Fun with Balloons!

Baby's first pulmonary mechanics

At some point—now that I’ve done things a bit bass-ackwards and covered trachs first—I should probably have a post about mechanical ventilation proper. But I’m still going to be a wee bit non-traditional, and start by describing how lungs work in the first place.

Your mouth connects to your pharynx¹ which connects to your trachea by way of your vocal cords, which are protected by a flap of cartilage called the epiglottis that closes every time you swallow. The trachea goes down to your lungs and splits into two bronchi at the carina. The two bronchi split again and again before terminating in these wrinkly bags of membranes and blood vessels (they look kind of like clusters of grapes) called alveoli. All that structure is supported, within the lungs, by a network of elastic fibers known as the parenchyma. The lungs are covered by a pleural membrane which anchors to the rib cage.²

Breathing is accomplished by the muscle known as the diaphragm, which lies at the base of both lungs. When it pulls down, the lungs and consequently the chest enlarge, which means they have lower pressure inside relative to the outside atmosphere, which means air rushes in from the trachea to equalize the pressure. Gas exchange occurs at the alveoli—oxygen in, CO2 out—and then whoooosh, the diaphragm relaxes and the lungs get smaller again. Higher pressure, everything reverses, air goes out. Note that the process of exhalation is normally passive; you can use various accessory muscles to force air out³, but normal quiet breaths are driven by your lungs’ natural elastic recoil, which in turn is driven almost entirely by the surface tension of water⁴.

Now you know all about how breathing is supposed to work. But when it matters most, some of what I just told you becomes kind of irrelevant. I’ve been describing negative-pressure ventilation, the way your body naturally works. Doctors used to employ negative-pressure ventilation via the famous “iron lung” used on polio patients. This had the advantage of allowing the patient to talk normally, and the considerable disadvantage of rendering the chest completely inaccessible behind a rigid metal breastplate. Want to listen to their heart or lungs, wash their torso, or do CPR since they just went into arrest? Gotta stop and open up the thing that helps them breathe first!

That’s why we’ve switched to positive-pressure ventilation, where a machine connects to a set of flexible plastic tubes, which connects to a narrower tube running through your mouth and vocal cords into your lower trachea just above the carina (the spot where the trachea diverges). The patient can still initiate breaths (depending on how you program the ventilator and how deeply you sedate them), but the ventilator is pushing air in, not helping them pull it.

So let’s stop talking about lungs, and start talking about balloons, because your lungs can be modeled as a pair of balloons, or even one balloon, stuck on the end of a straw and inflated by a man we’ll call Mr. Hamilton.⁵

You can think of the straw as the endotracheal tube, or as the trachea the tube runs through, or as the trachea plus all the bronchi and bronchioles branching off of it. Whatever it is, it’s what we RTs call “dead space.” It doesn’t participate in gas exchange, but it does use up some of the volume from every breath. Dead space is a kind of flat tax on breaths, which is why a few deep breaths are better than a bunch of shallow ones; if there’s a $20 income tax on every paycheck, you’re better off earning one thousand-dollar paycheck than ten hundreds.

Anyway, Mr. Hamilton is waiting for his orders. Blow in that straw, Mr. Hamilton! Watch him blow, folks. You see how the balloon inflates slowly at first? The air has to go through the narrow straw, and it doesn’t organize itself into a smooth stream right away. The molecules are all tumbling off each other and the sides of the straw. As he keeps blowing to overcome this resistance, the balloon inflates, then stays in place at one size.

Think about the pressures inside the balloon. They’ll be highest just before it settles down into that final, stable state, because the airflow is still turbulent, all the molecules jostling every which way. To an RT, that’s “peak inspiratory pressure,” or just “peak pressure.” The pressure just after, while Mr. Hamilton is forcing the balloon to stay inflated, is “plateau pressure.” If you calculate both peak and plateau pressure, the difference between them is defined as airway resistance. Resistance is caused by drag, turbulent flow, or (very frequently, in real lungs) secretions, AKA mucus.

The patient needs to exhale too, so Mr. Hamilton lets go, and the air goes pbbbt’ing out. As with real lungs, it’s an entirely passive process, though you could squeeze out the balloon if you wanted it to go faster. Normally, however, you just let it go, and exhalation is caused by the balloon’s elastance, as we call it. Elastance is the measurement of the balloon’s tendency to deflate. Its inverse or reciprocal is compliance (which we talk about rather more often in RT-land). Compliance is the measurement of how many units of pressure are needed to achieve a given volume inside the balloon. But elastance and compliance are two different ways of expressing the same thing: how stiff is this balloon? How hard am I going to have to blow to inflate it, and how hard will it push out when I let go?

