What was the mechanism of action of this maneuver?

Paramedics respond to a call regarding an 10-year-old male with severe sudden-onset heart palpitations. The patient reports symptoms of chest pain, fatigue, and dizziness. Upon examination, his heart rate is 175/min and regular. His blood pressure is 110/75 mm Hg. Gentle massage below the level of the left mandible elicits an immediate improvement in the patient, as his heart rate returns to 70/min. What was the mechanism of action of this maneuver?

1.Increasing the refractory period in ventricular myocytes
2.Increasing sympathetic tone in systemic arteries
3.Decreasing the length of phase 4 of the SA node myocytes
4.Impairing conduction in the AV node
5.Decreasing the firing rate of carotid baroreceptors

Summary

This patient has paroxysmal supraventricular tachycardia (PSVT), for which he is treated with carotid massage, which stimulates baroreceptors of the carotid body to increase parasympathetic tone and decrease sympathetic tone, lengthening AV nodal refractory period.
PSVT is characterized by its sudden onset and termination, atrial rates between 140 and 250 bpm, and a narrow QRS complex. The mechanism is most often re-entry through the AV node, atrium or accessory pathways. Acute treatment of PSVT is aimed at terminating re-entry by slowing conduction through the AV node. This can be accomplished by increasing vagal tone (carotid massage/valsalva) or by using intravenous adenosine which also impairs AV nodal conduction. Carotid massage mimics increased pressure in the carotid baroreceptors which increases signaling to the central nervous system and induces a parasympathetic response which results in slowed AV node conduction, among other effects.
Colucci et al report, that supraventricular tachycardias are typically caused by re-entry phenomenon that result in accelerated heart rates. This causes palpitations, chest pain, fatigue, dizziness, and dyspnea. Treat with calcium channel blockers or beta blockers.
Delaney et al state, PSVT can be treated equaly effectively with adenosine and verapamil. While verapamil is more likely to cause hypotension, adnosine has a higher rate of overall adverse effects, especially minor ones. Decision on treatment should be ideally made on a case-by-case basis with an informed discussion between doctor and patient.
Illustration A shows what a typical EKG of someone in PSVT would look like.