What is a vent?

At the end of the day, the ventilator is a giant air blower. It blows air at a set rate, sometimes with a set volume, and with a set flow. There are three main settings that make the blower blow.

The Physics of the System

There are three main things that make an air blower blow. All ventilators work with -

VOLUME - RATE - FLOW

• The machine delivers an amount of air (volume) at your desired amount of flow, at a potentially variable number of times per minute (rate) depending on the patients clinical status

when pt spontaneously breathing - vent can be Triggered by either flow or pressure is what turns it on (flow/pressure); your target is either volume or pressure

Volume

Lets talk more about volume. volume vs pressure cycled, patient factors (awake, paralyzed, etc)

Rate

parallel iwth volume - patient factors for rate. Awake and patient sets their rate vs not being awake and you set rate for them (awake vs comatose), make sure people understand - OR and paralyzed and you’re taking compelte control vs when theyre in ICU, glimpse of both worlds. Desired rates (what provider wants, RT wants - whoever is making vent settings) and what patient either needs or desires - either we’re taking conrol bc of ARDS or paralysis or what were really trying to do if pstient is semi awake is were trhying to mimic what patient physiolgoically deisres and what their brain wants - patient may wants faster flow, may be more comfortable for them, or they make have COPD and the want slower flow.

who is dictating rate - patient or provider? and which one is right or wrong (not that one is right or wrong but which one wins)? if patient is acidodic ,they wll want higher vt, and faster rate -and if you are taking control and not matching what patient wants, you can get into trouble - factor in both pH and overall acid/base status

Flow

primarily for comfort - comes into play when you get into trouble with either resitance or compliance problems (diff disease states)

Tubing + Cage discussion - tubes vs cage

When you think of a vent, think of a long tube entered an enclosed, relatively inflexible cage (similar to how we think of the skull with increased intracranial pressure). When you are pushing a blast of air through the tube into the cage, you will have to push the air through lots of things that are resisting its flow (the resistance). Then, once you get the air through the tube, you reach the alveoli. The alveoli are little balloons that expand and collapse; collapsed alveoli are derecruited while expanded alveoli are recruited (e.g. participating in air exchange). The alveoli can be affected by both things in and outside of the cage - in the case of something outside of the cage, think of large body mass or a distended abdomen pushing on the cage walls. For things that can affect alveoli inside the cage, think a pneumothorax/pleural effusion. The ability for alveoli to adjust to these changes is known as the compliance of the lungs. Having compliant lungs is important since we are pushing air into the balloons; if the balloon isn’t able to expand, it can pop. In the ICU we often see people with sick, inflamed and angry alveoli that are poorly compliant.

There are two main pressure readings to focus on when you are looking at a ventilator - peak pressure and plateau pressure.

• Peak pressure is increased by both resistance and compliance. It is affected by not only by how compliant the alveoli are, but also by the pressure in the upper airways, the mouth, the vent tubing (all the stuff that is resisting air flow). It is a constant measurement that changes second by second. You can see it on the ventilator and it is one of the most common alarms to hear.

• Plateau pressure is a reflection of the compliance. To check a plateau pressure, you do something called an inspiratory hold - the machine literally delivers a breath and holds it. By doing this, you take resistance out of the equation and can isolate what the pressure is at the alveolar level (how compliant the alveoli are). This is checked manually and is not a constant measurement.

My vent is alarming; is the issue resistance or compliance?

It’s important to recognize whether a change in resistance or a change in compliance is the problem; to do this, you follow the following steps:

1. What is the peak pressure? (can be elevated by changes in resistance or compliance)

2. What is the plateau pressure? (can only be changed by changes in compliance)

3. What is the difference between the two?

   • The two numbers are close together - resistance issue

     • Examples: Peak 30, plat 28; Peak 50; plat 48

     • Resistance issues are caused by anything resisting the flow of air. You can think of this starting at the vent itself and going towards the patient; too much condensation in the tubing, a kink in the tubing, the patient biting the ETT, the patient “bucking” the vent and spasming against the ETT

