The Stabbening

That ominous moment when the RT starts feeling for your pulse

Last time, I strongly implied that my next post would be about mechanical ventilators. As it turns out, I imp-lied. I want to go over something else first, just to establish important background.¹ You see, when somebody goes on mechanical ventilation—or when we’re concerned they might have to go on mechanical ventilation soon—there are certain parameters we start looking at to determine just how well they’re ventilating and oxygenating.²

I’m pretty sure I’ve used those two terms before on here. If you didn’t know, “ventilating” refers to the process of getting rid of CO2, whereas “oxygenating” refers to explaining words with obvious meanings. The two are naturally related, but they are distinct processes and you can be doing quite well at one and lousy at the other. Both can be determined very quickly by a simple blood test, but there’s one unfortunate hitch. We can’t just draw the blood we need off of an IV, because IV stands for “intra-venous,” ie in a vein. If you test a venous sample, it will tell you that there is very little oxygen in the blood. Well, duh. It’s from a vein. Your tissues are done with it, they took the oxygen out. Now it’s going back to the heart, which will send it to the lungs, etc. You have no way of knowing how much oxygen was in that blood in the first place.

You can probably see where I’m going with this. We need blood that your tissues haven’t had a chance to suck the oxygen out of yet. That means we need to, shall we say, “take your pulse in for questioning,” a process known as an arterial blood gas (ABG).

We generally draw blood from the radial (wrist on the thumb side), brachial (elbow on the pinky side) or femoral (right next to your groin) arteries, in descending order of preference. If you get stuck femorally, it’s probably because your circulation is simply trashed and you’re very close to death. I’ve tried to stick femoral precisely once, and missed. It was my first year out of school and circumstances were not favorable.

Wherever you stick, it’s done mostly by feel; you get your left/less-dominant hand on the pulse, then try to put your needle in as close to your finger as you can without stabbing yourself. The plunger on the needle is already drawn back to the amount you want to draw (generally about a milliliter), by the by. A blood gas is driven by blood pressure and fills itself once you find it.

Please note, incidentally, that your body does not like things damaging its arteries. It therefore puts nerves near those arteries so that, when something is about to damage them, you can yell “OW DAMMIT” and run away from or punch whatever is damaging the thing that famously gushes blood at an extravagant rate when severed. What I’m getting at is, ABGs typically hurt³. A lot. Patients wince, grumble, scream, and twist, and sometimes they twist with the needle inside their arm which is only going to make it hurt worse. It’s not something you should do casually, and I hate it when MDs push me to do unnecessary sticks on conscious and aware patients because they’re hyper-conservative and want to order every test to cover their back ends.

With that said, I’m going to elide a bunch of technical details here. I could write a whole other post about the art of the stick, not gonna right this second. Assuming you get the gas, the next step is to run it. That part is simple; there’s a dedicated machine that can suck up the sample and run it in somewhere under a minute. Modern machines can piggyback any number of tests on there, such that you can get a full set of lab chemistry results including sodium, potassium, lactic acid, and so on within five minutes of the needle going into the arm.⁴ A basic ABG, however, is concerned with four values: pH, CO2, O2, and HCO3 (also known as bicarbonate or “bicarb”)⁵.

Those four values are almost always listed in that order, but I’m going to do it differently for easier explanation. O2, the odd man out here, is technically PO2, partial pressure of O2 within the sample. This number is correlated to the SpO2 measured by hospital monitors and jogger pulse-ox probes, but in a confusing way. SpO2, also known as saturation or “sats,” is a measurement of the percentage of red blood cells which are bound to oxygen (the probes actually use lasers to measure the albedo, or reflectivity, of the cells, since oxygenated RBCs are shinier). It is always an estimate, and can be fooled or rendered misleading in various ways⁶. PO2 is the “real” value, but because of a complex and somewhat boring thing called the oxygen dissociation curve its values don’t correspond neatly to SpO2. A PO2 of 60 corresponds roughly to a saturation of 90%; a PO2 of 40 means you most likely hit a vein by accident. Or the patient is dead, or will shortly become so. One of those. But it can go up into the hundreds. We just want it to be somewhere north of sixty on the gas, most of the time.

The other three values—pH, CO2, and HCO3—are closely linked. An aggravating but necessary aside: most laymen do not realize that your lungs and your kidneys are best buddies and sworn wingmen. CO2, when dissolved in water, forms carbonic acid. The less effectively you’re ventilating, the more acid your blood becomes due to retained CO2. However, your kidneys will—after a while—notice that their pals the lungs aren’t doing so hot, and start trying to make the blood more basic again by increasing the amount of bicarb in it. This can take several hours even to start, but eventually, if the deficit isn’t too bad, healthy kidneys can compensate for bad lungs and get a patient back on an even keel. If, on the other hand, the kidneys go south, the lungs react much more quickly.

