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Difficulty: Very hardPharmacologic Interventions

A 21-year-old woman is brought to the emergency department by her roommate due to progressive confusion, nausea, vomiting, and ringing in her ears for the past 4 hours. The roommate reports finding several empty bottles of over-the-counter pain relievers in their shared bathroom. On arrival, her temperature is 38.1∘C38.1^\circ\text{C} (100.6∘F100.6^\circ\text{F}), blood pressure is 118/74 mmHg118/74\text{ mmHg}, pulse is 122/min122/\text{min}, and respirations are 28/min28/\text{min}. Physical examination reveals a diaphoretic, mildly agitated patient. Cardiopulmonary examination is unremarkable except for tachycardia and tachypnea.

Laboratory results are as follows:

Serum chemistry:
Sodium: 141 mEq/L141\text{ mEq/L}
Potassium: 3.2 mEq/L3.2\text{ mEq/L}
Chloride: 101 mEq/L101\text{ mEq/L}
Bicarbonate: 14 mEq/L14\text{ mEq/L}
BUN: 14 mg/dL14\text{ mg/dL}
Creatinine: 0.9 mg/dL0.9\text{ mg/dL}
Glucose: 88 mg/dL88\text{ mg/dL}

Arterial blood gas (room air):
pH: 7.457.45
pCO2: 19 mmHg19\text{ mmHg}
pO2: 98 mmHg98\text{ mmHg}

Serum salicylate level is 65 mg/dL65\text{ mg/dL} (therapeutic 15−30 mg/dL15-30\text{ mg/dL}). Acetaminophen level is undetectable.

The physician plans to initiate therapy to enhance the systemic elimination of the ingested toxin. Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. Intravenous sodium bicarbonate and potassium chlorideAnswer
  2. B
    Intravenous sodium bicarbonate alone
  3. C
    Intravenous acetazolamide
  4. D
    Intravenous fomepizole
  5. E
    Oral activated charcoal

Answer

The most appropriate next step is the concurrent administration of intravenous sodium bicarbonate and potassium chloride.
This patient is experiencing severe salicylate toxicity, evidenced by tinnitus, tachypnea, and a classic mixed acid-base disorder (high anion gap metabolic acidosis combined with a primary respiratory alkalosis). The cornerstone of pharmacologic management is enhanced elimination via urinary alkalinization using an intravenous sodium bicarbonate infusion. This process relies on 'ion trapping,' where the alkaline urine converts salicylic acid into its ionized, lipophobic form (salicylate), preventing its reabsorption across the renal tubular epithelium. However, successful alkalinization is physiologically impossible if the patient is hypokalemic. This patient's serum potassium is 3.2 mEq/L3.2\text{ mEq/L}. In the presence of a potassium deficit, the distal renal tubules will actively reabsorb potassium in exchange for hydrogen ions (H+H^+) via the H+/K+H^+/K^+-ATPase pump. This leads to paradoxical aciduria (acidic urine despite systemic alkalemia). Furthermore, administering sodium bicarbonate directly drives potassium into cells, which would dangerously exacerbate her hypokalemia. Therefore, aggressive concurrent repletion with intravenous potassium chloride is a mandatory requirement to ensure the efficacy and safety of the sodium bicarbonate therapy.

Step-by-Step Solution

1
Analyze the clinical and laboratory presentation.
The patient has the classic triad of tinnitus, hyperventilation, and mixed acid-base disorder (high anion gap metabolic acidosis with primary respiratory alkalosis) indicative of severe salicylate toxicity.
Accurate identification of the toxidrome dictates the specific elimination strategy.
2
Identify the primary mechanism for therapeutic intervention.
The goal is urinary alkalinization using intravenous sodium bicarbonate to induce 'ion trapping' of the salicylate molecule in the renal tubules.
Alkalinizing the urine to a pH > 7.5 converts salicylic acid to its lipophobic ionized form, preventing its reabsorption and enhancing excretion.
3
Evaluate the patient's current metabolic state for barriers to the planned intervention.
The patient has mild hypokalemia (3.2 mEq/L3.2\text{ mEq/L}).
Sodium bicarbonate administration shifts potassium intracellularly, which will worsen this hypokalemia if left uncorrected.
4
Determine the physiological consequence of untreated hypokalemia during alkalinization therapy.
In hypokalemic states, the kidneys attempt to conserve potassium by actively reabsorbing it in exchange for excreting hydrogen ions (H+H^+) via the H+/K+H^+/K^+-ATPase pump in the distal tubule.
This process, known as paradoxical aciduria, ensures the urine remains acidic despite the systemic bicarbonate infusion, completely neutralizing the therapeutic attempt at ion trapping.
5
Synthesize the requirements for the optimal pharmacologic order.
Potassium chloride must be administered concurrently with sodium bicarbonate.
To successfully alkalinize the urine and protect the patient from life-threatening arrhythmias associated with worsening hypokalemia.

Key Concept

Successful urinary alkalinization with sodium bicarbonate for salicylate toxicity requires concurrent potassium repletion, as underlying hypokalemia leads to paradoxical aciduria via distal tubular H+/K+H^+/K^+ exchange.
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Pharmacologic Interventions

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