ACID-BASE DISORDERS - CLINICAL CASE MCQs | MCQ Quiz | OmpathStudy Kenya

Question 1

A 62-year-old man with COPD is brought in with confusion. ABG: pH 7.28, PaCO₂ 64 mmHg, HCO₃⁻ 28 mmol/L. Which is the most likely primary disorder?

Accepted Answer

Chronic respiratory acidosis with metabolic compensation — pH is low (acidosis), PaCO₂ is markedly elevated (respiratory acidosis), and HCO₃⁻ is elevated above normal (24-26), indicating metabolic compensation. The presence of elevated bicarbonate suggests chronic respiratory acidosis with renal compensation, typical in COPD patients.

Question 2

A 28-year-old woman with vomiting presents with cramps. ABG: pH 7.54, PaCO₂ 48 mmHg, HCO₃⁻ 38 mmol/L. Which is the best interpretation?

Accepted Answer

Primary metabolic alkalosis with respiratory compensation — pH is high (alkalosis), HCO₃⁻ is markedly elevated (primary metabolic alkalosis from vomiting and loss of gastric acid), and PaCO₂ is appropriately elevated (respiratory compensation through hypoventilation to retain CO₂).

Question 3

A 45-year-old diabetic in DKA: ABG: pH 7.18, PaCO₂ 24 mmHg, HCO₃⁻ 9 mmol/L; Na⁺ 138, Cl⁻ 100, HCO₃⁻ 9. What is the most likely acid-base disorder?

Accepted Answer

High anion gap metabolic acidosis + respiratory alkalosis — AG = 138 - (100 + 9) = 29 (high). pH is low with low HCO₃⁻ (metabolic acidosis). Expected PaCO₂ by Winter's = 1.5(9) + 8 = 21.5 ± 2 (19.5-23.5). Actual PaCO₂ is 24, which is slightly higher than expected but close, suggesting possible mixed disorder with mild respiratory alkalosis component from Kussmaul breathing.

Question 4

A 70-year-old man with vomiting and nasogastric suction: ABG: pH 7.50, PaCO₂ 54 mmHg, HCO₃⁻ 40 mmol/L. Which is correct?

Accepted Answer

Metabolic alkalosis with expected respiratory compensation — pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting/NG suction), and PaCO₂ is appropriately elevated as respiratory compensation (hypoventilation to retain CO₂ and normalize pH).

Question 5

A trauma patient: ABG: pH 7.10, PaCO₂ 35 mmHg, HCO₃⁻ 10 mmol/L. Using Winter's formula, expected PaCO₂ for this HCO₃⁻ is approximately? (choose closest)

Accepted Answer

23 mmHg — Winter's formula: Expected PaCO₂ = 1.5(10) + 8 = 23 ± 2 (21-25 mmHg). The actual PaCO₂ of 35 is higher than expected, suggesting inadequate respiratory compensation or a mixed disorder with respiratory acidosis component.

Question 6

A patient's electrolytes: Na⁺ 140, K⁺ 4.2, Cl⁻ 102, HCO₃⁻ 8. Calculate anion gap (AG = Na - (Cl + HCO₃)). Which is the AG?

Accepted Answer

30 mmol/L — AG = 140 - (102 + 8) = 140 - 110 = 30 mmol/L. This is a markedly elevated anion gap (normal 8-12), indicating high anion gap metabolic acidosis from unmeasured anions like ketones, lactate, or toxins.

Question 7

A 50-year-old with chronic COPD: ABG: pH 7.36, PaCO₂ 56 mmHg, HCO₃⁻ 32 mmol/L. Best interpretation?

Accepted Answer

Chronic respiratory acidosis with metabolic compensation — pH is near normal (compensated), PaCO₂ is elevated (respiratory acidosis), and HCO₃⁻ is elevated (metabolic compensation). This represents chronic respiratory acidosis with complete renal compensation, typical in chronic COPD.

Question 8

A septic patient: ABG: pH 7.25, PaCO₂ 28 mmHg, HCO₃⁻ 12 mmol/L; electrolytes: Na 140, Cl 100, HCO₃ 12. AG = 28. Delta ratio: ΔAG = (28-12) = 16, ΔHCO₃ = (24-12) = 12. What is ΔAG/ΔHCO₃ and its interpretation?

