Every practice question answered, with the interpretation worked out step by step.
This is the handout's own opening list. Ask these five questions in this order, every time, before you look at the answer choices.
pH → CO₂ → HCO₃ → compensation → patient. Same five moves on every single ABG item.
📄 Note on the original file: the handout is mostly embedded images (a normal-value table and a tic-tac-toe diagram) that did not survive text extraction. The grids below are rebuilt from standard values — if her paper copy shows a figure that is not here, that is why.
You cannot interpret an ABG you cannot compare. Memorize these six lines; three of them do 90% of the work.
| Value | Below normal means… | Above normal means… |
|---|---|---|
| pH | < 7.35 = ACIDOSIS | > 7.45 = ALKALOSIS |
| CO₂ (acid) | < 35 = blowing acid off → alkalotic | > 45 = holding acid in → acidotic |
| HCO₃ (base) | < 22 = lost base → acidotic | > 26 = extra base → alkalotic |
CO₂ is backwards — a HIGH CO₂ means a LOW pH. Because CO₂ + water becomes carbonic acid; more CO₂ = more acid. HCO₃ is not backwards: high bicarb, high pH.
💡 If you only remember one thing: find the value that matches the pH's direction. That value names the problem. The other value is either normal or compensating.
Naming format the exam wants: [uncompensated / partially compensated / fully compensated] + [respiratory / metabolic] + [acidosis / alkalosis]. Say all three parts out loud and the right option usually reads itself off the page.
Draw three columns. Drop each number into the column it belongs to. Whatever lands in the same column as the pH is the primary problem.
| Put it here… | ACID column | NORMAL column | BASE column |
|---|---|---|---|
| pH | < 7.35 | 7.35 – 7.45 | > 7.45 |
| CO₂ (mm Hg) | > 45 | 35 – 45 | < 35 |
| HCO₃ (mEq/L) | < 22 | 22 – 26 | > 26 |
💡 The 7.40 split (only for a normal pH): 7.35–7.39 means the body started acidotic; 7.41–7.45 means it started alkalotic. The body compensates toward normal but never overshoots past the midline.
The two systems cover for each other. Whichever one is not broken is the one doing the compensating.
| System | Compensates for | How | Speed |
|---|---|---|---|
| 🫁 Lungs (CO₂) | Metabolic problems | Change rate & depth — blow off CO₂ in acidosis, hold CO₂ in alkalosis | Minutes — fast, but incomplete |
| 🫘 Kidneys (HCO₃) | Respiratory problems | Retain or dump bicarbonate and excrete H⁺ | Hours to 2–3 days — slow, but far more complete |
| State | pH | CO₂ & HCO₃ | Read it as |
|---|---|---|---|
| Uncompensated | Abnormal | One abnormal, one normal | Brand new — help hasn't arrived |
| Partially compensated | Still abnormal | Both abnormal | Help is on the way but losing |
| Fully compensated | NORMAL | Both abnormal | Chronic — the body caught up |
Normal pH + two abnormal values = fully compensated. It is never "normal ABG."
Never treat the compensation. Kussmaul breathing in DKA and a fast rate in metabolic acidosis are the patient's rescue — sedating or slowing that breathing removes the only thing holding the pH up.
Most of the questions below are really "name the cause." Learn these four columns and you can answer without an ABG at all.
| Imbalance | Mechanism | Classic causes |
|---|---|---|
| Respiratory acidosis ↑CO₂, ↓pH | HYPOventilation — not blowing CO₂ off | COPD · opioids / sedatives / anesthesia · pneumonia, atelectasis · chest trauma, flail chest · neuromuscular disease (MG, Guillain-Barré) · obesity hypoventilation, OSA · airway obstruction · post-op splinting |
| Respiratory alkalosis ↓CO₂, ↑pH | HYPERventilation — blowing too much CO₂ off | Anxiety / panic · pain · fever · early sepsis · pulmonary embolism · high altitude · over-set ventilator · early salicylate toxicity |
| Metabolic acidosis ↓HCO₃, ↓pH | Gained acid or lost bicarb | DKA · lactic acidosis / shock · renal failure · diarrhea, ileostomy, intestinal fistula · starvation · late salicylate toxicity |
| Metabolic alkalosis ↑HCO₃, ↑pH | Lost acid or gained base | Vomiting · NG suction · antacid / bicarb overuse · loop and thiazide diuretics · hypokalemia · steroids / Cushing’s · massive transfusion (citrate) |
The exam rarely hands you an ABG in the clinical items — it hands you a patient. Acidosis sedates. Alkalosis irritates.
