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Nursing Field Notes / Respiratory + Labs · ABG SERIES 1 OF 3 · Pathophysiology
🫁 RESPIRATORY + LABS SET NG-092 ABG · The Method NG-138 ABG · Compensation Practice NG-173 ABG · Answer Key NG-132 Anatomy of Breathing NG-170 Key Terms & Lung Sounds 🧪 By condition 🩺 Bedside tests

ABG Answers 🧪

The 3-step method that reads ANY arterial blood gas — then three worked answers

NG-092 LABS · ACID–BASE ADHD-friendly visual edition

An ABG looks like a wall of numbers. It isn't. There are only four possible answers, and you find yours with three questions asked in the same order every single time: which way did the pH go → which partner moved with it → is the other one trying to fix it. This page is the method and the three uncompensated gases from the answer sheet, worked out in full. Compensation gets its own two pages — NG-138 and NG-173.

📄 Simple Nursing original — opens in Drive →

1️⃣ pH FIRSTBelow 7.35 = acidosis. Above 7.45 = alkalosis. Nothing else names it.
2️⃣ FIND THE MATCHThe value that moved the SAME acid/base direction as the pH is the primary problem. CO₂ = lungs. HCO₃⁻ = kidneys.
3️⃣ COMPENSATED?pH still abnormal + partner normal = uncompensated. Partner moved = partial. pH back in range = full.
🧠 ROMERespiratory Opposite · Metabolic Equal. Compare the pH arrow with the partner arrow.
🧭

PART 1 · THE METHOD

3 STEPS, ALWAYS THE SAME ORDER

Follow the tree with your finger. You cannot get lost if you never skip a step.

🗺️ The ABG decision tree — the whole method in one picture

READ ANY ABG IN 3 STEPS — follow it with your finger Flowchart · read top to bottom · typical adult reference ranges, varies by lab KEY question acid base answer / result STEP 1 Look at the pH first. Which way did it move? The pH alone tells you acidosis or alkalosis — nothing else does. pH < 7.35 ACIDOSIS the blood is too acidic pH 7.35 – 7.45 NORMAL pH — but check both truly normal, OR fully compensated pH > 7.45 ALKALOSIS the blood is too alkaline STEP 2 Now find the MATCH — which partner AGREES with the pH? Check PaCO₂ (the lung number) and HCO₃⁻ (the kidney number). Whichever one agrees with the pH is the PRIMARY problem. PaCO₂ is the one that matches CO₂ is an ACID gas. High CO₂ = acid, low CO₂ = base. pH ↓ with CO₂ ↑ → RESPIRATORY ACIDOSIS pH ↑ with CO₂ ↓ → RESPIRATORY ALKALOSIS HCO₃⁻ is the one that matches HCO₃⁻ is a BASE. Low HCO₃⁻ = acid, high = base. pH ↓ with HCO₃⁻ ↓ → METABOLIC ACIDOSIS pH ↑ with HCO₃⁻ ↑ → METABOLIC ALKALOSIS “Respiratory Opposite · Metabolic Equal” — ROME · see the gauges below STEP 3 Is the OTHER value trying to fix it? That is compensation. The partner that did NOT cause the problem should drift the OPPOSITE way to drag the pH back toward 7.40. How far it got names the state. UNCOMPENSATED pH abnormal partner still NORMAL the rescue has not started PARTIALLY COMPENSATED pH STILL abnormal partner HAS moved the other way help arrived — not enough yet FULLY COMPENSATED pH back INSIDE 7.35–7.45 BOTH CO₂ and HCO₃⁻ abnormal balance restored the hard way THE ONLY 4 ANSWERS RESPIRATORY ACIDOSIS pH ↓ · CO₂ ↑ hypoventilation — CO₂ trapped in RESPIRATORY ALKALOSIS pH ↑ · CO₂ ↓ hyperventilation — CO₂ blown off METABOLIC ACIDOSIS pH ↓ · HCO₃⁻ ↓ acid gained or base lost METABOLIC ALKALOSIS pH ↑ · HCO₃⁻ ↑ base gained or acid lost

Read top to bottom. Step 1 sorts acid from base, step 2 names the organ, step 3 names the compensation state. Every ABG in your program ends in one of the four boxes at the bottom.

🧠 pH → partner → rescue. Three questions, four answers, no exceptions.

