A 76-year-old woman is brought to the emergency department by her son due to increasing ge… — USMLE Step 3

A $76$ -year-old woman is brought to the emergency department by her son due to increasing generalized weakness, confusion, and several episodes of nonbilious vomiting over the past $48$ hours. She has a history of ischemic cardiomyopathy with a left ventricular ejection fraction of $30\%$ , persistent atrial fibrillation, and chronic kidney disease. Her daily medications include lisinopril, bisoprolol, furosemide, apixaban, and digoxin. Four days ago, she was evaluated at an urgent care clinic for palpitations and was prescribed oral verapamil.

On physical examination, the patient is lethargic but arousable. Blood pressure is $82/48\text{ mm Hg}$ and pulse is $36\text{/min}$ . Cardiopulmonary examination reveals bradycardia with a regular rhythm and clear lungs. An electrocardiogram demonstrates a junctional escape rhythm at $36\text{/min}$ with frequent premature ventricular contractions.

Laboratory studies show:
Serum sodium: $136\text{ mEq/L}$
Serum potassium: $6.5\text{ mEq/L}$
Serum chloride: $101\text{ mEq/L}$
Serum bicarbonate: $22\text{ mEq/L}$
Blood urea nitrogen: $48\text{ mg/dL}$
Serum creatinine: $2.3\text{ mg/dL}$ (baseline $1.5\text{ mg/dL}$ )
Serum digoxin level: pending

In addition to discontinuing the offending medications, which of the following is the most appropriate next step in the pharmacologic management of this patient?

A
Administer intravenous calcium gluconate
Administer digoxin-specific antibody fragmentsAnswer
C
Administer intravenous regular insulin and dextrose
D
Administer intravenous glucagon
E
Administer intravenous atropine

Answer

The most appropriate next step is to administer digoxin-specific antibody fragments.

The patient presents with classic signs of acute-on-chronic digoxin toxicity, precipitated by a recent drug-drug interaction. Verapamil is a potent inhibitor of P-glycoprotein, the primary efflux transporter responsible for the renal and biliary clearance of digoxin. Co-administration of verapamil can rapidly double serum digoxin concentrations, leading to toxicity characterized by gastrointestinal distress, neurologic symptoms, and varied arrhythmias (e.g., severe bradycardia, junctional escape rhythms, premature ventricular contractions).

A critical marker of severe digoxin toxicity is hyperkalemia. Digoxin reversibly inhibits the Na+/K+-ATPase pump, preventing the intracellular influx of potassium. In the setting of digoxin toxicity with life-threatening hyperkalemia (

K^+ > 5.5\text{ mEq/L}

) and hemodynamic instability, the definitive and immediate pharmacologic intervention is the administration of digoxin-specific antibody fragments (DigiFab).

While standard Advanced Cardiac Life Support (ACLS) guidelines for hyperkalemia recommend membrane stabilization with intravenous calcium gluconate, calcium administration is historically contraindicated in the setting of digoxin toxicity. The inhibited Na+/K+-ATPase already causes a buildup of intracellular sodium, which reverses the Na+/Ca2+ exchanger, leading to dangerously high intracellular calcium. Administering exogenous calcium can theoretically precipitate irreversible myocardial contracture (known as 'stone heart'). Therefore, digoxin-specific antibody fragments must be administered promptly to bind free digoxin, reversing the pump inhibition and correcting both the arrhythmias and the hyperkalemia.

Step-by-Step Solution

Analyze the patient's symptoms and vital signs in the context of recent medication changes.

The combination of lethargy, vomiting, profound bradycardia (

36\text{/min}

), and hypotension indicates severe cardiovascular compromise following the addition of verapamil to a regimen containing digoxin.

Verapamil is a potent P-glycoprotein inhibitor that significantly decreases the renal clearance of digoxin, predictably leading to acute-on-chronic digoxin toxicity.

Interpret the laboratory findings to determine the severity and primary threat of the toxidrome.

The serum potassium is markedly elevated at

6.5\text{ mEq/L}

In digoxin toxicity, the Na+/K+-ATPase pump is inhibited, leading to extracellular potassium accumulation. Severe hyperkalemia is a critical marker of widespread pump poisoning and correlates closely with mortality.

Select the guideline-concordant pharmacologic intervention, noting any critical contraindications.

Digoxin-specific antibody fragments must be administered immediately.

While standard ACLS hyperkalemia management involves membrane stabilization with intravenous calcium, calcium is contraindicated in digoxin toxicity due to the risk of irreversible myocardial contracture. The definitive treatment for digoxin-induced life-threatening hyperkalemia is antibody fragments.

Key Concept

Management of life-threatening hyperkalemia in the setting of digoxin toxicity requires the administration of digoxin-specific antibody fragments, and standard calcium gluconate therapy is traditionally contraindicated.

Estimated Time:1m 30s

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