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Zorluk: OrtaNephrology

A 72-year-old man is brought to the emergency department from a long-term care facility due to a 4-day history of profuse, watery diarrhea and progressive lethargy. His medical history includes hypertension, benign prostatic hyperplasia, and moderate chronic obstructive pulmonary disease. His medications include amlodipine, tamsulosin, and an albuterol inhaler as needed. Temperature is 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), blood pressure is 98/62 mm Hg, pulse is 114/min, and respirations are 24/min. Physical examination shows dry mucous membranes, delayed capillary refill, and decreased skin turgor. Lungs are clear to auscultation bilaterally.

Laboratory studies show:
ParameterValueReference Range
Serum sodium136 mEq/L136 \text{ mEq/L}136−145 mEq/L136-145 \text{ mEq/L}
Serum potassium3.1 mEq/L3.1 \text{ mEq/L}3.5−5.0 mEq/L3.5-5.0 \text{ mEq/L}
Serum chloride115 mEq/L115 \text{ mEq/L}98−106 mEq/L98-106 \text{ mEq/L}
Serum bicarbonate10 mEq/L10 \text{ mEq/L}22−28 mEq/L22-28 \text{ mEq/L}
Blood urea nitrogen52 mg/dL52 \text{ mg/dL}7−18 mg/dL7-18 \text{ mg/dL}
Serum creatinine2.3 mg/dL2.3 \text{ mg/dL}0.6−1.2 mg/dL0.6-1.2 \text{ mg/dL}
Arterial blood gas (room air):
ParameterValueReference Range
pH7.207.35-7.45
pCO2\text{pCO}_232 mm Hg32 \text{ mm Hg}35−45 mm Hg35-45 \text{ mm Hg}
pO2\text{pO}_288 mm Hg88 \text{ mm Hg}75−105 mm Hg75-105 \text{ mm Hg}

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

  1. A
    Normal anion gap metabolic acidosis with concurrent respiratory alkalosis
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis with concurrent respiratory acidosis
  4. Normal anion gap metabolic acidosis with concurrent respiratory acidosisCevap
  5. E
    High anion gap metabolic acidosis with appropriate respiratory compensation

Cevap

Normal anion gap metabolic acidosis with concurrent respiratory acidosis
The patient has acidemia (pH = 7.20) with a low bicarbonate (10 mEq/L), indicating a primary metabolic acidosis. The next step is to calculate the serum anion gap to determine the etiology. The anion gap is calculated as Na+−(Cl−+HCO3−)\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-). For this patient, the anion gap is 136−(115+10)=11 mEq/L136 - (115 + 10) = 11 \text{ mEq/L}, which is within the normal reference range (8−12 mEq/L8-12 \text{ mEq/L}). This normal anion gap metabolic acidosis is consistent with severe bicarbonate loss from his profuse diarrhea. To assess for a mixed acid-base disorder, Winter's formula is used to calculate the expected respiratory compensation: Expected pCO2=(1.5×HCO3−)+8±2\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2. For this patient, the expected pCO2=(1.5×10)+8=23±2 mm Hg\text{pCO}_2 = (1.5 \times 10) + 8 = 23 \pm 2 \text{ mm Hg}. The patient's measured pCO2\text{pCO}_2 is 32 mm Hg, which is significantly higher than the expected range of 21-25 mm Hg. This inadequate respiratory compensation indicates a concurrent primary respiratory acidosis, which is likely secondary to respiratory muscle fatigue and his underlying chronic obstructive pulmonary disease (COPD).

Adım Adım Çözüm

1
Identify the primary acid-base disturbance.
The low pH (7.20) and low bicarbonate (10 mEq/L) indicate a primary metabolic acidosis.
Acidemia driven by a primary decrease in bicarbonate is the hallmark of metabolic acidosis.
2
Calculate the serum anion gap to determine the etiology of the metabolic acidosis.
Anion Gap = Na - (Cl + HCO3) = 136 - (115 + 10) = 11 mEq/L.
An anion gap of 11 mEq/L is within the normal range (8-12 mEq/L), confirming a normal anion gap metabolic acidosis, which is consistent with severe bicarbonate loss from diarrhea.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2 = (1.5 * 10) + 8 +/- 2 = 23 +/- 2 mm Hg (range 21-25 mm Hg).
This formula determines if the respiratory system is adequately compensating for the metabolic acidosis.
4
Compare the measured pCO2 to the expected pCO2 to identify any secondary disturbances.
The measured pCO2 of 32 mm Hg is higher than the expected range of 21-25 mm Hg.
An inappropriately elevated pCO2 in the setting of metabolic acidosis indicates relative hypoventilation, revealing a superimposed primary respiratory acidosis.

Anahtar Kavram

Mixed acid-base disorders are identified by determining the primary disturbance, calculating the anion gap, and using compensatory formulas such as Winter's formula to assess for secondary disturbances.
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