| ACIDOSIS — CNS depression | ALKALOSIS — CNS excitement | |
|---|---|---|
| Neuro | Headache, drowsy, confused, lethargic → coma | Anxious, lightheaded, dizzy, seizures |
| Neuromuscular | Weakness, flaccid, hyporeflexia | Tingling: circumoral, fingers, toes · twitching, tetany, carpopedal spasm, +Chvostek/Trousseau |
| Cardiac | Hyperkalemia → peaked T waves, wide QRS, bradycardia, VF | Hypokalemia → flat T waves, U waves, PVCs; tachycardia |
| Respiratory | Kussmaul (deep, rapid, labored) if metabolic; slow/shallow if respiratory | Fast and deep if respiratory; slow and shallow (holding CO₂) if metabolic |
| Skin | Warm, flushed (vasodilation) | Pale, may have carpal spasm |
Potassium follows the pH backwards: acidosis → hyperkalemia; alkalosis → hypokalemia. Because H⁺ and K⁺ trade places across the cell membrane. Alkalosis also drops ionized calcium, which is where the tingling and tetany come from.
Cover the reveal. Write your three-part name (compensation + system + acid/base) before you tap.
Answer: B — respiratory acidosis (uncompensated).
Reading the numbers:
pH 7.31 → below 7.35 → acidosis.
CO₂ 52 → above 45 → acidic, and it moved opposite the pH. Respiratory Opposite → this is the primary problem.
HCO₃ 24 → inside 22–26 → normal, so the kidneys have not started compensating → uncompensated.
Therefore: acidosis + CO₂ matches + bicarb untouched = uncompensated respiratory acidosis. Think hypoventilation — sedation, COPD, splinting, a suppressed drive.
Why the others are wrong:
A. Metabolic acidosis needs HCO₃ below 22. It is 24 — dead normal.
C. Respiratory alkalosis needs pH above 7.45 with CO₂ below 35. Both are the reverse here.
D. Metabolic alkalosis needs pH above 7.45 with HCO₃ above 26. Neither is true.
Answer: FALSE. Vomiting causes metabolic ALKALOSIS.
Why: gastric fluid is hydrochloric acid. Throwing it up — or suctioning it out through an NG tube — removes acid from the body, so the bicarbonate left behind is now in excess and the pH climbs. Expect ↑HCO₃, ↑pH, plus hypokalemia and hypochloremia that keep the alkalosis going until you replace K⁺ and Cl⁻ with normal saline.
Why TRUE is wrong: it reverses the belly-button rule. Losses from below the belly button (diarrhea, ileostomy, fistula) are bicarbonate-rich and cause metabolic acidosis. Losses from above are acid and cause alkalosis.
Answer: B — tingling around the mouth (circumoral paresthesia).
Chain of reasoning: panic → hyperventilation → CO₂ blown off → respiratory alkalosis → alkalosis makes calcium bind more tightly to albumin, so ionized calcium falls → neuromuscular irritability: tingling of lips, fingers and toes, lightheadedness, twitching, carpopedal spasm. Nursing action is coaching slow, controlled breathing, not oxygen.
Why the others are wrong:
A. Anxiety and a sympathetic surge cause tachycardia, not bradycardia.
C. Hyperventilation is increased rate and depth — decreased respirations describe the opposite problem (respiratory acidosis).
D. Alkalosis shifts K⁺ into the cells → hypokalemia. Hyperkalemia goes with acidosis.
Answer: A — respiratory alkalosis (uncompensated).
Reading the numbers:
pH 7.50 → above 7.45 → alkalosis.
CO₂ 30 → below 35 → alkalotic, opposite the pH → respiratory (ROME).
HCO₃ 24 → normal → uncompensated.
Therefore: uncompensated respiratory alkalosis — the patient is blowing off CO₂. Look for anxiety, pain, fever, early sepsis, PE, or a ventilator set too fast.
