Clinical Pearls: Ventilator Modes 101
The very first CME I attended as a new grad critical care nurse practitioner was put on by the Society of Chest on ventilator management. Still to this day it was the best one I’ve ever done. Critical care is a catch-all specialty but you gotta start somewhere and if nothing else, a decent ICU provider should have a darn good understanding of ventilators. There are a number of reasons why ventilators are confusing, and I’ll break down how to simplify these factors in this blog post. But the big takeaway is this: just because a patient seems controlled well on the vent does not mean he is; do not leave the task of interpreting the vent and the patient’s response to the vent to others. It is incumbent on you to educate yourself about what the knobs, numbers, and waveforms mean in order to better treat your patient. Let’s talk vent basics and it starts with understanding the mode.
This is a table comparing similar modes by brand and proprietary names.
Why Must It Be This Hard
In my mind there are two main reasons that interpreting ventilators is difficult. The first of which is that manufacturing companies intentionally obfuscate things by creating their own proprietary names for the same thing someone else is doing. I’m sure they do this so you get used to their brand and then request only this brand as you are familiar with and capable of managing it. Lo and behold, the other company has a vent that will do the EXACT. SAME. THING but you don’t know it because the name is different. 😡 It’s a racket. It doesn’t serve the patients or the providers who are learning. It is what it is, and you must be smarter than the marketing department. Understanding modes is how you break down this smokescreen. Understanding what the modes can and cannot control, what the targets and other variables are help you to classify different machines into similar modes of delivery.
The second reason is that waveforms are hard. Like, really hard to interpret in real time. You can grasp the concept fully, then go stand in a patients room, watch the graphics change and still struggle to interpret what is happening. That’s because it’s rare that something is textbook example. Think about EKGs. You know what 3rd degree block looks like. But the 12 lead in your hand is hard to interpret because there is also afib and ectopy, the baseline is off, you’re not sure if you are measuring from P to P or is that first P really a T? Life rarely looks textbook. Over time and repeated exposure it falls into place and you can weed out those PVCS, find the actual P waves and make your diagnosis.
A Few Generalities To Simplify Life
Independent variable: what you as the person controlling the vent set.
Dependent variable: what the patient controls in response to what you set.
Neural drive: the patients inherent desired breathing pattern.
Assist: a term referring to a breath that a patient initiates
Control: a term referring to something you are setting aka controlling on the vent.
Compliance: change in volume/change in pressure. This is a calculation. I find many nurses confuse this term with synchrony. You’ll hear “the patient is complaint with the vent.” Compliance is a number that tells you how sick the lungs are. The lower, the worse off it is.
The Big Picture
A ventilator is a machine used to deliver air to a patient via positive pressure (our inherent breathing is negative pressure). It’s important to point out because it is the opposite of what our bodies want. Therefore when we set variables to deliver breath at certain intervals, certain speeds, certain volumes, ending at a certain time, with a certain amount of pressure (or support) behind it we are making a guess at what that person really wants/needs. You account for the diagnosis and condition of the brain and lungs but again, you are guessing. That’s what leads to asynchrony. That’s the cause of what we’ve told our patients for years as nurses “being on a vent is uncomfortable” and this is why we sedate folks (for the most part). If you can do a better job interpreting the data you are seeing on the vent to match what your patient wants you will see less asynchrony and have to administer fewer sedatives. This is called neural matching. Assessing waveforms is a huge component of this and is beyond the scope of this post. But you can certainly start with choosing the optimal mode for your patient to limit discomfort and asynchrony.
