Examkin

Question

Difficulty: EasyNephrology

A 19-year-old college student presents to the student health clinic complaining of severe, watery diarrhea for the past 3 days after returning from a camping trip. He reports feeling profoundly weak and lightheaded when standing. His temperature is 37.2°C (99.0°F), blood pressure is 98/60 mm Hg while supine and 82/50 mm Hg while standing, and pulse is 115/min. Physical examination shows dry mucous membranes and delayed capillary refill. Laboratory results reveal:

Serum sodium: 138 mEq/L138 \text{ mEq/L}
Serum potassium: 3.1 mEq/L3.1 \text{ mEq/L}
Serum chloride: 112 mEq/L112 \text{ mEq/L}
Serum bicarbonate: 14 mEq/L14 \text{ mEq/L}

Arterial blood gas on room air:
pH: 7.28
pCO2pCO_2: 29 mm Hg
pO2pO_2: 95 mm Hg

Based on the laboratory findings, which of the following best characterizes this patient's acid-base status?

  1. A
    High anion gap metabolic acidosis with appropriate respiratory compensation
  2. B
    Normal anion gap metabolic acidosis with concurrent primary respiratory alkalosis
  3. Normal anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  4. D
    Normal anion gap metabolic acidosis with concurrent primary respiratory acidosis
  5. E
    Primary respiratory alkalosis with compensatory metabolic acidosis

Answer

Normal anion gap metabolic acidosis with appropriate respiratory compensation
The patient's presentation of severe diarrhea has led to a primary metabolic acidosis, evidenced by a pH of 7.28 and a low serum bicarbonate of 14 mEq/L14 \text{ mEq/L}. The anion gap is calculated as Na+−(Cl−+HCO3−)=138−(112+14)=12 mEq/LNa^+ - (Cl^- + HCO_3^-) = 138 - (112 + 14) = 12 \text{ mEq/L}, which is normal (reference range: 8-12 mEq/L). To determine if the respiratory compensation is appropriate, Winter's formula is applied: Expected pCO2=1.5×[HCO3−]+8±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. For this patient, 1.5×14+8±2=29±2 mm Hg1.5 \times 14 + 8 \pm 2 = 29 \pm 2 \text{ mm Hg}. The patient's actual pCO2pCO_2 is 29 mm Hg, which falls precisely within the expected range, indicating an appropriate respiratory compensation.

Step-by-Step Solution

1
Identify the primary acid-base disorder by evaluating the pH.
The pH is 7.28, indicating acidemia. The low serum bicarbonate (14 mEq/L) confirms a primary metabolic acidosis.
Establishing the primary driver of the pH abnormality is the first step in any acid-base interpretation.
2
Calculate the serum anion gap to classify the metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)=138−(112+14)=12 mEq/LNa^+ - (Cl^- + HCO_3^-) = 138 - (112 + 14) = 12 \text{ mEq/L}. This is a normal anion gap.
Metabolic acidosis must be divided into normal anion gap (hyperchloremic) and high anion gap categories to narrow the differential diagnosis.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=1.5×[HCO3−]+8±2=1.5×14+8±2=21+8±2=29±2 mm HgpCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 14 + 8 \pm 2 = 21 + 8 \pm 2 = 29 \pm 2 \text{ mm Hg}.
Winter's formula determines if the lungs are appropriately blowing off CO2CO_2 to compensate for the metabolic acidosis.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
The actual pCO2pCO_2 is 29 mm Hg, which is exactly within the expected range of 27-31 mm Hg.
Because actual matches expected, there is no secondary, concurrent respiratory disorder.

Key Concept

Patients with severe diarrhea lose bicarbonate in the stool, leading to a normal anion gap metabolic acidosis (NAGMA). Appropriate respiratory compensation is verified using Winter's formula: Expected pCO2=1.5×[HCO3−]+8±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2.
Rate this question

Topics

Nephrology

More questions from this topic