On the simple ventilator pressure waveform we have time on the x axis and pressure on the y axis. This is a square shaped waveform.

So on the first part of the waveform the pressure is at the baseline and nothing is happening, but then at (1) the pressure rises in a vertical fashion as air is entrained into the lung.

But what has happened to cause this to occur?

What has **triggered** the **start of inspiration** at (1)?

The **inspiration phase** is then at (2).

The breath then ends (3) at **end inspiration**.

Then we reach **baseline **or** expiratory phase** again (4).

This whole process takes place over a **total cycle time, **in other words how long does this whole breath take. The whole breath includes both the inspiration time, the expiration time and any pauses in between. In the diagram above the cycle starts at (1) and finishes at the end of (4).

So let us assume that the ventilator is giving 12 breaths every minute, a rate we can set. We need then to divide 60 seconds by 12 which will give us a **total cycle time** of 5 seconds.

From this we can work out how much of that **total cycle time** is inspiration and how much is expiration- this is the inspiration:expiration ratio or **I:E ratio**.

We can set our inspiratory time via the ventilator in many modes of ventilation. So let us assume that we set our inspiratory time or **T**_{i }to 1 second. So as our total cycle time was 5 seconds, we must have an expiratory time of 4 seconds. This will then give us an **I:E ratio** of 1:4.

So, in this example, we have four times as long in expiration as we do in inspiration.

So what will happen if we decrease the respiratory rate to 10 breaths per minute for example? This will mean that we now have 6 seconds per breath. If we don’t change the **T**_{i, }so that inspiration still takes 1 second then expiration will now take 5 seconds.

The **I:E ratio** has changed to 1:5. Expiration now takes five times longer than inspiration.

You can learn more about this from Ollie Pooles fabulous YouTube channel where he has a series of excellent videos breaking much of this down.

- Peak and Plateau Pressure
- AC versus SIMV mode
- Positive End Expiratory Pressure (PEEP)
- Increase the rate or tidal volume?
- Phases of a breath- I:E ratio and cycle time
- Intubation
- Ventilation screen- what do those numbers mean?
- Pressure Support
- Modes of ventilation I
- Modes of ventilation II
- Physiologic effects
- Physiologic goals
- How do I describe how my patients ventilation?
- Trigger, Limit and Cycle
- Pressure support ventilation graphs

- 6 ways to be better with Bag-Valve-Mask
- Terminology
- Phase Variables
- Airway Pressure Release Ventilation (APRV)
- Pressure Volume Loop
- Lung compliance in volume controlled ventilation
- Pressure/Volume/Flow graphs
- A-a gradient
- Goals and Indications
- Anatomy of the Endotracheal Tube
- Lung Compliance
- Ventilation/Perfusion V/Q matching
- Ventilator Induced Lung Injury (VILI)
- Ventilator Associated Pneumonia (VAP)
- Phase variables...again...

Having read the guidelines I made these infographics. They are FREE. Just click on the button below.

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