🏠 Study Hub 🖼️ Infographics
Nursing Field Notes / Respiratory · Acid–Base · Pathophysiology
🫁 ABG SERIES 1 · The method 2 · Practice gases 3 · Worked answers I 4 · Worked answers II 🧪 By condition 🩺 Bedside tests

ABG Compensation
Answers II 🧪

Set 2 — the harder gases: normal pH, partial compensation, and the mixed disorders

NG-193 RESPIRATORY · ACID-BASE ABG Series 4 of 4 ADHD-friendly visual edition

Set 1 gave you gases where the pH told you the answer. This set does not. Here the pH is often normal — and a normal pH does not mean a normal patient. It usually means the body has already fixed the problem, and your job is to work out which system caused it and which one did the fixing. When the pH is inside the range, the tie-breaker is 7.40. Above it, base is winning. Below it, acid is winning.

📄 Simple Nursing original — opens in Drive →

📏 RangespH 7.35–7.45 · PaCO₂ 35–45 · HCO₃⁻ 22–26 · PaO₂ 80–100 (typical adult; labs vary)
🧠 ROMERespiratory Opposite · Metabolic Equal. pH and CO₂ move opposite ways; pH and HCO₃⁻ move the same way.
⚖️ Normal pH tie-breakCompare to 7.40. Below = acid is winning. Above = base is winning.
🔀 Both abnormal, same directionThat is a mixed disorder — nobody is compensating.
🧭

THE METHOD — FIVE STEPS, EVERY TIME

STEP 1 · TOOL

Same five steps for every gas on this page. Do them in order and never skip step 4.

📏 The reference values you are comparing against

pH7.35–7.457.40 = midpoint
PaCO₂35–45mmHg · the LUNG value
HCO₃⁻22–26mEq/L · the KIDNEY value
PaO₂80–100mmHg · oxygenation only
⚠️ These are typical adult reference ranges and they vary between laboratories, with altitude, and with age. Always classify against the range printed on the report you are holding.
🧠 CO₂ is an acid; HCO₃⁻ is a base. High CO₂ pushes pH down. High HCO₃⁻ pushes pH up. Everything on this page follows from those two sentences.

🪜 The five steps

Look at the pH. Below 7.35 = acidemia. Above 7.45 = alkalemia. Inside the range = normal — but keep reading.
Look at the PaCO₂ — the respiratory arm. Above 45 = acid. Below 35 = base. Inside = normal.
Look at the HCO₃⁻ — the metabolic arm. Below 22 = acid. Above 26 = base. Inside = normal.
Match. Which arm points the same way the pH does? That arm is the primary problem. If the pH is normal, compare it to 7.40 to decide which side it leans. If both arms point the same way as each other, it is a mixed disorder.
Name the compensation. Other arm normal ⇒ uncompensated. Other arm abnormal but pH still outside the range ⇒ partially compensated. pH back inside the range ⇒ fully compensated.
🧠 Steps 1–3 are just reading. Step 4 is the thinking. Step 5 is the label. Students lose marks by jumping from step 1 to step 5.

📉 The pH ruler — with the compensation zones marked

THE pH RULER the whole normal range is only 0.10 pH units wide — that green sliver is what the body defends Gas 9 · 7.04 Gas 5 · 7.22 Gas 3 · 7.26 Gas 6 · 7.47 Gas 4 · 7.53 Gas 10 · 7.55 6.90 7.35 7.45 7.70 ACIDEMIA · pH below 7.35 ALKALEMIA · pH above 7.45 NORMAL ZOOM · the normal band magnified — four of the eleven gases live in here 7.357.407.45 acid-leaning half base-leaning half Gas 1 & 8 · 7.37 Gas 11 · 7.39 Gas 7 · 7.42 Gas 2 · 7.43 pH outside the range pH inside the range Gas 1 and Gas 8 share a pH and have opposite causes — pH alone never finishes the job.
🧠 Gas 1 and Gas 8 sit on the exact same pH (7.37) but have opposite primary problems. The pH alone can never finish the job.