In RT school we go over other values too—the little bit of air that stays inside the balloon after you let go and it’s done deflating, for example, might represent your lungs’ “residual volume.”⁶ These are most relevant, professionally, for RTs who work in pulmonary function testing, a thing I’ve never done. PFT is an important diagnostic tool, which is why I mention it, but when I’m at work I don’t tend to worry much about a lot of those little terms, and you probably shouldn’t either⁷.

Do remember compliance, though—that’s a critical factor⁸. Mechanical ventilation is easier to understand if you first grasp that there’s a close relationship between pressure and volume. So much pressure will yield so much volume, period. If the patient is obese, you need more pressure to get more volume, because the weight of the chest presses down. If the patient has icky pneumonia, all that snot will either slow down the air or block off sections of the airway; either way, effective compliance goes down. If the patient has ARDS—the rigid-lung condition we saw in so many COVID cases a few years back—compliance goes WAY down and whoa nelly is it dangerous. Sometimes, it’s simply not practical to inflate a set of lungs above a given volume, regardless of pressure. And sometimes, if the pressure gets high enough, your balloon pops⁹.

Whatever the problem is, a mechanical ventilator has a few variables to manipulate (pressure, volume, flow, time), and managing them deftly is the way we get patients stable and hopefully off the ventilator again. That’s it for now—but I think we’ll be seeing Mr. Hamilton again next time.

1. That awkward no-man’s-land between your mouth, nose, and throat.

2. I should probably have illustrations for all this stuff, but I’m paranoid about navigating the legalities of borrowing somebody else’s pictures for this Substack, which I am running for free but might eventually, I guess, try a subscription model for? This author is simply too lazy (at the present time) to bother over the nuisance of copyright law and figuring out attributions for creative commons pics from Wikipedia etc. That may change later as I get accustomed to Substacking. Sorry.

3. For birthday candles, recovering from exertion, compensating for advanced emphysema, what-have-you.

4. Huh? Yep, water. Your airways are lined by a thin layer of liquid secretions, and as seen in raindrops on car windshields, or the bulging meniscus atop a totally full glass, water likes to stick to itself. Without something holding it all open, all that water naturally wants to draw itself closer together, and it yanks your lungs along with it. In fact, your body produces surfactant (a soaplike substance) to decrease the surface tension of the fluid so the recoil force won’t be too strong to handle. Premature babies might not have lung surfactant yet, which is why they usually wind up on the ventilator. Sometimes we give them artificial surfactant to compensate. This may be the last time you hear a baby-related fact from me, because I don’t do pediatrics, neonatal, or any other such cursed subject. Babies are creepy, disturbingly fragile things that go blind if you give them too much oxygen (really) and they’re surrounded by hyper-hormonal nurses and an ominous cloud of heightened legal liability and dysfunctional mamas with drug habits popping out preemie after preemie and UGH. No thank you, I’ll stick to adults, whose horrible medical problems are typically either the result of normal old age or their own bad decisions.

5. He could also be Mr. Draeger, Mr. Servo, Mr. Puritan-Bennett, or any of a number of other fine mechanical ventilator brands, but I’ve never worked at a facility rich enough to run a fleet of Hamilton vents and I want to feel fancy.

6. Provided you tried to squeeze out the expiratory reserve volume first. The residual volume is just the tiny little bit that can’t be squeezed out, but still technically counts towards total lung capacity, etc. If you’re bored already, don’t worry, this is all I’m going to say about that. Like, ever.

7. Until some other nerd starts pulmonaryFUN!ctiontesting.substack.com, that is.

8. In fact, we tend to think of all lung diseases in terms of compliance; at the broadest and most abstract level, problems are classified as restrictive (compliance too low, balloon too stiff, can’t get fresh air in), or obstructive (compliance too high, balloon too floppy, can’t get the stale air out—as in Chronic Obstructive Pulmonary Disease). These are, again, very broad generalities, and potentially misleading if you take them literally, but you can think of them that way.

9. Because the balloon is inside a cage of meat and bone, it will only have a hole in its exterior, leading to the surrounding pleural space I mentioned near the start of this post. This is known as a pneumothorax, or sometimes a “pneumo” for short, though laymen sometimes call it a “collapsed lung.”