A 23-year-old man comes to the emergency department with sudden onset of heart palpitations that started while he was at his desk at work. The patient has no known medical problems and does not use tobacco or illicit drugs. He drinks alcohol occasionally on the weekends. Initial blood pressure is 110/70 mm Hg and pulse is 160/min and regular. Gentle neck massage just below the angle of the right mandible provides immediate improvement of his condition. His blood pressure is now 120/80 mm Hg and pulse is 75/min. Which of the following mechanisms is responsible for improvement of this patient's condition?

Question

A 23-year-old man comes to the emergency department with sudden onset of heart palpitations that started while he was at his desk at work.  The patient has no known medical problems and does not use tobacco or illicit drugs.  He drinks alcohol occasionally on the weekends.  Initial blood pressure is 110/70 mm Hg and pulse is 160/min and regular.  Gentle neck massage just below the angle of the right mandible provides immediate improvement of his condition.  His blood pressure is now 120/80 mm Hg and pulse is 75/min.  Which of the following mechanisms is responsible for improvement of this patient's condition?

Quizplus · Accepted Answer

11ec160b_3ca4_3b68_9249_63355e8ae821_MD0001_00 This patient's presentation with sudden onset of palpitations and rapid regular tachycardia is consistent with paroxysmal supraventricular tachycardia (PSVT), which is most often due to a reentrant impulse traveling circularly between the slowly and rapidly conducting segments of the atrioventricular (AV) node. Vagal maneuvers such as carotid sinus massage, Valsalva, and cold water immersion can be used to acutely terminate PSVT. The carotid sinuses are bulges in the internal carotid artery with baroreceptors located immediately distal to the bifurcation of the common carotid artery. The carotid sinus reflex has an afferent limb that arises from the baroreceptors in the sinus and travels to the vagal nucleus and medullary centers via the glossopharyngeal nerve (cranial nerve [CN] IX), whereas the efferent limb carries parasympathetic impulses to the sinoatrial (SA) and AV nodes via the vagus nerve (CN X). Carotid sinus massage leads to increased afferent firing from the carotid sinus, which in turn, increases vagal parasympathetic tone. This slows conduction through the AV node and prolongs the AV node refractory period, helping to terminate the reentrant tachycardia. (Choices A, B, and C) Carotid sinus massage stimulates the baroreceptors and increases the firing rate from the carotid sinus, leading to an increase in parasympathetic output and withdrawal of sympathetic output to the heart and peripheral vasculature. This leads to slowing of the heart rate and AV conduction, along with a decrease in systemic vascular resistance. (Choice E) Vagal efferent nerves only sparsely innervate ventricular myocardium, and carotid sinus massage does not cause any significant change in the ventricular refractory period. Educational objective: Carotid sinus massage leads to an increase in parasympathetic tone causing temporary inhibition of sinoatrial node activity, slowing of conduction through the atrioventricular (AV) node, and prolongation of the AV node refractory period. It is a useful vagal maneuver for termination of paroxysmal supraventricular tachycardia. __________ References: Vagal techniques for termination of paroxysmal supraventricular tachycardia. (http://www.ncbi.nlm.nih.gov/pubmed/7416025)

A 23-Year-Old Man Comes to the Emergency Department with Sudden