🧪 The five numbers you are handed — and what each one is for

pH7.35–7.45< 7.35 acid · > 7.45 base
PaCO₂35–45mmHg · the LUNG number
HCO₃⁻22–26mEq/L · the KIDNEY number
PaO₂80–100mmHg · oxygenation
SaO₂95–100%saturation

These are typical adult reference ranges and vary by lab — always read the range printed on your facility's own report.

  • pH, PaCO₂ and HCO₃⁻ answer the acid–base question. Those three are all you need for steps 1–3.
  • PaO₂ and SaO₂ answer a completely separate question: is this person oxygenated? A gas can be perfectly balanced and still show dangerous hypoxemia.
🧠 Two questions, not one: “what's the acid–base problem?” and “is the O₂ okay?”

1️⃣ Step 1 — look at the pH and nothing else

↓
pH below 7.35 → ACIDOSIS. The blood is too acidic. Say the word out loud before you look at anything else.
↑
pH above 7.45 → ALKALOSIS. The blood is too alkaline.
=
pH 7.35–7.45 → normal. Do not stop. A normal pH with two abnormal partners means fully compensated, and you still have to name what it was compensating for.
🧠 The pH is the verdict; the partners are the evidence. Verdict first.

2️⃣ Step 2 — find the partner that MATCHES the pH

Ask each partner “are you acid or base right now?” and keep the one that agrees with the pH.

  • PaCO₂ is an ACID gas. High CO₂ = acidic. Low CO₂ = alkaline. (It reads backwards — that is why ROME exists.)
  • HCO₃⁻ is a BASE. High HCO₃⁻ = alkaline. Low HCO₃⁻ = acidic. (It reads the same way as the pH.)
  • The one that agrees is the primary problem. CO₂ agrees → respiratory. HCO₃⁻ agrees → metabolic.
🧠 “Who's on the pH's side?” That one caused it. The other one is either innocent or rescuing.

🧠 ROME — Respiratory Opposite, Metabolic Equal

ROME — the arrow trick that names the disorder Memory device (not anatomy) · compare the pH arrow with the partner arrow KEY ▲ value HIGH · ▼ value LOW gold arrow = pH · colored arrow = partner red plate = acid result · blue plate = alkaline R O — RESPIRATORY OPPOSITE pH and PaCO₂ point OPPOSITE ways M E — METABOLIC EQUAL pH and HCO₃⁻ point the SAME way pH 7.25 PaCO₂ 55 OPPOSITE RESPIRATORY ACIDOSIS CO₂ went UP, pH went DOWN — opposite arrows, and CO₂ is the lung number. ▲▼ compare the two arrows above pH 7.57 PaCO₂ 25 OPPOSITE RESPIRATORY ALKALOSIS CO₂ went DOWN, pH went UP — opposite again. Blowing off acid gas. ▲▼ compare the two arrows above pH 7.21 HCO₃⁻ 19 SAME METABOLIC ACIDOSIS Both arrows DOWN — equal. Base was lost, so the pH fell with it. ▲▼ compare the two arrows above pH 7.55 HCO₃⁻ 35 SAME METABOLIC ALKALOSIS Both arrows UP — equal. Base was gained, so the pH rose with it. ▲▼ compare the two arrows above 🧠 ROME: Respiratory = Opposite · Metabolic = Equal. Line the pH arrow up beside the partner arrow — if they disagree it is a LUNG problem; if they agree it is a KIDNEY / metabolic problem.

Draw two arrows: one for the pH, one for the partner. If they point opposite ways it is a lung problem. If they point the same way it is a metabolic problem.

🧠 R–O–M–E. Say it as you draw the arrows and step 2 takes about two seconds.

3️⃣ Step 3 — is the other value trying to rescue the pH?

Answer first: look at the value that did not cause the problem.

  • Still normal → UNCOMPENSATED. Nobody has come to help yet.
  • Abnormal, in the opposite acid/base direction, but pH still out of range → PARTIALLY compensated.
  • Abnormal and the pH has crawled back inside 7.35–7.45 → FULLY compensated.
🧠 The source's phrase is perfect: compensation is the body “finding common ground.”