Why the others are wrong:
B. Metabolic alkalosis requires HCO₃ > 26; this bicarb is 24.
C. Respiratory acidosis requires pH < 7.35 and CO₂ > 45 — both are the opposite.
D. Metabolic acidosis requires pH < 7.35 and HCO₃ < 22 — neither is true.
Answer: FALSE. The kidneys are the slow system.
Why: renal compensation means retaining or dumping bicarbonate and excreting hydrogen ions — that takes hours to begin and 2–3 days to be complete. The lungs are the fast responders: change the rate and depth and the CO₂ moves within minutes. The trade-off is that the slow system is the thorough one, which is why a chronic COPD patient can end up with a fully normal pH.
Why TRUE is wrong: it swaps the two systems. Fast = lungs = CO₂. Slow = kidneys = HCO₃.
Answer: C — high CO₂.
Why: COPD traps air and destroys alveolar surface, so the patient cannot get CO₂ out → CO₂ retention → respiratory acidosis. Rising CO₂ is a cerebral vasodilator and a sedative — increasing lethargy in a COPD patient is CO₂ narcosis until proven otherwise, and it is an early warning of respiratory failure. Do not treat a sleepy COPD patient as "resting comfortably."
Why the others are wrong:
A. A low CO₂ means the patient is blowing it off — the opposite of the COPD problem, and it would not cause lethargy.
B. The HCO₃ is elevated in chronic COPD (renal compensation), but the word "only" makes it wrong — the CO₂ is high too, and the CO₂ is what is making her lethargic.
D. Chronic CO₂ retention pushes the pH down or, once compensated, back to low-normal — not high.
Answer: C — metabolic acidosis.
Why: intestinal and pancreatic secretions are rich in bicarbonate. Severe diarrhea flushes that base out of the body before it can be reabsorbed → ↓HCO₃ → ↓pH. Expect hypokalemia at the same time, since stool carries potassium out too.
Why the others are wrong:
A. Metabolic alkalosis is the vomiting / NG suction answer — acid lost from above the belly button.
B. Respiratory acidosis requires hypoventilation; diarrhea does not change ventilation.
D. Respiratory alkalosis requires hyperventilation. (If it appeared later, it would be compensation, not the primary problem.)
Answer: TRUE.
Why: Kussmaul respirations are deep, rapid, labored breaths. When the pH falls, chemoreceptors drive the lungs to blow off CO₂ (carbonic acid) to pull the pH back up. It is the textbook finding in DKA and in severe metabolic acidosis of any cause.
The exam trap: Kussmaul breathing is compensation, not the emergency. Never give a sedative or try to slow that breathing. Fix the acidosis (insulin, fluids, perfusion) and the breathing settles on its own.
Answer: B — metabolic alkalosis (uncompensated).
Reading the numbers:
pH 7.48 → above 7.45 → alkalosis.
CO₂ 38 → sits inside 35–45 → normal, so the lungs are not the cause and are not yet compensating.
HCO₃ 31 → above 26 → alkalotic, and it moved the same way as the pH. Metabolic Equal → primary problem.
Therefore: uncompensated metabolic alkalosis. Ask about vomiting, NG suction, diuretics, or antacid use.
Why the others are wrong:
A. Respiratory alkalosis would show CO₂ < 35; 38 is normal.
C. Respiratory acidosis needs pH < 7.35 and CO₂ > 45.
D. Metabolic acidosis needs pH < 7.35 and HCO₃ < 22 — this bicarb is high, not low.
Answer: C — the patient with a respiratory rate of 6 after an opioid.
Why: ABCs. A rate of 6 is not just an abnormal number — it is failing ventilation. CO₂ climbs by roughly 3–6 mm Hg per minute of apnea/hypoventilation, so the pH is dropping right now and respiratory arrest follows. Act: stimulate, open the airway, apply oxygen, bag-mask if needed, hold the opioid, and give naloxone per protocol — then keep watching, because naloxone wears off sooner than the opioid does.
Why the others are wrong:
A. Panic-driven hyperventilation causes respiratory alkalosis — frightening and uncomfortable, but self-limiting; coach slow breathing. Fast breathing is far safer than no breathing.
