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Zorluk: OrtaNephrology

A 21-year-old female university student is evaluated in the urgent care clinic after her roommates noticed her acting increasingly confused and breathing rapidly over the past 4 hours4\text{ hours}. She has a history of major depressive disorder and recently failed an important exam. She complains of a constant "buzzing" sound in her ears. Her temperature is 37.9∘C37.9^\circ\text{C} (100.2∘F100.2^\circ\text{F}), pulse is 118/min118\text{/min}, respirations are 30/min30\text{/min}, and blood pressure is 112/68 mm Hg112/68\text{ mm Hg}. Physical examination reveals an agitated, tachypneic patient with clear lung fields and no focal neurologic deficits.

Laboratory studies reveal the following:

Laboratory TestResult
Sodium140 mEq/L140\text{ mEq/L}
Potassium3.5 mEq/L3.5\text{ mEq/L}
Chloride104 mEq/L104\text{ mEq/L}
Bicarbonate13 mEq/L13\text{ mEq/L}
Blood urea nitrogen18 mg/dL18\text{ mg/dL}
Creatinine0.9 mg/dL0.9\text{ mg/dL}
Arterial Blood GasResult
pH7.487.48
PaCO2_218 mm Hg18\text{ mm Hg}
PaO2_298 mm Hg98\text{ mm Hg}

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

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

Cevap

High anion gap metabolic acidosis and concurrent respiratory alkalosis
The correct answer identifies that this patient has a mixed acid-base disorder characterized by both a high anion gap metabolic acidosis and a primary respiratory alkalosis, which is the classic presentation of acute salicylate (aspirin) toxicity. The clinical context of a suicide attempt, tinnitus ('buzzing' in ears), fever, and tachypnea strongly suggest this diagnosis. The first step in evaluating this patient's acid-base status is calculating the serum anion gap: Na+−(Cl−+HCO3−)=140−(104+13)=23 mEq/L\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (104 + 13) = 23\text{ mEq/L}. Since the normal range is 10−14 mEq/L10-14\text{ mEq/L}, a high anion gap metabolic acidosis (HAGMA) is present. Salicylates cause this by uncoupling oxidative phosphorylation and interfering with the Krebs cycle, leading to the accumulation of organic acids. The second step is to use Winter's formula to determine if the respiratory response is an appropriate physiological compensation or if a concurrent respiratory disorder exists. Expected PaCO2=1.5×[HCO3−]+8±2\text{PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2. For this patient, 1.5×13+8=27.5±2 mm Hg1.5 \times 13 + 8 = 27.5 \pm 2\text{ mm Hg} (range 25.5−29.5 mm Hg25.5 - 29.5\text{ mm Hg}). The patient's measured PaCO2\text{PaCO}_2 is 18 mm Hg18\text{ mm Hg}, which is substantially lower than the expected range. This excessive hyperventilation indicates a concurrent primary respiratory alkalosis, caused by salicylates directly stimulating the medullary respiratory center. Therefore, the patient has a mixed HAGMA and respiratory alkalosis.

Adım Adım Çözüm

1
Calculate the serum anion gap to evaluate for a primary metabolic acidosis.
Anion Gap = Na - (Cl + HCO3) = 140 - (104 + 13) = 23 mEq/L.
An anion gap > 14 mEq/L establishes the presence of a High Anion Gap Metabolic Acidosis (HAGMA), a key component of salicylate toxicity.
2
Use Winter's formula to determine the expected respiratory compensation (expected PaCO2) for the calculated metabolic acidosis.
Expected PaCO2 = (1.5 * HCO3) + 8 +/- 2 = (1.5 * 13) + 8 +/- 2 = 27.5 +/- 2 mm Hg (Range: 25.5 to 29.5 mm Hg).
Winter's formula determines if the patient's breathing pattern is an appropriate physiologic response to the acidosis or if a second primary acid-base disorder is present.
3
Compare the patient's measured PaCO2 to the expected PaCO2 range.
The measured PaCO2 is 18 mm Hg, which is significantly lower than the expected minimum of 25.5 mm Hg.
Because the measured PaCO2 is lower than what would be expected from compensatory hyperventilation alone, a concurrent primary respiratory alkalosis must also be present.

Anahtar Kavram

Patients with salicylate (aspirin) toxicity classically present with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation).
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