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Difficulty: Very hardNephrology

A 7676-year-old woman is brought to the emergency department by her daughter due to worsening confusion and rapid breathing that started earlier today. The daughter notes the patient has severe osteoarthritis and recently started taking a new "over-the-counter powder" for joint pain in addition to her prescribed medications. Temperature is 38.4∘C38.4^\circ\text{C} (101.1∘F101.1^\circ\text{F}), blood pressure is 110/70 mm Hg110/70 \text{ mm Hg}, pulse is 112/min112/\text{min}, and respiratory rate is 32/min32/\text{min}. Physical examination shows a confused, elderly woman. Mucous membranes are dry. Cardiopulmonary examination reveals clear lungs bilaterally and tachycardia without murmurs. Laboratory studies show:

Leukocyte count: 14,500/mm314,500/\text{mm}^3
Serum sodium: 140 mEq/L140 \text{ mEq/L}
Serum potassium: 3.8 mEq/L3.8 \text{ mEq/L}
Serum chloride: 104 mEq/L104 \text{ mEq/L}
Serum bicarbonate: 12 mEq/L12 \text{ mEq/L}
Blood urea nitrogen: 32 mg/dL32 \text{ mg/dL}
Serum creatinine: 1.5 mg/dL1.5 \text{ mg/dL}
Thyroid-stimulating hormone (TSH): 0.2 μU/mL0.2 \ \mu\text{U/mL} (Normal: 0.5−5.00.5-5.0)
Free T4: 0.6 ng/dL0.6 \text{ ng/dL} (Normal: 0.9−1.70.9-1.7)

Arterial blood gas analysis on room air:
pH: 7.507.50
pCO2pCO_2: 16 mm Hg16 \text{ mm Hg}
pO2pO_2: 98 mm Hg98 \text{ mm Hg}

Which of the following is the most appropriate next step in management?

  1. Intravenous administration of sodium bicarbonateAnswer
  2. B
    Empiric administration of intravenous ceftriaxone and azithromycin
  3. C
    Rapid intravenous bolus of 0.9% normal saline
  4. D
    Intravenous administration of levothyroxine
  5. E
    Administration of intravenous furosemide

Answer

Intravenous administration of sodium bicarbonate
The correct answer addresses the specific toxidrome revealed by the patient's arterial blood gas. The patient's acid-base status demonstrates a primary high anion gap metabolic acidosis (AG = 2424) and a primary respiratory alkalosis (actual pCO2pCO_2 of 1616 is lower than the Winter's formula expected range of 24−2824-28). The alkalemic pH (7.507.50) confirms the respiratory alkalosis is a primary, dominant process rather than merely compensation. This specific mixed acid-base disorder, especially accompanied by fever, confusion, and a history of unprescribed analgesic powder use, is pathognomonic for salicylate (aspirin) toxicity. Salicylates directly stimulate the medullary respiratory center (causing respiratory alkalosis) and uncouple oxidative phosphorylation (leading to lactic acidosis and ketoacidosis, causing HAGMA). Intravenous sodium bicarbonate is the most appropriate next step; it alkalinizes the serum to reduce salicylate entry into the central nervous system and alkalinizes the urine to promote salicylate excretion via ion trapping.

Step-by-Step Solution

1
Analyze the pH to determine the dominant primary disorder.
The pH is 7.507.50 (alkalemia), indicating that a primary alkalosis is the dominant driving force.
Compensation for an acid-base disorder never overcorrects the pH past the normal range (7.35−7.457.35-7.45).
2
Calculate the Anion Gap (AG) to assess for metabolic acidosis.
AG=Na−(Cl+HCO3)=140−(104+12)=24\text{AG} = Na - (Cl + HCO_3) = 140 - (104 + 12) = 24.
A high anion gap (>12>12) confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the overall pH.
3
Apply Winter's formula to determine the expected respiratory compensation for the HAGMA.
Expected pCO2=(1.5×HCO3)+8=(1.5×12)+8=26±2 mm HgpCO_2 = (1.5 \times HCO_3) + 8 = (1.5 \times 12) + 8 = 26 \pm 2 \text{ mm Hg}.
This establishes what the pCO2pCO_2 should be if the HAGMA was the only primary disorder.
4
Compare actual pCO2pCO_2 to expected pCO2pCO_2.
The actual pCO2pCO_2 (16 mm Hg16 \text{ mm Hg}) is significantly lower than the expected range (24−28 mm Hg24-28 \text{ mm Hg}).
This proves there is a superimposed, independent primary respiratory alkalosis.
5
Synthesize the clinical presentation with the acid-base diagnosis.
A mixed HAGMA and respiratory alkalosis in an elderly patient with fever, confusion, and use of an unknown analgesic powder is the classic presentation of salicylate (aspirin) toxicity.
Salicylates directly stimulate the medullary respiratory center and uncouple oxidative phosphorylation.
6
Determine the appropriate management.
Administer intravenous sodium bicarbonate.
Sodium bicarbonate alkalinizes the serum (keeping salicylate out of the CNS) and alkalinizes the urine (promoting salicylate excretion via ion trapping).

Key Concept

Mixed Acid-Base Disorders in Salicylate Toxicity

Alternative Method

When evaluating an ABG, checking the pH first immediately reveals the primary disorder. A pH of 7.50 means alkalosis is the primary driving force. Seeing a low HCO3 (12) means metabolic acidosis is present. A patient cannot have a primary metabolic acidosis driving an alkaline pH. Therefore, the low pCO2 (16) MUST be the primary driver of the alkalemia. The simultaneous presence of an anion gap of 24 proves HAGMA is also present. Mixed HAGMA + Resp Alk = Salicylates.
Estimated Time:2m 0s
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Topics

Nephrology

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