Examkin

Question

Difficulty: MediumNephrology

A 68-year-old man is brought to the emergency department due to worsening shortness of breath, a productive cough, and progressive lethargy over the past 3 days. He has a history of severe chronic obstructive pulmonary disease (COPD) and heart failure with a reduced ejection fraction. One week ago, his primary care physician doubled his daily dose of oral furosemide due to increased lower extremity edema. On physical examination, his temperature is 37.2°C (99.0°F), blood pressure is 110/70 mm Hg, pulse is 98/min, and respiratory rate is 14/min. He is somnolent but arousable. Lung auscultation reveals diffuse expiratory wheezes and prolonged expiration. There is no lower extremity edema.

Arterial blood gas on room air:
pH: 7.37
pCO2pCO_2: 80 mm Hg
pO2pO_2: 55 mm Hg

Serum chemistry:
Sodium: 137 mEq/L
Potassium: 2.9 mEq/L
Chloride: 80 mEq/L
Bicarbonate: 45 mEq/L

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

  1. Mixed respiratory acidosis and metabolic alkalosisAnswer
  2. B
    Fully compensated chronic respiratory acidosis
  3. C
    Mixed metabolic alkalosis and respiratory alkalosis
  4. D
    Compensated metabolic alkalosis
  5. E
    Mixed respiratory acidosis and high anion gap metabolic acidosis

Answer

Mixed respiratory acidosis and metabolic alkalosis
This patient presents with a pH of 7.37 and a pCO2pCO_2 of 80 mm Hg, indicating a primary respiratory acidosis driven by his severe COPD exacerbation. To determine whether this is a simple or mixed disorder, one must calculate the expected renal compensation. In chronic respiratory acidosis, the kidneys retain bicarbonate to compensate, typically increasing serum HCO3−HCO_3^- by roughly 4 mEq/L for every 10 mm Hg increase in pCO2pCO_2. Since this patient's pCO2pCO_2 is 40 mm Hg above normal (80 - 40), his maximal expected HCO3−HCO_3^- should be approximately 24+(4×4)=4024 + (4 \times 4) = 40 mEq/L. His measured HCO3−HCO_3^- of 45 mEq/L significantly exceeds this physiological limit. This reveals a second, independent process that is actively raising the bicarbonate: a primary metabolic alkalosis. This secondary disorder was precipitated by the recent doubling of his furosemide dose, which led to volume depletion, chloride loss, and severe hypokalemia (contraction alkalosis).

Step-by-Step Solution

1
Determine the primary acid-base disorder by looking at the pH and pCO2.
The pH is 7.37 (< 7.40), indicating acidemia. The pCO2pCO_2 is elevated at 80 mm Hg. Because the pCO2pCO_2 movement matches the acidic pH, the primary disorder is a respiratory acidosis.
Establishing the primary disorder dictates which compensation formula to use.
2
Calculate the expected metabolic compensation for the respiratory acidosis.
This patient has severe COPD, implying a chronic state. For chronic respiratory acidosis, serum HCO3−HCO_3^- increases by approximately 3.5 to 4 mEq/L for every 10 mm Hg increase in pCO2pCO_2 above 40 mm Hg.
Kidneys compensate for chronic hypercapnia by retaining bicarbonate, but this compensation has predictable physiological limits.
3
Compare the patient's actual bicarbonate to the expected calculated bicarbonate.
The pCO2pCO_2 is 40 mm Hg above normal (80 - 40). The expected HCO3−HCO_3^- increase is 4×4=164 \times 4 = 16 mEq/L. Thus, maximum expected HCO3−HCO_3^- is 24+16=4024 + 16 = 40 mEq/L. The patient's actual HCO3−HCO_3^- is 45 mEq/L.
An actual HCO3−HCO_3^- higher than the maximum expected compensation indicates an additional, independent primary metabolic alkalosis.

Key Concept

Identifying mixed acid-base disorders by applying compensation rules for chronic respiratory acidosis.
Rate this question

Topics

Nephrology

More questions from this topic