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Difficulty: MediumGeneral Internal Medicine

A 5555-year-old man is evaluated in the step-down unit due to progressively worsening lethargy and tachypnea over the last 44 hours. He was admitted two days ago for a severe intra-abdominal infection following a perforated appendix and is receiving intravenous broad-spectrum antibiotics. His past medical history is significant for severe chronic obstructive pulmonary disease. Vital signs show a temperature of 38.9∘C38.9^\circ\text{C} (102.0∘F102.0^\circ\text{F}), blood pressure of 86/5086/50 mm Hg, pulse of 118118/min, and respiratory rate of 2828/min.

Laboratory and arterial blood gas results on room air are as follows:

TestResult
Serum sodium (Na+Na^+)138138 mEq/L
Serum chloride (Cl−Cl^-)9898 mEq/L
Serum bicarbonate (HCO3−HCO_3^-)1010 mEq/L
Arterial pH7.157.15
PaCO2PaCO_23535 mm Hg
PaO2PaO_26262 mm Hg

Based on these findings, which of the following best describes the patient's primary acid-base disturbances?

  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
    Non-anion gap metabolic acidosis and concurrent respiratory acidosis
  5. E
    Primary respiratory acidosis with appropriate metabolic compensation

Answer

The patient is experiencing a primary high anion gap metabolic acidosis combined with a concurrent respiratory acidosis.
The patient's initial acid-base status reveals a primary metabolic acidosis, evidenced by the low pH (7.157.15) and low serum bicarbonate (1010 mEq/L). The anion gap is calculated as Na+−(Cl−+HCO3−)=138−(98+10)=30Na^+ - (Cl^- + HCO_3^-) = 138 - (98 + 10) = 30 mEq/L. Because this is markedly greater than the normal value (typically 8−128-12 mEq/L), there is a primary high anion gap metabolic acidosis (HAGMA), likely secondary to lactic acidosis from systemic hypoperfusion (septic shock).

To determine if a mixed acid-base disorder is present, Winter's formula is applied to calculate the expected respiratory compensation: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, the expected PaCO2PaCO_2 is (1.5×10)+8±2=23±2(1.5 \times 10) + 8 \pm 2 = 23 \pm 2 mm Hg.

Because the patient's actual measured PaCO2PaCO_2 (3535 mm Hg) is significantly higher than the expected compensatory range (21−2521-25 mm Hg), the patient is hypoventilating relative to the severity of their metabolic acidosis. This indicates a concurrent, superimposed respiratory acidosis. This mixed disorder (HAGMA and respiratory acidosis) frequently occurs in critically ill patients, especially those with underlying COPD who develop respiratory muscle fatigue when attempting extreme compensatory hyperventilation.

Step-by-Step Solution

1
Determine the primary acid-base disorder.
The arterial pH is 7.157.15 (acidemia) and serum HCO3−HCO_3^- is severely depressed at 1010 mEq/L, establishing a primary metabolic acidosis.
Identifying the primary disturbance dictates the subsequent calculations required for complete interpretation.
2
Calculate the anion gap (AG).
AG=Na+−(Cl−+HCO3−)=138−(98+10)=30AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (98 + 10) = 30 mEq/L.
An elevated AG (normal 8−128-12 mEq/L) confirms the presence of a high anion gap metabolic acidosis, which in this patient is likely due to lactic acidosis from septic shock.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient: (1.5×10)+8=23±2(1.5 \times 10) + 8 = 23 \pm 2 mm Hg (range 21−2521-25 mm Hg).
Winter's formula determines the appropriate physiological respiratory response (hyperventilation) to a primary metabolic acidosis.
4
Compare the expected PaCO2PaCO_2 to the measured PaCO2PaCO_2.
The measured PaCO2PaCO_2 is 3535 mm Hg, which is significantly higher than the expected range of 21−2521-25 mm Hg.
When the measured PaCO2PaCO_2 is greater than the expected PaCO2PaCO_2, it indicates an inability to adequately ventilate, diagnostic of a superimposed respiratory acidosis.

Key Concept

In patients presenting with a primary metabolic acidosis, Winter's formula must be utilized to determine if the measured PaCO2 represents appropriate respiratory compensation or if an additional concurrent respiratory acid-base disorder is present.

Alternative Method

A quick bedside approximation for expected PaCO2 is that it should roughly equal the decimal digits of the arterial pH (e.g., if expected PaCO2 is 23, the pH should be roughly 7.23). In this patient, the pH is 7.15, but the PaCO2 is 35, confirming a gross mismatch indicative of a mixed disorder.
Estimated Time:1m 30s
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General Internal Medicine

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