Accepted Answer

16/12 = 1.33 → pure HAGMA — Delta ratio = 16/12 = 1.33. A ratio between 1-2 suggests pure high anion gap metabolic acidosis (HAGMA). The rise in anion gap equals the fall in bicarbonate, consistent with lactic acidosis in sepsis.

Question 9

A patient with salicylate overdose: early ABG: pH 7.55, PaCO₂ 26 mmHg, HCO₃⁻ 22 mmol/L. Which best describes this?

Accepted Answer

Primary respiratory alkalosis with early metabolic acidosis developing → mixed disorder — Salicylates initially stimulate the respiratory center causing respiratory alkalosis (low PaCO₂, high pH). HCO₃⁻ is starting to drop (22, slightly below 24) indicating developing metabolic acidosis. This represents the early mixed disorder typical of salicylate toxicity.

Question 10

ABG: pH 7.10, PaCO₂ 70 mmHg, HCO₃⁻ 22 mmol/L. What is the best interpretation?

Accepted Answer

Mixed respiratory acidosis and metabolic acidosis — pH is severely low (acidosis), PaCO₂ is markedly elevated (respiratory acidosis), but HCO₃⁻ is normal/slightly low. The severe acidemia with inadequate bicarbonate elevation suggests mixed respiratory and metabolic acidosis rather than chronic respiratory acidosis with compensation.

Question 11

Patient with severe diarrhea: ABG: pH 7.32, PaCO₂ 34 mmHg, HCO₃⁻ 17 mmol/L. Which is most likely?

Accepted Answer

Normal anion gap metabolic acidosis (NAGMA) with respiratory compensation — Diarrhea causes loss of bicarbonate-rich intestinal fluid, leading to normal anion gap metabolic acidosis (NAGMA). pH is low, HCO₃⁻ is low, and PaCO₂ is appropriately reduced as respiratory compensation.

Question 12

Post-operative patient receiving large IV normal saline: ABG: pH 7.32, HCO₃⁻ 18 mmol/L, Na 140, Cl 108. What acid-base disturbance is likely?

Accepted Answer

Normal anion gap metabolic acidosis due to hyperchloremia (dilutional) — Large volumes of normal saline (0.9% NaCl) contain high chloride (154 mmol/L), causing hyperchloremic metabolic acidosis. The elevated Cl⁻ (108) and low HCO₃⁻ with normal anion gap confirms NAGMA from dilutional hyperchloremia.

Question 13

A patient with known COPD presents with acute worsening and fever. ABG: pH 7.22, PaCO₂ 81 mmHg, HCO₃⁻ 34 mmol/L. Which is the best interpretation?

Accepted Answer

Acute on chronic respiratory acidosis (acute rise on chronic) — Elevated baseline HCO₃⁻ (34) suggests chronic respiratory acidosis with compensation. However, pH is significantly acidotic (7.22) with very high PaCO₂ (81), indicating an acute exacerbation on top of chronic disease - acute-on-chronic respiratory acidosis.

Question 14

ABG: pH 7.43, PaCO₂ 30 mmHg, HCO₃⁻ 20 mmol/L. Which is correct?

Accepted Answer

Primary respiratory alkalosis with renal compensation — pH is slightly alkalotic, PaCO₂ is low (respiratory alkalosis), and HCO₃⁻ is appropriately reduced as renal compensation. The pH being on the alkalotic side suggests the primary disorder is respiratory alkalosis with compensation.

Question 15

A 65-year-old with acute pulmonary edema: ABG: pH 7.25, PaCO₂ 52 mmHg, HCO₃⁻ 22 mmol/L. Most likely?

Accepted Answer

Primary respiratory acidosis with some metabolic acidosis → mixed disorder — Pulmonary edema impairs ventilation causing respiratory acidosis (high PaCO₂). The HCO₃⁻ is normal/low, which doesn't show appropriate elevation for pure respiratory acidosis. The severe acidemia suggests coexisting metabolic acidosis (possibly from hypoperfusion/lactic acidosis) - a mixed disorder.