Thus the pH is the consequence of a cooperative balancing act between lungs and kidneys, and you can generally assess what’s going on (in a rough way) by looking at all three together. Is the pH low (meaning more acid), while the CO2 is elevated and the bicarb is normal? This is probably a relatively recent failure to ventilate, assuming healthy kidneys, in that no compensation has taken place. In cases where they’ve been ventilating poorly for a while, you’d expect to see the pH at least closer to normal and the bicarb high. If the pH, bicarb, and CO2 are all low, the patient is desperately trying to compensate for kidney failure by blowing off excess CO2. You often see an elevated PO2 in these cases, since you can’t breathe out without breathing in and healthy lungs can take in oxygen just fine at a normal rate.⁷

So, what are normal ranges? In order:
pH should be 7.35 to 7.45
CO2 should be 35 to 45
PO2 should be over 60⁸
HCO3 … depends who you ask, but generally 22 to 26ish.

Before I go any farther, it’s a very common mistake for inexperienced doctors to want to treat the CO2. High CO2 absolutely is a problem, don’t get me wrong … if it’s making the pH low. But many patients, particularly COPDers, will have adjusted to truly silly amounts of CO2 retention. They’ll have some terrifying load like 60 or 70 … and a bicarb of around 40 keeping it under control⁹. A new doctor or NP can look at that top number, freak out, and want to intubate them right now. They disregard the pH of 7.35. Making the patient ventilate normally will only push them out of their personal equilibrium and make them alkalotic. Bad idea. It’s also generally a bad idea to try and put your thumb on the scales by, say, trying to compensate for elevated CO2 by injecting the patient with bicarb¹⁰. You are treating the pH, and the CO2 and bicarb tell you how to treat it.

How you treat it is a question for next time. Unless I’m lying again.

1. Also I can’t think of a cutesy name for a post about ventilators—always a key consideration for me.

2. The order I list them in is significant; your body is actually more sensitive to excess CO2 than it is to insufficient O2. The hypoxia sense is more of a backup. This can be a problem with certain COPD patients.

3. I once mentioned this on a Facebook respiratory therapy group, and somebody sniffed, “if performed correctly, an ABG should not hurt at all.” This is correct in that sometimes, if you’re very good or very lucky, you slide right in like a ghost and the patient doesn’t even notice. I’ve done it myself, especially on shallow pulses and when I use my favorite petite 25-gauge needles (which I call my “little ninjas”). Patient asks “when are you going to do it?” and I hold up a full syringe. That’s fun.

Nevertheless, that was a fairly obnoxious thing to say, and I wish I’d replied, “‘If you employ correct crossbow technique, Mr. Tell, your son should be completely unharmed while the apple is struck in the precise center.’” Real world, we’re not all William Tell, and the damn needle usually hurts.

4. This has led to the infuriating but near-ubiquitous tendency of ER docs to order full-panel ABGs on patients who have no apparent respiratory problems; we tend to be much quicker than the lab techs and they want results now, not later. This is a terrible reason to puncture an artery but it totally happens.

5. Sodium bicarbonate (just stick an “Na” in front of the HCO3) is common baking soda. It doesn’t make any sense for it to be called “bicarbonate” when it only has one carbon atom, does it? Wiki says it was coined in 1814. It contains twice as much carbon by weight as a substance called sodium carbonate, so it’s bicarbonate. Outdated name that stuck. Meh.

6. Among others: it can’t read a patient with poor circulation or low temperature; it can be fooled by dark skin tones; it can’t distinguish effective oxygenation from massive carbon monoxide poisoning (CO binds to hemoglobin even better than O2, and doesn’t like to let go!); and if you’re catastrophically anemic it doesn’t matter that your, like, two remaining red blood cells both have oxygen attached, but that’s still 100% saturation as far as the sat probe is concerned.

7. You might have noticed I’m focusing on cases of acidosis, low blood pH, here. This is because it’s by far the more common and serious problem from respiratory’s perspective. If the patient is ventilating too effectively, either they have anxiety or pain or the problem is metabolic or neural, etc. Whatever it is, the problem is caused by something else and the low CO2 is incidental to that root problem. There really isn’t a lung problem that makes you too good at blowing off waste, to my knowledge. Alkalosis, the opposite of acidosis, is absolutely a problem. It’s just not MY problem.

8. More or less; the doctor may want higher or lower oxygenation depending on the patient’s condition. I shrug at anything over sixty, personally. If it’s really high it means I can turn down their oxygen, but it takes a long time for excess oxygen to become really toxic so it’s not a crisis.

9. I’ve seen this situation tons of times, but I still can’t recall exactly what CO2 balances against what HCO3 at what pH. But it’s something like that. You see a bicarb around 40 a lot.

10. Sounds like a clever trick, but I understand that, for some damn reason, the HCO3 just turns into more CO2 if they’re in specifically respiratory acidosis. I don’t know why. Just don’t do it.