Arterial Blood Gases don’t have to feel like secret code. In this episode of Think Like a Nurse, created by Brooke Wallace (20-year ICU nurse, transplant coordinator, and clinical instructor), we break down ABG interpretation in plain English. Find out not just what the numbers mean, but why we even care! Learn how to connect pH, CO₂, and bicarb values to what’s actually happening at the bedside — from opioid overdoses and COPD to panic attacks, DKA, and vomiting. Understand the “why,” not just the numbers — and start thinking like a nurse. Visit ThinkLikeANurse.org for study resources and NCLEX-style practice questions.
Why ABG interpretation feels like decoding secret language — and how to simplify it.
02:30 – Foundations of Acid-Base Balance
CO₂ = acid (lungs control it)
Bicarb = base (kidneys control it)
The body maintains balance (homeostasis) through both systems.
06:00 – Respiratory Acidosis
Cause: CO₂ retention (slow, shallow breathing)
Examples: Opioid overdose, pneumonia
ABG Pattern: ↓ pH, ↑ CO₂, normal bicarb (acute)
Symptoms: Confusion, hypoventilation, drowsiness
Chronic case: COPD → compensated by high bicarb
11:30 – Respiratory Alkalosis
Cause: Blowing off too much CO₂ (hyperventilation)
Examples: Panic attack, pulmonary embolism, pregnancy
ABG Pattern: ↑ pH, ↓ CO₂, normal bicarb
Symptoms: Dizziness, tingling fingers/lips, lightheadedness
16:45 – Metabolic Acidosis
Cause: Too much acid or loss of base
Examples: DKA, renal failure
ABG Pattern: ↓ pH, ↓ bicarb, ↓ CO₂ (compensation)
Signs: Deep, rapid (Kussmaul) respirations, fruity breath, fatigue
22:15 – Metabolic Alkalosis
Cause: Loss of acid or gain of base
Examples: Prolonged vomiting, NG suctioning
ABG Pattern: ↑ pH, ↑ bicarb, ↑ CO₂ (compensation)
Symptoms: Shallow breathing, muscle cramps, EKG changes
27:00 – The Big Nursing Pearl
“You treat the patient, not the paper.”
ABG numbers mean nothing in isolation — always match them to your patient’s story and symptoms.
29:00 – Critical Thinking Challenge
How do shifts in pH affect electrolytes like potassium and calcium?
Recognize how alkalosis causes low potassium and low calcium, showing up as cramps or EKG changes.
30:00 – Wrap-Up & CTA
Visit ThinkLikeANurse.org for more NCLEX practice and weekly episodes that help you think like a nurse — not just study like one.
Welcome to Think Like a Nurse. This is where we tackle those really complex kind of dense nursing topics. Yeah. And we work to make them easier to understand. Uh giving you more confidence right there at the bedside. We know that looking at lab values, especially something like an AG, arterial blood gas. It can feel like trying to read, I don't know, secret code sometimes. Absolutely. And our goal today really is to help you crack that code. Right. And if you're new here, just a reminder, this whole concept comes from Brooke Wallace. She's a 20-year ICU nurse, organ transplant coordinator, clinical instructor, and uh a published author, too.
Our focus is always on helping you really get the why behind things, the physiology, not just memorizing facts for a test. Exactly. Understanding what's happening with a patient. And hey, for more resources, more in-depth stuff, you can always check out think like a nurse.org after you listen. Okay. So, today, like we said, we're diving into arterial blood gas interpretation, AGS, but we're going beyond just, you know, the Rom pneummonic. That's atory opposite metabolic equal. It's helpful, but it's not the whole story.
No, not at all. We want to connect those numbers, the pH, the CO2, the by carb directly to what you actually see, you know, the patient's signs and symptoms. The shallow breathing, maybe tingling fingers, those deep small breaths. What does that patient look like when their chemistry is off? That's the key. Okay, so let's just quickly lay the groundwork. The body needs balance, right? Homeostasis and it uses buffers. We've got two big players. the lungs, the respiratory system controlling CO2. That's our acid we can breathe off.
And then the kidneys, the metabolic system controlling bicarbonate or HCO3. That's our base. And the ABG, that's what tells us, you know, which system maybe caused the imbalance and which one is trying to compensate, trying to fix it. Yeah. And that's where ROM gives you a starting point. Respiratory opposite metabolic equal tells you if the arrows for pH and CO2 or PH and by carb are going in opposite or equal directions. Exactly. Ry's like the map legend. But we need to understand on the actual landscape, the clinical picture.
So, let's start with the lungs causing the problem. Respiratory issues mean ventilation's gone wrong, either too slow and shallow or way too fast. Okay, first up, respiratory acidosis. This is always always about holding on to too much CO2, CO2 retention. And the classic like super acute example, think about someone with severe CNS depression, maybe in an opioid overdose or way too much sedation. Yeah. They're confused, maybe hard to wake up. And when to check their breathing. It's slow, dangerously slow and shallow. Maybe like eight breaths a minute. They're basically trapping all that CO2 inside.