Question 16

ABG after vomiting & diuretic use: pH 7.56, PaCO₂ 54 mmHg, HCO₃⁻ 45 mmol/L. Which is the best statement?

Accepted Answer

Metabolic alkalosis with appropriate respiratory compensation (hypoventilation) — pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting and diuretics causing H⁺ and Cl⁻ loss), and PaCO₂ is appropriately elevated as respiratory compensation through hypoventilation.

Question 17

An intoxicated patient: ABG pH 7.20, PaCO₂ 18 mmHg, HCO₃⁻ 7 mmol/L. Which is correct?

Accepted Answer

High anion gap metabolic acidosis with respiratory compensation (likely pure metabolic acidosis) — pH is very low (severe acidosis), HCO₃⁻ is very low (metabolic acidosis), and PaCO₂ is appropriately low (respiratory compensation via hyperventilation). Using Winter's: expected PaCO₂ = 1.5(7) + 8 = 18.5, which matches actual. This is appropriate compensation for severe metabolic acidosis, likely from toxic alcohol ingestion.

Question 18

ABG: pH 7.39, PaCO₂ 48 mmHg, HCO₃⁻ 29 mmol/L. Patient asymptomatic. Interpretation?

Accepted Answer

Chronic respiratory acidosis with metabolic compensation (compensated) — pH is normal (compensated), PaCO₂ is elevated (respiratory acidosis), and HCO₃⁻ is elevated (metabolic compensation). This represents fully compensated chronic respiratory acidosis, typical in asymptomatic chronic lung disease patients.

Question 19

A patient: Na 138, Cl 90, HCO₃ 8, AG = 40. ΔAG = 28 (40-12), ΔHCO₃ = 16 (24-8). ΔAG/ΔHCO₃ = ? (select closest) and interpretation.

Accepted Answer

1.75 → HAGMA + metabolic alkalosis or chronic respiratory acidosis — Delta ratio = 28/16 = 1.75. A ratio 1.6-2.0 suggests HAGMA with concurrent metabolic alkalosis or chronic respiratory acidosis. The rise in anion gap exceeds the fall in bicarbonate, indicating another process is maintaining bicarbonate levels.

Question 20

ABG: pH 7.30, PaCO₂ 26 mmHg, HCO₃⁻ 12 mmol/L. Which best describes respiratory compensation? (expected PaCO₂ using Winter's formula for HCO₃ 12)

Accepted Answer

Expected PaCO₂ ≈ 26 mmHg → appropriate respiratory compensation (pure metabolic acidosis) — Winter's formula: Expected PaCO₂ = 1.5(12) + 8 = 26 ± 2 (24-28 mmHg). The actual PaCO₂ of 26 matches expected, indicating appropriate respiratory compensation for pure metabolic acidosis without mixed disorder.

Question 21

A 30-year-old with repeated vomiting: ABG pH 7.58, PaCO₂ 60 mmHg, HCO₃⁻ 48 mmol/L. Which is most likely among choices?

Accepted Answer

Metabolic alkalosis with appropriate respiratory compensation (hypoventilation) — pH is high (alkalosis), HCO₃⁻ is markedly elevated (metabolic alkalosis from vomiting with loss of gastric HCl), and PaCO₂ is appropriately elevated (respiratory compensation through hypoventilation to retain CO₂ and normalize pH).

Question 22

Post-op patient given large amount of lactated Ringer's now with hypotension: ABG pH 7.34, HCO₃⁻ 20 mmol/L, Na 135, Cl 100. Likely disturbance?

Accepted Answer

Mild metabolic acidosis; consider lactic acidosis (HAGMA) in sepsis/hypoperfusion — AG = 135 - (100 + 20) = 15 (mildly elevated). With hypotension suggesting hypoperfusion, lactic acidosis (HAGMA) is most likely. The mild metabolic acidosis with elevated AG in the setting of shock points to tissue hypoperfusion and lactate accumulation.

Question 23

ABG: pH 7.15, PaCO₂ 20 mmHg, HCO₃⁻ 6 mmol/L. Which is the most likely cause?

