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Difficulty: MediumNephrology

A 27-year-old female with a history of type 1 diabetes mellitus and asthma is evaluated in the emergency department for a three-day history of worsening cough, fever, and shortness of breath. She has been unable to eat and has missed several doses of her basal insulin. On physical examination, her temperature is 38.5∘C38.5^{\circ}\text{C} (101.3∘F101.3^{\circ}\text{F}), blood pressure is 105/65 mmHg, pulse is 118/min, and respirations are 32/min. The patient appears lethargic and is using accessory muscles for respiration.

Laboratory studies reveal the following:

TestResult
Sodium136 mEq/L136 \text{ mEq/L}
Potassium4.8 mEq/L4.8 \text{ mEq/L}
Chloride100 mEq/L100 \text{ mEq/L}
Bicarbonate12 mEq/L12 \text{ mEq/L}
Blood urea nitrogen28 mg/dL28 \text{ mg/dL}
Creatinine1.2 mg/dL1.2 \text{ mg/dL}
Glucose512 mg/dL512 \text{ mg/dL}

Arterial blood gas analysis on room air:

TestResult
pH7.187.18
PaCO2PaCO_235 mmHg35 \text{ mmHg}
PaO2PaO_270 mmHg70 \text{ mmHg}

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

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

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
This patient presents with diabetic ketoacidosis (DKA) triggered by an underlying respiratory infection, leading to a primary high anion gap metabolic acidosis. The anion gap is elevated at 24 mEq/L24 \text{ mEq/L} (136−(100+12)136 - (100 + 12)). In a primary metabolic acidosis, the lungs should compensate by hyperventilating to blow off CO2CO_2 (Kussmaul respirations). The expected PaCO2PaCO_2 can be calculated using Winter's formula: Expected PaCO2=1.5×[HCO3−]+8±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2. For this patient, the expected PaCO2PaCO_2 is (1.5×12)+8=26±2 mmHg(1.5 \times 12) + 8 = 26 \pm 2 \text{ mmHg}. The patient's actual PaCO2PaCO_2 is 35 mmHg35 \text{ mmHg}, which is substantially higher than the expected target range of 24−28 mmHg24 - 28 \text{ mmHg}. This relative CO2CO_2 retention indicates a concurrent primary respiratory acidosis. Clinically, this is consistent with respiratory muscle fatigue secondary to her asthma exacerbation and severe work of breathing, preventing her from achieving the necessary compensatory hyperventilation.

Step-by-Step Solution

1
Calculate the anion gap to evaluate the metabolic acidosis.
Anion gap = Na+−(Cl−+HCO3−)=136−(100+12)=24 mEq/LNa^+ - (Cl^- + HCO_3^-) = 136 - (100 + 12) = 24 \text{ mEq/L}.
An anion gap >12 mEq/L>12 \text{ mEq/L} confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), consistent with her diabetic ketoacidosis.
2
Determine the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×12)+8±2=18+8±2=26±2 mmHgPaCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 \text{ mmHg} (Range: 24−28 mmHg24 - 28 \text{ mmHg}).
In a primary metabolic acidosis, the lungs should compensate by hyperventilating to lower PaCO2PaCO_2. Winter's formula determines if this response is adequate.
3
Compare the expected PaCO2PaCO_2 to the patient's actual PaCO2PaCO_2.
The actual PaCO2PaCO_2 is 35 mmHg35 \text{ mmHg}, which is significantly higher than the expected maximum of 28 mmHg28 \text{ mmHg}.
Because the actual PaCO2PaCO_2 is higher than the calculated compensatory target, there is relative CO2CO_2 retention, diagnosing a concurrent primary respiratory acidosis superimposed on the HAGMA.

Key Concept

Mixed Acid-Base Disorders: Utilizing Winter's Formula in High Anion Gap Metabolic Acidosis
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Topics

Nephrology

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