ABGs stop being a “secret code” when you read them in order: pH → CO2 → bicarb. In this Think Like a Nurse episode, Brooke Wallace walks you through the 4-step method, ROME, and Tic-Tac-Toe, then connects each imbalance to causes, bedside cues, and priority actions. Perfect for med-surg, ICU, and NCLEX prep.
What You’ll Learn
The 5 ABG parts you must know cold
Three proven interpretation methods (4-step, ROME, Tic-Tac-Toe)
How to spot respiratory vs. metabolic problems fast
Compensation (uncompensated, partial, full) and what it tells you about timing
Nursing priorities for each imbalance (what to do now vs. what to fix next)
Key ABG Components and “Normal” Anchors
pH: 7.35–7.45 (acidic vs. alkaline)
CO2: 35–45 (respiratory acid; high = retention, low = blowing off)
Bicarb: 22–26 (metabolic base)
Oxygen: 80–100 (oxygenation; interpret separately from acid–base)
Base excess: −2 to +2 (overall base deficit/excess)
The 3 Methods You Can Trust
1) 4-Step Systematic
pH: low = acidosis, high = alkalosis
CO2: opposite pH → respiratory
Bicarb: same direction as pH → metabolic
Decide compensation (none, partial, full)
2) ROME
Respiratory Opposite, Metabolic Equal
pH high + CO2 low → Respiratory Alkalosis
pH low + CO2 high → Respiratory Acidosis
pH high + bicarb high → Metabolic Alkalosis
pH low + bicarb low → Metabolic Acidosis
3) Tic-Tac-Toe (Visual Grid: Acid | Normal | Base)
Place pH, CO2, bicarb in their columns
Two in a row = primary imbalance; the third value shows compensation
The 4 Acid–Base Imbalances at a Glance
Respiratory Acidosis: pH low, CO2 high
Common causes: hypoventilation (COPD flare, opioids, pneumonia, asthma)
Clues: somnolence, confusion, dyspnea
Priority: airway and ventilation (suction, bronchodilators, BiPAP, intubation if needed)
Respiratory Alkalosis: pH high, CO2 low
Causes: hyperventilation (anxiety, pain, fever, early sepsis)
Clues: dizziness, tingling, lightheadedness
Priority: treat the trigger (calm environment, pain/fever control, guided breathing)
Metabolic Acidosis: pH low, bicarb low
Causes: DKA, lactic acidosis, diarrhea, renal failure
Clues: fatigue, confusion, Kussmaul breathing
Priority: treat the cause (fluids, insulin for DKA, shock management), monitor potassium
Metabolic Alkalosis: pH high, bicarb high
Causes: vomiting, NG suction, diuretics, excessive base intake
Clues: muscle cramps, arrhythmias
Priority: stop the loss or over-base, replace electrolytes (especially potassium, chloride)
Compensation — What It Tells You About Time
Uncompensated: primary system abnormal; pH abnormal
Partially compensated: both systems abnormal; pH still abnormal
Fully compensated: both systems abnormal; pH back in range
A fully compensated respiratory acidosis usually means a chronic issue (like COPD)
Nursing Pearls
Always read in order: pH → CO2 → bicarb
Never interpret an ABG in isolation — check the patient first
Think beyond the label: cause and correction
Check oxygen last: it affects stability but not the acid–base label itself
Quick Practice (from the Transcript Flow)
pH 7.30, CO2 60, bicarb 30 → partially compensated respiratory acidosis
pH 7.20, CO2 25, bicarb 12 → partially compensated metabolic acidosis
pH 7.50, CO2 25, bicarb 23 → uncompensated respiratory alkalosis
Welcome to Think Like a Nurse. This is the show created by Brooke Wallace. She's a 20-year ICU nurse, organ transplant coordinator, clinical instructor, and a published author. Our mission here is pretty straightforward. We take those really complex, sometimes overwhelming nursing topics, and well, we make them easier to grasp, giving you real confidence right there at the bedside. Absolutely. And today, uh, we are jumping into what can honestly feel like the most intimidating set of labs for a lot of students and frankly even new nurses. arterial blood gases, ABGs. So, we'll focusing this session on how you can interpret these numbers quickly, clearly, and yeah, with confidence.