B. Vomiting with cramps is metabolic alkalosis with hypokalemia and low ionized calcium — genuinely needs attention, but the cramps are not immediately lethal. Assess second.
D. An SpO₂ of 91% is expected in a chronic CO₂ retainer, where the target is often 88–92%. It is a baseline, not a change.
Answer: TRUE.
Why: CO₂ dissolved in water becomes carbonic acid (CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻). Retained CO₂ therefore means retained acid, and the pH falls. That is exactly why CO₂ reads backwards next to the pH — the higher the CO₂, the lower the pH.
Why FALSE is wrong: it would mean CO₂ behaves like a base, which is only true of bicarbonate. CO₂ up = acid up = pH down, every time.
Answer: B — the lungs are compensating for the acidosis.
Chain of reasoning: no insulin → fat is burned for fuel → ketoacids accumulate → bicarbonate is consumed buffering them → ↓HCO₃, ↓pH = metabolic acidosis → chemoreceptors trigger Kussmaul respirations → CO₂ is blown off → the pH is dragged back toward normal. The breathing is the fix, not the failure.
Why the others are wrong:
A. The CO₂ is low, but that is compensation. The pH is still acidic, so the primary problem cannot be an alkalosis. (You only call it respiratory alkalosis when the low CO₂ is the thing driving a high pH.)
C. Kidneys do not handle CO₂ — they handle bicarbonate and H⁺. CO₂ belongs to the lungs.
D. DKA lowers bicarbonate. Metabolic alkalosis is the opposite picture and would not produce Kussmaul breathing.
Answer: D — prolonged vomiting.
Why: vomiting drains hydrochloric acid out of the stomach. Losing acid raises the pH, and the bicarbonate that remains is now relatively excessive → ↑HCO₃, ↑pH. The accompanying volume depletion, hypochloremia and hypokalemia keep the kidney from correcting it, which is why treatment is 0.9% saline plus potassium replacement.
Why the others are wrong:
A. A panic attack → hyperventilation → respiratory alkalosis (right pH direction, wrong system).
B. COPD → CO₂ retention → respiratory acidosis.
C. Diarrhea → bicarbonate loss → metabolic acidosis (right system, exact opposite direction — this is the distractor she must not fall for).
Answer: TRUE.
Why: bicarbonate is the body's main base. HCO₃ below 22 mEq/L means the buffer has been lost (diarrhea, renal wasting) or used up neutralizing acid (DKA, lactic acidosis), so the pH falls with it. Metabolic Equal: pH ↓ and HCO₃ ↓ together.
Why FALSE is wrong: it would put high bicarb with acidosis. High HCO₃ (> 26) is metabolic alkalosis, or the kidney's compensation for chronic respiratory acidosis.
Answer: B — partially compensated metabolic acidosis.
Reading the numbers:
pH 7.32 → below 7.35 → acidosis, and still abnormal.
HCO₃ 18 → below 22 → acidic, same direction as the pH → metabolic is the primary problem (ME).
CO₂ 29 → below 35 → alkalotic, which does not match the pH — so it is not causing the problem, it is fixing it. The lungs are hyperventilating to blow off acid.
Both values abnormal + pH still out of range = partially compensated. Picture DKA or lactic acidosis with Kussmaul breathing.
Why the others are wrong:
A. "Fully compensated" requires the pH to be back inside 7.35–7.45. It is 7.32 — the lungs have not caught up yet.
C. If respiratory alkalosis were the primary problem the pH would be high, not low. A low CO₂ with an acidic pH is always compensation.
D. Uncompensated respiratory acidosis would need CO₂ high and HCO₃ normal. Here the CO₂ is low and the bicarb is low — the exact reverse.
Answer: B — respiratory acidosis.
Why: opioids blunt the brainstem's response to rising CO₂, so the patient simply stops breathing enough. Hypoventilation → CO₂ retention → carbonic acid → ↓pH. Expect ↑CO₂ with a normal HCO₃ (uncompensated — this happened in minutes, and the kidneys need days).
Priority actions: stimulate and reposition, oxygen, bag-valve-mask if the rate keeps falling, withhold further opioid, naloxone per protocol, and continued monitoring — because naloxone's duration is shorter than most opioids and she can re-sedate.
