DIP Episode 65 - Ventilator Physiology For The USMLEs (an overview)

Okay, welcome. My name is Ed Devine. I am a transitional year resident that's going to going into radiology. I'm welcome to episode 65 of the Divine Intervention Podcasts. This will probably be the shortest podcast on the website because all I'm going to talk about these vents as relevant to the US, USML is step one, two, and three exams. The vents later I'll probably have like a more extended discussion about this later. But really, if you understand what's discussed here, you should be good to go for pretty much all the vent questions you'll see on your exam. The thing I would say though is apologize for not making any podcasts in the last probably like two weeks. Currently on a nice zero rotation and it's been it's been quite the experience. It's been a wonderful experience, but just I literally had no time to make any podcasts. But this is something that I feel like it's just a simple thing I can do right off the cuff and just get it over with. So basically, the kinds of vent questions you should expect on your exams, right, should basically revolve around one of two problems, right? And I'm going to give you an easy way to analyze these things, right? So you can get either a problem with oxygen or you can get a problem with carbon dioxide. Okay, so you can either get an oxygen problem or CO2 problem. Whenever you have oxygen problems, well, that means you have issues with oxygenation. That's very hard to tie together. I'm just kidding.

But when you have problems with CO2, you have ventilation problems. Okay, and the thing is, under those circumstances, there are four parameters you can adjust to deal with those kinds of issues, right? So for example, the file say the four big parameters you probably want to know is one, the respiratory rate to the tidal volumes, three, the peep, and four, the FIO2. Okay, so you may want to adjust respiratory rates, tidal volumes, peep and FIO2. Whenever you adjust respiratory rates and tidal volumes, you can deal with your CO2 problem. Whenever you can, whenever you have problems with oxygenation, you adjust your FIO2 or your peep. Okay, and the good thing about all of this is these things, micological sense. So let's talk about the ventilation problems first, CO2. Okay, so if for example, right, they give you a question stem about a patient that has asthma, right? One classic scenario they love to put on examples is they'll talk about a patient that has asthma, right? And then they may show you like his CO2 tension, like three hours ago, and he's CO2 tension now. And you may see that the CO2 tension is rising. Whenever you see that, that's an indication for intubation, right? Because the thing is, if a person has asthma, usually those people hyperventilate, so their CO2 tension should be low, right? They should be hypokapnic.

But if you notice that the CO2 tension is beginning to rise in an asthmatic, it means that they're beginning to the irrespectorial muscles are beginning to tire out. And under those circumstances, you should the next step in management, it will always be the correct answer is to go ahead and into be that patient. And then just one real quick thing again, before I talk about the CO2 issues, one other classic exam question is to try to give you, if actually there will be two things that are going to here. So I'm sorry, I'm sort of going off on these targets, but they're high old things that pop up often or not, if I think I should talk about them, right? So the another classic one is a patient that's having a PE, right? So the thing is usually patients that are having P Es, right? Again, they tend to, because if you're trying to catch your breath, right, you sort of try to hyperventilate, okay? So the patient is having a PE, you should expect hypokapnea, not hyper capnea, okay? You should expect the low CO2 tension. And then if a patient is a chronic COPD, right? So if they have like chronic COPD, I mean, COPD is obviously a chronic disease, those people you should expect a high CO2 tension, because those people are chronic CO2 retainers, right? So again, that's a classic exam question. Then just give you a run of the male patient with COPD. And hopefully you should expect that with your CO2 being high, the kidneys will try to respond by giving rise to metabolic alkalosis.

