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These settings can be used for almost any patient as a baseline set up, there are some exceptions (such as the patient with obstruction pathology and those with COVID-19 who are not coping with the basic settings). Patients who do not fit into “baseline” or starting ventilation should not be treated without expert consultation.
This post will help you to understand the basics of ventilation, and safe ventilation parameters for just about everyone.
For obstructed patients there will be an approach published soon, and for the COVID specific patient you can access our COVID-19 ventilation thoughts (bearing in mind that there is not great evidence for ventilation in the COVID-19 patient yet).
First the Basics
Really simply, ventilated lungs can be thought of as balloons on the end of a straw. The tube represents all the tubing from the ventilator, the endotracheal tube and the major airways (all the spaces where air travels but is not able to distend the tissue it travels in).
The balloon represents the expandable alveoli at the end of the conducting airways.
In order for the balloon (alveoli) to inflate, the pressure that is applied must be greater than the resistance in the straw (the airways). Gas only flows when there is a pressure gradient, and so pressure is required to open the alveoli.
Peak inspiratory pressure is the pressure that we apply to the airways to allow for the gas to flow down towards the alveoli.
The pressure inside the alveoli can be divided into two pressures:
The pressure that is “baseline” (PEEP) and the pressure that is achieved after the driving pressure is applied (Plateau). PEEP is shown here:
Plateau pressure is shown below, this is measured at the end of inspiration, and depends on the compliance (stretch) of the alveoli, the volume delivered to the alveoli and the baseline pressure (PEEP).
This is the pressure difference between Plateau Pressure and PEEP. The higher the driving pressure, the greater the shear forces on the alveoli, we want to maintain this pressure difference at no more than 13-15cmH20 (Matthay et al., 2020).
The higher the driving pressure, the higher the risk of mortality (this may not be as a result of the driving pressure but more to do with the fact that the more damaged the lungs are, the higher this pressure will tend to go, and thus lead to a higher mortality). We cant set the actual driving pressure on the vent, we can only measure it, and try not allow it to exceed the recommended values.
Its important to note that all the parameters for ventilation that will be discussed are interlinked, there can be no change in one parameter without affecting one or more of the others. In order to apply principles of ventilation we need to know that everything works in a system and cannot be isolated out.
No matter what kind of ventilator you have, you will be able to use these three parameters to control the basic ventilation for any patient. Pressure, Flow and Volume.
The basic Rules for Ventilating ANYONE (normal lungs)
These rules apply to the patient who presents with lungs that are not obstructive (by this we mean air trapping pathologies like asthma or exacerbation of COPD).
The recommendations in this article have been adapted for the patient who has been intubated for respiratory failure/ARDS.
Choose a mode for ventilation:
The simplest setting for the patient is one that you are familiar with, pressure controlled modes are better for patient synchrony and comfort, but volume control modes are more freely available and very easy to set up. Choose one that you can work easily.
The following 5 rules for basic ventilation have been adapted from this resource.
Set tidal volume between 4 and 8ml/kg
The lungs of any patient who has been intubated for ventilation in the emergency setting should be thought of as “baby lungs”, there will inevitably be shunting present (spaces that are horribly ventilated but well perfused, and spaces that are well ventilated and horribly perfused). Functional residual capacity will be reduced due to position, movement and illness. The lungs of the COVID-19 patient even more so, with infiltrates and damage to the elastic recoil of the lungs, we don’t want to deliver more tidal volume than is safe.
Starting at 8ml/kg initially might be an option for most patients but will likely be too high for the patient with ARDS or severely damaged lungs.
6ml/kg ideal body weight is a good start
This can be titrated down later if needed.
Ideal body weight should be calculated based on the patient’s height, a calculator can be found here:
PREDICTED BODY WEIGHT
Inspiratory Flow is for patient comfort
When we breathe normally, we like to have a lot more flow at the start of inhalation, and less flow towards the end of inhalation as the lung fills. Many ventilators will not allow you to set this parameter, however it’s something to think about if the patient is not comfortable. Pressure control modes of ventilation allow this flow pattern to be achieved, while volume control does not (see the images below). Volume control applies a set flow through the entire phase of inhalation.
