A 25-year-old woman is evaluated for occasional episodes of palpitations. She describes the episodes as a sudden, abrupt onset of fast and regular heartbeats that feel like "her heart is racing in her chest." The episodes are commonly unprovoked and are not related to exertion. During one episode, her blood pressure was recorded as 100/60 mm Hg and her heart rate was 185/min and regular. She has no significant past medical history and does not take any medications. She has no family history of heart disease. Physical examination is unremarkable, including no heart murmurs or additional sounds. She also reports that she has been self-treating by immersing her face in cold water during the palpitation episodes. Generally, she says, cold-water immersion relieves her symptoms abruptly within several minutes. This maneuver terminates the palpitations by directly altering which of the following?

Question

A 25-year-old woman is evaluated for occasional episodes of palpitations.  She describes the episodes as a sudden, abrupt onset of fast and regular heartbeats that feel like "her heart is racing in her chest."  The episodes are commonly unprovoked and are not related to exertion.  During one episode, her blood pressure was recorded as 100/60 mm Hg and her heart rate was 185/min and regular.  She has no significant past medical history and does not take any medications.  She has no family history of heart disease.  Physical examination is unremarkable, including no heart murmurs or additional sounds.  She also reports that she has been self-treating by immersing her face in cold water during the palpitation episodes.  Generally, she says, cold-water immersion relieves her symptoms abruptly within several minutes.  This maneuver terminates the palpitations by directly altering which of the following?

Quizplus · Accepted Answer

This patient's clinical presentation - abrupt onset of regular tachycardia that resolves with cold-water immersion - is consistent with paroxysmal supraventricular tachycardia (PSVT). Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common form of PSVT and frequently develops in young patients with a structurally normal heart. Palpitations are the most common presentation, but some patients may have dizziness, shortness of breath, or chest pain. AVNRT is due to the presence of 2 conduction pathways (slow and fast) in the AV node. Normally, sinus beats are conducted in an antegrade direction through the fast pathway, as antegrade conduction through the slow pathway is extinguished due to the presence of a refractory period. If an atrial premature beat occurs at a critical time when the fast pathway is refractory but the slow pathway is not, it can initiate AVNRT that is then sustained by a reentry mechanism: the slow and the fast pathways form a looped circuit, with impulses traveling in an antegrade direction through the slow pathway and returning through the fast pathway. Vagal maneuvers (eg, carotid sinus massage, cold-water immersion or diving reflex, Valsalva maneuver, eyeball pressure) increase parasympathetic tone in the heart and result in a temporary slowing of conduction in the AV node and an increase in the AV node refractory period, leading to termination of AVNRT. (Choices B, E, and F) Vagal stimulation has minimal effect on the His-Purkinje system conduction or myocardial contractility. Furthermore, the Purkinje fibers and the myocardium are not involved in the reentrant pathway responsible for initiation of AVNRT. Increased parasympathetic activity increases peripheral vascular capacitance and decreases venous return to the heart, but this decreased venous return does not directly affect the AV node. (Choice C) Although vagal maneuvers decrease sinoatrial (SA) node automaticity and cause slowing of the rate of impulse formation from the sinus node, AVNRT termination is due to effects on the reentrant mechanism in the AV node rather than to changes in SA node automaticity (as the SA node is not part of the reentrant pathway). (Choice D) Cold-water immersion can increase vascular tone by causing vasoconstriction of blood vessels in the face; however, this is not the underlying mechanism responsible for termination of PSVT. Educational objective: Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common form of paroxysmal supraventricular tachycardia and is caused by a reentry mechanism due to the presence of a dual electrical pathway (slow and fast pathway) in the AV node. Vagal maneuvers increase parasympathetic tone and result in a temporary slowing of conduction in the AV node and an increase in the AV node refractory period, leading to termination of AVNRT. References: Clinical practice. Evaluation and initial treatment of supraventricular tachycardia. (http://www.ncbi.nlm.nih.gov/pubmed/23050527) ACC/AHA/ESC guidelines for the management of patients with supraventricular arrhythmias - executive summary. (http://www.ncbi.nlm.nih.gov/pubmed/14557344)

A 25-Year-Old Woman Is Evaluated for Occasional Episodes of Palpitations