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Difficulty: MediumNephrology

Match each clinical presentation with its most likely corresponding arterial blood gas and serum electrolyte panel.

  • A 22-year-old man with polyuria, generalized weakness, and a fruity breath odorpH 7.28, PaCO2 26 mmHg, HCO3- 12 mEq/L, Na+ 132 mEq/L, K+ 5.5 mEq/L, Cl- 98 mEq/L
  • A 38-year-old woman with generalized weakness after 5 days of high-volume, watery diarrheapH 7.30, PaCO2 29 mmHg, HCO3- 14 mEq/L, Na+ 138 mEq/L, K+ 2.9 mEq/L, Cl- 114 mEq/L
  • A 67-year-old man with a chronic smoker's cough presenting with acute-onset confusion and severe wheezingpH 7.26, PaCO2 68 mmHg, HCO3- 30 mEq/L, Na+ 140 mEq/L, K+ 4.1 mEq/L, Cl- 100 mEq/L
  • A 45-year-old man with recurrent postprandial vomiting for 4 days due to suspected gastric outlet obstructionpH 7.51, PaCO2 48 mmHg, HCO3- 37 mEq/L, Na+ 139 mEq/L, K+ 2.8 mEq/L, Cl- 91 mEq/L

Answer

Diabetic ketoacidosis pairs with the high anion gap metabolic acidosis; diarrhea pairs with the normal anion gap metabolic acidosis; COPD exacerbation pairs with the primary respiratory acidosis; and vomiting pairs with the metabolic alkalosis.
Each clinical presentation leads to a classic acid-base and electrolyte disturbance. The patient with a fruity breath odor has diabetic ketoacidosis, producing an elevated anion gap metabolic acidosis (AG = 22 mEq/L). The patient with severe diarrhea loses substantial bicarbonate in the stool, producing a normal anion gap metabolic acidosis (AG = 10 mEq/L) along with severe hypokalemia. The patient with a COPD exacerbation retains CO2, leading to a primary respiratory acidosis with a significantly elevated PaCO2. Finally, the patient with gastric outlet obstruction loses hydrochloric acid through vomiting, resulting in a hypochloremic, hypokalemic metabolic alkalosis with compensatory CO2 retention.

Step-by-Step Solution

1
Identify the primary acid-base disorder for each laboratory panel by evaluating the pH, PaCO2, and HCO3- values.
The first two panels represent metabolic acidosis (low pH, low HCO3-). The third panel represents respiratory acidosis (low pH, high PaCO2). The fourth panel represents metabolic alkalosis (high pH, high HCO3-).
Establishing the primary disorder is the critical first step in interpreting acid-base derangements.
2
Calculate the serum anion gap (Na+−[Cl−+HCO3−]Na^+ - [Cl^- + HCO_3^-]) for the two metabolic acidoses.
The first panel has an anion gap of 22 mEq/L (High AG). The second panel has an anion gap of 10 mEq/L (Normal AG).
The anion gap distinguishes between different etiologies of metabolic acidosis.
3
Match the clinical scenarios to their corresponding laboratory findings based on pathophysiology.
DKA causes ketone accumulation (high anion gap acidosis). Diarrhea causes direct bicarbonate loss (normal anion gap acidosis). COPD causes ventilation failure (respiratory acidosis). Vomiting causes acid loss (metabolic alkalosis).
Each clinical etiology produces a distinct signature on a basic metabolic panel and arterial blood gas.

Key Concept

Acid-Base Derangements and Electrolyte Abnormalities

Alternative Method

You can also use Winter's formula (PaCO2=1.5×HCO3−+8±2PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2) to confirm that the respiratory compensation for the metabolic acidoses in the provided correct panels is appropriate, further reinforcing your diagnostic match.
Estimated Time:2m 0s
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Topics

Nephrology

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