🌳 The decision tree — trace it with your finger

ABG DECISION TREE start at the top · follow one arrow at a time · finish at a gold box START · look at the pH normal range 7.35 – 7.45 pH < 7.35 · ACIDEMIA pH 7.35 – 7.45 · NORMAL pH > 7.45 · ALKALEMIA CO₂ HIGH? (> 45 · acid) the lungs did it HCO₃⁻ LOW? (< 22 · acid) the kidneys / metabolism did it RESPIRATORY ACIDOSIS METABOLIC ACIDOSIS BOTH acid ⇒ MIXED ACIDOSIS (Gas 9) Are CO₂ AND HCO₃⁻ both normal? YES ⇒ acid–base status is normal NO ⇒ compare the pH to 7.40 pH < 7.40 acid arm is primary pH > 7.40 base arm is primary …and the pH is normal ⇒ FULLY COMPENSATED CO₂ LOW? (< 35 · base) blowing it off HCO₃⁻ HIGH? (> 26 · base) too much base RESPIRATORY ALKALOSIS METABOLIC ALKALOSIS BOTH base ⇒ MIXED ALKALOSIS (Gas 10) LAST STEP · name the compensation other arm normal ⇒ NONE pH still abnormal ⇒ PARTIAL pH normal ⇒ FULL

🪢 Why "who's winning the tug of war?" actually works

THE TUG OF WAR lungs hold the acid end · kidneys hold the base end · the knot is the pH LUNGS pull with CO₂ — the ACID end fast: minutes KIDNEYS pull with HCO₃⁻ — the BASE end slow: hours to days normal band 7.35–7.45 7.40 = center line pH Knot is left of center ⇒ ACID is winning ⇒ the acid value is the PRIMARY problem (even though the knot is still inside the green band)
🧠 Compensation never overshoots. The body pulls the pH back toward 7.40 but does not drag it past. That is precisely why the side of 7.40 identifies the culprit.

🧭 ROME, drawn — and the right way round

ROME · Respiratory Opposite · Metabolic Equal each arrow shows which way that value moves — read the pair together RESPIRATORY = OPPOSITE PaCO₂ ⇒ pH PaCO₂ ⇒ pH the arrows point in OPPOSITE directions CO₂ is an acid — more of it means a lower pH METABOLIC = EQUAL HCO₃⁻ ⇒ pH HCO₃⁻ ⇒ pH the arrows point in the SAME direction HCO₃⁻ is a base — more of it means a higher pH
⚠️ Check you have ROME the right way round. Respiratory = Opposite (pH and CO₂ disagree). Metabolic = Equal (pH and HCO₃⁻ agree). Getting these swapped flips every answer on the page.

✅ Write it down in this order, every single time

  • 1️⃣ pH: low / normal / high — and if normal, above or below 7.40.
  • 2️⃣ CO₂: acid / normal / base.
  • 3️⃣ HCO₃⁻: acid / normal / base.
  • 4️⃣ Primary: the arm that agrees with the pH direction.
  • 5️⃣ Compensation: none / partial / full.
  • 6️⃣ Then — separately — the PaO₂.
🧠 Six lines on scrap paper beats trying to hold it all in your head under exam pressure.

❌ The four mistakes that cost the most marks

  • ❌ Stopping at "pH is normal, so this gas is normal."
  • ❌ Forgetting to compare a normal pH to 7.40.
  • ❌ Calling a mixed disorder "partially compensated."
  • ❌ Never looking at the PaO₂.
⚠️ Three of those four appear somewhere in the eleven gases below. They are deliberate.
🧮

WORKED GASES — SET 2

STEP 2 · PRACTICE

Cover the verdict, work the five steps yourself, then uncover. Every answer here was derived from the numbers, not copied.

🧪 Gas 1 from the source sheet · Question 3

pH7.37normal — acid side
PaCO₂52HIGH = acid
HCO₃⁻32HIGH = base
PaO₂78borderline low
pH 7.37 sits inside 7.35–7.45 ⇒ no acidemia or alkalemia is being reported. Keep going.
PaCO₂ 52 is above 45 ⇒ the respiratory arm is acidic.
HCO₃⁻ 32 is above 26 ⇒ the metabolic arm is alkalotic.
Both arms are abnormal and pointing opposite ways, so one is the problem and the other is the fix. The pH is 7.37, which is below 7.40 ⇒ acid is winning ⇒ the acid value (PaCO₂) is the primary. ROME check: respiratory opposite — CO₂ up while pH drifts down. ✓
The pH has made it all the way back inside the normal range ⇒ fully compensated. The raised HCO₃⁻ is the kidneys' work.
AnswerRespiratory acidosis — fully compensated
🧠 Typical patient: a long-standing CO₂ retainer whose kidneys have had weeks to build up bicarbonate. Chronic, not acute.