And that CO2 builds up fast. It mixes with water, makes carbonic acid, and boom, the pH drops immediately. So, the ABG reflects that crisis right away. You'll see something like pH7.25, so acidic. CO2 maybe 60. That's really high. Yeah. And by carb 26. Okay. Cause there. Why is the byarb 26 still pretty much normal? That's key for students. It's about time, isn't it?
Exactly. Time. time. The kidneys, they're powerful, but they're slow. It takes them, you know, 24, 48 hours, sometimes even longer to really crank up or down the bicarb levels. So, in a sudden crisis like that overdose or maybe someone with sudden really bad pneumonia where they just can't breathe effectively, right? The kidneys haven't had a chance to react yet. They haven't started compensating. So, you see that acute picture. Maybe for that pneumonia patient, struggling to breathe, crackles everywhere, using accessory muscles, your AG might look similar. PH 7.28, CO 258 bicarb 26. It's immediate ventilatory failure urgent.
Okay. Now, let's contrast that with someone who's adapted. Let's talk chronic respiratory acidosis. Ah, okay. So, this is maybe your patient with severe long-term COPD. Exactly. They live with high CO2 levels all the time. They might look tired, maybe a little dusky or cyanotic, always coughing, doing that purse lip breathing, but their body has had time to adjust, right? This is where you see that compensation kick in because they've had high CO2 for months, maybe years, the kidneys have stepped up. They're holding on to extra by carb like crazy to buffer that acid to protect the pH. So, when you get their AVG, it tells a different story, a story of adaptation. The CO2 is still high, maybe CO2 58, but the pH
the pH is actually almost normal, maybe pH 7.38, and the bicard is high, maybe 32. So, the pH is back in a safer range, even though the underlying lung problem, the high CO2 is still there. They're compensated. Mhm. But living right on the edge, that near-n normal pH, despite a really high CO2, that's the sign of a chronic compensated state. And that explains why, you know, a COPD patient might walk around with a CO2 of 58 and be relatively okay, while that overdose patient with a CO2 of 60 is unconscious. Different time frames, different compensation.
Precisely. Okay, let's flip the switch. Let's talk about the opposite problem, respiratory alkalossis. Blowing off too much CO2. Yep. And the first thing that probably comes to mind is hyperventilation, right? Often from a panic attack, severe anxiety. Oh, yeah. The patient's breathing super fast, maybe 32 breaths a minute. They feel dizzy. They complain about that weird tingling. Paristhesia. Yeah. Pins and needles often around the lips or in the fingers because they're blowing off all that CO2. They're getting rid of acid too quickly. So, the pH goes up, becomes alkaline.
And the AG shows it pH 7.52 high, CO2 30 low. And the bicarb is still normal, maybe 23. It's acute, uncompensated. Okay. But why? Why the dizziness and tingling? That's a connection students need to make. Good question. It's physiology. That low CO2 actually causes cerebral vasoc constriction. Ah, so the blood vessels going to the brain tighten up.
Exactly. Less blood flow to the brain leads to that dizziness, the lightheadedness, the altered sensation, which unfortunately can just make the anxiety worse. It's a cycle. Okay. But it's not just anxiety. We see this in really sick patients too, right? Definitely. Think about a trauma patient. Maybe after a car crash, you're worried about a pulmonary embolism (PE). PE they've got chest pain, they're anxious, breathing really fast, maybe unequal chest expansion. So in a PE there's that VQ mismatch, ventilation profusion mismatch. Yeah. Blood isn't getting oxygenated properly,
right? The body senses the low oxygen, the hypoxia, and the immediate reaction is breathe faster, get more air in. But the fast breathing doesn't really fix the oxygen problem in that PE situation. Not immediately, no. But what it does do is blow off a ton of CO2. So their AG might show pH 7.4. alkaline CO2 32 low by carb 22 normal it's an uncompensated respiratory alkyossis again yeah their main issue is the PE and the hypoxia but the fast breathing creates this secondary acidbased problem it's the body trying maybe over trying to compensate for the lack of oxygen interesting is there any other situation where we see this
well there's one more kind of a unique one late pregnancy oh right physiology not pathology exactly progesterone the hormone actually stimulates breathing. So someone in their third trimester might feel a bit breathless, maybe laded, breathing faster, say 28 breaths per minute, and their AG would reflect that. Yeah, it might be mildly alkaline, maybe pH 7.48, CO2 30 by carb 20. But you have to know the context. This is expected. It's physiologic driven by hormones, not necessarily a sign something is wrong. It's their baseline then.
