A 22-year-old woman complains of palpitations and has a regular heartbeat at a rate of 170/min, with a blood pressure of 110/70 mm Hg. The rate abruptly changes to 75/min after applying carotid sinus pressure. Which of the following is the most likely diagnosis?
Correct Answer: paroxysmal supraventricular tachycardia (PSVT)
Description: The patient most likely has PSVT since the tachycardia terminates after carotid sinus massage (CSM). CSM increases vagal tone (parasympathetic) which decreases AV nodal conduction and terminates AV node re-entry arrhythmias. Sinus tachycardia differs from PSVT tachycardia in that it does not start or stop abruptly. In PSVT, the QRS is usually narrow without clearly discernible P waves. A wide QRS in PSVT can result from a preexisting bundle branch block, or a functional bundle branch block secondary to the tachycardia. This can make the distinction from a ventricular arrhythmia quite difficult.
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