🧪 Gas 2 from the source sheet · Question 4

pH7.43normal — base side
PaCO₂43normal (top end)
HCO₃⁻33HIGH = base
PaO₂92normal
pH 7.43 is inside the range ⇒ no acidemia or alkalemia reported.
PaCO₂ 43 is inside 35–45 ⇒ the respiratory arm is normal.
HCO₃⁻ 33 is above 26 ⇒ the metabolic arm is alkalotic.
Only one arm is abnormal, so it must be the primary. The pH is 7.43, above 7.40 ⇒ base is winning, which matches the raised bicarbonate. ROME check: metabolic equal — HCO₃⁻ up, pH up. ✓ ⇒ metabolic alkalosis.
The pH is inside the normal range ⇒ by the standard rule this is fully compensated. Note that PaCO₂ 43 sits at the top of normal — exactly the direction respiratory compensation moves (hypoventilating to hold CO₂ in) — but it has not yet left the range, so the compensation is early or minimal.
AnswerMetabolic alkalosis — fully compensated (pH restored to normal)
⚠️ Be ready for either wording. Some texts call this "fully compensated" because the pH is normal; others say "compensation is minimal" because the PaCO₂ is still inside its range. The primary disorder — metabolic alkalosis — is not in dispute, and that is what the question is really testing.

🔍 A note on the numbers, so nothing surprises you later

On a real analyzer the three values are mathematically linked — pH, PaCO₂ and HCO₃⁻ always hang together, because the bicarbonate is calculated from the other two. Teaching sheets often round each number independently, so a practice trio can be a little "off" from a physiologically exact gas.

That does not change anything you do. The classification method is what is being tested, and it gives the same answer either way. Gases 3 to 11 below have been checked to hang together properly.

🧠 If a real gas ever looks internally impossible, question the sample (venous? air bubble? delay?) rather than your method.

🧪 Gas 3 COPD exacerbation

pH7.26LOW = acidemia
PaCO₂62HIGH = acid
HCO₃⁻27slightly HIGH = base
PaO₂52LOW = hypoxemia
pH 7.26 is below 7.35 ⇒ acidemia. The pH itself already names the direction.
PaCO₂ 62 is well above 45 ⇒ respiratory arm is acidic — it points the same way as the pH.
HCO₃⁻ 27 is just above 26 ⇒ metabolic arm is mildly alkalotic — the opposite way.
The arm that matches the pH is the respiratory one ⇒ respiratory acidosis. ROME: CO₂ up, pH down — opposite. ✓ The mildly raised bicarbonate is the kidneys trying to help.
Compensation has started (HCO₃⁻ has left its range) but the pH is still abnormal ⇒ partially compensated.
AnswerRespiratory acidosis — partially compensated, with hypoxemia
⚠️ The PaO₂ 52 is a separate problem and, clinically, often the more urgent one. Never report an ABG without looking at the oxygen.

🧪 Gas 4 acute anxiety / panic, rapid deep breathing

pH7.53HIGH = alkalemia
PaCO₂30LOW = base
HCO₃⁻24normal
PaO₂101normal / high-normal
pH 7.53 is above 7.45 ⇒ alkalemia.
PaCO₂ 30 is below 35 ⇒ respiratory arm is basic (CO₂ has been blown off).
HCO₃⁻ 24 is inside 22–26 ⇒ metabolic arm is normal.
Only the respiratory arm is abnormal and it points the same way as the pH ⇒ respiratory alkalosis. ROME: CO₂ down, pH up — opposite. ✓
The metabolic arm has not moved at all ⇒ uncompensated. This makes sense: renal compensation takes hours to days, and this problem is minutes old.
AnswerRespiratory alkalosis — uncompensated
🧠 Uncompensated usually means acute. A completely normal partner value is a clock: the problem has not been going on long enough for the kidneys to respond.

