Transcript for:
Respiratory Management in Anesthesia

respiratory rate alone. You really have to look at the ventilatory quality. Okay, and this is one of those ones where I have to get numbers involved. So if you took two patients who are totally the same, and one has rapid shallow respiration, the other has slow deep respirations, and we just count, and there's a couple things I'm going to just have to go over a little bit, dead space, but if we just counted respiratory rate, I'm pretty sure most of you would say that the patient that's breathing rapidly, you'd say, well, he's breathing better, right?

He's breathing 30 breaths per minute. How could he be hypoventilating? Right? No? Good.

Some of you know this concept already. You don't know necessarily because one of the things that you guys have to realize is when we take a breath, part of the breath that we take is wasted ventilation. Right? It ventilates dead space.

And I'm talking about the dead space in our bodies. So everything from my lips down to my bronchioles where gas exchange occurs, that's considered dead space. Physiological, anatomical dead space.

And that's fixed. It doesn't change. It doesn't matter how big my breath is. or how small my breath is, that's fixed.

So that means every breath I take, and I'm just using these numbers as estimates, but every breath that this patient takes, 150 mils of that is wasted ventilation. It's just ventilating dead space. It's not contributing to gas exchange at all. And now if we look at these two patients and this guy that's breathing rapidly and shallow, he's doing 300 mils per breath. The slow deep patient is doing 600 mils per breath.

If we actually multiply that through and calculate out the minute ventilation, which is the amount of air that we exchange in a minute, they're exactly the same, right? So shouldn't the CO2 or the gas exchange be exactly the same? No, because if you actually go one step further and look at alveolar ventilation, how much ventilation actually went to the alveoli with each breath, you have to get rid of that 150 dead space because it didn't contribute to it, right? So this patient, every time he takes a breath, 150 mL gets down to the alveoli, whereas this guy 450 mL gets down to the alveoli.

And this is what ultimately determines whether your patient is exchanging CO2 with the environment. And you can see now that this patient with the slow deep respirations, he's actually ventilating much much better than this patient who's breathing rapidly and shallow. So respiratory rate alone doesn't tell you the whole picture.

You also have to look at the quality of respiration, the depth of respiration. And that's one of the reasons that we choose an appropriate size rebreathing bag, right? It's not just so, you know, the patient has a reservoir to breathe from.

Part of it helps us estimate the tidal volume that the patient's taken, right? Because you can use, like honestly, and this is what I tell the techs, is I'm like, yeah, I don't care. You can use a 5-liter rebreathing bag on a 1-kilo dog if you want.

You're not going to see the bag move at all when the patient takes a breath. You can do it. It's not going to hurt the patient, but you're not going to be able to estimate tidal volume from it. You're much further ahead to use a half liter bag or something.

So that's why we do that. As far as what we define hypoventilation as, really, again, you can only define hypoventilation, true hypoventilation, if you're measuring CO2. You cannot tell from just respirations alone.

You can get an assessment. You can make guesses. But unless you're measuring end tidal CO2 or arterial CO2, it's really hard to say for sure that your patient's hypoventilating.

And I usually consider a CO2 of greater than 60 to be clinically significant hypoventilation. And we measure end-tidal CO2 in all our patients. And normal end-tidal CO2 should be around 35 to 45 in an awake patient. But in an anesthetized patient, it would be exceedingly rare to have an end-tidal CO2 that low.

It's usually hanging around 50 to 55. The only patients where we'll sometimes see it down at around awake values are those patients with epidurals who are exceptionally light during anesthesia because we really are just... having them sleep. They're not really just the heads asleep and the anesthesia is being controlled through the epidural. Hypoventilation can lead to acidemia, which will then lead to potentially arrhythmias. It can cause tachycardia.

It can lead to peripheral vasodilation, which then it may make your patient a little bit hypotensive. You don't get as good an anesthetic uptake because you're not getting as much gas down to the alveoli. And generally what I find, even if you don't have a capnograph, If I have a patient and someone is struggling with it or things aren't making sense, the animal's tachycardic, hypotensive, breathing a little bit funny, not staying in an appropriate depth of anesthesia, sometimes I'll put a capnograph on and it's like, oh wow, your CO2's way up.

And then we bring the CO2 back into a normal range and suddenly the patient just becomes a whole different patient. And so how do you bring CO2 back into the normal range? Whoops, I'm going the wrong way. Let's go this way.

Oh, there's the causes. I'm not going to go through those causes. The biggest cause during anesthesia, you guys, is the CNS depression.

All these are causes for hypoventilation that would be more likely in an awake patient, other than maybe an obstruction. But it's usually just inhalant-induced depression, CNS depression, that's responsible for the hypoventilation. Treatment. IPPV, bag the patient, that's easy. Put it on a ventilator if you have one.

So it's very easy to treat hypoventilation. But again, you want to look at the reason for the hypoventilation. If the patient's too deep, lighten them up, get them to where they need to be.

A lot of anesthesia is about making sure your patient's at the appropriate depth of anesthesia and also providing good analgesia so you're not using just an inhalant to maintain the anesthesia because I can guarantee you your anesthetic plane is going to do one of these if it's just an inhalant versus an inhalant, local anesthetic techniques, analgesics and sedatives and so forth. Another thing that happens very commonly, and this happens usually with new techs, is what I call pseudo-hypoventilation. And it's a situation that happens right after you induce the patient. So usually what happens is I'll have a tech and they're induced the patient and the patient doesn't breathe. And so immediately, oh my god he's too deep let me lighten him up.

