Ventilators and ventilation re: covid

a deep sciency dive on ventilation and ventilators

Today I'm going to explain oxygenation and ventilation in layman's terms, so y'all can play the home game! (This is a recap of a prior post based off a viral tweet I had in spring. I sent it to my author mailing list a bit ago, so if you’re on both lists and this feels familiar, that’s why!)

At the time I wrote it it was more to explain why NYC needed so many vents so quickly — I’ve tweaked it now to explain more about why people with covid might be on a vent.

People on the internet have been throwing around terms like ventilators, non-invasive ventilators, respirators, sharing pictures of modified snorkles, 3D printed things, and sharing hacks where you can run four people off of the same ventilator. My intention here is to help break that down so that everyone can understand what we mean when we talk about those terms and why ventilation is so important, particularly in Covid cases. If you find this email helpful at all, feel free to pass it along. You can even post it on facebook as long as you give me credit and link here, 'k?

Let's begin at the beginning -- what is oxygen and why do you need it? Oxygen is part of the Krebs cycle, which is a biochemical process that produces ATP, the building block of energy for your body. Your body gets the oxygen it needs out of your lungs. When you drown, your lungs are filled with water, and this oxygen transfer can't occur. No ATP = death. (Randomly, because if you're reading this far, we're all nerds -- the reason cyanide kills you is it blocks another function in the Krebs cycle, which also = no ATP = death.)

So! There are several different levels of oxygenating people. We'll go from lightest to strongest.

The first is a nasal canula -- you've seen these on TV. They've got the two prongs that fit into your nose (which don't create a perfect seal, we'll come back to seals in a bit here) -- they basically goose your lungs with a little extra oxygen.

If that's not enough we can move through a series of more intense face masks till we get to a non-rebreather. You've probably seen these on TV too, they're the cone-mask looking ones with the inflatable air bags behind them. Like you see on the drawings of 'in case of loss of pressure' on an airplane. These have a better seal (to keep that oxygen in!) and can provide 100% FiO2 (a term we'll break down later). 

We usually use non-rebreathers during an emergency. If you're in a situation where you need 100% oxygen, you are doing very poorly. (Atmospheric oxygen concentrations are 21%, so you can see, us giving you 100% oxygen is a lot more than that.) It's also a bad idea to blast people with 100% oxygen for too long, it represses your respiratory drive to breathe (via a biochem thing I won't get into.). 

[As an aside, a respirator is like what people use to paint cars. It's not a mechanical implement, it's like a fancy gas mask.]

So if a 100% rebreather's not cutting it -- there's two possible reasons why:

1) Your airway is blocked. You could be drunk, passed out, a stroke victim, choking due to something, apnea -- at this stage we say you're 'not protecting your airway'. This makes sense because if the oxygen isn't getting into your lungs, it means there's a blockage somewhere, right?

2) Your airway is fine, but your lungs are damaged. 

Your lungs are tissue-paper-thin organs that inflate and deflate a kajillion times over the course of your life. Over time, they lose elasticity and compliance, especially as you age. You can go to the gym all you want to, but you can't really keep your lungs 'youthful' indefinitely. 

The functional unit of your lungs is an air sac called an alveoli. They're lined with a surfactant that keeps them open/inflatable. (It's the lack of this surfactant before a certain gestational week that makes life so dangerous/hard for preemies.) Inside your alveoli is where the magic happens -- because the entire job of your lungs is to serve as the intersection between the outside world's oxygen and your internal body's blood supply. Your alveoli are wrapped in miles of tiny capillaries, tissue against tissue, and that's where this oxygen transfer occurs. You breathe in (your normal 21% air in normal life), your alveoli inflate, and the tension between that inflation, sticking the lung tissue to the capillaries is when/where that oxygen jumps from the air into your blood supply, to be whisked around and used to eventually make energy. 

Does that make sense? I hope so! (Have I almost typed ravioli twenty times already? Yes.)

So! 

You need air to live! How's it gonna get in there if you can't breathe?

Sometimes just adding pressure helps. You probably know all about this because of your/your husband's/your mom's CPAP machine. That's what we call 'Non-Invasive Ventilation' -- because we're not using an 'invasive breathing tube'. The CPAP machine creates a seal (important to keep pressure up and air in!) around a person's face and shoves that normal 21% atmospheric oxygen in there to help get around whatever's blocking your airway. (This is why doctors are turning snorkel masks turning into CPAP machines on the internet.)

These are nice to use in the hospital if you can, because intubating someone (we're getting there) is highly invasive. However, with Covid patients, CPAP and their cousin BiPAP machines are dangerous because they've got exhalation valves that aerosolize the Covid and spray it out everywhere. So the patient's better off because yay breathing! But the health care workers helping them in their rooms have to be extra careful. 

We also have ‘High Flow’ machines that are like aggressive and thick nasal canulas, that are almost a BiPAP/CPAP intermediary, they’re two pronged, one to each nostril, but they jet air in at a strong rate and can be cranked up to 100% FiO2. These are usually what we use right now before intubation — we’ll see if someone can keep their oxygen saturation up this way, and especially say if the person has a DNI (do not intubate) order — this is going to be their last chance to stay alive.

But sometimes people's lungs need higher concentrations of oxygen, due to damage/age/pre-existing conditions like COPD, or they can't protect their airways because they're so exhausted from trying to breathe they're passed out, etc.