🧪 Gas 5 diabetic ketoacidosis

pH7.22LOW = acidemia
PaCO₂20LOW = base
HCO₃⁻8VERY LOW = acid
PaO₂100normal
pH 7.22 is below 7.35 ⇒ acidemia, and a marked one.
PaCO₂ 20 is below 35 ⇒ respiratory arm is basic — the opposite direction to the pH, so it is not the cause.
HCO₃⁻ 8 is far below 22 ⇒ metabolic arm is strongly acidic — same direction as the pH.
The metabolic arm matches ⇒ metabolic acidosis. ROME: metabolic equal — HCO₃⁻ down, pH down. ✓ The very low CO₂ is the lungs compensating by hyperventilating (this is what Kussmaul respirations look like on paper).
Compensation is vigorous but the pH is still well outside the range ⇒ partially compensated.
AnswerMetabolic acidosis — partially compensated
⚠️ Do not read the low CO₂ as "the patient is breathing too fast, slow them down." That deep rapid breathing is the only thing keeping the pH as high as 7.22. Treat the cause, not the respiratory rate.

🧪 Gas 6 three days of vomiting / continuous NG suction

pH7.47HIGH = alkalemia
PaCO₂48HIGH = acid
HCO₃⁻34HIGH = base
PaO₂86normal
pH 7.47 is above 7.45 ⇒ alkalemia.
PaCO₂ 48 is above 45 ⇒ respiratory arm is acidic — the opposite direction to the pH.
HCO₃⁻ 34 is above 26 ⇒ metabolic arm is basic — the same direction as the pH.
Metabolic arm matches ⇒ metabolic alkalosis (hydrogen ion and chloride lost in the vomit). ROME: metabolic equal — HCO₃⁻ up, pH up. ✓ The raised CO₂ is the lungs hypoventilating on purpose to hold acid in.
The other arm has clearly moved but the pH is still outside the range ⇒ partially compensated.
AnswerMetabolic alkalosis — partially compensated
🧠 Respiratory compensation for a metabolic alkalosis is limited — the patient still has to breathe. That is why full compensation here is uncommon.

🧪 Gas 7 long-standing hyperventilation (e.g. chronic hypoxic drive)

pH7.42normal — base side
PaCO₂30LOW = base
HCO₃⁻19LOW = acid
PaO₂96normal
pH 7.42 is inside the range ⇒ no acidemia or alkalemia reported. This is where students stop too early.
PaCO₂ 30 is below 35 ⇒ respiratory arm is basic.
HCO₃⁻ 19 is below 22 ⇒ metabolic arm is acidic.
Both arms abnormal, opposite directions. Tie-break on 7.40: the pH is 7.42, above it ⇒ base is winning ⇒ the base value (the low CO₂) is primary ⇒ respiratory alkalosis. ROME: CO₂ down, pH drifting up — opposite. ✓
pH is back inside the range ⇒ fully compensated. The low bicarbonate is the kidneys excreting base to match.
AnswerRespiratory alkalosis — fully compensated
🧠 Only 0.02 pH units decided this. Nothing else in medicine turns on such a small number — which is exactly why you write down whether the pH is above or below 7.40 before you do anything else.

🧪 Gas 8 chronic kidney disease

pH7.37normal — acid side
PaCO₂32LOW = base
HCO₃⁻18LOW = acid
PaO₂92normal
pH 7.37 is inside the range — the same pH as Gas 1, and the answer will be completely different.
PaCO₂ 32 is below 35 ⇒ respiratory arm is basic.
HCO₃⁻ 18 is below 22 ⇒ metabolic arm is acidic.
Both abnormal, opposite ways. Tie-break: pH 7.37 is below 7.40 ⇒ acid is winning ⇒ the acid value is primary. Here the acid value is the bicarbonate, not the CO₂ ⇒ metabolic acidosis. ROME: metabolic equal — HCO₃⁻ down, pH down. ✓ The low CO₂ is respiratory compensation.
pH is inside the range ⇒ fully compensated.
AnswerMetabolic acidosis — fully compensated
⚠️ Compare Gas 1 and Gas 8 side by side. Identical pH of 7.37. In Gas 1 the acid value was the CO₂, so it was respiratory. Here the acid value is the bicarbonate, so it is metabolic. The pH never tells you which arm — it only tells you which direction.
🔥

THE HARD ONES — MIXED & TRICKY

STEP 3 · LEVEL UP

When both arms point the same way, nobody is compensating — and that is its own diagnosis.

