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Difficulty: MediumNephrology

A 42-year-old man is brought to the emergency department by his partner due to profound weakness and lethargy. For the past four days, he has experienced profuse, watery diarrhea with up to 10 large bowel movements daily. He has not been able to tolerate oral intake. His temperature is 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}), pulse is 112/min112/\text{min}, respirations are 22/min22/\text{min}, and blood pressure is 98/64 mm Hg98/64\text{ mm Hg}. Physical examination reveals dry mucous membranes, flat neck veins, and decreased skin turgor. Neurologic examination shows generalized muscle weakness with strength rated 3/53/5 in both upper and lower extremities. Laboratory studies show the following:

TestResult
Serum sodium134 mEq/L134 \text{ mEq/L}
Serum potassium2.4 mEq/L2.4 \text{ mEq/L}
Serum chloride114 mEq/L114 \text{ mEq/L}
Serum bicarbonate10 mEq/L10 \text{ mEq/L}
Blood urea nitrogen42 mg/dL42 \text{ mg/dL}
Serum creatinine1.8 mg/dL1.8 \text{ mg/dL}
Arterial blood gas analysis on room air reveals:
TestResult
pH7.227.22
pCO2_232 mm Hg32 \text{ mm Hg}
pO2_294 mm Hg94 \text{ mm Hg}

Based on the laboratory findings, which of the following is the most accurate characterization of this patient's acid-base status?

  1. Normal anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Normal anion gap metabolic acidosis and concurrent respiratory alkalosis
  4. D
    High anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    High anion gap metabolic acidosis and concurrent respiratory acidosis

Answer

The patient has a normal anion gap metabolic acidosis due to gastrointestinal bicarbonate loss, complicated by a concurrent respiratory acidosis, likely secondary to hypokalemia-induced respiratory muscle weakness.
The correct answer accurately synthesizes the three necessary calculations for this patient. First, the low pH (7.227.22) and low bicarbonate (10 mEq/L10 \text{ mEq/L}) establish a primary metabolic acidosis. Second, the anion gap is normal at 10 mEq/L10 \text{ mEq/L} (134−[114+10]134 - [114 + 10]), which is consistent with bicarbonate loss from severe diarrhea. Third, Winter's formula dictates that the expected pCO2 should be (1.5×10)+8=23±2 mm Hg(1.5 \times 10) + 8 = 23 \pm 2 \text{ mm Hg}. The patient's actual pCO2 is 32 mm Hg32 \text{ mm Hg}. Because 3232 is greater than the expected maximum of 2525, the patient is inadequately ventilating to compensate for the acidosis, confirming a concurrent respiratory acidosis. This patient's profound hypokalemia (2.4 mEq/L2.4 \text{ mEq/L}) has led to skeletal muscle weakness, which often includes the muscles of respiration, blunting the compensatory hyperventilation.

Step-by-Step Solution

1
Identify the primary acid-base disturbance.
The pH is 7.227.22 (acidemia) and serum HCO3−\text{HCO}_3^- is 10 mEq/L10 \text{ mEq/L} (low).
Low pH with low bicarbonate confirms a primary metabolic acidosis.
2
Calculate the serum anion gap.
Anion Gap=Na+−(Cl−+HCO3−)=134−(114+10)=134−124=10 mEq/L\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 134 - (114 + 10) = 134 - 124 = 10 \text{ mEq/L}.
An anion gap of 10 mEq/L10 \text{ mEq/L} is within the normal range (8−12 mEq/L8-12 \text{ mEq/L}), identifying the primary disorder as a normal anion gap metabolic acidosis (NAGMA), typical of severe diarrhea.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×HCO3−)+8±2=(1.5×10)+8±2=15+8±2=23±2 mm Hg\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 10) + 8 \pm 2 = 15 + 8 \pm 2 = 23 \pm 2 \text{ mm Hg}.
Winter's formula determines the expected pCO2 to see if the respiratory response to the metabolic acidosis is appropriate.
4
Compare the expected pCO2 to the measured pCO2.
The measured pCO2 is 32 mm Hg32 \text{ mm Hg}, which is significantly higher than the expected range of 21−25 mm Hg21-25 \text{ mm Hg}.
Because the measured pCO2 is higher than expected, there is relative hypoventilation, diagnosing a superimposed respiratory acidosis (likely due to severe hypokalemia causing respiratory muscle weakness).

Key Concept

In patients with severe metabolic acidosis, the actual pCO2 must be compared to the expected pCO2 calculated by Winter's formula. If the measured pCO2 is higher than the expected value, a concurrent respiratory acidosis is present, which may be caused by hypokalemia-induced respiratory muscle fatigue.
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Topics

Nephrology

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