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Difficulty: MediumNephrology

A 74-year-old man is brought to the emergency department from a skilled nursing facility due to severe abdominal pain, bloody diarrhea, and altered mental status. His medical history includes peripheral artery disease, hypertension, and a myocardial infarction one year ago. His temperature is 37.8°C (100.0°F), pulse is 118/min, blood pressure is 88/50 mm Hg, and respirations are 28/min. Physical examination shows a lethargic patient taking shallow, rapid breaths. The abdomen is distended and diffusely tender with involuntary guarding.

The following laboratory results are obtained:

TestResult
Sodium (Na+Na^+)138 mEq/L
Potassium (K+K^+)4.8 mEq/L
Chloride (Cl−Cl^-)98 mEq/L
Bicarbonate (HCO3−HCO_3^-)10 mEq/L
Blood urea nitrogen45 mg/dL
Creatinine2.1 mg/dL
Lactic acid7.2 mmol/L

Arterial blood gas analysis on room air reveals:

TestResult
pH7.15
PaCO2PaCO_230 mm Hg
PaO2PaO_265 mm Hg

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

  1. A
    Primary high anion gap metabolic acidosis with appropriate respiratory compensation
  2. Primary high anion gap metabolic acidosis and a concurrent respiratory acidosisAnswer
  3. C
    Primary high anion gap metabolic acidosis and a concurrent respiratory alkalosis
  4. D
    Primary normal anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Primary respiratory acidosis with compensatory metabolic alkalosis

Answer

Primary high anion gap metabolic acidosis and a concurrent respiratory acidosis
The correct answer identifies a mixed acid-base disorder. First, the patient has acidemia (pH 7.15) driven by a very low bicarbonate (10 mEq/L), which establishes a primary metabolic acidosis. The anion gap is calculated as 138−(98+10)=30138 - (98 + 10) = 30 mEq/L. Because the anion gap is > 12 mEq/L, this is a high anion gap metabolic acidosis, perfectly consistent with severe lactic acidosis (7.2 mmol/L) secondary to acute mesenteric ischemia. Next, Winter's formula must be used to check for adequate respiratory compensation: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Using the patient's values, Expected PaCO2=(1.5×10)+8±2=23±2PaCO_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 mm Hg. The patient's actual measured PaCO2PaCO_2 is 30 mm Hg. Since the actual PaCO2PaCO_2 is significantly higher than the expected range of 21-25 mm Hg, the patient has inadequate respiratory compensation, diagnosing a superimposed, concurrent respiratory acidosis. Clinically, this occurs because the patient is lethargic and fatigued, taking shallow breaths that fail to adequately clear carbon dioxide.

Step-by-Step Solution

1
Analyze the pH to determine the primary status.
The pH is 7.15.
A pH < 7.35 indicates primary acidemia.
2
Identify the primary disturbance causing the acidemia.
The HCO3−HCO_3^- is extremely low at 10 mEq/L.
A low bicarbonate matching an acidic pH defines a primary metabolic acidosis.
3
Calculate the serum anion gap.
Anion Gap = Na+−(Cl−+HCO3−)=138−(98+10)=30Na^+ - (Cl^- + HCO_3^-) = 138 - (98 + 10) = 30 mEq/L.
An anion gap > 12 mEq/L confirms the presence of a high anion gap metabolic acidosis (HAGMA), in this case due to lactic acidosis from ischemic bowel.
4
Apply Winter's formula to determine the expected respiratory compensation.
Expected PaCO2=(1.5×10)+8±2=23±2PaCO_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 mm Hg.
Winter's formula calculates the expected PaCO2PaCO_2 response to a primary metabolic acidosis.
5
Compare measured PaCO2PaCO_2 to expected PaCO2PaCO_2.
Measured PaCO2PaCO_2 (30 mm Hg) is greater than the expected range (21-25 mm Hg).
When the measured PaCO2PaCO_2 is higher than the calculated compensatory expected value, a concurrent respiratory acidosis is present, likely due to respiratory muscle fatigue in this critically ill patient.

Key Concept

In a patient with a primary metabolic acidosis, calculating the anion gap and applying Winter's formula are essential to identify mixed acid-base disorders such as a concurrent respiratory acidosis.
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Topics

Nephrology

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