🧪 Gas 9 cardiac arrest / prolonged shock

pH7.04VERY LOW = acidemia
PaCO₂58HIGH = acid
HCO₃⁻15LOW = acid
PaO₂42SEVERELY LOW
pH 7.04 ⇒ profound acidemia.
PaCO₂ 58 is high ⇒ respiratory arm is acidic.
HCO₃⁻ 15 is low ⇒ metabolic arm is acidic too.
Both arms point the same way as the pH. There is no "other arm" left to be the compensator — both systems are contributing ⇒ this is a mixed (combined) acidosis. Ventilation has failed and tissue hypoperfusion is generating lactic acid.
Compensation is absent by definition: a mixed disorder has no compensating arm. Do not label it partial or full.
AnswerMixed (combined) respiratory and metabolic acidosis — no compensation possible
⚠️ This is the sickest gas on the page. Severe acidemia depresses myocardial contractility and blunts the response to catecholamines. It is an airway, breathing and circulation emergency, not a calculation exercise.

🧪 Gas 10 vomiting plus over-ventilation

pH7.55HIGH = alkalemia
PaCO₂33LOW = base
HCO₃⁻28HIGH = base
PaO₂97normal
pH 7.55 ⇒ marked alkalemia.
PaCO₂ 33 is below 35 ⇒ respiratory arm is basic.
HCO₃⁻ 28 is above 26 ⇒ metabolic arm is basic as well.
Both arms are pushing the pH the same way ⇒ mixed (combined) alkalosis. In practice: acid lost from the stomach and CO₂ blown off — for example a ventilated post-operative patient on continuous NG suction.
No compensating arm exists ⇒ compensation absent.
AnswerMixed (combined) respiratory and metabolic alkalosis
🧠 Mixed disorders produce the most extreme pH values, because nothing is pulling the other way. An unusually far-out pH should make you check for two problems, not one.

🧪 Gas 11 the distractor

pH7.39normal
PaCO₂41normal
HCO₃⁻24normal
PaO₂66LOW = hypoxemia
pH 7.39 inside the range.
PaCO₂ 41 inside 35–45 ⇒ respiratory arm normal.
HCO₃⁻ 24 inside 22–26 ⇒ metabolic arm normal.
Nothing is abnormal and nothing is compensating ⇒ the acid–base status is normal. There is nothing to compensate for, so no compensation label applies.
But look at the fourth number: PaO₂ 66 is well below 80 ⇒ hypoxemia. This patient has a real, treatable problem that the acid–base analysis completely missed.
AnswerNormal acid–base status with hypoxemia
⚠️ Acid–base and oxygenation are two separate questions on one report. Answer both, every time.

🗺️ All eleven gases on one plot

CO₂ vs HCO₃⁻ · the four corners same direction on both axes = MIXED · opposite directions = one is compensating for the other both values normal PaCO₂ (mmHg) → 15354570 HCO₃⁻ (mEq/L) → 5222640 LOW CO₂ · HIGH HCO₃⁻ both base ⇒ MIXED ALKALOSIS HIGH CO₂ · HIGH HCO₃⁻ compensated resp. acidosis or metabolic alkalosis LOW CO₂ · LOW HCO₃⁻ compensated resp. alkalosis or metabolic acidosis HIGH CO₂ · LOW HCO₃⁻ both acid ⇒ MIXED ACIDOSIS 123 456 78 91011 fully compensated partially compensated uncompensated mixed fully compensated (metabolic-side lean) normal acid–base
🧠 Diagonal = compensation. Same corner = mixed. If a point sits top-right or bottom-left, one system is helping the other. If it sits top-left or bottom-right, both systems are pushing the same way and nobody is helping.

📊 The three compensation states, side by side

UNCOMPENSATED → PARTIAL → FULL green band = normal pH · gold pin = where the pH is · the two chips under each panel are the two arms UNCOMPENSATED pH outside the band problem arm other arm NORMAL nobody has started helping usually means ACUTE (Gas 4) PARTIALLY COMPENSATED pH moving back, still outside problem arm other arm MOVED help has started, not finished Gases 3, 5, 6 FULLY COMPENSATED pH back inside the band problem arm other arm MOVED both still abnormal — pH fixed Gases 1, 2, 7, 8
🧠 "Full" does not mean "better." A fully compensated gas often means the disorder has been present for a long time. The patient is stable, not cured.
🩺

SO WHAT? — WHAT THE GAS MEANS FOR THE PATIENT

STEP 4 · ACT

Naming the disorder is half the answer. The other half is which system to look at and how fast.

