Examkin

Question

Difficulty: EasyNephrology

A 45-year-old woman is evaluated in the emergency department for 4 days of severe, voluminous watery diarrhea. She reports significant weakness and lightheadedness when standing. Her medical history is unremarkable, and she takes no medications. Vital signs show a temperature of 37.2°C (99°F), blood pressure of 96/60 mm Hg, heart rate of 114/min, and respiratory rate of 22/min. Physical examination shows dry mucous membranes and decreased skin turgor.

Laboratory studies show the following:

TestValue
Sodium138 mEq/L
Potassium3.2 mEq/L
Chloride115 mEq/L
Bicarbonate14 mEq/L
Blood urea nitrogen38 mg/dL
Creatinine1.1 mg/dL

Arterial blood gas (room air):

TestValue
pH7.27
pCO2pCO_229 mm Hg
pO2pO_296 mm Hg

Which of the following best describes this patient's current acid-base status?

  1. A
    Elevated anion gap metabolic acidosis with appropriate respiratory compensation
  2. B
    Non-anion gap metabolic acidosis with a concomitant respiratory alkalosis
  3. Non-anion gap metabolic acidosis with appropriate respiratory compensationAnswer
  4. D
    Non-anion gap metabolic acidosis with a concomitant respiratory acidosis
  5. E
    Primary respiratory alkalosis with a compensatory metabolic acidosis

Answer

Non-anion gap metabolic acidosis with appropriate respiratory compensation
The patient's arterial blood gas shows a pH of 7.27, indicating acidemia. The low serum bicarbonate (14 mEq/L) identifies the primary disorder as a metabolic acidosis, which is consistent with severe diarrhea (loss of bicarbonate in the stool). The anion gap is calculated as Sodium - (Chloride + Bicarbonate) = 138 - (115 + 14) = 9 mEq/L, which falls within the normal range (8-12 mEq/L), indicating a non-anion gap metabolic acidosis. To determine if 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. In this patient, the expected pCO2pCO_2 is (1.5×14)+8=29±2(1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The patient's actual pCO2pCO_2 is 29 mm Hg, which exactly matches the expected value. Therefore, this is a simple, fully compensated non-anion gap metabolic acidosis.

Step-by-Step Solution

1
Determine the primary acid-base disturbance based on pH and bicarbonate.
pH is 7.27 (acidemia) and bicarbonate is 14 mEq/L (low).
Low pH with a low bicarbonate indicates a primary metabolic acidosis.
2
Calculate the serum anion gap.
Anion gap = 138−(115+14)=9138 - (115 + 14) = 9 mEq/L.
Classifying the metabolic acidosis as non-anion gap or elevated anion gap is required to determine the etiology.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=1.5×14+8=29±2pCO_2 = 1.5 \times 14 + 8 = 29 \pm 2 mm Hg.
To determine if there is a secondary superimposed respiratory disorder.
4
Compare the expected pCO2pCO_2 to the actual measured pCO2pCO_2.
The measured pCO2pCO_2 (29 mm Hg) falls perfectly within the expected range (27-31 mm Hg).
Since the measured value matches the expected value, the respiratory compensation is appropriate without any superimposed respiratory disorders.

Key Concept

Acid-base interpretation of a simple metabolic acidosis requires establishing the primary disorder, calculating the anion gap, and using Winter's formula to verify appropriate respiratory compensation.
Estimated Time:1m 0s
Rate this question

Topics

Nephrology

More questions from this topic