Good point. Always context. Okay, let's shift gears now to the metabolic side. These can be tougher to fix quickly. Mhm. Now we're talking metabolic acidosis. Either the body is making too much acid or it's losing too much base, the by carb. And the classic example here has to be DKA, right? Diabetic keto acidosis. Absolutely. That patient looks sick. You might smell that fruity acetone breath. Their blood sugar is skyhigh, maybe 450. They might have belly pain. And the breathing, those cusolo respirations, deep, fast, almost sighing breaths.
Okay, so the root problem is the diabetes is out of control. The body is making tons of acidic ketones because it can't use sugar for energy. These ketones just chew up all the available by carb. So, the metabolic system is in crisis. The AG shows a major problem. PH7.22, very acidic. CO2 is low, maybe 28. And the by carb is critically low, maybe 12. Right. That by carb of 12 tells you the metabolic system is overwhelmed. But look at the CO2 again. Why is it low? That's the condensation. The lungs are trying to help.
Exactly. This is where you see that amazing interplay. The lungs aren't the primary problem, but They sense that bodywide acid crisis, so they kick into high gear. Cousmal breathing is the lungs working flat out, blowing off as much CO2, the acid they can control as possible, trying desperately to pull that pH back up from dangerous levels. That low CO2, that 28, is the sign of the lungs trying to save the day because the metabolic system failed. Heroic effort. We see a similar low by carb problem in renal failure too, don't we?
We do, but the cause is different here. It's not overprouction of acid like ketones. It's that the kidneys themselves are failing. They just can't get rid of the normal daily acid waste products, uric acids. So those acids build up again consuming by carp, right? The patient presentation is different. Maybe fatigue, swelling, nausea, high creatinine levels, but the body's response can be similar. You might still see those deep sighing breaths. The lungs trying to compensate for the kidneys not doing their job.
Exactly. You know, their AG might also show that metabolic disaster pattern. PH7.25, CO2 lowish, say 30, and by carb really really low again maybe 14 lungs compensating for kidney failure. Okay, one more category to cover the opposite metabolic problem metabolic alkalossis too much base or more commonly losing too much acid and the classic scenario for losing acid severe vomiting or maybe someone getting continuous NG tube suctioning pulling out all that stomach acid.
Okay, so picture someone who's been vomiting non-stop for say 3 days they feel weak might have muscle cramps. Yeah. And you might even see EKG changes, irregular heart rhythms. That's important. Because when they lose all that hydrochloric acid, the H+ ions from the stomach, the body is left with relatively too much by carb, right? So the AG reflects that pH750 alkaline. The CO2 might be high, say 50, and the by carb is high, maybe 30. Okay, wait. High by carb makes sense. That's the primary problem. But why is the CO2 high this time?
Ah, that's the compensation again. It's the opposite of DKA. In DKA, the lungs breathed fast to blow off acid CO2. So here in metabolic alkalossis. The body wants to keep acid. Exactly. It tries to hold on to CO2 to counteract that high by carb. And how do the lungs hold on to CO2? By breathing slower. Mhm. Shallower. Hypoventilation.
Bingo. That elevated CO2 of 50 is the lungs deliberately slowing down, taking shallower breaths, trying to let CO2 build up and bring the pH back down towards normal. So those shallow breaths are the nurse's clue at the bedside. And you mentioned the muscle cramps, the EKG changes. Yeah. Sever Alkalossis, especially combined with fluid loss from vomiting, really messes with electrolytes, particularly potassium and calcium that often causes the cramps and arhythmias. It's all connected.
Wow. Okay, that really brings it full circle, doesn't it? The absolute key takeaway here is that AG numbers mean nothing in isolation. Absolutely nothing. You have to match them to the patient. What's their history? What do they look like right now? If the AG shows severe metabolic acidosis, like that pH 7.22 and by carb 12, you better be seeing or expecting those deep rapid small breaths. And if you see respiratory acidosis, pH7.25 and CO260, you should be assessing for hypoventilation, slow, shallow breathing, maybe altered mental status. The numbers have to fit the clinical picture. You treat the patient, not the paper.
So true. Which kind of leads us to a final thought, maybe something for you listeners to chew on as you keep learning. Yeah. Think about this. We've seen how acid base balance shifts things. How might those big shifts, especially going into alkalossis, affect electrolytes. Think specifically about potassium and calcium. Oo, good one. Because electrolytes move in and out of cells depending on pH, right? And how quickly could those electrolyte changes like low potassium or low calcium from alkalossis show up on your patient's cardiac monitor? Like changes in the T- waves or maybe a prolonged QT interval. That's integrating everything. Chemistry, breathing, heart function. That's really thinking like a nurse, seeing those connections. Acid base and electrolytes are tied together.
Fantastic point to end on. Thank you so much for joining us for this discussion on ABG interpretation here on Think Like a Nurse. We really hope this helped connect some of those dots for you and makes you feel a bit more confident looking at those numbers. Remember to check in for more conversations like this each week. And definitely visit think like nurse.org for more support, more resources. We'll talk to you next time.