Examkin

Soru

Zorluk: OrtaNephrology

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 alkalosisCevap
  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

Cevap

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).

Adım Adım Çözüm

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.

Anahtar Kavram

Identifying mixed acid-base disorders by applying compensation rules for chronic respiratory acidosis.
Bu soruyu puanla

Konular

Nephrology

Bu konudan daha fazla soru