator delivers a preset tidal volume at a set time-triggered frequency. Basically, the ventilator controls both the rate and tidal volume which means that it’s in total control of the minute ventilation.

This mode should only be used on patients who are fully sedated and have been administered neuromuscular blocking agents.

That is also the biggest hazard of using this mode because, since the patient is fully dependant on the machine for ventilation and oxygenation, it could be devastating if they were to become disconnected.

Airway Pressure Release Ventilation (APRV)

A mode of mechanical ventilation in which two levels of continuous positive airway pressure are applied with an intermittent release phase for spontaneous breaths. This mode is often recommended to improve oxygenation and treat refractory hypoxemia.

Other indications for APRV include an Acute Lung Injury (ALI), Acute Respiratory Distress Syndrome (ARDS), and Severe Atelectasis.

Settings for APRV:

  • High Pressure
  • Low Pressure
  • High Time
  • Low Time

Mandatory Minute Ventilation (MMV)

This is a feature of some ventilators that causes an increase in the mandatory breaths that are delivered when the patient’s spontaneous breathing level becomes inadequate.

So basically, if the patient’s spontaneous breathing decreases, the ventilator compensates in order to make sure that a safe minimal minute ventilation is delivered.

MMV is often an additional function of the SIMV mode and is intended to prevent hypercapnia.

Inverse Ratio Ventilation (IRV)

This is a mode that uses an inverse I:E ratio to improve oxygenation and gas exchange. It’s been shown to decrease shunting, improve V/Q mismatching, and decrease deadspace ventilation.

IRV is commonly recommended for patients with Acute Respiratory Distress Syndrome (ARDS). 

This mode causes auto-PEEP (intrinsic PEEP) which is actually what helps improve the patient’s oxygenation and reduce shunting.

Pressure Regulated Volume Control (PRVC)

A mode of mechanical ventilation that provides volume-controlled breaths with the lowest pressure possible. It does so by altering the flow and inspiratory time. This mode is used to keep the peak airway pressure at the lowest possible level.

This mode is volume-cycled and can be patient triggered-or time-triggered.

Proportional Assist Ventilation (PAV)

This is a mode of mechanical ventilation where the machine uses variable pressure to provide pressure support for a patient’s spontaneous breaths. The level of pressure support is adjusted depending on the patient’s work of breathing.

PAV is either pressure-triggered or flow-triggered, and the breathing cycle ends once the patient’s volume or flow demands are met.

One thing to keep in mind about this mode is that, if the patient’s lungs show rapid improvement, overdistention or barotrauma could occur because too much pressure would be delivered.

Adaptive Support Ventilation (ASV)

A mode of ventilation that changes the number of mandatory breaths and pressure support level according to the patient’s breathing pattern.

Adaptive Pressure Control (APC)

A pressure-controlled mode that utilizes a closed-loop control of the pressure setting in order to maintain a minimum delivered tidal volume.

With that said, in this mode, the delivered tidal volume will vary depending on the patient’s lung compliance.

Volume-Assured Pressure Support (VAPS)

A mode of ventilation that provides a stable tidal volume by incorporating inspiratory pressure support ventilation along with conventional volume-assisted cycles. It’s only available on certain ventilators.

This mode can cause a prolonged inspiratory time, so patients with an obstructive disease should be monitored closely in order to prevent air trapping or other cardiovascular effects.

Neurally Adjusted Ventilatory Assist (NAVA)

A ventilator mode that uses the patient’s electrical activity of the diaphragm to guide the functionality of the ventilator.

A catheter with electrodes is positioned in the patient’s esophagus at the level of the diaphragm, and that is how the electrical activity is picked up from the phrenic nerves. Then, the ventilator uses this information to ventilate the patient.

Automatic Tube Compensation (ATC)

While not technically a ventilator mode, this is a setting on some ventilators that offsets and compensates for the airflow resistance that is imposed by the endotracheal tube or artificial airway.

High-Frequency Oscillatory Ventilation (HFOV)

A type of mechanical ventilation that delivers very small tidal volumes at an extremely fast rate which minimizes the chances of a lung injury. 

This mode has been shown to improve oxygenation in severe cases, such as with refractory hypoxemia.

Adjusting the Setting in HFOV:

  • Ventilation – can be increased by increasing the Amplitude (Power) or by decreasing the frequency.
  • Ventilation – can be decreased by decreasing the Amplitude (Power) or by increasing the frequency.
  • Oxygenation – can be increased by increasing the Mean Airway Pressure setting or by increasing the FiO2.
  • Oxygenation – can be decreased by decreasing the Mean Airway Pressure setting or by decreasing the FiO2.

This mode is also indicated to provide mechanical ventilatory support in neonates with conditions such as congenital diaphragmatic hernia, diffuse alveolar disease, and pulmonary hypoplasia.

Dr Rohit Bhaskar, Physio
Dr Rohit Bhaskar, Physio Dr. Rohit Bhaskar, Physio is Founder of Bhaskar Health and Physiotherapy and is also a consulting physiotherapist. He completed his Graduation in Physiotherapy from Uttar Pradesh University of Medical Sciences. His clinical interests are in Chest Physiotherapy, stroke rehab, parkinson’s and head injury rehab. Bhaskar Health is dedicated to readers, doctors, physiotherapists, nurses, paramedics, pharmacists and other healthcare professionals. Bhaskar Health audience is the reason I feel so passionate about this project, so thanks for reading and sharing Bhaskar Health.

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