6.5 Pressure Support: Settings and How to Set Them
Normal pressure supports fall between [latex]5\text{ - }20\text{ cmH}_2\text{O}[/latex] depending on the patient’s needs. In most cases, the pressure support is never lowered below [latex]5\text{ cmH}_2\text{O}[/latex]. The minimum of [latex]5\text{ cmH}_2\text{O}[/latex] is widely considered as necessary to overcome the resistance that has been added artificially by the endotracheal tube.
Object Lesson
Being intubated is akin to “breathing through a straw”. Imagine trying to breathe for longer than a couple minutes—in and out—through a straw. It might become difficult after a while to get the air in fast enough to fill your lungs adequately.
The added pressure support of [latex]5\text{ cmH}_2\text{O}[/latex] gives a little “push” to the air to help get the air in quicker and negate the added difficulty the endotracheal tube adds. This low limit is often termed “minimal settings” and therefore, we do not go below [latex]\text{PS 5 cmH}_2\text{O}[/latex] unless instructed by a physician.
Since PSV is meant to allow the patient to initiate their own breaths at their own speed, monitoring how they are breathing is the first step in monitoring tolerance. As patients are tiring out or experiencing increased WOB, their respiratory rates normally increase and their tidal volumes may be insufficient for their physiologic needs. These effects can be seen quite soon after any changes are made. If the respiratory rate climbs outside the physiologic averages [latex](12\text{ - }25\text{ bpm})[/latex], this may be indicative of a patient not tolerating the support level given. They may benefit from increasing the set pressure support.
In addition, look at the tidal volumes the patient is getting. If they are a lot lower than the normal tidal volumes [latex](6\text{ - }8\text{ mL/Kg})[/latex] for their ideal body weight, this also is indicative of needing additional support. Additional indicators that the patient may require more support from the ventilator are the heart rate and blood pressure. If there is an increase by [latex]20\%[/latex] from the initial baseline when the change is made, this may indicate the patient is not tolerating the change.
Once the pressure support is increased, the patient can be observed for improvements in their work of breathing, a slowing of their respiratory rate or an increase in their tidal volumes. ABGs should also be considered to ensure that the patient is improving.
Conversely, if the pressure support is higher than needed, this can be seen in respiratory rates that may be too low (less than [latex]12\text{ bpm}[/latex]) or tidal volumes that fall above safe volume ranges of [latex]4\text{ - }8\text{ mL/Kg}[/latex]. Refer to ideal body weight (IBW) calculations to check appropriate tidal volumes. Pressure support should be titrated up or down quickly within a few minutes until tidal volumes are within the normal range for the patient’s IBW and respiratory rates are within normal limits. Pressures should not be left higher than necessary for a long time, as this can lead to over-support, or in extreme cases could result in asynchrony or missed breaths.
Note: If the patient is pulling large volumes and you decrease the pressure support, do not go lower than the set minimum of [latex]5\text{ cmH}_2\text{O}[/latex]. Once you get this far, leave it. The pressures experienced by the lungs are very low and safe. Even if the tidal volume is still too high, this is an indication the patient may be ready for extubation, removal of the endotracheal tube (ETT), and discontinuation of mechanical ventilation.
Pressure Support Initial Settings
| Setting | Starting Point | Additional Instructions |
|---|---|---|
| PEEP | Use the same PEEP that was utilized on the previous control mode. or Start at PEEP [latex]\small{5\text{ cmH}_2\text{O}}[/latex] and titrate up if needed to assist with oxygen delivery by [latex]\small{\text{1 - 2 cmH}_2\text{O}}[/latex].NOTE: [latex]\small{\mathit{5\;cmH}_{\mathit 2}\mathit{O}}[/latex] is the minimum, PEEPs higher than [latex]\small{\mathit{10\;cmH}_{\mathit 2}\mathit{O}}[/latex] should be cleared with a physician. |
Changes to PEEP should be done in tandem with FiO2 and approximately every 30 minutes. PEEP changes take time for the impacts to be seen, as changes to distending pressure recruit collapsed alveoli over time. |
| FiO2 | Use the same FiO2 as the previous control mode. or Start at [latex]100\%[/latex] and wean down quickly to SpO2.If the patient did not require high oxygen before initiation, start at [latex]50\%[/latex] and titrate up or down to target SpO2 [latex]92\text{ - }99\%.[/latex] |
Changes to FiO2 can be done within 3-4 minutes after watching SpO2. You want to give the minimum amount of FiO2 possible to target SpO2 within the normal range. |
| PS (Pressure Support) | A starting point of [latex]10\text{ cmH}_2\text{O}[/latex] is reasonable with titrating up or down within a few minutes after watching the [latex]RR[/latex] and [latex]V_T[/latex] the patient is getting and ensuring they are within normal limits as described above. | Note: minimum PS of 5 to overcome ETT resistance. Do not go below this level unless instructed by a physician. Always check a PS change with an ABG within 30-60 min to ensure the patient is tolerating it. |
| Trigger, exp [latex]\%[/latex], other settings | [latex]25[/latex] by default. Can be adjusted to allow for shorter or longer exhalation | These settings are defaulted to fit [latex]95\%[/latex] of patients. |
Let’s look at some cases together!
Case Study A
A patient has been ventilating on AC/VC (volume control). The clinicians have been weaning the sedation and want to try changing the patient to a spontaneous mode to start weaning the patient off mechanical ventilation. The volume control settings are as follows:
- [latex]RR=14\text{ bpm}[/latex] (patient breathing above at a rate of [latex]18\text{ bpm}[/latex] total)
- [latex]V_T=380\text{ mL }(6\text{ mL/Kg})[/latex]
- [latex]\text{PEEP}=8\text{ cmH}_2\text{O}[/latex]
- [latex]\text{FiO}_2=0.40[/latex]
Is this patient a candidate for PSV?