Accepted Answer

Severe metabolic acidosis (e.g., DKA, toxins) with respiratory compensation — pH is very low (severe acidosis), HCO₃⁻ is critically low (severe metabolic acidosis), and PaCO₂ is appropriately very low (respiratory compensation). Using Winter's: expected = 1.5(6) + 8 = 17 ± 2, actual is 20 (close). This represents severe metabolic acidosis with appropriate compensation, likely DKA, lactic acidosis, or toxic alcohol.

Question 24

A patient's labs: Na 142, Cl 108, HCO₃ 22, AG = 12. ABG pH 7.41, PaCO₂ 38, HCO₃ 24. What does this ABG suggest?

Accepted Answer

Essentially normal acid-base status (AG ~ normal) — AG = 142 - (108 + 22) = 12 (normal). All ABG values are within normal ranges: pH 7.35-7.45, PaCO₂ 35-45, HCO₃⁻ 22-26. This represents normal acid-base status.

Question 25

A 55-year-old with liver failure: ABG pH 7.29, PaCO₂ 28 mmHg, HCO₃⁻ 12 mmol/L; Na 138, Cl 96. AG = ? (choose closest) and likely disorder.

Accepted Answer

AG ≈ 30 → HAGMA from lactic acidosis (with appropriate respiratory compensation) — AG = 138 - (96 + 12) = 30 (markedly elevated). Liver failure patients can develop lactic acidosis from impaired lactate clearance and tissue hypoperfusion. The HAGMA with appropriate respiratory compensation (low PaCO₂) is consistent with lactic acidosis.

Question 26

ABG: pH 7.44, PaCO₂ 32 mmHg, HCO₃⁻ 22 mmol/L. Which is best?

Accepted Answer

Mild primary respiratory alkalosis with near-normal pH (compensation) — pH is at upper limit of normal (7.44), PaCO₂ is mildly reduced (mild respiratory alkalosis), and HCO₃⁻ is at lower limit of normal (renal compensation beginning). This represents mild compensated respiratory alkalosis.

Question 27

A patient with CHF on loop diuretics: ABG pH 7.50, PaCO₂ 46 mmHg, HCO₃⁻ 37 mmol/L. Most likely?

Accepted Answer

Metabolic alkalosis due to diuretics with respiratory compensation — Loop diuretics cause loss of H⁺, Cl⁻, and K⁺, leading to metabolic alkalosis (contraction alkalosis). pH is high, HCO₃⁻ is elevated (metabolic alkalosis), and PaCO₂ is appropriately elevated (respiratory compensation through hypoventilation).

Question 28

ABG: pH 7.02, PaCO₂ 40 mmHg, HCO₃⁻ 9 mmol/L. Interpretation?

Accepted Answer

Severe metabolic acidosis with inadequate respiratory compensation (or mixed respiratory acidosis) — pH is severely low (severe acidosis), HCO₃⁻ is very low (metabolic acidosis). Expected PaCO₂ by Winter's = 1.5(9) + 8 = 21.5 ± 2. Actual is 40 mmHg, which is much higher than expected, indicating inadequate respiratory compensation or coexisting respiratory acidosis - a life-threatening situation.

Question 29

A patient: ABG pH 7.36, PaCO₂ 30 mmHg, HCO₃⁻ 17 mmol/L. Which is most plausible?

Accepted Answer

Primary metabolic acidosis with respiratory compensation (acute) — pH is at lower limit of normal, HCO₃⁻ is low (metabolic acidosis), and PaCO₂ is appropriately low (respiratory compensation). The near-normal pH suggests good compensation. This represents compensated metabolic acidosis.

Question 30

ABG: pH 7.22, PaCO₂ 24 mmHg, HCO₃⁻ 9 mmol/L; electrolytes: Na 140, Cl 104. Calculate AG and pick the most likely scenario (normal AG assumed 12).

Accepted Answer

AG = 27 → high AG metabolic acidosis (likely combined with respiratory compensation) — AG = 140 - (104 + 9) = 27 (markedly elevated). This is HAGMA with appropriate respiratory compensation (PaCO₂ 24, expected by Winter's = 1.5(9) + 8 = 21.5 ± 2). The high AG suggests unmeasured anions like ketones, lactate, or toxins.

ACID-BASE DISORDERS - CLINICAL CASE MCQs | MCQ Quiz | OmpathStudy Kenya

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