Oh my god he's awake. Oh my god now I just gave him more induction drug and now he's apneic again and now you know. and it's a seesaw.

One of the things that happens very... Like, what stimulates you to breathe? CO2 or hypoxia or some kind of physiological event happening in your brain where your brain's like, take a breath or don't breathe or whatever, right?

So it's normally CO2. Well, what do you do at the beginning of your anesthesia usually? You bag the crap out of them. is what you do, right?

Because what we do at our practice is we bag them to check for leaks in the cuff, right? So someone's listening, someone's bagging, we're listening for a leak, we inflate until we don't hear the leak. Well, you just gave them a big breath. And then we also auscultate both sides of the chest, bagging, big breath, big breath. Well, guess what?

We just took that patient's CO2. It was at, I don't know, 45 when we induced him, and now we brought it down to 30. Well, yeah, he's not going to take a breath for a while at all. And if he doesn't take a breath, he's not going to stay anesthetized because soon your injectable drug is going to wear off, and you don't have any inhalant on board, and they're awake. So this pseudo-hypoventilation happens pretty commonly initially for people. And I'm the big person and advocate of be patient, relax, let the patient do its work.

Don't you do the work for the patient. But you do have to give them some breaths during this period. Because yes, we bagged them too much and now the CO2 is lower.

But we also have to give them some breaths not only to maintain oxygenation, but also to get the inhalant into the patient so that they stay anesthetized, right? So this is a very common thing to happen and it's always fun. Hypoxia, hopefully you... never have this happening during your anesthesia, it's really, really rare to have hypoxia unless you have underlying lung disease during anesthesia.

And even if you have underlying lung disease, it's still unusual to have hypoxia because we use 100% oxygen. So we super oxygenate our patients, right? I mean, right now you and I are sitting in this room breathing 21% air and none of us are hypoxic, nor should we be.

And if I hold my breath for 30 seconds, I still won't be hypoxic. And if I could hold my breath for two seconds, I still wouldn't be hypoxic. two minutes I still wouldn't be hypoxic.

It takes a long time to become hypoxic even on room air, okay? But when you put them on 100% oxygen it takes even longer to become hypoxic because now you're super oxygenating them and you can overcome a lot of the causes of hypoxia. So hypoxia is very very rare during anesthesia and if it does happen to you there's probably something very very wrong happening. And these are the physiological reasons for hypoxia which I'm not going to go through in a lot of detail because probably the one one that's most likely to cause hypoxia in a patient getting 100% oxygen during anesthesia is it's not getting oxygen is the number one cause. Low inspired O2.

So the biggest cause for that is a failed intubation. That is probably the number one cause of hypoxia during anesthesia is a failed intubation. And a lot of times that happens because we're rushing to get the tube in because the patient's a little light. We don't use a laryngoscope in experience. We don't check to ensure that the tube is actually in place by esculpting lungs.

sounds on both sides of the chest or using a capnograph, something like that. An inadvertent extubation can happen because a lot of times we move patients from machine to machine, from place to place in the hospital. And I've definitely had calls from referring vets and just vets in general asking questions where a patient gets extubated, someone shoves the tube back in, no one really says anything or recognizes it. Next thing you know, they're in the next room. Now the patient's blue.

And then it's like, okay. And then I asked you. usually a few questions, I'm like, did you bag the patient?

Yup, was the machine on? Yup, was it connected? Yup, did you see the chest move? Yup. Was your oxygen on?

Yup. And they tell me yup yup yup yup yup to everything. I'm like it wasn't intubated.

There's no way that patient was intubated. If you bag it, your machine's on and the chest moves There's no way it could be blue. And I also asked, did it have a heart rate? If it had a heart rate, right?

It really can't be blue. So usually, if you bag a patient and they don't respond to you, they've either got a primary lung disease that's really significant that we probably would have had some clinical signs of, or you're not intubated. And one of the things is, if you ever have a patient that's hypoxic, you should immediately ask yourself, is my tube in? Okay, that's the number one thing to ask yourself.

Because the other thing that happens is an animal will arrest under a situation like that. We start doing CPR. Well, if we didn't check to make sure our endotracheal tube was even in place, your CPR, I can guarantee you, is not going to be very successful. So just always double check.

I can't stress enough. And again, these are sort of the reasons, right? Rushing to intubate a patient who's too late is probably the number one thing that happens with...

inexperienced people because they're very unsure of the anesthetics. I saw this a lot with students. They're like, oh my God, he's this close to death. If I don't, if I give him too much, he's going to, which could, but most patients, they, you can give them enough anesthetic to intubate them.

They're not going to die. brachycephalics are another one and even experienced people that we run into these problems i mean there's been patients where you know they've just got a really challenging airway and i cannot see anything or i can remember we had a cat that had a massive polyp hanging down the back of the throat where it completely occluded this cat's laryngeal opening you couldn't even see it it basically had to do sort of a blind technique i've got five minutes yeah see i knew i was going to run way way too long so it's all right though but at least hopefully the stuff that we've gone through you guys understand and this will go on to the website here eventually so um so confirm all intubations i have to say gastroesophageal reflex is another one that i think we don't pay enough attention to but we should pay attention to because if you don't manage gastroesophageal reflex appropriately when it happens during anesthesia you can end up with strictures in patients and patients who end up with strictures sometimes don't have very positive outcomes so really pay attention to gastroesophageal reflex and we have a lot more of it that happens than we actually know occurs Okay, so a lot of the times it can be silent.