If the ventilator allows you to set the inspiratory flow, set it to around 60L/min and assess if they are comfortable (looking for signs of air hunger, increased work of breathing at the start of inhalation), if these are present, increase the flow and assess again.
Respiratory rate is for titrating CO2 levels (for controlling VENTILATION, not oxygenation)
Release of carbon dioxide relies on the movement of air through the airways and creation of a concentration gradient for CO2 movement from the alveoli to the larger airways and further to the outside environment.
CO2 removal is a factor of the following:
Minute Volume = (Tidal volume – Dead Space) x Respiratory Rate
Using the equation above, if we are struggling to remove CO2, we can increase the tidal volume (but we can’t because we set it to safe parameters at the start), or we can increase the rate of breathing, or decrease dead space (dead space could be decreased by decreasing the shunt present in the system, or physically removing obsolete tubing on the circuit).
Respiratory rate should only be titrated UP to decrease CO2, if you are thinking of titrating a NORMAL RATE DOWN STOP!! Look for other reasons for why the CO2 could be low!
- Consider starting the respiratory rate for the COVID-19 patient on the higher end of the physiological norm for the patient’s age, and then assessing how they do with the view to titration down to norms later.
- Respiratory rates of up to 35b/min may be required (this would be in extremis)
- If your patient came in with a high respiratory rate, it may need to be matched initially until the underlying issue has been sorted out
There are some problems with high respiratory rates:
- The time for inspiration decreases with higher respiratory rates
- Leads to increased pressure in the system
- Long term the aim should be to return the patient to a physiologically normal rate, once all the underlying issues have been corrected
PEEP and FIO2 are for OXYGENATION
The longer the alveolar surface is under pressure, the better the exchange of oxygen will be. PEEP allows for a continuous exposure of the alveolar surface to pressure, even when the patient exhales, thus maximizing the time for oxygenation, decreasing collapse of the alveoli and decreasing the pressure required to reopen the alveoli for the next breath.
- PEEP allows us to achieve ‘OPEN LUNG VENTILATION”, less trauma to the tissues, less movement of the alveoli and better oxygenation with less “drug” (oxygen)
- This becomes especially important for patients who present with traditional ARDS, and a decease in compliance.
Normally the suggestion is to titrate oxygen down rapidly in the emergency ventilation of a patient, and to titrate the PEEP up to achieve adequate oxygenation of the patient. There are specifica ARDS guidelines for the adjustment of FiO2 and PEEP values to maximise the oxygenation status of the patinet. These values and tables are ideal for the patient with traditional ARDS, and act as a good starting point for the patient who needs ventilation in the Emergency setting. These are not neccesarily the best settings for the COVID-19 patient who now needs to be ventilated (more on this in the
Some things about PEEP:
Some videos to show the effect of PEEP
CALL FOR HELP EARLY as more advanced ventilation strategies may be required to achieve oxygenation.
Make sure the alveoli are protected by checking the following pressures:
- Peak inspiratory pressure
- Plateau pressure
Remembering from the notes right at the start of this post, PIP (peak inspiratory pressure ) is the pressure that is required to overcome the resistance in the larger airways to allow for inflation of the alveoli.
- Use whatever inspiratory pressure is required to achieve the tidal volume you set, and watch that the plateau pressure remains within safe parameters (the lower the driving pressure, the better)
- Try to limit the Driving Pressure to no more than 13-15cmH20 (Matthay et al., 2020) remember you can calculate this by the following formula: Driving Pressure = Plateau pressure – PEEP
- Plateau pressure MAX: 30cmH20 (measure this by pushing the inspiratory hold button and reading the pressure in the system at the time if your vent doesn’t provide this reading automatically)
If the plateau pressure is too high you will need to slowly decrease tidal volume until the plateau pressure drops below 30cmH20, this can result in tidal volumes as low as 4ml/kg in some cases. CALL FOR HELP EARLY!
The chart below can be used as a basis for setting up the ventilator for the basic ventilation of almost any patient in the emergency medicine setting. The ventilation settings for the COVID-19 patient can be found in the discussion on COVID-19 ventilation.
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