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Zorluk: OrtaNephrology

An 18-year-old female university student is evaluated in the emergency department after her roommate found her to be acutely disoriented. Over the last 12 hours, the patient has experienced continuous nausea, episodes of vomiting, and a persistent buzzing sensation in her ears. The roommate suspects she ingested a large quantity of an over-the-counter medication following a personal dispute. Her temperature is 37.8∘C37.8^\circ C (100∘F100^\circ F), blood pressure is 114/72114/72 mm Hg, pulse is 118118/min, and respirations are 2626/min. Physical examination shows a diaphoretic, agitated young woman with deep, rapid breathing.

Laboratory results are as follows:

Laboratory TestResult
Serum sodium140140 mEq/L
Serum potassium3.83.8 mEq/L
Serum chloride104104 mEq/L
Serum bicarbonate1212 mEq/L
Blood urea nitrogen1818 mg/dL
Serum creatinine1.01.0 mg/dL
Arterial pH7.427.42
Arterial PaO2PaO_29898 mm Hg
Arterial PaCO2PaCO_21919 mm Hg

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

  1. Mixed high anion gap metabolic acidosis and primary respiratory alkalosisCevap
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Primary respiratory alkalosis with appropriate metabolic compensation
  4. D
    Mixed normal anion gap metabolic acidosis and primary respiratory alkalosis
  5. E
    Mixed high anion gap metabolic acidosis and metabolic alkalosis

Cevap

Mixed high anion gap metabolic acidosis and primary respiratory alkalosis
The correct answer identifies a mixed high anion gap metabolic acidosis and a primary respiratory alkalosis. This patient's clinical presentation (tinnitus, tachypnea, nausea, vomiting, agitation) is highly suspicious for salicylate toxicity (aspirin overdose). The acid-base workup reveals a severely low bicarbonate (1212 mEq/L) with an elevated anion gap of 2424 mEq/L, confirming a high anion gap metabolic acidosis. Utilizing Winter's formula (Expected PaCO2=(1.5×12)+8=26±2PaCO_2 = (1.5 \times 12) + 8 = 26 \pm 2), the expected PaCO2PaCO_2 for respiratory compensation is 24−2824-28 mm Hg. The actual PaCO2PaCO_2 is 1919 mm Hg, which is lower than expected, confirming an independent, concurrent primary respiratory alkalosis.

Adım Adım Çözüm

1
Evaluate the patient's arterial blood gas to identify the predominant pH direction and primary disturbances.
The pH is 7.427.42, which is normal but slightly alkalemic. The PaCO2PaCO_2 is severely low (1919 mm Hg), indicating respiratory alkalosis, and the HCO3−HCO_3^- is significantly low (1212 mEq/L), indicating metabolic acidosis.
Understanding the individual components helps recognize that two opposing primary processes are occurring simultaneously, which is characteristic of salicylate toxicity.
2
Calculate the serum anion gap to classify the metabolic acidosis.
Anion Gap =Na+−(Cl−+HCO3−)=140−(104+12)=24= Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 12) = 24 mEq/L. This reveals a high anion gap metabolic acidosis (HAGMA).
A high anion gap confirms the presence of unmeasured anions, which in this case are organic acids (lactic acid and ketoacids) secondary to salicylate-induced uncoupling of oxidative phosphorylation.
3
Apply Winter's formula to calculate the expected respiratory compensation for the degree of metabolic acidosis.
Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Calculation: (1.5×12)+8=26±2(1.5 \times 12) + 8 = 26 \pm 2 mm Hg. The expected range is 24−2824-28 mm Hg.
Winter's formula determines if the patient's respiratory drive is appropriately compensating for the metabolic acidosis.
4
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2 range.
The actual PaCO2PaCO_2 (1919 mm Hg) is significantly lower than the expected compensatory range (24−2824-28 mm Hg).
A PaCO2PaCO_2 lower than expected indicates excessive hyperventilation, diagnosing an independent, concurrent primary respiratory alkalosis due to direct salicylate stimulation of the medullary respiratory center.

Anahtar Kavram

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

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