Paramedics transport an 82-year-old female to the acute care area following a syncopal epi… — USMLE Step 3

Paramedics transport an $82$ -year-old female to the acute care area following a syncopal episode at her residence. Her family reports she complained of severe dizziness and chest heaviness immediately before losing consciousness. Her medical history is significant for hypertension, hyperlipidemia, and a myocardial infarction three years ago. Upon arrival, the patient is lethargic and profoundly diaphoretic. Vital signs show a temperature of $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), blood pressure of $72/40\text{ mmHg}$ , pulse of $28/\text{min}$ , and respiratory rate of $18/\text{min}$ . Oxygen saturation is $94\%$ on room air. Physical examination reveals cool, clammy extremities with a capillary refill time of $4\text{ seconds}$ . Jugular venous distention is absent, and lung fields are clear to auscultation bilaterally. A $12$ -lead electrocardiogram displays regular P waves at a rate of $90/\text{min}$ and regular, broad QRS complexes ( $0.16\text{ sec}$ ) at a rate of $28/\text{min}$ with complete dissociation between the atria and ventricles. Two large-bore intravenous lines are secured. Which of the following is the most appropriate next step in the stabilization of this patient?

Immediate application of transcutaneous pacing padsAnswer
B
Bolus injection of atropine sulfate
C
Bedside transthoracic echocardiography
D
Delivery of a metoprolol tartrate bolus
E
Expedited transport to the cardiac catheterization suite

Answer

Immediate application of transcutaneous pacing pads

The patient is in cardiogenic shock secondary to a third-degree atrioventricular block with a wide QRS escape rhythm. According to Advanced Cardiovascular Life Support (ACLS) guidelines, unstable bradycardia requires immediate intervention to restore perfusion. While atropine is often the first-line pharmacologic agent for unstable bradycardia, it works by inhibiting vagal tone primarily at the AV node. In complete heart block with a wide QRS complex, the block is located below the AV node (in the His-Purkinje system). Atropine is generally ineffective for infranodal blocks and may even induce paradoxical slowing. Therefore, immediate electrical stabilization with transcutaneous pacing is the most appropriate and direct next step to re-establish cardiac output while preparing for transvenous pacemaker placement.

Step-by-Step Solution

Interpret the clinical presentation and vital signs.

The patient is in cardiogenic shock (hypotension, altered mental status, diaphoresis, delayed capillary refill) driven by profound bradycardia (pulse

28/\text{min}

Identifying the presence of shock and its primary driver is the first step in the ACLS bradycardia algorithm.

Analyze the electrocardiogram (ECG) findings.

The ECG shows AV dissociation with P waves faster than QRS complexes, confirming third-degree (complete) AV block. The broad QRS (

0.16\text{ sec}

) indicates an escape rhythm originating below the AV node (infranodal or His-Purkinje system).

The level of the block dictates the appropriate therapeutic intervention.

Determine the most appropriate immediate stabilization therapy.

Because the block is infranodal, atropine is unlikely to be effective and transcutaneous pacing must be initiated immediately to restore cardiac output.

ACLS guidelines state that atropine should not be relied upon for wide-complex complete heart blocks; electrical pacing or chronotropic infusions are the standard of care for these unstable patients.

Key Concept

In patients with hemodynamically unstable third-degree atrioventricular block featuring a wide QRS complex (infranodal block), immediate transcutaneous pacing is the preferred initial stabilization step because atropine is generally ineffective at the His-Purkinje level.

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