Examkin

Question

Difficulty: Very hardNephrology

An obtunded 68-year-old man is brought to the emergency department by paramedics following a two-day history of worsening shortness of breath, productive cough, and fever. His medical history is significant for severe chronic obstructive pulmonary disease (COPD) requiring 2 L/min home oxygen, and cor pulmonale. His temperature is 38.6°C (101.5°F), pulse is 112/min, blood pressure is 90/60 mm Hg, and respirations are 26/min. Physical examination reveals a somnolent patient utilizing accessory muscles of respiration. Auscultation of the lungs reveals diffuse expiratory wheezing and coarse crackles in the right lower lobe.

Laboratory results from a routine outpatient clinic visit one month ago, when the patient was at his clinical baseline, showed:

TestResult
Arterial blood gas (baseline, room air)
pHpH7.387.38
pCO2pCO_256 mm Hg56 \text{ mm Hg}
pO2pO_262 mm Hg62 \text{ mm Hg}
HCO3−HCO_3^-32 mEq/L32 \text{ mEq/L}

Current laboratory results obtained upon arrival are as follows:

TestResult
Serum chemistry
Sodium140 mEq/L140 \text{ mEq/L}
Potassium4.2 mEq/L4.2 \text{ mEq/L}
Chloride96 mEq/L96 \text{ mEq/L}
Bicarbonate14 mEq/L14 \text{ mEq/L}
Blood urea nitrogen30 mg/dL30 \text{ mg/dL}
Creatinine1.5 mg/dL1.5 \text{ mg/dL}
L-lactate5.5 mmol/L5.5 \text{ mmol/L}
Arterial blood gas (room air)
pHpH7.157.15
pCO2pCO_242 mm Hg42 \text{ mm Hg}
pO2pO_248 mm Hg48 \text{ mm Hg}

Which of the following best characterizes this patient's current acid-base status?

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

Answer

High anion gap metabolic acidosis and respiratory acidosis
This patient has a primary high anion gap metabolic acidosis (HAGMA) due to sepsis-induced lactic acidosis, evidenced by an anion gap of 30. Using Winter's formula (Expected pCO2 = 1.5 × 14 + 8 = 29 ± 2 mm Hg), the actual pCO2 of 42 mm Hg is significantly higher than expected, indicating a concurrent respiratory acidosis. Finally, calculating the delta-adjusted bicarbonate yields 32 mEq/L (Measured HCO3 [14] + Delta AG [18]). Because 32 mEq/L perfectly matches the patient's documented chronic baseline bicarbonate (a compensatory metabolic alkalosis from chronic COPD), there is no newly superimposed metabolic disorder.

Step-by-Step Solution

1
Calculate the serum Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=140−(96+14)=30AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (96 + 14) = 30.
An AG>12AG > 12 indicates the presence of a primary high anion gap metabolic acidosis (HAGMA), which in this case is driven by severe lactic acidosis (5.5 mmol/L5.5 \text{ mmol/L}) secondary to sepsis.
2
Calculate expected respiratory compensation using Winter's formula.
Expected pCO2=1.5×HCO3−+8±2=1.5×(14)+8=29±2 mm HgpCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = 1.5 \times (14) + 8 = 29 \pm 2 \text{ mm Hg}.
This establishes the target pCO2pCO_2 for a pure metabolic acidosis. Because the patient's actual pCO2pCO_2 is 42 mm Hg42 \text{ mm Hg} (which is >31 mm Hg> 31 \text{ mm Hg}), he has a concurrent, superimposed respiratory acidosis due to COPD preventing adequate hyperventilation.
3
Calculate the Δ\Delta Gap to evaluate for a third, hidden metabolic disorder.
ΔAG=Actual AG−Normal AG=30−12=18\Delta AG = \text{Actual } AG - \text{Normal } AG = 30 - 12 = 18. The Δ\Delta-adjusted HCO3−HCO_3^- = Measured HCO3−+ΔAG=14+18=32 mEq/LHCO_3^- + \Delta AG = 14 + 18 = 32 \text{ mEq/L}.
The Δ\Delta-adjusted HCO3−HCO_3^- reveals what the patient's bicarbonate would be if the high anion gap acids were removed.
4
Compare the Δ\Delta-adjusted HCO3−HCO_3^- to the patient's proper baseline.
The calculated Δ\Delta-adjusted HCO3−HCO_3^- (32 mEq/L32 \text{ mEq/L}) is identical to the patient's known chronic baseline HCO3−HCO_3^- (32 mEq/L32 \text{ mEq/L}).
Because the adjusted value matches his chronic baseline, there is NO additional metabolic acid-base disorder. Comparing it to the population normal (24 mEq/L24 \text{ mEq/L}) would falsely suggest a metabolic alkalosis.

Key Concept

When analyzing mixed acid-base disorders in a patient with a known chronic acid-base abnormality (e.g., chronic respiratory acidosis), the delta-adjusted bicarbonate must be compared to the patient's established baseline bicarbonate, not the standard population normal of 24 mEq/L.
Estimated Time:2m 0s
Rate this question

Topics

Nephrology

More questions from this topic