📏 The pH scale — where each compensation state actually sits

THE pH SCALE — and what “compensated” looks like on it One axis · left = acid, right = base · arterial pH, typical adult reference 7.35–7.45 (varies by lab) ACIDOSIS ALKALOSIS NORMAL 7.35–7.45 6.80 6.90 7.00 7.10 7.20 7.30 7.35 7.45 7.50 7.60 7.70 7.80 ← about 6.8 = lower limit compatible with life about 7.8 = upper limit → TARGET — pH 7.40 Everything below is dragged back toward this line ▶ pH 7.22 ① UNCOMPENSATED pH is OUT of range. The partner value has NOT moved yet — no rescue in progress. This is the fresh, untreated problem. pH 7.31 ② PARTIALLY COMPENSATED pH is STILL out of range, but the partner HAS moved the opposite way and is dragging the pH back toward 7.40. pH 7.37 ③ FULLY COMPENSATED pH made it back INSIDE 7.35–7.45 — but BOTH CO₂ and HCO₃⁻ are abnormal. Balance was bought, not restored. ⚠️ Full compensation NEVER overshoots. The body drags the pH back toward 7.40 but never pushes it past. So a fully compensated pH of 7.37 started as an ACIDOSIS, and a fully compensated 7.43 started as an ALKALOSIS.

One axis, three zones, three states. Notice that full compensation lands the pH back inside the green window but never pushes it past 7.40 to the other side.

🧠 7.40 is the magnet. Compensation drags toward it and stops.

⚠️ The trap that catches everyone: a normal pH

A normal pH does NOT mean a normal gas. Check whether CO₂ and HCO₃⁻ are both abnormal.

  • pH normal + CO₂ normal + HCO₃⁻ normal = a genuinely normal ABG.
  • pH normal + both partners abnormal = fully compensated — there is still a real disorder underneath.
  • To name it: which side of 7.40 is the pH on? 7.35–7.39 came from an acidosis; 7.41–7.45 came from an alkalosis.
🧠 “Normal pH, abnormal partners = the body already fixed it — badly.”

✅ Read them in the same order every time

  • pH — acid or base?
  • PaCO₂ — does it match the pH?
  • HCO₃⁻ — does it match the pH?
  • Name it — respiratory or metabolic + acidosis or alkalosis.
  • Compensation — none, partial or full?
  • Then look at PaO₂ / SaO₂ and ask the oxygenation question separately.
🧠 A fixed order beats cleverness. Same six moves, every gas, every time.
🫁

PART 2 · WHO FIXES WHAT

LUNGS vs KIDNEYS

Two organs, two dials, two completely different speeds — and that speed difference is half the exam.

🫁 The two organs that control your pH — and how fast each one moves

WHO FIXES WHAT — and how fast Anterior view of the trunk · skin and ribs removed · read top (lungs) to bottom (kidneys) KEY CO₂ — acid gas, leaves on the breath HCO₃⁻ — base, returned to the blood arterial blood (aorta, renal arteries) CO₂ CO₂ CO₂ CO₂ CO₂ exhaled CO₂ — acid leaving trachea carina heart (behind) cardiac notch RIGHT 3 lobes LEFT 2 lobes LUNGS control PaCO₂ breathe faster → CO₂ ↓ → pH ↑ breathe slower → CO₂ ↑ → pH ↓ DIAPHRAGM the floor of the chest aorta renal arteries cortex (outer rim) medullary pyramid renal pelvis ureter → bladder KIDNEYS control HCO₃⁻ keep HCO₃⁻ + dump H⁺ → pH ↑ dump HCO₃⁻ + keep H⁺ → pH ↓ HCO₃⁻ reclaimed → back to blood HOW FAST EACH ONE WORKS LUNGS — minutes start within 1–2 minutes · near-maximal within hours · fast, but it is a stop-gap KIDNEYS — hours to days begin within hours · maximal at roughly 3–5 days · slow, but powerful and lasting

The lungs sit above the diaphragm and blow off CO₂ (acid). The kidneys sit below it and handle HCO₃⁻ (base). Structural airway and alveolar anatomy is drawn in full on NG-004 · Anatomy of the Lungs.

SEE ALSO Lobes, pleura, the alveolus and the respiratory membrane live on NG-004 Anatomy of the Lungs; the mechanics of the breath itself are on NG-132 Anatomy of Breathing.
🧠 Lungs = fast and temporary. Kidneys = slow and permanent. Minutes vs days.