🫁🫘 Who fixes what, and how fast

THE TWO COMPENSATORS left: alveolus with its capillary, cutaway  ·  right: kidney tubule cell, cutaway CO₂ crosses into the air sac … … and is exhaled LUNGS — the fast compensator breathe faster/deeper ⇒ CO₂ falls ⇒ pH rises breathe slower/shallower ⇒ CO₂ rises ⇒ pH falls = CO₂ molecule tubule lumen (urine) tubule cell blood HCO₃⁻ reabsorbed (base kept in the body) H⁺ secreted (acid leaves in urine) KIDNEYS — the slow compensator keep or dump bicarbonate · excrete or retain acid SPEED: lungs: minutes kidneys: hours to days ⇒ full compensation = chronic
🧠 Speed is a diagnostic clue. A fully compensated respiratory problem has been there for days. An uncompensated one started today.

🫁 Respiratory acidosis — what you actually do

The problem is ventilation. CO₂ is not being blown off.

  • 🛏️ Position upright; encourage deep breathing and effective coughing.
  • 🫧 Look for the cause: airway obstruction, sedation/opioids, atelectasis, pneumonia, COPD exacerbation, chest-wall or neuromuscular weakness.
  • 📈 Monitor level of consciousness — rising CO₂ makes patients drowsy and confused before they arrest.
  • 🆘 Escalate for non-invasive or invasive ventilatory support when the pH keeps falling.
⚠️ In a chronic CO₂ retainer, oxygen is titrated carefully to the target the team sets — never simply turn it up to maximum without reassessing ventilation.

🌬️ Respiratory alkalosis — what you actually do

The problem is over-ventilation. CO₂ is being blown off faster than it is made.

  • 🧘 Coach slow, controlled breathing; stay with the patient; reduce stimulation.
  • 🔎 Rule out the dangerous causes before calling it anxiety: hypoxemia, pain, fever, sepsis, pulmonary embolism, salicylate toxicity, and over-set ventilator rates.
  • 🖐️ Expect tingling around the mouth and in the fingers, light-headedness, carpopedal spasm.
🧠 Hyperventilation is a symptom. Ask what the body is trying to fix before you try to slow it down.

🧪 Metabolic acidosis — what you actually do

Acid is being made or base is being lost.

  • 🍬 Think DKA, lactic acidosis from shock or hypoxia, kidney failure, severe diarrhea (bicarbonate lost in stool), some poisonings.
  • 💨 Kussmaul respirations — deep, rapid, sighing breaths — are compensation, not distress to be suppressed.
  • 🧪 Check potassium: acidosis tends to shift potassium out of cells, so the serum value can look normal or high while total body potassium is low.
  • 💧 Treat the cause — fluids, insulin, perfusion, dialysis as indicated.

🤮 Metabolic alkalosis — what you actually do

Acid is being lost or base is being gained.

  • 🤢 Think vomiting, prolonged NG suction, excessive diuretic use, potassium depletion, excessive alkali intake.
  • 🧪 Watch potassium, chloride, magnesium and volume status — these usually need correcting for the alkalosis to resolve.
  • 😖 Expect tingling, muscle twitching, tremor, tetany, and confusion in severe cases.
  • 🫁 Compensation is limited: the patient cannot hypoventilate indefinitely.

✅ Getting the sample right — the part that ruins gases

  • 🩸 Arterial sample, drawn per policy; apply firm pressure afterwards for the time your facility specifies, and longer if the patient is anticoagulated.
  • 🫧 Expel air bubbles immediately — trapped air changes the PaO₂ and PaCO₂.
  • ⏱️ Send it straight away; delay allows continued cell metabolism and shifts the values.
  • 📝 Document the oxygen delivery and flow rate at the moment of sampling — a PaO₂ means nothing without it.
  • 🖐️ Check circulation to the hand before and after a radial puncture, per policy.
⚠️ A venous sample interpreted as arterial will look like a respiratory acidosis that is not there. If the gas does not fit the patient in front of you, question the sample.