Yeah, ABGs can look like some kind of secret code, can't they? But they are truly the most reliable snapshot you can get of your patients immediate physiological state. Our goal today is simple. Break down these numbers using those gold standard evidence-based methods. We're talking about the systematic approaches endorsed by groups like the American Thoracic Society, Nurse Labs, Stat Pearls, you know, the stuff you really need for Medser, ICU, and definitely for ENLEX prep. Exactly. We want to give you that mental blueprint, that framework, so you can look at an AG and know exactly what to do next. And just a reminder, if you want more practice, uh maybe some scenarios or study guides, you can always find those over at the think like a nurse.org.
Okay, perfect. Let's start right at square one. Before we can even think about diagnosing an imbalance, we've got to lock down the five key parts of an AVG and their normal ranges. These are like our anchors, right? Couldn't agree more. They are the foundation. So, the first three are really the core four acidbased players. First up, the ultimate referee. PH, its normal range is super super tight. 7.35 to 7.45. This number tells you the final verdict. If it's low, below 7.35, your patient's acidotic. If it's high, above 7.45, they're alkalotic. Simple as that.
Okay, so pH sets the overall scene. Acidic or basic. Now, what about the body systems that actually control this? The regulator, right? So, the first regulator is CO2, carbon dioxide. This is our respirator. piece and you need to think of CO2 as an acid. The normal range here is 35 to 45. Now, if you see this number creep up, say above 45, it means the lungs aren't getting rid of enough acid. They're retaining it. And if it's low under 35, well, that means they're probably hyperventilating, blowing off way too much acid.
Gotcha. So, CO2 is a fast responder because, well, you can change how you breathe pretty instantly. But then we have the other side of the coin, the metabolic side, which is maybe slower, but just as power. powerful. That's exactly right. That brings us to bicarb or HCO3 bicarbonate. This is the metabolic component and it acts as a base. Its normal range is 22 to 26. This value really reflects how the kidneys are managing base levels in the body. And you know, unlike the lungs, the kidneys take their time, hours, sometimes days to make significant adjustments.
Okay. PH, CO2, by carb, the big three. What about the other two values you often see on an AG report? Good question. We also need to look at oxygen. The PAO2 normal is t ally 80 to 100. Now this is obviously critical, right? It tells us about oxygenation, tissue profusion, but and this is important, we sort of interpret it separately from the acid base balance itself. You can have a perfectly normal pH but be dangerously hypoxic. And finally, there's the base excess. Usually ranges from minus2 to plus2. Think of it as a quick check. A positive number suggests too much base, often metabolic. A negative number like minus4 suggests a deficit of base or an excess of acid.
Okay, knowing the number numbers is step one, absolutely essential. But when you're under pressure, maybe in a code situation or just with a rapidly declining patient, you need a fast, reliable way to put these numbers together. So, let's talk about those triedand-true interpretation techniques, the ones that always work. Yeah. And the great thing is there are a few really solid methods, and they all lead to the same answer. So, you can pick the one that clicks best for you. Probably the most uh thorough is the four-step systematic method. This is the one often highlighted by groups like the American Thoracic Society. Very methodical.
Okay. Walk us through the first few steps of that one. How does it work? Sure. Step one, look at the pH. Is it low acidosis or high alkalossis? That's your starting point. Step two, check the CO2. Now, here's the key. If the CO2 is moving in the opposite direction of the pH abnormality, like pH is low but CO2 is high, then your primary problem is respiratory. Step three, check the bicarb. If the bicarb is moving in the same direction as the pH, like pH is low and by carb is also low, then the primary problem is metabolic.
That's atic way is great, very logical. But let's be honest, we all love a good shortcut, especially when things are moving fast. And that brings us to probably the most famous pneumonic in nursing school. Rome. Ah, Rome. Yes. Respiratory opposite metabolic equal. It's brilliant because it helps you quickly link the pH to the main driver. If your pH and CO2 are moving in opposite directions, it's respiratory. So, high pH, low CO2, respiratory alkalossis, low pH, high CO2, respiratory acidosis. You found your primary issue.
And the other half metabolic equal. Exactly. If the pH and the by carb are moving in the equal or same direction, it's metabolic. Low pH, low by carb, metabolic acidosis, high pH, high barbar, metabolic alkyossis. It's just super fast for identifying that primary problem. Love it. Okay, so we have the systematic approach. We have the rone pneummonic. What about visual learners? Is there something for them? Absolutely. For those who like to see it laid out, there's the tic-tac-toe method. This one's fantastic for making sure you consider all the pieces, including compensation.