⚖️ Why CO₂ counts as an acid at all

WHY CO₂ COUNTS AS AN ACID — the buffer, drawn as a balance Schematic (not anatomy) · the lungs work the left-hand end of the reaction, the kidneys the right CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ carbon dioxide + water ⇄ carbonic acid ⇄ hydrogen ion + bicarbonate ▲ the LUNG end ▲ the KIDNEY end LUNGS KIDNEYS Breathe FASTER → blow CO₂ off acid pan LIGHTER → pH rises Breathe SLOWER → hold CO₂ in acid pan HEAVIER → pH falls Kidney KEEPS / makes HCO₃⁻ base pan HEAVIER → pH rises Kidney DUMPS HCO₃⁻ in urine base pan LIGHTER → pH falls pH 7.40 the pH the body defends CO₂ H⁺ ACID SIDE more of these → pH FALLS HCO₃⁻ BASE SIDE more of these → pH RISES ⭐ One sentence: CO₂ dissolved in blood BECOMES acid — so a rising CO₂ always drags the pH down, and bicarbonate is the body’s stock of base that mops that acid back up.

CO₂ dissolves in plasma, becomes carbonic acid, and splits into H⁺ and HCO₃⁻. That single reversible reaction is why the lung number and the kidney number end up on the same scale.

🧠 CO₂ + water = acid. Every time you hold your breath you are making acid.

🫁 The lungs' dial: respiratory rate and depth

  • Breathe more (faster / deeper) → blow off CO₂ → PaCO₂ falls → pH rises.
  • Breathe less (slower / shallower) → trap CO₂ → PaCO₂ rises → pH falls.
  • Response begins in 1–2 minutes and is near-maximal within hours.
  • This is why Kussmaul respirations appear in metabolic acidosis — deep, rapid breathing is the lungs frantically blowing off acid.
🧠 “Breathe it off.” The lung's only tool is the CO₂ tap.

🫘 The kidneys' dial: keep or dump bicarbonate

  • Acidosis → kidneys reclaim HCO₃⁻ and excrete H⁺ (acidic urine) → HCO₃⁻ rises → pH rises.
  • Alkalosis → kidneys dump HCO₃⁻ and hold H⁺ → HCO₃⁻ falls → pH falls.
  • Starts within hours, maximal at roughly 3–5 days.
  • Because it is slow, a fully compensated respiratory problem tells you the problem is chronic — this is the classic COPD picture.
🧠 Kidneys are the slow, stubborn friend. Late to arrive, but they stay.

🔁 Who compensates for whom — the one rule

Primary problemWho compensatesWhat they doHow fast
Respiratory acidosis (CO₂ ↑)KidneysRetain HCO₃⁻ → HCO₃⁻ riseshours → days
Respiratory alkalosis (CO₂ ↓)KidneysDump HCO₃⁻ → HCO₃⁻ fallshours → days
Metabolic acidosis (HCO₃⁻ ↓)LungsHyperventilate → CO₂ fallsminutes
Metabolic alkalosis (HCO₃⁻ ↑)LungsHypoventilate → CO₂ risesminutes

The organ that did not cause the problem is the one that comes to the rescue.

🧠 “You broke it, I'll fix it.” Lungs rescue kidney problems; kidneys rescue lung problems.
🧪

PART 3 · THREE WORKED GASES

THE METHOD, APPLIED

The three practice gases from the answer sheet — plotted, reasoned and named. All three are uncompensated.

⭐ How to use these three

Cover the answer bar at the bottom of each picture. Work the three steps yourself, out loud, then uncover. All three of these are uncompensated on purpose — the rescuing partner is still sitting in the normal range, which makes step 3 easy and lets you concentrate on steps 1 and 2.

🧠 Practice the METHOD here. Practice COMPENSATION on NG-138.

🧪 Practice 1 — pH 7.25 · PaCO₂ 55 · HCO₃⁻ 25

PRACTICE GAS 1 — plotted on the A/B scales Three scales, one pin each · acid side vs base side KEY A = acid zone normal B = base zone gold pin + dot = THIS patient’s value dashed white box = the normal window pH ref 7.35–7.45 typical adult, varies by lab A ACID BASE B NORMAL 7.00 7.10 7.20 7.30 7.35 7.45 7.50 7.60 7.70 7.80 7.25 ACID (A) PaCO₂ (lungs) ref 35–45 mmHg typical adult, varies by lab B BASE ACID A NORMAL 20 25 30 35 45 50 55 60 65 70 55 ACID (A) HCO₃⁻ (kidneys) ref 22–26 mEq/L typical adult, varies by lab A ACID BASE B NORMAL 10 15 20 22 26 30 35 40 45 50 25 NORMAL – no match RESPIRATORY ACIDOSIS · UNCOMPENSATED ANSWER ▶

The pH pin and the PaCO₂ pin both land in a red ACID zone — that is the match. The HCO₃⁻ pin is still inside its green normal window, so no rescue has started.