Solution
Yes! They are triggering above the set RR and have an intact drive to breathe. Before switching to PSV, the practitioner should ensure the patient’s CO2 levels are within normal limits by checking the ABG. If the ABG is normal and the patient has stable vital signs, then the patient is deemed a candidate.
What settings do you start with?
Solution
Change the patient to pressure support ventilation by switching the mode. Before accepting changes, select the chosen settings for PEEP, FiO2 and PS. For this patient, initial PSV settings could be as follows:
- [latex]\text{PEEP}=8\text{ cmH}_2\text{O}[/latex] (same as previous control mode)
- [latex]\text{FiO}_2=0.40[/latex] (same as previous control mode)
- [latex]\text{PS}=10\text{ cmH}_2\text{O}[/latex].
Don’t forget: Every patient has their own “normal.” Before changing to PSV, look at their WOB, [latex]RR\;(18\text{ bpm})[/latex] and vital signs (heart rate and blood pressure) for a baseline.
What do you do now?
Solution
Watch the patient for about 5 minutes. What is the RR? What are the tidal volumes? What is their WOB? What are their vitals?
The clinician should target [latex]RR\;12\text{ - }25\text{ bpm}[/latex], tidal volumes: around the same as they had on AC/VC. Watch for signs of increased WOB and watch the vitals for any changes [latex]20\%[/latex] or greater than baseline. If patient is getting worse, increase the PS by [latex]1\text{ - }2\text{ cmH}_2\text{O}[/latex] and watch again if an improvement. If their tidal volumes are higher than when in AC/VC or their RR is on the low range of normal, try decreasing the PS value by [latex]1\text{ - }2\text{ bpm}[/latex] and assess the same things above. Remember! We want to set the lowest tolerated PS that keeps the patient breathing comfortably without increased WOB.
The patient seems to be tolerating the change. What now?
Solution
Repeat an ABG after 30 minutes and if everything looks good, consider decreasing the PS within a few hours until you get to the minimum settings—at which point the patient may be a candidate for extubation.
Case Study B
A patient with profound hypoxia is being intubated electively due to increasing oxygen requirements. After initial intubation, before being put on the ventilator, they are waking up and triggering breaths. The respiratory therapist (RT) tried to put the patient on pressure control ventilation (ACPC), but the patient is double triggering and uncomfortable. Instead of fully sedating the patient, the RT attempts to put them on Pressure Support Ventilation to see if they can keep breathing on their own.
Is this patient a candidate for PSV?
Solution
Maybe. They have a drive to breathe and would probably be more comfortable on pressure support, but their hypoxia might mean they will not be stable on a spontaneous mode. ABGs and oxygenation status must be monitored closely and if they deteriorate, the patient should be sedated and fully ventilated for better control over their ventilation.
What settings do you start with?
Solution
Since the patient is being intubated for mostly oxygen issues, FiO2 should be started at [latex]1.0[/latex] with a higher PEEP—if acceptable by the physician—probably [latex]10\text{ cmH}_2\text{O}[/latex] to start. The PS can be started at [latex]10\text{ cmH}_2\text{O}[/latex] and then adjusted up if the RR is high or the tidal volume is low.
What do you do now?
Solution
FiO2 can be titrated down by watching the SpO2 over the next few minutes and the patient’s vitals and RR and WOB should be monitored to see if higher pressure support is needed. If their volumes are too high and they are tolerating PS, the PS can be decreased until the volumes are within normal [latex]4\text{ - }8\text{ mL/Kg}[/latex], as long as they do not show signs of intolerance such as increased WOB (remember the DiapHRaGM mnemonic).
The patient seems to be tolerating. Now what?
Solution
ABGs should be checked for overall tolerance. If there are any changes to patient status, settings should be increased to the previous better-tolerated level; or, if there is a significant change, consider sedation and full control ventilation if the ABGs are poor.
It is important to remember that, because pressure support is a patient-triggered breath, changing the level of pressure support will not necessarily change the volumes or respiratory rate of the patient. Sometimes, the patient’s neurological control to breathe will remain the same. Here are two common situations that fit this scenario:
Example #1: Diabetic Ketoacidosis (DKA)
These patients tend to be tachypneic—breathing rapidly—with a high minute volume (amount of air exhaled in a total minute, as calculated by multiplying the tidal volume by the respiratory rate). Some clinicians might see tachypnea and assume the patient has an increased work of breathing. Increasing the pressure support will not decrease the respiratory rate at all. These patients will most likely remain tachypneic no matter what mode or setting you have them in. This is not a sign of failure. Instead, the tachypnea is a symptom of the pathophysiology of their illness.
Example #2: Oversedation/Waking up from anaesthesia
Patients that have had a large amount of sedation or narcotics usually breathe very slow with large tidal volumes. At times, their respiratory rates can be lower than [latex]10\text{ bpm}[/latex] with tidal volumes well above their IBW range. Decreasing the pressure support to minimum pressures can sometimes still reveal tidal volumes that are too large and respiratory rates that are below normal range. These effects will normally resolve when sedation wears off. Consider decreasing sedation further if able, or allow the patient to wake up more.
Back Up Ventilation!
For safety reasons, in PSV ventilation (or any other spontaneous mode), back up ventilation must be available in case the patient stops breathing. This is often called Apnea Ventilation on most ventilators and entails a controlled mode that the ventilator will initiate if the patient does not trigger a breath in a specific time interval. By default, most ventilators use a time interval of 20 seconds.
“Pressure Support: Settings and How to Set Them” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