Tic-tac-toe. How does that work? work with ABGs. You literally draw a tic-tac-toe grid, but just the three columns, label them, acid, normal base. Then you take your patient's pH, CO2, and by carb values, and place each one in the column that matches its status. Is the pH acidotic, normal, or alkalotic? Place it there. Same for CO2 and by carb. Remembering CO2 is backwards high, CO2 goes in the acid column, low CO2 with the base column. The rule is simple. Whichever column has two values lined up tells you the primary imbalance. So if pH and CO2 both land in the acid column. Bingo. Respiratory acidosis. PH and by carb land in the base column. Metabolic alkyossis. And that third value, the one that's off on its own, that usually tells you about the body's attempt to compensate.
Okay, this is perfect because it flows right into the next critical piece. Getting the label respiratory acidosis metabolic alkyossis is important, but it's useless unless it tells you what to do for the patient. So, let's connect these labels to what's actually happening with the patient and what your nursing priorities are. Great point. Let's start with respiratory acidosis. Remember, that's low pH, high CO2. The core issue here is simple. The lungs aren't breathing off enough CO2. It's building up, making the blood acidic.
So, we're thinking about patients who are hypoventilating, right? Maybe someone who got too much sedation or opioids or a patient with severe COPD having an exacerbation or even someone with bad pneumonia. And clinically, they might look uh confused, maybe really sleepy, struggling to breathe. Exactly. That confusion and somalance that sleepiness is a big clue. High CO2 acts as a vasod diilator in the brain. It can increase incraanial pressure. So your immediate nursing priority. It's all about the airway and breathing. Improve that gas exchange. Maybe it's suctioning. Maybe it's respiratory treatments like nebulizers. Maybe they need BPAP or even intubation. You have to fix the breathing problem first.
Okay, makes sense. Now flip side, respiratory alkalossis. That's high pH, low CO2. Here they're breathing too much, blowing off excess CO2. Yeah, classic hyperventilation. And the causes are often things affecting the brain's respiratory center. Think significant pain, high fever, severe anxiety or panic attacks, even early sepsis can trigger it. Patient might complain of feeling dizzy, lightaded, maybe tingling around their mouth or in their fingertips. That tingling happens because the alkalossis affects how calcium binds in the blood.
Interesting. So the nursing priority here isn't about making them breathe more. Obviously, it's about finding and treating that underlying cause, isn't it? Like get the fever down, manage the pain effectively, create a calm environment, maybe coach breathing breathing for the anxious patient. Got to fix the trigger. Precisely. Okay, moving over to the metabolic side. Let's talk metabolic acidosis. That's low pH and low by carb. This usually means the body's either making too much acid or it's losing too much base.
right? And the causes here can be really serious systemic problems like diabetic keto acidosis, DKA, or severe diarrhea where you're literally losing bicarbonate or shock leading to lactic acidosis, kidney failure, too, right? Yeah. All major causes. Clinically, these patients often look quite sick, confused, very fatigued. But the physiological sign you might see is kusal breathing. Those are those really deep, rapid, almost gasping breaths. It's the lungs trying desperately to compensate by blowing off as much CO2, that respiratory acid, as possible.
Wow. So, nursing priorities here are immediate and often involve multiple things. If it's DKA, they need fluids, insulin, careful monitoring. If it's shock causing lactic acidosis, you treat the shock. And you absolutely have to Watch electrolytes, especially potassium, right? Because acidosis makes potassium shift out of cells into the blood. So, their serum potassium might look normal or high, but their total body potassium could actually be dangerously low. Excellent point about potassium. Super critical. Okay, last one of the four. Metabolic alkalossis, high pH, high by carb.
This one often feels like it's caused by something we did or by GI losses. Like maybe someone's been vomiting excessively or they have an NG tube pulling out lots of stomach acid or maybe aggressive diuretic use. Exactly. Those are very common it's losing acid or gaining base. Clinically, signs can be a bit more vague, maybe muscle cramps, some confusion, but the big danger is arrhythmias because of the electrolyte shifts, particularly potassium. Your priority is to stop the cause if possible, maybe anti-imetics for vomiting, maybe reassessing that NG suction, and aggressively replace electrolytes, especially potassium and chloride. Alkalossis drives potassium into the cells, making hypocalemia, low potassium a major risk. Monitor that K+ very closely. That really highlights how knowing the label isn't enough. You have to know the cause and the right action. Okay, this brings us beautifully to that final step in the four-step process and honestly the part that often trips people out, compensation. How do we figure out if the body is trying to fix the problem?
