Ventilation in Metabolic Acidosis (Video 2)
Let’s talk about another issue that I see often. Let’s talk about metabolic acidosis. So, a patient comes in with metabolic acidosis. If you remember how to deal with metabolic acidosis, you remember the Winter’s Formula. It’s just put up a blood gas here, 7.2/1/21/70/8. As you recall, this is the pH and this is the pCO2; pO2 is this third one, and then finally this is the bicarbonate.
让我们谈谈我经常看到的另一个问题。让我们谈谈代谢性酸中毒。因此，患者会出现代谢性酸中毒。如果您记得如何应对代谢性酸中毒，那就记得温特的配方。它只是在这里放了一个血气，7.2 / 1/21/70/8。您还记得，这是pH值，这是pCO2。 pO2是第三个，最后是碳酸氢盐。
So here, clearly, we’ve got a metabolic acidosis, and if you don’t remember how to do that, really quickly, if these two (pH and pCO2) are going in the same direction, then it is metabolic, and of course, they’re both down, it is a metabolic problem and that is an acidosis.
The other thing that’s a little pearl if you will, inside of a pearl, is if these two digits after the decimal point of the pH equal the digits of the pCO2 or very close to it, that usually means that it’s a pure metabolic acidosis.
So, let’s say you get a blood gas on a patient. And this is what it shows and here’s the bicarb, it’s pretty low, trying to compensate for that. And so, you know that this is metabolic acidosis. Your bicarb has dropped, trying to buffer whatever metabolic acidosis that was, and as a result of that, you’re trying to get rid of the carbon dioxide; it’s a Lewis acid.
And in doing so, you are able to buffer your pH so that it is not so low. Okay, so you’ve got this, you’ve got the blood gas. You see the patient beforehand, and you say, you know, he’s breathing in the 30s or in the 40s, we need to intubate this patient.
Okay, maybe it’s DKA (Diabetic ketoacidosis), and the DKA has just gone on for long enough and the patient’s metabolic acidosis becomes just so severe that the patient cannot through his respirations, keep up with the work of breathing that he’s needing to do to compensate for this metabolic acidosis.
But for whatever reason, you take a look at this patient, and he’s breathing hard, so his respiratory rate is in the 30s. Let’s say it’s 30, and you’re able to guesstimate his tidal volume at about 400 based on how much he’s moving air. So, you do a little calculation and you come up with 30 breaths per minute times 400cc each breath, and you come up with about 12 liters of air movement per minute. So that’s his minute ventilation.
So then you go and intubate the patient. And you know your protocol, or whatever your baseline is, states where you ventilate the patient, so you decide and you dial into the ventilator, AC of 12, tidal volume of 450. let’s say you put in a peep of 5 and an FIO2 of 100%. Since you’re just intubating the patient.
So, to review what we talked about already, we’ve already said that this patient is going from negative pressure ventilation to positive pressure ventilation, even with a little peep of 5 on it. So, we already know that we’re going to expect our venous return to drop, our blood pressure could drop, so we need to make sure that we give plenty of fluids which were probably already doing in a patient with metabolic acidosis, whether it’s due to sepsis, or DKA because both of those involve fluid.
The thing, though, that you may not realize is that this patient is actually trying to compensate for that metabolic acidosis. And by intubating, the patient usually has to give sedatives, versed, morphine, propofol, etomidate. Sometimes even paralytics are used, which means that you’ve just basically taken away their ability to compensate for this metabolic acidosis.
So instead of this being a backup rate of 12, this is going to be the respiratory rate of 12. And the patient is going to get, you know, 450 per minute. So, what you end up having here is 12 times 450. So, the respiratory rate 12, and the tidal volume is going to be 450, and you multiply those two together, and you come up with something that’s close to about, you know, 5.4 liters per minute.
So, you can see what’s happening here, the metabolic acidosis hasn’t gone away. You’ve knocked out the patient’s drive, you’ve intubated them. Yes, but you are not meeting their metabolic demand. They’re not meeting their ventilation demand. You’ve gone from about 12 liters per minute to 5.4 liters per minute. And as a result of that, this CO2 is not going to be driven down low to compensate for this low bicarb. And what you’re going to get is the pH is going to drop even lower. And you know folks, I’ve seen it where patients have been intubated in the emergency room and they crashed afterwards just because of this.
So, here’s the pearl. The point is look and see what kind of minute ventilation is occurring before you intubate. If there is compensation occurring, try to simulate that with your new vent settings, so instead of 12, you know, maybe go to 20, or even higher.
I just saw a patient today actually who came into the emergency room with severe DKA, I mean pH of 6.8. And they intubated him, and I knew full well that this patient was going to be set on on settings close to this (20), and I wanted to make sure that that wasn’t. They actually did a good job and set the AC rate at 20. I actually set it to 30 because this patient had no COPD. She was very young, and she wasn’t breast stacking.
As it turned out, we just barely kept up with the metabolic acidosis. The repeat blood gas, which I recommend doing afterwards was still around 6.8, 6.9. So we gave some bicarbonate and eventually the patient’s pH improved and we’re able to pull back.
But that’s the other side of this. If you’re going to set these vent settings high, like to AC rate of 20 and an AC rate of 5 or TV of 500, you’re purposely hyperventilating these patients, and you’re doing so for a very good reason: you’ve got a pretty severe metabolic acidosis.
The thing that you’ve got to remember is as you start to fix the metabolic acidosis, you’re gonna have to re-check your vent settings. Otherwise, you can just over ventilate these patients and they’re going to have pH of 7.5 and 7.6. So, in the case of DKA, you know, the insulin drip is working, it’s starting to reduce the amount of beta-hydroxybutyrate. We’re giving bicarbonates, we are flushing the
kidney, we’re sailing to get rid of the keto acids. And so, within maybe 3, 4, 5 hours, we were able to take a pH of 6.8 and bring it up to 7.4. And we know that we needed start backing off in the ventilation, something of the body that would normally do but because now the patients on the ventilator, we have to do that for the patient.
So, second pearl is, take-home message is if you have a patient with metabolic acidosis who’s compensating above and beyond what normally would be seen,
make sure you simulate that minute ventilation when you put the patient on the ventilator, and then check it very soon afterwards to make sure that you’re keeping up with those demands. If you don’t do it, you’re going to get more acidotic and the patient is going to unravel. Okay, especially if you’ve sedated the patient for ventilation and especially if you’ve paralyzed the patient for ventilation, so just be aware of that.