   • The two numbers are far apart - compliance issue

     • Examples: Peak 21, plat 10; Peak 50, plat 15

     • Compliance issues are at the alveolar level, and as we said have to do with the ability of the alveoli to adjust to changes in the “cage”. Think pneumonia, pleural effusion, ARDS (non-cardiogenic pulmonary edema), cardiogenic pulmonary edema, pneumothorax

The Two Most Important Things

Intubating someone and putting an ETT in their throat is the first step. Once you have a patient on the vent, you hook them up to the ventilator, and you must ask yourself:

1. What am I telling the ventilator to do? (what vent mode, what settings)

2. What is the ventilator communicating to me? (what are the alarms set at)

If you do not have alarms that are set at appropriate levels, the vent won’t know when to be concerned. If you don’t have the patient in an appropriate ventilator mode, it can’t adequately blow its air.

Which mode should I use?

At the end of the day, all the vent modes are like quintuplet set of sisters but all have different outfits and styles. You can change stuff and make one match the other as long as you know what settings to change. Therefore, the best vent mode to use is the one that you know best. Because when the alarm goes off and the patient stops getting a tidal volume, you need to know how to troubleshoot the machine.

Common Types of Vent Settings

The first ventilators invented there wasn’t any brain in the machine - the machine would just give you a certain amount of air at “X” amount seconds, no matter what the consequences could be for the patient. This resulted in a lot of barotrauma and subsequent pneumothoraces; and it quickly became apparent that we needed to make our machines smarter. Each mode varies in the amount it allows a patient to do - a range from less support (more patient work) to more support (less patient work).

IMV - intermittent mandatory ventilation - super brutal, the first mode

This is the brutal mode ever. It’s the first vent mode created that says- “Okay, you’re getting a breath every five seconds” and doesn’t care if you want to try to breathe. If you tried to breathe in between breaths, you would breathe against a closed valve (like a straw with a cap at the end). It doesn’t care if the pressure is high - it will deliver that full volume. It was used in the 60s/70s but is not used anymore.

CPAP - you don’t have to breathe if you don’t wanna, I’ll just give you a continuous pressure to stent open your airways

This is often used for vent weaning; it is one continuous pressure that doesn’t change. So during both inhalation and exhalation the pressure is the same.

Bilevel/Pressure support - you don’t have to breathe if you don’t wanna, but I’ll help you a whole lot (during expiration AND inspiration) if you do

Pressure support, or bilevel support, is kind of like a BiPAP (ubiquitous for a machine that is commonly used) on a non-vented patient. It adjusts the flow to a set pressure during inhalation and ventilation. So you have a pressure (called PEEP/EPAP/CPAP) you set during exhalation and then you have a pressure during inhalation (this pressure is called IPAP/inspiratory pressure/pressure control level); this pressure on inhalation is added to the PEEP/EPAP/CPAP from your previous exhalation. It’s an apneic mode of ventilation - you don’t set a rate. So the patient completely has independence on when they take a breath but they get some help if they do.

SIMV - Synchronized Intermittent Mandatory Ventilation - mandatory breaths, but if you wanna do some I’ll help as much as the providers allow me to do

This was an upgrade from IMV. It’s a volume control mode of ventilation where the vent delivers a MANDATORY set number of breaths with a set volume while at the same time allowing spontaneous breaths. If you trigger the machine close enough to the scheduled breath you will get a machine breath. If you breathed over and weren’t in that window you would get a breath based on whatever support you were given - either no pressure support (PEEP - you breathing through a straw on your own) or pressure support (think a pressure support of 5 or 10, like a CPAP machine in OSA patients). When you add the pressure support, this is considered to be SIMV-PS.

SIMV-PS is a lot more common. If you have enough pressure support in SIMV to support a full tidal volume (same as set on the vent) and rate (same as set on the vent), it’s the exact same as AC/PC. It’s common in the operating room, or in patients coming out of the OR. For patients on it longer term you have to keep an eye out constantly make sure that the pressure support is enough to support the spontaneous breaths to the fullest extent.

