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Difficulty: MediumNephrology

Match each clinical presentation with its corresponding classic arterial blood gas and basic metabolic panel profile.

  • A 28-year-old healthy pregnant woman at 32 weeks gestation presents for a routine prenatal check-up.pH 7.44, pCO2pCO_2 28 mmHg, HCO3−HCO_3^- 18 mEq/L
  • A 50-year-old man presents with profound weakness and severe, intractable vomiting over the past 4 days due to gastric outlet obstruction.pH 7.52, pCO2pCO_2 45 mmHg, HCO3−HCO_3^- 36 mEq/L, Urine Cl−Cl^- 8 mEq/L
  • A 35-year-old man with progressive ascending muscle weakness over the past week is brought to the emergency department exhibiting shallow breathing and poor diaphragmatic excursion.pH 7.22, pCO2pCO_2 65 mmHg, HCO3−HCO_3^- 26 mEq/L
  • A 20-year-old man with type 1 diabetes mellitus presents with diffuse abdominal pain, polyuria, and deep, rapid Kussmaul respirations.pH 7.24, pCO2pCO_2 28 mmHg, HCO3−HCO_3^- 12 mEq/L, Serum Anion Gap 24 mEq/L

Answer

Pregnancy matches with chronic respiratory alkalosis; vomiting matches with saline-responsive metabolic alkalosis; neuromuscular respiratory failure matches with acute respiratory acidosis; and diabetic ketoacidosis matches with high anion gap metabolic acidosis.
Each clinical vignette presents a classic etiology for one of the four primary acid-base disorders. Accurate diagnosis requires matching the clinical mechanism (hyperventilation, acid loss, hypoventilation, or acid overproduction) to its expected mathematical footprint on the blood gas panel.

Step-by-Step Solution

1
Analyze the pregnant patient's expected physiology
Identify chronic respiratory alkalosis
Progesterone directly stimulates the medullary respiratory center. The resulting hyperventilation lowers pCO2pCO_2, and the kidneys compensate over days by decreasing HCO3−HCO_3^- reabsorption.
2
Evaluate the patient with intractable vomiting
Identify metabolic alkalosis with volume contraction
Loss of gastric H+H^+ raises serum pH. Concurrent loss of volume and Cl−Cl^- leads to a saline-responsive state where the kidneys avidly retain Na+Na^+ at the expense of K+K^+ and H+H^+, yielding a urine Cl−Cl^- below 10 mEq/L.
3
Assess the patient with ascending weakness and shallow breathing
Identify acute respiratory acidosis
Neuromuscular weakness impairs ventilation, causing an acute buildup of CO2CO_2. The HCO3−HCO_3^- is only slightly elevated because renal metabolic compensation has not had time to occur.
4
Examine the patient with type 1 diabetes and rapid breathing
Identify high anion gap metabolic acidosis with respiratory compensation
Insulin deficiency leads to ketogenesis. The unmeasured ketones increase the anion gap and consume HCO3−HCO_3^-. The rapid, deep Kussmaul respirations represent respiratory compensation (lowering pCO2pCO_2).

Key Concept

Acid-Base Disorders and Compensatory Mechanisms
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Topics

Nephrology

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