I'll give you my impression. Ventilators are simultaneously simple and complex, but for the ED purposes we don't need to get into the complexity of them. The goal is to appropriately oxygenate/ventilate without causing harm. Don't worry about any of the rest of it. I'll type out some things below which may go beyond what you're asking. But if you want to understand ventilators a bit better, it may help.
Disclaimer: there are many ways to manage a vent. There are several correct ways, and there are wrong ways. Different people may have different ways of doing the same thing, it doesn't mean one is right and the others are wrong. It means there are many safe/correct ways to manage it. Below you'll find the way I learned, and the way I think about vents.
It's a method, not the method.
Essentially, a mode of ventilation is broken down into 3 pieces: [Control Variable] + [breath sequence] + [targeting scheme] = mode of ventilation. For the ED you really need to understand only the first 2, and maybe just the first one.
For the next bit keep this equation in mind (you don't have to do math, it will just help your conceptual understanding:
Pressure (vent) + Pressure (muscle effort) = Elastance*Volume + Resistance*Flow
- Control Variable is what you control. On a vent it's only ever 1 of 2 things, either pressure or volume. No matter what fancy mode of ventilation you put someone into, at the foundation, you are only ever controlling one of these variables. From the equation you can see that pressure and volume are on opposite sides. If you are controlling pressure, the actual volume delivered will be varied depending on the patient's pulmonary mechanics (their intrinsic resistance and compliance [compliance = 1/elastance]. If you control volume, the pressures the ventilator requires to deliver those volumes will depend on the patient's pulmonary mechanics. The resistance and elastance will change with different disease processes. This is important for the ED physician to understand. You can either use pressure or volume and often accomplish the same thing.
- Breath sequence. This is an intermediate area for you to understand. We define breaths on a ventilator by inspiration only. Expiration is passive. The start of inspiration we call 'trigger', and the end of inspiration we call 'cycle.' Cycle is not the end of the breath, it is the end of
inspiration. Now, for each trigger / cycle event it can be broken down into who or what is causing that.
- The trigger can be = patient initiated or machine initiated (ie, the patient can trigger a breath if they're active, or if they're passive the ventilator can trigger it)
- The cycle can be either from the patient or machine
Let's look at this table for all possible combinations:
| Trigger | Cycle |
| Patient | Patient |
| Patient | Machine |
| Machine | Patient |
| Machine | Machine |
Anytime you see "machine" we will define that breath as a "mandatory" breath. Therefore, the only spontaneous breath in mechanical ventilation is one that is
both patient triggered and patient cycled. I say this because a lot of beginners see the patient trigger a breath and say that's a spontaneous breath. This is not so.
A spontaneous mode of ventilation will be something like pressure support. Which is identical to either CPAP/BiPAP depending on if you set a different inspiratory pressure from PEEP. If your patient is paralyzed and you put them on a spontaneous mode, they will die because they cannot initiate a breath. (Actually the vent will likely go into a backup mode that you set, but you get the idea).
Now, onto the breath sequence. Since a breath can either be spontaneous or mandatory, there are 3 possible combination of the sequence of those breaths. What I mean is, if you look at 5, 10, 100, 1000 breaths in a row - that is a sequence of breaths. The three possibilities are:
- Every single breath is spontaneous = continuous spontaneous ventilation (CSV)
- Every single breath is mandatory = continuous mandatory ventilation (CMV)
- There is a mix of spontaneous and mandatory breaths. (any combination)= intermittent mandatory ventilation (IMV)
- The third box is the targeting scheme. Don't worry about this. It has to do with how the ventilator will adjust things you set as targets based on feedback it gets from a variety of inputs. This is beyond the ED. You can learn it if you want, and if you want I'll go into more detail. But let's leave it off for now.
So now a mode of ventilation may look something like: PC-CSVs
This is pressure control, continuous spontaneous ventilation, with a set-point targeting scheme (again this part is beyond this post).
Basically this is pressure support. You set an inspiratiory/expiratory pressure, some other stuff that we don't need to go into, and the patient is on the vent breathing for themselves. Similar to BiPAP/CPAP
How should you set the ventilator?
Like everything in medicine - it depends.