Assist Control - either Volume Control or Pressure Control - Mandatory breaths, and if you wanna do some I have your back 100%

With assist control ventilation, the machine delivers a breath at a set rate and you can choose whether the machine prioritizes pressure or volume. It guarantees this volume or pressure and the patient chooses whether to participate or initiate. If the machine prioritizes volume, then this is AC/VC - you set the volume, and the pressure varies. If the machine prioritizes pressure, then you set a max/minimum pressure of the vent and the volume varies. Either way, in each assist control mode, the set rate is delivered BUT the patient has the choice to breathe if they would like to do so and the patient will skip the breath. If the patient does choose to breathe, then the machine will support them through this breath with either a blast of volume (in AC/VC) or a blast of pressure (AC/PC).

PRVC - Pressure regulated volume control, smart AC without a conscious - Mandatory breaths, I have your back 100%, will give you a guaranteed volume and I can adjust my flow based on your previous breaths

In a lot of newer machines, you will see this as the default mode on the ventilator. It is like a smarter version of assist control - but without any conscious. It was designed to try to give the comfort of pressure control but have a guaranteed volume of volume control - the best of both worlds. In some machines, it’ll take the average flow rate of the last three breaths and adjust the flow as needed.

Imagine you have a patient on a vent and they have worsening pulmonary edema overnight. As the compliance of the lungs decreases and the resistance increases, the peak pressure alarms will start ringing off the chain - the nurse will go into the room, notice that the vent is alarming at them, and they get the RT. Then the RT can adjust the vent settings or call the MD/APP to come re-evaluate the patient.

In PRVC, the machine is allowed to change the flow without alarming and it adjusts with the patient. Rather than screaming and sending off a distress signal, the machine senses that the resistance has increased/the compliance has decreased (e.g. the peak pressures are going up) and it’s smart self changes flow to adjust for this. This is all cool and all, but the machine does NOT care that the patient is getting worse. Because of this, it can sometimes mask a worsening clinical picture if nobody is paying attention. At the end of the day, will you have less alarms? Absolutely. But in a busy unit this also prevents the machine from yelling at you - “My patients getting worse!” This doesn’t mean it necessarily has no utility, but it is something you need to be aware of when a patient is on this mode. It can often be more comfortable for patients.

APRV - Inverse Ratio Ventilation - the oxygenation Hail Mary of ventilator settings, extreme version of I:E inversion

APRV is also known as inverse ratio ventilation. We normally breathe at a rate of 1:5 (inhalation: exhalation), with exhalation being much longer than inhalation. In inverse ratio ventilation it is what it sounds like - this ratio is flipped, and inhalation ends up being much longer than exhalation. But why would we want this? There are two things that help with oxygenation in a patient on a vent- FiO2 (fractioned amount of O2, the amount of O2 actually going into the vent tubing) and the mean airway pressure (which indirectly we change with PEEP).

You can think of your mean airway pressure like your mean arterial pressure. When you see a patient in septic shock, you don’t necessarily focus on the systolic or diastolic pressures - you focus on the MAP (MAP of 65 will keep a patient alive!). This is similar to the vent - you focus on your mean airway pressure to guarantee flow is reaching the alveoli. A higher mean airway pressure implies more constant flow.

By increasing the amount of time you are inhalation, you are indirectly doing an inspiratory hold (like you do when you check plateau pressures) and increasing the overall mean airway pressure to help with oxygenation. In doing this, you sacrifice some component of ventilation, but it is worth it to help the oxygenation. This is great for an ARDS patient on the vent - where you want to increase the mean airway pressure without increasing the PEEP (since increasing PEEP could result in barotrauma). But for anyone that’s awake, it’s can be uncomfortable (try it at home - breathe in five seconds, out one second, then again). This mode only works if you actually INCREASE the mean airway pressure compared to the previous mode.

The two questions you should think of in your head when you hear someone is on APRV are - “did my mean airway pressure increase?” and “are they adequately ventilating (since I recognize my expiration time is much shorter)?”

Luckily the mean airway pressure can be seen on most vents (denoted as p-mean), so if you change from AC/PC to APRV and the mean airway pressure changes from 20 to 25, that is a good thing and APRV may help. If the mean airway pressure changes from 20 to 5, you are - as Dr. Carrick would say it - “shooting yourself in the alveolus” and you should switch back to AC/PC.