That's when we intubate you.

Intubation is when they put a breathing tube through your mouth down to where your lungs branch. How do we keep that air in there? Great question! There's an inflatable ring around the tube that we inflate once the tube is in position, allowing us to keep the higher concentrations of oxygen you need in there without leaking, and allowing us to give you oxygen at higher pressures to keep your lungs inflated.

Okay, you're tubed -- now what? 

Now, we put that tube to a ventilator. A ventilator is a machine that is capable of managing all sorts of factors that go into breathing for you, according to what the doctor's think will be most therapeutic for your lung conditions. (Respiratory therapists are some of the most unsung heroes during all of this -- they're going into rooms to assess and make changes all the time, they work hand in hand with RNs and MDs to make sure you get your air.) 

The most important things a ventilator can do are: 

1) Control your percent of oxygenation. (Higher means you're worse off, alas.)
2) Control the number of breaths you take (sometimes with the help of sedation, more shortly)
3) Control the pressure with which you receive your oxygen
4) Control the volume of oxygen that you're given (measured in mls, heh)

[As our imaginary patient was getting intubated, they were sedated prior to that -- and paralyzed so they wouldn't fight. Because getting a breathing tube put in you and then breathing through that tube (which I've heard is like breathing through a straw) is almost universally upsetting. So we typically keep people on a little sedation after the tube is in, just so they won't be freaked out or remember things as clearly.]

When you get sick from Covid, a lot of things happen to your body, but the worst of them is this: ARDS. Stands for Acute Respiratory Distress Syndrome. 

This is why NYC is so in need of ventilators, and how come the need for them is likely to wash across the nation as the (hopefully we've flattened it some I pray to god) wave hits. [AUTHORS NOTE FROM PRESENT TIMES: HAHAHAHAHAHAHAHA, OH FUCK, WE’RE SO FUCKED, LOLSOB…..]

ARDS is hard to survive under the best conditions. It's got wicked mortality rates that only increase as you get older (and lose that lung elasticity/compliance like I mentioned before.) 

ARDS gets you a couple different ways -- due to inflammation on a cellular level, the surfactant layer inside your alveoli breaks down so you can't transfer oxygen across to your capillaries. Your alveoli fill up with mucus, so you can't transport oxygen. And the final kicker is that that inflammation also causes the interstitial space between where your alveoli and your capillaries should touch to widen -- also preventing oxygen transfer. 

How a ventilator helps is this: 

We can give you a higher concentration of oxygen (while remembering that riding straight 100% for too long is bad). We can help you to breathe more effectively via controlling your breaths (and sedation). We can increase the pressure with which you receive your oxygen, helping to keep what functional alveoli you have working, and we can control the volume of oxygen we given you (also to keep the alveoli inflated.) 

I don't know if you saw those photos of ICU patients in Italy -- there's a reason they're all on their stomachs. You have most of your lung tissue against your back, because there are less other organs in the way. So when things are dire, we 'prone' you, flipping you over so that all the fluid in your lungs sloshes forward (and everyone watching panics briefly, waiting for your oxygen saturation numbers to hopefully come back up as your alveoli come back online). If you're proned, you're also paralyzed (pain control and neuromuscular blockade sedation) because it's uncomfortable, and if you're that badly off, we just need to take your lungs 'offline' so that the ventilator can do their job for them. We don't want you wasting any precious energy (literally precious, because oxygen = energy, remember?) on the work of breathing. We want to control all of that for you.  

We do that, and then we judiciously manage the ventilator to give you the greatest chance at survival by keeping the good parts of your lungs good, if we can. But everything has its limits, alas. 

We can't over inflate your lungs, they'll pop. We can't give you too much pressure, see = popping. 100% oxygenation can be bad long term, and it doesn't matter how many breaths-per-minute we give you if the lung tissue just isn't able to take the oxygen out of it, you know? 

Other challenges — you might remember from my (very cursing and angry) ICU Capacity post on here that we can’t keep people on ventilators indefinitely. Unfortunately, long term ventilator use has consequences — a breathing tube can cause erosion inside your throat and mouth (it’s a foreign object in there, after all) — and people who get tracheostomies (the breathing hole in your throat, which is normally part of our stepdown process for people on vents) can’t typically have high PEEP (the measure of the pressure we put in.)

So with these people on vents long term, if they don’t get better, we’re in between a rock and a hard place — keeping the breathing tube in is untenable, they can’t handle transitioning to a trach because they still require too high pressure of air to make their lungs function, and so they’re in limbo.

If you've come this far with me, you are in for the full science ride, but content warning death ahead -- this is why these people with covid are dying in the ICU. Because, like I said, ARDS is extremely dangerous under optimal conditions. How do you die of ARDS? Basically...you wind down due to lack of oxygen/ATP. And internally, you drown.

I wish I had a positive note to end this on, but (writing this part from the now of my couch) I’ve gotten four emails, two voicemails, and three texts asking me to come in in the past 24 hours. I’ve already signed up to work two sixteen hour shifts, one tomorrow, and one on Christmas. I can’t believe there’s another holiday here that’s gonna bone us — we’re not done clearing out patients from Thanksgiving….

I don’t really know what to say about things anymore, other than to ask you to stay home if at all possible and to wear a mask. Drowning in your own fluids is no way to go, and no way to take out other people with you.

— Cassie