pCO2

First thing that you would do is establish what your baseline ABG is, because what you consider to be your threshold values beyond which a patient should breathe does depend on where they're starting from.

So generally, this is a pH of 7.35 to 7.45 and a PaCO2 of 35 to 45 millimeters of mercury, unless they have a documented different baseline, right? Your, your sort of chronic respiratory acidosis patients.

And you also have to preoxygenate them, uh, at 100% FiO2 for 10 minutes before starting your apnea test.

Then what?

Got it.

And what does a positive test look like?

So a positive test is a pH of under seven point three and a PaCO2 of at least sixty millimeters of mercury and at least twenty milliliters, millimeters of mercury higher than their pre-test baseline with no respirations.

So they can't breathe, and their CO2 has to go up, and their pH has to go down.

If we're not giving patients enough time to exhale, they will trap an auto-PEEP, and that certainly can make the situation worse in terms of a respiratory acidosis.

And then when it comes to the PaCO2, we're not chasing a normal number at the expense of lung protection.

ARDS Net recommends targeting a pH of seven point three to seven point four five.

And in patients who have really sick lungs, we're gonna be even more tolerant of a severe acidemia.

this?The guidelines would conditionally suggest VV ECMO in selected patients with severe ARDS, and we have a whole topic dedicated to this in the Critical Care series.

I think the EOLIA trial gives us really useful escalation thresholds.

So, a PF less than fifty for more than three hours, below eighty for more than six hours, or a pH below seven point two five with a PaCO2 of at least sixty for more than six hours despite optimized conventional ventilation.

I'm not at an ECMO center, so I kinda tend to use those as referral triggers.

So in a mild, moderate asthma exacerbation, blood gas is typically not needed.

But if you do get a blood gas, typically you'll have a decreased PaCO2.

So you'll see a respiratory alkalosis.

It is a red flag if you have a patient with dyspnea, with tachypnea, and they have a normal or even elevated PCO2.

Severe asthma thresholds would be an FEV1 less than 25% predicted, and that also is indicative of impending respiratory failure.

Capnographs can be used in evaluation of asthma, but really the only thing a PA or an entitled CO2 can tell you, say it's elevated.

So the PaCO2, if you see it's at 50.

You know that their PaCO2 is at least that high, but it can't reliably distinguish exactly where it is.

It asks you to look at a patient, a pH.

A PCO2 and a bicarb tell the difference between a system that is compensating appropriately and one that is quietly failing in two directions.

That is the seesaw.

One side is metabolic, run by the kidney over hours to days.

First, look at the pH and decide whether the patient is academic or alkalemic.

This is your anchor.

Second, look at the PCO2 and the bicarb and decide which one explains the pH.

If the bicarb is low and the pH is low, you're looking at a metabolic acidosis.

And the really easy way to think about that is your lungs are doing everything they can, huge minute volumes, just to keep the pH at, say, 7.

And so you may say, well, the PCO2 only went from 15 to 25.

It's not even high.

But it's delta 10 higher than it was.

That's basic ICU understanding.

PaCO2, 35 to 45, again, classic.

Bicarbonate is 22 to 26 milliequivalents per liter.

Base excess is approximately minus 2 to plus 2 milliequivalents per liter.

What could cause that in anesthesia? residual neuromuscular blockade, opioids, airway obstruction, severe COPD with CO2 retention, inadequate minute ventilation.

Anything that decreases effective alveolar ventilation can push CO2 upward.

So if I see a low pH and a high PaCO2, my brain immediately asks, why isn't this patient ventilating? And that is what we have to get down.

We have to investigate further at that point.

There are a couple of different ways of thinking about this, but I think the number one thing we should think about is look at the patient that's being frequently admitted to the hospital for not only COPD exacerbation with increased use of medications, but also seeing a bump in their CO2.

And the qualifying factor being our PCO2 greater than 52 is kind of a cutoff where we say, okay, hey, you reached the threshold for qualifying for this device.

However, I like to take the stance of thinking that just because a patient is seen to have compensated hypercapnia, we shouldn't be looking at that as a rule.

kind of like a disqualifier for this device.

And, you know, you need to wear the BiFab S for 61, no sooner than 61 days, repeat an ADG, repeat a sleep study before you can step up to a BiFab ST. They've really smoothed that out and made it so it's a lot more up to the subscriber.

And there are a few stipulations there.

So again, with the COPD population specifically, again, if you have a PCO2 greater than 52, if you have a diagnosis of COPD, you're kind of open the doors and saying you qualify.

A lot of times in our practice, we're moving right for that BiPAP ST.

But the other thing, respiratory acidosis, that we do need to keep on there, especially when we're thinking about how to fix it with regard to event management, is physiologic dead space, right? Right.

where if we, for example, see the end tidal CO2 is fine, but then we check an ABG and our PaCO2 is 75, but our end tidal was 25, where we just can't get the CO2 to the alveoli because of the physiologic dead space.

So a respiratory acidosis, it can be the lungs, it can be the thoracic cavity, or it can be the capillaries and physiologic dead space.

metabolic acidosis.

Okay, let's do it.

So we're going to start with a 64-year-old lady who has obesity, obstructive sleep apnea, and chronic opioid use, who's admitted after abdominal surgery.

Several hours later, she becomes somnolent with shallow respirations, and her ABG shows us a pH of 7.18, a PaCO2 of 72, a bicarbonate of 26, and a PaO2 for what it's worth in this case.

of 68.

The visible scene is drips forming mineral rings, calcite depositing at a stalactite tip, dry air moving through a passage, and delicate crystals under stable humidity.

Beneath it are carbon dioxide degassing, evaporation, saturation state, calcite precipitation, cave air PCO2, ventilation, drip rate, and speleothem fabric.

A cave is not a sealed museum room.

It is an atmosphere inside rock, connected to the surface through entrances, fractures, soil, water, and passages whose full geometry may still be unknown.