A Tale Of Two Variables
If you can grasp this key concept, ventilators will become less intimidating. There are certain variables I can control (set) on a ventilator, and there is an equal and opposite reaction the patient can control as a dependent response. The machine can do what it’s told but the patient will respond based on their inherent needs and problems. These are the independent (I set) variable and the dependent (pt dictates) variable. You can control one but not the other. The primary variables involved with breath delivery are volume and pressure. Ventilators have advanced greatly over the years and now offer synchronized modes (some ventilator, some patient) and adaptive modes (continuously changing targets based on patient response). Additionally settings can be modified to target a certain goal but limited to prevent damage/complications. These factors add in all the confusing lingo. But try to boil it down to the basics first: how are you attempting to deliver air to the patient - via a set volume or a set pressure?
Ventilator physics involve these three main components of air delivery. And it starts with targets.
Target: What you tell (independent variable) the machine to use as a goal for delivering a breath.
Volume targeted: You set a certain volume of air to be delivered. In some patients this requires very little pressure (this would be your compliant patient) and in others it requires a great deal of pressure (sick lungs, biting the tube, coughing, bronchospasm, etc). The problem with high pressures is barotrauma. Because this is a big problem we want to avoid, most vents require pressure limits. The RT has to dial in a max pressure limit. This tells the machine that if it gets too high to stop delivering air. This leads to poor ventilation and low tidal volumes. This is colloquialy referred to as “peaking out.” This is the pt who has a very poor minute ventilation (think 2-4ish) and if you get an ABG they will likely be acidotic.
Modes: VCAC, CMV, VC, ACVC, AC-V
Synchronized modes: SIMV-V, SIMV-VC
Pressure targeted: You set a certain amount of inspiratory pressure to deliver the breath. The machine says ok, I will blow air into this person until I reach this amount of pressure. The dependent variable is volume. So if it requires large tidal volumes to get to a set pressure so be it. The problem with this arises when you want to employ a low tidal volume ventilation strategy. Or if you want to correct an abnormal carbon dioxide level, you have less control over this in PC modes. This mode is generally more comfortable for patients.
Modes: PCAC, (P) CMV, ACPC, PC, PC-V
Synchronized: SIMV-P, SIMV-PC
Weaning modes: Pressure support, CPAP, SPONT. These are modes designed for people breathing independently and the machine simply offers a bit of pressure to make the work of breathing easier - invision a CPAP mask on a pt. It’s the same thing, just into the airway itself. A pressure support of 5 is very different than a pressure support of 20 - just FYI.
2. Trigger: What tells the machine to start delivering a breath. These can be either something you set, or something the patient initiates, or a combination of both.
Patient initiated: A sensitivity level is set on the machine. When the machine senses that the patient is making an effort to inhale (applies negative pressure or triggers a flow rate) it either allows the patient to breath all on their own (SIMV) or assists it with some applied force or pressure.
Machine triggered: a certain amount of time elapses and the vent delivers a breath. The variable here is time. (RR, I:E)
3. Cycle: What tells the machine to stop delivering a breath. This is machine driven.
A certain amount of time elapses.
The set amount of volume is reached.
The set amount of pressure is reached.
Numbers 2 and 3 are the primary driving factors in weird waveforms (asynchrony) and are problems best interpreted from the graphics.
In most ICUs we start with a volume targeted mode as this allows us to more tightly regulate a low tidal volume strategy. It bears noting that either mode can achieve this. But in a pressure targeted mode the dependent variable is volume so it will be an ever changing number based on the patient’s response. A diligent ventilator controller can adapt for this, but it requires diligence (or adaptive modes that constantly change based on an algorithm - which is probably where we are heading in the future for most people).
If the pt has discomfort, or lots of asynchrony the RT will typically start by switching to a pressure control mode. For those that don’t have large tidal volume concerns or conversely, aren’t breathing much this may be perfectly fine. Recall, the independent variable of volume so if a pt is over-sedated or inherently not breathing for some reason this will lead to acidosis 2/2 lack of ventilation. Most machines allow you to set back up rates or volume guaranteed delivery (PCV-VG).
Alright - I think that’s as deep as I want to take you in a ventilator intro discussion. LMK if this is helpful to your practice - or if I missed anything!
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