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Difficulty: MediumNephrology

A 62-year-old man is brought to the emergency department after falling from a 10-foot ladder while cleaning his gutters. He states he was lying on the ground for approximately 12 hours before a neighbor found him. He complains of severe, sharp pain in his right lower chest that significantly worsens when he takes a deep breath. His temperature is 36.8°C (98.2°F), blood pressure is 115/75 mmHg, pulse is 112/min, and respiratory rate is 16/min with shallow breaths. Physical examination reveals bruising and marked point tenderness over the right 7th and 8th ribs. Decreased breath sounds are noted at the right lung base.

Laboratory results are as follows:

Serum ChemistryValue
Sodium140140 mEq/L
Potassium5.25.2 mEq/L
Chloride104104 mEq/L
Bicarbonate1212 mEq/L
Blood urea nitrogen5555 mg/dL
Creatinine3.13.1 mg/dL
Creatine kinase18,50018,500 U/L
Arterial Blood Gas (Room Air)Value
pH7.167.16
PaCO2P_aCO_23535 mmHg
PaO2P_aO_26868 mmHg

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

  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
    Mixed high anion gap metabolic acidosis and metabolic alkalosis

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
This patient has a mixed acid-base disorder: a high anion gap metabolic acidosis (HAGMA) and a concurrent respiratory acidosis. The primary disturbance is a metabolic acidosis, evidenced by severe acidemia (pH 7.167.16) and low bicarbonate (1212 mEq/L). The anion gap is elevated at 2424 mEq/L (140−[104+12]140 - [104 + 12]), confirming a HAGMA. Clinically, this is likely secondary to acute kidney injury from rhabdomyolysis (markedly elevated CK and creatinine) following his prolonged immobilization. To assess respiratory compensation, Winter's formula must be applied: ExpectedPaCO2=1.5×HCO3−+8±2Expected P_aCO_2 = 1.5 \times HCO_3^- + 8 \pm 2. For this patient, the expected PaCO2=1.5×12+8=26±2P_aCO_2 = 1.5 \times 12 + 8 = 26 \pm 2 mmHg (range: 24−2824-28 mmHg). The patient's actual PaCO2P_aCO_2 is 3535 mmHg. Because the actual PaCO2P_aCO_2 is significantly higher than the expected range, he has inadequate respiratory compensation, defining a concurrent respiratory acidosis. This hypoventilation is clinically consistent with splinting due to severe pain from his rib fractures.

Step-by-Step Solution

1
Determine the primary acid-base disturbance.
The pH is 7.167.16 (acidemia) and HCO3−HCO_3^- is 1212 mEq/L (low), indicating a primary metabolic acidosis.
The direction of the pH change matches the direction of the HCO3−HCO_3^- change, defining a metabolic primary process.
2
Calculate the anion gap (AG) to classify the metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=140−(104+12)=24AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 12) = 24 mEq/L.
A normal AG is 10−1410-14 mEq/L. An AG of 2424 indicates a high anion gap metabolic acidosis (HAGMA), likely due to uremia secondary to rhabdomyolysis-induced acute kidney injury.
3
Apply Winter's formula to determine the expected respiratory compensation.
ExpectedPaCO2=1.5×(HCO3−)+8±2Expected P_aCO_2 = 1.5 \times (HCO_3^-) + 8 \pm 2. Thus, 1.5×(12)+8=26±21.5 \times (12) + 8 = 26 \pm 2 mmHg (range: 2424 to 2828 mmHg).
Winter's formula predicts the appropriate compensatory hyperventilation (decrease in PaCO2P_aCO_2) for a given severity of metabolic acidosis.
4
Compare the actual PaCO2P_aCO_2 to the expected PaCO2P_aCO_2.
The actual PaCO2P_aCO_2 is 3535 mmHg, which is significantly higher than the expected range of 24−2824-28 mmHg.
When actual PaCO2P_aCO_2 is higher than expected, it indicates inadequate ventilation and a concurrent respiratory acidosis (due to pain splinting from rib fractures in this patient).
5
Check the delta-delta ratio for any occult metabolic alkalosis.
ΔAG=24−12=12\Delta AG = 24 - 12 = 12. ΔHCO3−=24−12=12\Delta HCO_3^- = 24 - 12 = 12. Ratio = 12/12=1.012 / 12 = 1.0.
A ratio between 11 and 22 implies a pure HAGMA without an additional metabolic component.

Key Concept

Patients with a primary metabolic acidosis should hyperventilate to decrease PaCO2P_aCO_2. If the actual PaCO2P_aCO_2 is higher than the expected PaCO2P_aCO_2 calculated by Winter's formula (1.5×HCO3−+8±21.5 \times HCO_3^- + 8 \pm 2), a concurrent respiratory acidosis is present.
Estimated Time:2m 0s
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Nephrology

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