🧍 What acidosis and alkalosis look like at the bedside

CLINICAL PICTURE · acidosis vs alkalosis anterior view · the signs overlap, so always confirm with the gas ACIDOSIS Drowsy, confused headache · CNS depression Kussmaul breathing deep, rapid — compensation Warm, flushed skin weakness, fatigue, nausea K⁺ shifts OUT of cells serum K⁺ may read high ALKALOSIS Light-headed, anxious confusion · irritability Tingling round the mouth a very early, very typical sign Tingling fingers, twitching tremor, tetany, spasm K⁺ shifts INTO cells serum K⁺ may fall Acidosis sedates. Alkalosis excites. Both move potassium — check the level and the rhythm.
🧠 "Down and drowsy, up and twitchy." A falling pH depresses the nervous system; a rising pH makes it irritable.
⚡

QUICK RECALL

SAY IT OUT LOUD
🧠 ROMERespiratory Opposite · Metabolic Equal
⚖️ Normal pH?Compare to 7.40. Below = acid winning. Above = base winning.
🕰️ SpeedLungs compensate in minutes. Kidneys take hours to days.
🔀 Same direction = MIXEDBoth arms acid, or both base ⇒ two problems, no compensator.

📋 The answer key for this set — cover it and rebuild it

GaspH / CO₂ / HCO₃⁻Primary disorderCompensation
17.37 / 52 / 32Respiratory acidosisFully compensated
27.43 / 43 / 33Metabolic alkalosisFully compensated (pH normal; CO₂ only at the top of range)
37.26 / 62 / 27Respiratory acidosisPartially compensated (+ hypoxemia)
47.53 / 30 / 24Respiratory alkalosisUncompensated
57.22 / 20 / 8Metabolic acidosisPartially compensated
67.47 / 48 / 34Metabolic alkalosisPartially compensated
77.42 / 30 / 19Respiratory alkalosisFully compensated
87.37 / 32 / 18Metabolic acidosisFully compensated
97.04 / 58 / 15Mixed acidosisNone possible
107.55 / 33 / 28Mixed alkalosisNone possible
117.39 / 41 / 24Normal acid–basen/a — but PaO₂ 66 = hypoxemia
🎯 Cover & check — 8 rapid-fire questions
Q1: State ROME correctly and say what each half means.
Respiratory Opposite, Metabolic Equal. In a respiratory disorder the pH and the PaCO₂ move in opposite directions. In a metabolic disorder the pH and the HCO₃⁻ move in the same direction.
Q2: The pH is 7.38, PaCO₂ is 50 and HCO₃⁻ is 30. What is it?
pH is normal but below 7.40, so acid is winning. The acid value is the PaCO₂, so the primary problem is respiratory acidosis, and because the pH is back in range it is fully compensated.
Q3: What does a completely normal partner value tell you about how long the problem has existed?
That it is probably acute. Uncompensated means the other system has not responded yet — the lungs respond in minutes, so an uncompensated metabolic problem is very new, and an uncompensated respiratory problem is newer than the hours to days the kidneys need.
Q4: Both PaCO₂ and HCO₃⁻ are low and the pH is 7.20. What is happening?
The low bicarbonate matches the low pH, so it is a metabolic acidosis. The low PaCO₂ is respiratory compensation. Since the pH is still abnormal, it is partially compensated.
Q5: Both PaCO₂ and HCO₃⁻ are high and the pH is 7.10. What is happening?
The high PaCO₂ is acid and matches the low pH; the high bicarbonate is base and is the compensation. This is a partially compensated respiratory acidosis — the pH is still far outside the range.
Q6: When is it correct to call a gas "mixed"?
When the PaCO₂ and the HCO₃⁻ both push the pH in the same direction — both acid, or both base. There is no compensating system left, so no compensation label applies.
Q7: Why does a normal pH not mean a normal patient?
Because full compensation restores the pH while both underlying values remain abnormal. The disorder is still there — often chronic — and the oxygenation may be poor as well.
Q8: Which single number tells you which side of "normal" the pH is leaning, and why does it matter?
7.40, the midpoint. Compensation pulls the pH back toward 7.40 but never past it, so the side the pH sits on identifies which value is the primary problem.

🧠 Take these four lines into the exam

  • ROME — Respiratory Opposite, Metabolic Equal.
  • "7.40 breaks the tie." Normal pH? Look which side of 7.40 it sits on.
  • "Same direction = mixed." Both values acid, or both base, means two problems.
  • "Minutes vs days." Lungs compensate fast; kidneys compensate slowly — so full compensation means chronic.
🧠 Then finish the set: NG-092 (the method) · NG-138 (practice gases) · NG-173 (worked answers I).