Mr. Hamilton Takes Over
The long-promised introduction to mechanical ventilation
This is a followup to Fun with Balloons from … uh, a month ago. Took a while to get up the gumption, sorry.
The patient’s ABG was bad. They’ve been emergently intubated. Now they’re on the ventilator. What kind of settings should they be on? The ventilator comes with a number of preset modes to choose from, each of which will have a variety of individual settings you can tweak. Before I get into those modes and settings, though, I’m going to start off with some more basics1.
The ventilator itself is a computerized box connected to (usually) a dual-limb circuit—that is, there’s a long floppy loop of corrugated plastic tubing that comes out of one part of the ventilator, circles around by the patient’s mouth where it connects to the ET tube, and goes back into a different part of the ventilator where the built-in expiratory cassette hopefully filters out the worst of the germs from the patient’s exhalations2. The ventilator’s job, from its own point of view, is to continuously blow through this loop of plastic, maintaining a basic level of pressure inside it, occasionally sending through pulses of higher pressure and measuring what effects they have on the whole system. It then comes to conclusions about what the patient is doing based on those measurements, tries to compensate for that, and occasionally3 sounds alarms.
Let’s start with that basic, background level of flow and pressure that it always keeps going. In a ventilator, that baseline pressure is known as PEEP4, positive end-expiratory pressure. It’s normal for your lungs to go back to slightly higher than atmospheric pressure, because they’re exhaling against the bottleneck of the vocal cords which creates some backwash and turbulence. PEEP is a very useful thing, since it encourages the alveoli in your lungs to stay open. Raising the PEEP is one of two major ways, along with raising the FiO25, to improve oxygenation.
But PEEP is just the baseline. To make them actually breathe, ventilators have a small number of variables they can manipulate: pressure, flow, volume, and time6. A given increase flow will generate a given increase in pressure, delivering a given volume depending on the compliance7 of the patient’s lungs. The vent can blow a little harder until it has reached a certain level of flow (in milliliters of air per second), pressure (like PEEP, this is in centimeters of H2O, the same unit used to measure car tire pressure), or volume (in mL which it believes the patient has inhaled). Or it can cut off blowing after a given time. That said, I have never seen “flow-trigger” ventilation at work, nor time. We essentially always tell the vent to blow until it reaches a given pressure or a given volume, then stop.
Pressure and volume being so closely linked, even those are pretty close in practice; you can control for pressure or for volume, but whether you choose one or the other is a question of what you’re prioritizing. If you’re nervous about pressure getting too high and damaging the lungs, you control for pressure; otherwise, most of the time, you control for volume. But you can look at a patient on volume control, and see that their peak pressure is 28 with a PEEP of 5 to get a volume of about 500 mL. Switch to pressure control of 23 above PEEP, and you should still see them taking breaths of about 500 mL, because their lung compliance hasn’t changed. If the pressure’s making you nervous, you also have the option to reduce tidal volumes8, which will reduce pressure. You might drop it to 450 mL and see peak pressures drop to, oh, 24 or 259. Switch to pressure control of 19 above PEEP and you’ll see volumes clustering around 450.
Volume control is the norm, because you want the patient to breathe a certain amount, and pressure is just a hazard to manage. Pressure control’s main value, the way I see it, is as a hedge against sudden reduction in compliance. Anyway most vents these days have something like PRVC, pressure-regulated volume control—that’s what my hospital’s Servo vents call it, but every vent brand has its own trademarked name for the spiffier modes. We put almost everyone on PRVC, which will aim for a target volume but stop if the pressure nears a limit set by the vent’s alarm parameters. That is to say, if you tell the vent to sound the alarm when peak pressures hit 40, the vent might notice pressure getting to 35 or so and cut the patient off. Raise the limit to 45 and it might let them get closer to 40. Switching to pressure control from there is … yeah, there’s honestly something kind of fetishistic about that, to me. Tomato, tomahto. I picture a lawyer frowning approvingly when we do it.
Very well, you say. But how do I know what volume to aim for in the first place? That’s based on ideal body weight10, which is calculated based on the patient’s height and, to a limited degree, their sex. There are all sorts of formulae out there for calculating IBW, so you can pick one. My hospital spoils me and autocalculates IBW for me, with the result that I’ve forgotten how to do it myself. In practice you can often eyeball the patient and say, “eh, she’s a little thing, let’s go for a VT of 400 to start.” You’ll draw another ABG after an hour or so on the vent, and adjust based on that.
Generally, you want a volume of 6-8 mL per kg of IBW. My IBW as a six-foot man is, per some random internet site, about 78 kg. Sure, let’s go with that. So you might start me at 500 or 550 mL. Please note that my weight has nothing to do with it; if my actual weight is 250 pounds, or even 350, I should still only get between five and six hundred CCs of air per breath. In fact, you might even go for lower volumes, because overweight patients are prone to higher pressures. But every now and then you’ll have a doctor or nurse practitioner who swears you can’t give that huge patient such a tiny volume. Yes, you can! The brain and vital organs get first dibs anyway; nobody cares if adipose tissue goes hypoxic. At least, I don’t. Is that even a thing? Hypoxic fat injury? Hell if I know.