🔎 Practice 1 — the reasoning, step by step

1
pH 7.25 is below 7.35 → ACID. We are looking for an acidosis.
2
PaCO₂ 55 is above 45 → ACID. Match! And CO₂ is the lung number → respiratory acidosis.
HCO₃⁻ 25 sits inside 22–26 → normal, no match.
3
Is the pH back in range? No — 7.25 is nowhere near normal, and HCO₃⁻ has not moved at all → UNCOMPENSATED.

Respiratory acidosis, uncompensated

🧠 pH down, CO₂ up — opposite arrows = Respiratory (the R–O of ROME).

🩺 Practice 1 — who is this patient, and what do you do?

A retained-CO₂ picture. Something is stopping air from moving out.

  • Typical causes: opioid or sedative over-sedation, COPD exacerbation, atelectasis or pneumonia, chest-wall or neuromuscular weakness, airway obstruction, post-op splinting from pain.
  • Expect: drowsiness, confusion, headache, flushed warm skin — CO₂ is a CNS depressant and a vasodilator.
  • Nursing: improve ventilation — sit upright, wake and coach deep breathing, incentive spirometer, treat pain so they can breathe, suction if secretions, escalate for possible non-invasive ventilation.
  • Do not simply crank up the oxygen and walk away — this is a ventilation problem, not purely an oxygen problem.
🧠 “CO₂ narcosis.” A sleepy patient with a rising CO₂ is not resting — they are failing.

🧪 Practice 2 — pH 7.57 · PaCO₂ 25 · HCO₃⁻ 22

PRACTICE GAS 2 — plotted on the A/B scales Three scales, one pin each · acid side vs base side KEY A = acid zone normal B = base zone gold pin + dot = THIS patient’s value dashed white box = the normal window pH ref 7.35–7.45 typical adult, varies by lab A ACID BASE B NORMAL 7.00 7.10 7.20 7.30 7.35 7.45 7.50 7.60 7.70 7.80 7.57 BASE (B) PaCO₂ (lungs) ref 35–45 mmHg typical adult, varies by lab B BASE ACID A NORMAL 20 25 30 35 45 50 55 60 65 70 25 BASE (B) HCO₃⁻ (kidneys) ref 22–26 mEq/L typical adult, varies by lab A ACID BASE B NORMAL 10 15 20 22 26 30 35 40 45 50 22 NORMAL – no match RESPIRATORY ALKALOSIS · UNCOMPENSATED ANSWER ▶

Both the pH and the PaCO₂ pins land in blue BASE zones. HCO₃⁻ 22 is at the very bottom edge of its normal window — still normal, so it does not count as a match.

🔎 Practice 2 — the reasoning, step by step

1
pH 7.57 is above 7.45 → BASE (alkalosis).
2
PaCO₂ 25 is below 35 → BASE. Match, and it is the lung number → respiratory alkalosis.
HCO₃⁻ 22 is the low end of 22–26 but still inside it → normal, no match.
3
pH is still far outside the range and HCO₃⁻ has not moved out of normal → UNCOMPENSATED.

Respiratory alkalosis, uncompensated

🧠 A value sitting on the edge of normal is still normal. Don't promote it.

🩺 Practice 2 — who is this patient, and what do you do?

A blown-off-CO₂ picture. Something is making them breathe too much.

  • Typical causes: anxiety or panic, pain, fever, early sepsis, pulmonary embolism, hypoxia, high altitude, over-set mechanical ventilation.
  • Expect: light-headedness, tingling around the mouth and fingers, carpopedal spasm, tetany — alkalosis lowers ionized calcium and makes nerves twitchy.
  • Nursing: find and treat the cause. Coach slow breathing, sit with them, treat pain and fever, and rule out the dangerous causes (PE, sepsis, hypoxia) before you call it anxiety.
🧠 Acidosis = sleepy. Alkalosis = twitchy. Opposite CNS effects.