Why the others are wrong:
A. Respiratory alkalosis requires hyperventilation; this patient is doing the opposite.
C. Metabolic alkalosis comes from acid loss or base gain — vomiting, NG suction, antacids. No such history here.
D. Metabolic acidosis would require lost bicarb or gained fixed acid. The failure here is ventilation, so the disorder is respiratory. (A prolonged arrest eventually adds a lactic acidosis on top — but the first and "most likely" answer is respiratory.)
Answer: FALSE.
Why: hyperventilation blows CO₂ off — the more air moved, the more CO₂ exhaled → ↓CO₂ → ↑pH → respiratory alkalosis. It is HYPOventilation (slow, shallow, obstructed) that retains CO₂ and produces respiratory acidosis.
Why TRUE is wrong: it confuses the prefixes. Hyper → off → alkalosis. Hypo → held → acidosis. Read those two words carefully on the exam — they are one letter apart and flip the whole answer.
Answer: B — potassium 6.1 mEq/L.
Why: in acidosis, excess H⁺ moves into the cells and K⁺ is pushed out into the bloodstream. A potassium of 6.1 (normal 3.5–5.0 mEq/L) can produce peaked T waves, a widening QRS, bradycardia, and ventricular fibrillation or asystole — a rhythm problem, not a lab problem. Get the patient on a monitor, obtain an ECG, and anticipate IV calcium gluconate to protect the heart, then insulin with dextrose to move K⁺ back inside.
Why the others are wrong:
A. Deep rapid (Kussmaul) respirations are expected compensation — the thing keeping her alive. Concerning would be if they stopped.
C. Mild nausea is common in acidosis and is not urgent.
D. A respiratory rate of 20 is at the top of the normal range (12–20) and completely reasonable in someone compensating.
Answer: B — fully compensated respiratory acidosis.
Reading the numbers:
pH 7.40 → normal. A normal pH sitting next to two abnormal values never means "normal ABG" — it means full compensation.
CO₂ 50 → above 45 → acidic.
HCO₃ 32 → above 26 → alkalotic.
The two are pulling in opposite directions and have cancelled out. The primary disorder is the one that came first: the retained CO₂. The kidneys then held onto bicarbonate over days to bring the pH home — the classic chronic COPD gas.
How to break the tie: normally you use the 7.40 split — 7.35–7.39 means it started acidotic, 7.41–7.45 means it started alkalotic. Here the pH is exactly 7.40, dead on the line, so the split cannot decide it. Use the pattern instead: a high CO₂ with a compensating high bicarb is chronic respiratory acidosis, and it is the only option offered that describes two abnormal values with a normal pH.
Why the others are wrong:
A. "Uncompensated" would require an abnormal pH and a normal HCO₃. Both parts fail here.
C. Respiratory alkalosis requires a low CO₂ (< 35) and a high pH. This CO₂ is 50.
D. Metabolic alkalosis as a primary problem would show a pH above 7.45; and here the elevated bicarb is the response, not the cause — the kidney does not spontaneously retain bicarb without a reason.
Answer: TRUE.
Why: each system covers the other. When the problem is metabolic, the lungs adjust rate and depth to move CO₂ within minutes — breathe faster and deeper to blow acid off in metabolic acidosis (Kussmaul), breathe slower and shallower to hold CO₂ in metabolic alkalosis. When the problem is respiratory, the kidneys take over with bicarbonate, over hours to days.
Why FALSE is wrong: it would leave metabolic acidosis with no fast defence at all — and would make Kussmaul respirations in DKA impossible to explain.
Twenty questions, two of them are actually emergencies. These are the ones that become "which patient first" items.
Never leave a sedated patient with a falling respiratory rate to "check on them later."
⚠️ Also on the danger list: lethargy in a COPD patient (Q6) — that is CO₂ narcosis, an early sign of respiratory failure, not a patient finally resting.
If she reads nothing else the morning of the exam, she reads this.
pH → CO₂ → HCO₃ → compensation → patient. Five moves, every time.
💬 Ask Claire: “Give me five random ABG sets with a one-line patient story, make me name compensation + system + acid/base, and tell me which one I got wrong and why.”