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.
Answer 1: The refractory period of ventricular myocytes does not change under the influence of sympathetic/parasympathetic discharge.
Answer 2: Carotid massage decreases stimulation by the sympathetic nervous system and therefore has a vasodilatory effect on systemic arteries.
Answer 3: Decreased sympathetic and increased parasympathetic discharge slows heart rate and therefore serves to prolong the length of phase 4 in the SA node myocytes.
Answer 5: Carotid massage mimics an increase in pressure in the carotid artery which is sensed by baroreceptors. This increases discharge from these receptors stimulating an increase in parasympathetic tone and a decrease in sympathetic tone.
Colucci RA, Silver MJ, Shubrook J. Common types of supraventricular tachycardia: diagnosis and management. Am Fam Physician. 2010 Oct 15;82(8):942-52. Review.
PMID:20949888 (Link to Abstract)
Delaney B, Loy J, Kelly AM. The relative efficacy of adenosine versus verapamil for the treatment of stable paroxysmal supraventricular tachycardia in adults: a meta-analysis. Eur J Emerg Med. 2011 Jun;18(3):148-52. Review.
PMID:20926952 (Link to Abstract)
USMLE World Step 1 QBank Question #1515. Copyright © USMLEWorld, LLC 2012