🧪 Practice 3 — pH 7.21 · PaCO₂ 39 · HCO₃⁻ 19

PRACTICE GAS 3 — plotted on the A/B scales Three scales, one pin each · acid side vs base side KEY A = acid zone normal B = base zone gold pin + dot = THIS patient’s value dashed white box = the normal window pH ref 7.35–7.45 typical adult, varies by lab A ACID BASE B NORMAL 7.00 7.10 7.20 7.30 7.35 7.45 7.50 7.60 7.70 7.80 7.21 ACID (A) PaCO₂ (lungs) ref 35–45 mmHg typical adult, varies by lab B BASE ACID A NORMAL 20 25 30 35 45 50 55 60 65 70 39 NORMAL – no match HCO₃⁻ (kidneys) ref 22–26 mEq/L typical adult, varies by lab A ACID BASE B NORMAL 10 15 20 22 26 30 35 40 45 50 19 ACID (A) METABOLIC ACIDOSIS · UNCOMPENSATED ANSWER ▶

This time the CO₂ pin sits inside the green window — the lungs are innocent. The pH and the HCO₃⁻ pins both land on their red ACID sides, and they are on the same end of their scales.

🔎 Practice 3 — the reasoning, step by step

1
pH 7.21 is below 7.35 → ACID.
2
PaCO₂ 39 is inside 35–45 → normal, no match.
HCO₃⁻ 19 is below 22 → low base = ACID. Match, and it is the kidney number → metabolic acidosis.
3
pH still badly abnormal and the lungs have not started to blow off CO₂ yet → UNCOMPENSATED.

Metabolic acidosis, uncompensated

🧠 pH down, HCO₃⁻ down — same arrows = Metabolic (the M–E of ROME).

🚨 Practice 3 — who is this patient, and what do you do?

Acid gained, or base lost. This is the sickest-looking of the three.

  • Acid gained: DKA (ketoacids), lactic acidosis from shock or sepsis, renal failure (can't excrete acid), certain poisonings.
  • Base lost: severe or prolonged diarrhea — intestinal fluid is bicarbonate-rich.
  • Expect Kussmaul respirations — deep, rapid, sighing breaths as the lungs try to blow off acid. Also watch for hyperkalemia.
  • Nursing: treat the cause — fluids, insulin for DKA, perfusion for shock, stop the diarrhea losses. Monitor potassium and cardiac rhythm.
🧠 Deep and fast in an acidosis is a rescue, not a symptom. Never sedate it away.
⚠️

PART 4 · TRAPS, SAFETY & DRILL

DON'T LOSE EASY MARKS

The mistakes that cost marks on paper and cost time at the bedside.

❌ The five classic ABG mistakes

  • Reading CO₂ as if it behaved like the pH. It is an ACID — high CO₂ means acidic.
  • Calling a normal pH a normal gas. Check both partners before you say “normal”.
  • Promoting an edge value. HCO₃⁻ of 22 is normal. PaCO₂ of 45 is normal.
  • Naming the compensator as the problem. The rescuer moves the opposite way to the pH.
  • Answering the O₂ question with the acid–base answer. They are separate. Look at PaO₂ and SaO₂ too.
🧠 “Match names it, mismatch rescues it.”

🚨 What the pH actually does to your patient

Acidosis (pH ↓)Alkalosis (pH ↑)
CNS depressed — drowsy, confused, headache, coma CNS irritable — anxious, light-headed, seizures
Potassium tends to shift out of cells → hyperkalemia Potassium tends to shift into cells → hypokalemia
Warm, flushed skin (in respiratory acidosis) Tingling lips/fingers, carpopedal spasm, tetany
Kussmaul breathing if metabolicNumbness, muscle cramps, arrhythmias

Acidosis puts them to sleep. Alkalosis winds them up.

🧠 “Down and drowsy · up and uptight.”

✅ Drawing and handling the sample

  • Arterial blood, usually radial — a modified Allen test is commonly performed first to check collateral circulation. Follow your facility's policy.
  • Heparinised syringe, expel air bubbles — air in the sample falsely alters the values.
  • Hold firm pressure for at least 5 minutes after the puncture — longer if the patient is on anticoagulants or has a bleeding disorder.
  • Get it to the lab fast; follow local policy on icing if there will be a delay.
  • Document the oxygen delivery and FiO₂ at the time of the draw — the numbers mean nothing without it.
🧠 No bubbles · press 5 · say what O₂ they were on.