So they are by-carb will be high, okay? Do not be surprised when you see, you just see like scenarios like this, and you see them as, as a, a, BG values or pH values that you need to interpret on the exam. Okay, so back to vents, right? So the thing is, if a patient has CO2 retention, if they're hypercapnic, you can deal with that problem by increasing the respiratory rate on the ventilator, because if you increase the respiratory rate, right, you'll breathe faster and blow off that CO2, okay? Alternatively, if a patient is retaining CO2, one other way you can also deal with it is to increase their tidal volumes. If you increase tidal volumes, you can get rid of that CO2 problem. And then basically you just do the reverse of all those things. If a person is hypercapnic, right? So if your CO2 is too low, you can try to sort of speed that processor, you can try to retain CO2 by turning down the respiratory rate or turning down the tidal volumes, okay? And again, before I jump to the oxygenation problem, another high yield, and again, sorry, I honestly did not plan for any of this. This is just coming to mind as I'm talking about these things. But another high CO2 scenario is a person that goes to a higher elevation, right? So let's say you travel to Colorado, or you go to the top of a mountain, for instance, when you go to higher elevations, you become hypoxic. And the reason you become hypoxic, right?

Is because your oxygen tension, remember, as you go higher in the air, as a human being, you're supporting less weight of air, it is like, oh, if you at the bottom of a rock, and you're carrying that rock on your back, you'll feel a lot more pressure than if you're on the top of the rock, right? You're not carrying as much of the rock on your back if you may. So the same thing happens as you go to higher elevations, atmospheric pressure goes down. So it lets us you meet 760 millimeters of mercury at sea level, or ground level, or whatever, when you go to higher elevations, it becomes a smaller number, see like 750 millimeters of mercury, right? The thing is the FI U2 does not change. It will still be 21% of oxygen in the atmosphere, but 21% of a smaller number means that the oxygen that's been delivered to you of your life goes down, okay? And whenever you have those situations, your body tries to compensate by hyperventilating, right? Because if you hyperventilate, you can bring in more oxygen. But the thing is as you hyperventilate, you blow off CO2, okay? So those people will be hypocapnic, and again, over time, the kidneys can respond by, because if you are hypocapnic, right, you develop a respiratory alkalosis because you're blown off CO2. So your kidneys try to respond by causing, um, by, um, getting rid of bycarb, right? So that you can create a metabolic acidosis to counterbalance that, okay?

Remember, you can speed up that process with a carbonic and hydrism inhibitor, like acetylzolomide. Okay. So, and then please also do not forget, right? The deflapition overduces on, on a aspirin, right? It increases your respiratory rate, right? So they get a respiratory alkalosis, but they also get a metabolic acidosis with that, right? Because aspirin is broken down to acidol, salicylic acid, right? Okay. Now, if you, um, if you've dealt with your carbon dioxide problem, let's talk about how you deal with oxygenation problems. This is kind of easy, right? So if a patient has an oxygenation problem, you can basically deal with it in one of two ways, right? So one way you can deal with it is to increase peep, right? Because the thing is, by increasing peep, you basically keep our viola open, right? You prevent them from collapsing, right? So you keep our viola open, right? And that promotes gas exchange, right? promotes gas exchange, because you're almost sort of like recruiting more, um, our viola to help with a gas exchange. So that's when you can increase oxygenation. Um, and that's commonly what's done in ARDS, right? Remember ARDS, very, very high yield, one of the key diagnostic criteria, you sort of want to keep at the back of your mind for tests is that, um, the pulmonary capillary wedge pressure will be less than 18, right? So ARDS is an example of, um, uh, it's an example of a non cardiogenic pulmonary edema, okay?

A non cardiogenic pulmonary edema, very high yield to know that, right? Because again, if you look at the lungs on imaging, you see a ton of fluid, but that fluid is not coming because it's not building up because the heart is not functional, right? If a person had a cardiogenic cause of pulmonary edema, the pulmonary capillary wedge pressure will be more than 18, but in ARDS, again, super high yield to remember that number, the pulmonary capillary wedge pressure is less than 18. And then another high yield thing you want to keep at the back of your mind is if you want to improve oxygenation, well, you can just crank up the oxygen, right? You literally just increase the FIO to, right? Instead of going from like 21% oxygen to 100% oxygen, I mean, instead of being a 21% oxygen, if you bump up to 100% oxygen, well, obviously, they'll deal with your oxygenation, they'll deal with your oxygenation problem. Again, super, super high yield to know that. And another thing, I guess I would like to touch on is remember, and again, I'll have a USML step one review that focuses solely on pulmonology, like I already have for biochemistry and reno. But one thing I think you just probably want to know is that if a patient is getting a 100% oxygen right, and that's not improving their oxygen status, right? That tells you right of the bad that, okay, this person likely has a shunt, right?