Ahem. That’s volume settled. What about rate? Normal human respiratory rate is 12-20 per minute, hence the famous RT joke that every non-ventilated patient in the hospital breathes sixteen times a minute on every visit—it’s hard to count breaths without staring and making them breathe funny, so if you’re not breathing fast many of us will just chuck a 16 or 18 down in our charting as shorthand for “yeah, they’re breathing normal.” So a standard hypothetical PRVC vent setting might be 500 tidal volume, rate of sixteen, PEEP of five, and … oh, 40% FiO2. In practice you just titer that until the patient’s sat is something you can live with.
But say I’m on the vent for hypercapneic respiratory failure—that is, I have far too much CO2 in me? We want to blow that off faster, so let’s get me breathing faster than normal. Rate of 20 or 24. No problems there, but if you go much faster, you’re going to start running into problems. Can you see why? One of the variables the vent has to manage is time, and exhalation is passive. The vent can’t suck; it can blow and blow harder and then stop blowing so hard, and trust that you’re matching it on the other end. But exhalation takes longer than inhalation, and the patient is presumably unconscious, maybe paralyzed. If you try to get the patient breathing thirty-five times a minute passively, they won’t have time to exhale completely, and exhalation is how the CO2 gets out! You’ll get air-trapping, which defeats the whole purpose. Now, you can adjust the amount of time the vent allows for inhalation to buy yourself room, and you have to with higher rates, but there’s a limit there. And, as we established in Fun with Balloons, it’s better to breathe a few times deeply than to pant like a dog, on account of dead space.
Physics dictates the limited realm of possibilities: you can breathe deeper until the pressure gets too high, and faster until you don’t have time to exhale anymore. You can raise the PEEP to help with oxygenation—volume and rate mostly have an effect on ventilating CO2—but again, pressure. In a sense, oxygenation and ventilation are in competition, so if you have issues with both, you have to decide whether PEEP or peak inspiratory pressure gets a bigger slice of the pie. Adjusting the inhalation time likewise biases for ventilation or oxygenation; lower i-time means more time to exhale and remove CO2, but less time for the fresh oxygen to cross the barrier into the blood11.
This is a full-sized post now, I’d say, and I’ve really only scratched the surface here. I can go into more detail later. For now, we’re going to leave our hypothetical patient sedated and paralyzed on the ventilator, a passive object for the machine to manipulate on bog-standard PRVC. That’s the beginning. Our next problem: getting them off it. Which, I think, will be the subject of the next post in this series. Whenever that happens …
We’ve had one set of basics, yes. What about say-cond basics?
A “single-limb” circuit, typically found on BIPAPs, just goes straight to the mask, with a little hole near the end for exhalation to vent into the room’s air.
By which I mean “quite frequently”
That same basic concept of “baseline pressure” can also be known as EPAP (expiratory positive airway pressure) in a BIPAP, or CPAP (continuous positive airway pressure) for a … CPAP machine, of the kind used by sleep apnea patients. This is confusing but it works for us. I like to hope that these little touches of obfuscation help intimidate nurses into not touching our stuff when they don’t know what they’re doing.
Fraction of inspired oxygen. 21% to 100%.
The last variable is of course FiO2, which is so simple and straightforward that it’s boring. The only question there is whether the vent needs hoses for air and O2 or if it has an internal turbine to suck in room air. The process of mixing 21% oxygen from room air with 100% oxygen from the wall or tank to achieve a given FiO2, well, that involves some math, but the vent does it invisibly and effortlessly so we don’t need to care about it now.
Which rounds to “flexibility,” if you forgot from last month’s post.
I don’t believe we’ve used the term tidal volume (abbreviated VT) yet; it just means the volume of a typical breath, without deliberate forceful inhalation or exhalation.
Note that none of these values are constant between breaths. Whether you’re controlling for pressure or volume, any one breath might be 20% higher or lower than the target just because of all the variables like secretions, what the patient is doing, their position in the bed, whatever. Since every breath is aiming for the same target, they all cluster around the same average point and it works out fine. Patient is on a volume of 500 and they get 463, 508, 477, 520 …
IBW is “ideal” for the purposes of making the math work, in the same way physicists like frictionless vacuums. If you actually weighed your ideal body weight you’d be rather skinny and possibly not that attractive depending on how much muscle/curvature you’re going for. I just checked and I weight less than my IBW, but I’m rail-thin with negligible musculature. Back in respiratory school my prof introduced the concept with a disclaimer about how it’s just a name. One of the girls, who was fairly thin, still got indignant at the math calling her a fatty.
Note, however, that i-time is somewhat deep magic, a little trick you can fiddle with to eke out a tiny edge one way or another. At shift change we say “patient is a 7.5 at 24, PRVC 500, rate of sixteen, PEEP of 5, 40%.” Rate, PEEP, FiO2, and volume or pressure are the big four.