🫁 The oxygenation question — asked separately

  • PaO₂ 80–100 mmHg and SaO₂ 95–100% are typical adult reference values (vary by lab, and by the patient's baseline).
  • Low PaO₂ = hypoxemia — not enough oxygen in the blood.
  • Chronic CO₂ retainers often live with abnormal values; treat the patient and their baseline, not the printout.
  • A gas can read “respiratory acidosis, uncompensated” and show a normal PaO₂ — ventilation and oxygenation are different failures.
🧠 Ventilation moves CO₂. Oxygenation moves O₂. Two problems, two answers.

🗣️ How to say it out loud on the ward

Report the gas in the order you read it. It makes you sound like you know exactly what you are doing — because you will.

Weak: “The gas is bad.”
Strong: “pH 7.25, PaCO₂ 55, HCO₃⁻ 25 on 2 liters nasal cannula — uncompensated respiratory acidosis. He's drowsy and hard to rouse. I've sat him up and I'm calling you now.”

🧠 Numbers, name, patient, action. Four beats.

🎯 Cover & check — eight rapid-fire questions

Q1 · pH 7.30, PaCO₂ 50, HCO₃⁻ 24. Name it. pH acid; CO₂ high = acid → match in the lungs. HCO₃⁻ normal → no rescue. Respiratory acidosis, uncompensated.
Q2 · pH 7.50, PaCO₂ 40, HCO₃⁻ 30. Name it. pH base; CO₂ normal; HCO₃⁻ high = base → match in the kidneys. Metabolic alkalosis, uncompensated.
Q3 · Which value is the LUNG value, and which way does it read? PaCO₂. It is an acid gas, so it reads backwards compared with the pH — high CO₂ = acidic.
Q4 · Which organ compensates for a metabolic acidosis, and how fast? The lungs, by hyperventilating to blow off CO₂ — starting within minutes.
Q5 · What does “uncompensated” actually mean? The pH is abnormal and the partner value that did not cause the problem is still sitting in its normal range. No rescue has begun.
Q6 · A patient's pH is 7.38 but CO₂ is 60 and HCO₃⁻ is 34. Normal? No. Both partners are abnormal, so this is fully compensated. The pH is on the acidic side of 7.40, so the original problem was an acidosis — with a high CO₂, a respiratory acidosis (a classic chronic COPD picture).
Q7 · Why does deep, rapid breathing appear in metabolic acidosis? It is compensation — Kussmaul respirations blow off CO₂ (acid) to pull the pH back up. Treat the acidosis, never the breathing pattern.
Q8 · Which two numbers answer the oxygenation question? PaO₂ (typical adult 80–100 mmHg) and SaO₂ (95–100%) — read separately from the acid–base numbers.

🔗 The ABG series — all three pages, in order

PageWhat it gives you
NG-092 · ABG Answers (1 of 3) THE METHOD — the 3-step read, ROME, the decision tree, the pH scale, which organ fixes what, and three fully worked uncompensated gases.
NG-138 · Compensation Questions (2 of 3) PRACTICE — what compensation actually is, uncompensated vs partial vs full, and four compensation gases plotted for you to answer before you peek.
NG-173 · Compensation Answers (3 of 3) ANSWER KEY — the filled-in decision path for all four gases, plus the causes of each disorder, the expected-compensation check, oxygenation, and the traps.
🧠 Method → Practice → Key. Do them in that order and the fourth ABG stops being scary.
🚨 pH FIRST, ALWAYSBelow 7.35 acid · above 7.45 base. The pH is the verdict; everything else is evidence.
🫁 CO₂ = LUNGSAn ACID gas. High CO₂ = acidic. Reference 35–45 mmHg (typical adult, varies by lab).
🫘 HCO₃⁻ = KIDNEYSA BASE. Low HCO₃⁻ = acidic. Reference 22–26 mEq/L (typical adult, varies by lab).
🧠 ROMERespiratory Opposite · Metabolic Equal. Arrows disagree = lungs. Arrows agree = metabolic.
⏱️ MINUTES vs DAYSLungs compensate in minutes. Kidneys take hours to days. A fully compensated respiratory problem is a CHRONIC one.
⚠️ NORMAL pH ≠ NORMAL GASIf both partners are abnormal it is fully compensated. Which side of 7.40 tells you what it started as.