And in an example, and basically right, shunt physiology operates whenever you have ventilation of a given part of the lung without perfusion, right? So if for example, a person has a PFO, a P10 for immunovall, right? Blood is going from the right side of the heart to the left side of the heart. So it's bypassing the lungs. So it's not getting oxygenated, right? So again, but again, it's usually hard to get a complete shunt. Most times people have like partial shunt, okay? So hopefully you found this podcast to be helpful, like really to be perfectly honest, if you know this, you know, you probably know like 99.9% of what you potentially see on a US Emily exam relating to ventilators. I don't mean if you have like an ICU shelf exam, this is obviously not going to cut all of it. And again, I'll have a discussion on ventilators, but that's something that has to be a little more carefully planned. And I have a lot of good stuff coming down the pike in January. I'll be off my ICU rotation then. So I should have a considerably more time to make sort of makeup for this time that I've been on a long stretch of a ICU. So to round this up, I just want to highlight that I offer tutoring for many exams that people take. So I offer tutoring for the US Emily step one, step two CK and step three exams. And also the US Emily step two CS exams, right? And I've had a tremendous success.

Thankfully, again, I'm not saying this with any elements of pride, but with all utmost humility, I've had a lot of success tutoring hundreds and thousands of people actually for these exams that have ended up doing pretty well. And then I also offer tutoring for the medicine in training exam. That's something that's more tuned to residents. And I also tutor to the MCAT, but mostly in the realm of organic chemistry. I'll say like college courses, I tutor mostly organic chemistry and physics. So if you have anyone that's taking any of those exams, please feel free to send them my way. I also prepare people for interviews and I prepare like eras and med school applications. I do like mock interviews and all that stuff. I mean, I've had many opportunities to be intimately involved with the admissions processes at medical schools. So I am very, I'm very well aware of how that process goes. And I have helped again, tons of people with like consulting and advising on other circumstances. So I wish all the best. I hope you have a wonderful Sunday. I will be spending my Sunday in the ICU. And I will see you in the next podcast, episode 66. God bless and good night. Thank you.

Oxygenation problems or $\text{CO}_2$ (ventilation) problems.

Increasing PEEP (to keep alveoli open) or increasing $\text{FiO}_2$.

Hypocapnia (low $\text{PCO}_2$), because they tend to hyperventilate.

High $\text{PaCO}_2$ (hypercapnic) and Metabolic Alkalosis (due to renal compensation).

The kidneys excrete bicarbonate ($\text{HCO}_3^-$), leading to metabolic acidosis.

PCWP $< 18$ mm Hg.

ARDS typically presents with a PCWP less than 18 mm Hg (non-cardiogenic).

Decreased atmospheric pressure leads to lower $\text{PO}_2$, causing compensatory hyperventilation.

It keeps alveoli open (recruits collapsed units), promoting gas exchange.

They will cause metabolic alkalosis by retaining bicarbonate ($\text{HCO}_3^-$).

Respiratory alkalosis (due to increased RR) AND Metabolic acidosis (because aspirin metabolizes into salicylic acid/acidol).

A shunt (ventilation without perfusion).

ARDS typically presents with a PCWP less than 18 mm Hg (non-cardiogenic).

Decreased atmospheric pressure leads to lower $\text{PO}_2$, causing compensatory hyperventilation.

It keeps alveoli open (recruits collapsed units), promoting gas exchange.

They will cause metabolic alkalosis by retaining bicarbonate ($\text{HCO}_3^-$).

Respiratory alkalosis (due to increased RR) AND Metabolic acidosis (because aspirin metabolizes into salicylic acid/acidol).

A shunt (ventilation without perfusion).