For undifferentiated respiratory failure you can set either VC / PC. Target a tidal volume of ~6-8cc/kg of ideal body weight. This is easy to do, measure the patient and look at the ARDSnet chart. You can find it on google images in about 1 second by typing in "ideal body weight ardsnet". I have a picture folder on my phone that I keep peep tables, ideal body weight charts, common vent names ➔ mode descriptions.
Set the minute ventilation as you see appropriate based on the reason they were intubated. If bad COPD / Asthma set a slightly higher tidal volume if needed, but a very low RR to allow the longest expiration time to ensure no autoPEEP. Remember, you can adjust flow, i-time, ramp to your hearts content, but the undisputed winner of longer e-time is the lower RR. You can see autoPEEP in the expiratory limb of the flow waveform.
Get an ABG 30 minutes-1 hour after intubation and initial settings. Adjust the RR to normalize the pH (unless they have ARDS then we'll tolerate pH down to 1.15-1.2 (depending on your comfort) and tolerate pCO2 higher than that. Or if they have bad COPD and you see their HCO3 is 38, then trying to normalize the pCO2 will not help as they live higher, just normalize the pH at that point and admit them to the ICU.
Patient dyssynchrony comes in many shapes and forms. It's too much to go into here, but if you think this is helpful I can type out a post on waveforms and a post on dyssynchrony and what you may do about it.
Pressure alarms. Everyone always worries about the peak pressure, and while it can be important, it often doesn't tell you what you think it's telling you. When I get called for peak alarms there are a number of things that go through my head. If it's someone who has been fine the first thing I do is check the plateau pressure (inspiratory pause maneuver). If the peak is elevated and the plateau is normal then it rules out all of the bad stuff (worsening ARDS, tension pneumothorax, usually right mainstem...). The peak pressure is the pressure at the airway opening, and is the sum of the airway and alveolar pressure. We care mostly about alveolar pressure. If you have normal alveolar pressure, you aren't going to cause barotrauma with high peaks. Think about things like: bronchospasm, mucous plugging, small ETT, patient biting the tube...
- In this case, suction the patient's ETT, look and see if they're biting the tube, listen for wheezing ➔ albuterol/duonebs etc. Just fix what's in front of you, but don't worry about high peak pressures here. It's not the pressure the alveoli are seeing.
If you get high peak and high plateau pressures, this is when you really need to worry. Something has gone wrong. If they were previously normal then think about a new right mainstem, tension pneumothorax, rapidly worsening pulmonary compliance (edema, effusion, consolidation, fibrosis whatever). Some of those things likely won't happen within minutes, so if they're normal and rapidly worsen then you likely mainstem the tube or the patient has a pneumothorax. Otherwise, get a CXR and rule things out.
PEEP can be titrated depending on what's wrong with the patient. But if you have no idea then use the ARDSnet PEEP table, either the high or low is fine. For severe asthma there is a great paper by Caramez on PEEP phenomenon in obstruction. But what you have to know in the ED is that dynamic hyperinflation can kill. Recognize it, fix it. You can recognize it on the ventilator flow waveform and by measuring total peep with an end-expiratory maneuver. If the total peep is >~3 of what you set, you need to start fixing it. If it's higher than that then you should not leave the bedside until you've stabilized the patient because make no mistake, they are unstable at that point. If they are collapsing then disconnect from the vent and let them exhale. If not then turn the RR waaaay down and follow the flow waveform. You may need to increase tidal volume to keep up with your minute ventilation. Follow the gases here and adjust accordingly. This is usually in severe obstructive disease, so start nebulizing albuterol + give systemic steroids etc.
There's so much more we could talk about. But hopefully the above will give you an understanding of the background / basics, as well as some ways to initially set up a vent. Synchrony is a big issue but beyond the scope here, in a nutshell the more awake the patient is and the more control they have (ie spontaneous ventilation) the more synchronous they'll be. If they are awake and in a controlled mode of ventilation you may have to change trigger/cycle variables, flow ramps, inspiratory time etc to make them more comfortable. That's beyond the scope of this though. Your other option for dyssynchrony in the ED is just to sedate the heck out of them.
Hope it helps. Let me know if I can clarify anything. Again, this is
a way, not
the way.