Compensation is just the body's natural response. The system that isn't the primary cause of the problem tries to step up and push the pH back towards normal. We usually talk about it in three stages. The simplest is uncompensated. This means only the primary system is out of whack. For example, the CO2 is high causing respiratory acidosis, but the bicarb is still totally normal. The other system hasn't reacted yet. And importantly, the pH is definitely abnormal.
Okay, uncompensated is straightforward. What about partially compensated? Partially compensated means the other system has started to react. So, using that respiratory acidosis example again, the CO2 is high and the bicarb has started to rise. The kidneys are trying to hold on to base to buffer the acid, but And this is key for partially. The pH is still abnormal, still outside that 7.35 7.45 range. The compensatory effort has begun, but it hasn't been enough to fix the pH yet.
Right. So, both systems are involved, but the pH hasn't normalized, which leads us to the goal state, I guess, fully compensated. Exactly. Fully compensated is when both systems are abnormal. CO2 is high, by carb is high, but their combined effort has successfully pushed the pH back into the normal range somewhere between between 7.35 and 7.45. Getting a fully compensated result is actually a really significant clinical finding.
Why is that so significant? Doesn't it just mean the body fixed it? Well, yes, but how it got fixed tells you a lot about time. Remember how the lungs compensate quickly within minutes to hours, but the kidneys take days. So, if you see a fully compensated metabolic acidosis, you know, the lungs kicked in fast to blow off CO2. That can be relatively acute. But you think it if you see a fully compensated respiratory acid dosis meaning the pH is normal despite high CO2 because the byarb is also high you know that didn't happen overnight that tells you the kidneys have had days to retain that extra by cararb.
ah so a fully compensated respiratory acidosis points towards a chronic problem like longstanding COPD maybe not an acute overdose that just happened precisely it tells you this patient has likely had this underlying respiratory issue for a while long enough for the slow methodical kidneys to adapt that context completely changes how you I think about their condition and treatment. It's that deeper level of understanding. That is absolutely thinking like a nurse. Okay. Fantastic. So, let's wrap this up. For the nurse heading on to the floor tonight, maybe feeling a bit shaky about ABGs. What are the top, say, three practical tips or pearls to remember. Okay. Three key takeaways. First, read the AG systematically. Always look at pH first, then CO2, then by carb. That order helps prevent confusion. Second, never ever interpret the AB PG in isolation. Look at your patient. If the numbers look terrifyingly bad, but your patient is sitting up talking, looks okay. Question the sample. Maybe it was Venus. Maybe it sat too long. Your patient's clinical presentation is the ultimate truth.
So true. Numbers are important, but the patient comes first. What's the third pearl? Third, think beyond the label. Think cause and correction. Okay, it's metabolic acidosis. Why is it DKA, diarrhea, renal failure? The label just points you in the direction. Your job is to find the root cause and help. Fix that. Perfect. Cause and correction. And maybe a little bonus one. Check the oxygen last. Yes, P2 is vital for assessing stability and oxygenation needs, but it doesn't determine the acidbased diagnosis itself. Focus on pH, CO2, by carb first for that label, then factor in oxygen for the overall picture.
Excellent. So, just to recap, we've gone through the essential ABG values, those building blocks. We've looked at handy tools like the Rome pneummonic and the tic-tac-toe method. And we've really dug into the four major imbalances, what causes them, what they look like, and crucially, the nursing actions you need to take. And understanding that compensation piece, uncompensated, partially, fully, that really gives you insight into how acute or chronic the patients situation is. It helps you anticipate what might happen next, and that is truly thinking like a nurse.
Could have said it better. Thank you so much for joining us for this really important conversation. We genuinely hope this makes you feel way more confident next time you're faced with interpreting an AG result. Please do check in for more cont. conversations like this each week. And don't forget to visit think likelan nurse.org for loads of study guides, practice questions, all the resources you need to really master these concepts. We'll see you next time.