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Difficulty: HardNephrology

A 72-year-old man with a history of severe chronic obstructive pulmonary disease (COPD) and heart failure with preserved ejection fraction is brought to the emergency department with worsening shortness of breath and lower extremity edema over the past week. Three days ago, his outpatient physician doubled his daily dose of oral furosemide. On examination, the patient is alert but in mild respiratory distress. Vital signs are: temperature 37.1∘C37.1^\circ\text{C}, blood pressure 110/70 mmHg110/70 \text{ mmHg}, heart rate 95/min95/\text{min}, and respiratory rate 22/min22/\text{min}. Pulse oximetry is 89%89\% on room air.

Initial laboratory studies on room air show:

TestValue
Arterial pHpH7.397.39
Arterial PaCO2PaCO_260 mm Hg60 \text{ mm Hg}
Arterial PaO2PaO_258 mm Hg58 \text{ mm Hg}
Serum Na+Na^+138 mEq/L138 \text{ mEq/L}
Serum K+K^+3.1 mEq/L3.1 \text{ mEq/L}
Serum Cl−Cl^-92 mEq/L92 \text{ mEq/L}
Serum HCO3−HCO_3^-36 mEq/L36 \text{ mEq/L}
Blood urea nitrogen38 mg/dL38 \text{ mg/dL}
Serum creatinine1.5 mg/dL1.5 \text{ mg/dL}

Which of the following is the most accurate assessment of this patient's current acid-base status?

  1. Chronic respiratory acidosis and superimposed metabolic alkalosisAnswer
  2. B
    Fully compensated chronic respiratory acidosis
  3. C
    Primary metabolic alkalosis with compensatory respiratory acidosis
  4. D
    Acute respiratory acidosis and superimposed metabolic alkalosis
  5. E
    Chronic respiratory acidosis and superimposed metabolic acidosis

Answer

The patient has a primary chronic respiratory acidosis (due to underlying COPD) with a superimposed metabolic alkalosis (driven by aggressive furosemide therapy leading to contraction alkalosis and hypokalemia).
The correct interpretation involves identifying the primary disturbance and checking compensation. A pH of 7.39 and PaCO2 of 60 mm Hg denotes primary respiratory acidosis. Because the patient has severe COPD, this is chronic. The renal compensation for chronic respiratory acidosis is an increase in HCO3- of 3.5 mEq/L for every 10 mm Hg rise in PaCO2. Here, PaCO2 is 20 above normal (60 - 40). The expected HCO3- = 24 + (2 * 3.5) = 31 mEq/L. The patient's actual HCO3- is 36 mEq/L. Since actual HCO3- > expected HCO3-, there is a superimposed metabolic alkalosis, clinically explained by the recent escalation of loop diuretic therapy causing volume contraction and hypokalemia.

Step-by-Step Solution

1
Determine the primary acid-base disturbance by looking at the pH and PaCO2.
The pHpH is 7.397.39 (acidemic side of 7.407.40), and PaCO2PaCO_2 is high at 60 mm Hg60 \text{ mm Hg}. This combination defines a primary respiratory acidosis.
The direction of the pH variation from the absolute normal (7.407.40) indicates the primary disorder. High PaCO2PaCO_2 drives the pH down.
2
Determine whether the respiratory acidosis is acute or chronic based on clinical history and standard compensation formulas.
The patient has severe COPD, suggesting chronicity. In chronic respiratory acidosis, HCO3−HCO_3^- increases by 33 to 4 mEq/L4 \text{ mEq/L} for every 10 mm Hg10 \text{ mm Hg} increase in PaCO2PaCO_2 above 4040.
Renal compensation for respiratory acidosis takes 3-5 days to fully manifest. Chronic CO2 retainers rely on this mechanism.
3
Calculate the expected compensatory HCO3−HCO_3^- level.
PaCO2PaCO_2 has risen from 4040 to 60 mm Hg60 \text{ mm Hg} (an increase of 2020). The expected increase in HCO3−HCO_3^- is 2×3.5=7 mEq/L2 \times 3.5 = 7 \text{ mEq/L}. Expected HCO3−HCO_3^- is 24+7=31 mEq/L24 + 7 = 31 \text{ mEq/L}.
Calculating expected compensation is necessary to unmask any hidden secondary or tertiary acid-base disorders.
4
Compare actual HCO3−HCO_3^- to expected HCO3−HCO_3^-.
The actual HCO3−HCO_3^- is 36 mEq/L36 \text{ mEq/L}, which is significantly higher than the expected 31 mEq/L31 \text{ mEq/L}.
A higher-than-expected bicarbonate level indicates a concurrent, primary process that generates bicarbonate—specifically, a metabolic alkalosis.
5
Correlate with the clinical presentation.
The recent doubling of furosemide caused volume depletion (elevated BUN/Cr ratio of >20:1) and hypokalemia (3.1 mEq/L3.1 \text{ mEq/L}), both of which independently generate and maintain a metabolic alkalosis.
Matching calculated lab abnormalities with clinical interventions confirms the diagnosis.

Key Concept

In mixed acid-base disorders, if the actual serum bicarbonate is significantly higher than the expected compensatory bicarbonate for a primary respiratory acidosis, a superimposed metabolic alkalosis is present.
Estimated Time:2m 0s
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Nephrology

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