Which of the following arrhythmias qualifies as a medical emergency?

• A. Premature atrial beats
• B. Unsustained sinus tachycardia
• C. Junctional escape rhythm
• D. Third-degree atrioventricular block

Answer: (D)

In a complete heart block, there is no conduction from the atria to the ventricles, so the atrial and ventricular rhythms run independently. The ventricles rely on a slow escape rhythm, which typically causes marked bradycardia and reduced cardiac output. This combination can lead to dizziness, hypotension, syncope, and even hemodynamic collapse if not treated promptly, making it a medical emergency. Emergency management usually involves urgent monitoring and temporary pacing (often transcutaneous, with plans for definitive pacing) and addressing reversible causes.

Premature atrial beats are common and usually benign. Unsustained sinus tachycardia self-resolves and is not typically an emergency. A junctional escape rhythm can occur as a slower backup rhythm and may be tolerated if stable, but it does not imply complete block and is not in itself an emergency like a complete block. The key distinction is the complete dissociation between atrial and ventricular activity with severe bradycardia and instability in the block scenario.