WBR0501

Author	[[PageAuthor::Gonzalo A. Romero, M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathology, MainCategory::Pathophysiology, MainCategory::Pharmacology, MainCategory::Physiology
Sub Category	SubCategory::Cardiology, SubCategory::Endocrine
Prompt	[[Prompt::A 66-year-old man presents to the cardiology clinic with complaints of constipation, decreased libido and excessive hair loss. He has a three year history of atrial fibrillation that is well controlled on medical therapy. Physical examination demonstrates dry, coarse skin with a distinct bluish-grey discoloration of the nose, cheeks, and lips. Which of the following electrophysiological changes is characteristic of the drug most likely to be used in this case?]]
Answer A	AnswerA::Decreased conduction velocity, increased ERP and PR interval
Answer A Explanation	AnswerAExp::These electrophysiological changes are observed with calcium channel blockers.
Answer B	AnswerB::Decreased AP duration with activity more significant in ischemic and depolarized ventricular tissue
Answer B Explanation	AnswerBExp::These electrophysiological changes are observed with class IB antiarrhythmics.
Answer C	AnswerC::Increased repolarization phase, AP duration, ERP, and QT interval
Answer C Explanation	AnswerCExp::This mechanism of action corresponds to amiodarone, the drug most likely to be causing this patient's symptoms.
Answer D	AnswerD::Decreases slope of phase 0, Minimal effect on AP duration
Answer D Explanation	AnswerDExp::These electrophysiological changes are observed with class IC antiarrhythmics
Answer E	AnswerE::Decreased SA and AV nodal activity, Increased PR interval, Decreased slope of phase 4
Answer E Explanation	AnswerEExp::These electrophysiological changes are observed with beta blockers.
Right Answer	RightAnswer::C
Explanation	[[Explanation::Amiodarone is a class III antiarrhythmic drug that has effects of class Ia, II, III, and IV antiarrhythmics. Electrophysiological effects of amiodarone include prolongation of phase 3 of the action potential (repolarization) leading to an increase in AP duration, and increase of the refractory period causing a decrease in the heart rate. Amiodarone also increases the QT interval, however, it is not associated with an increase risk of torsade de pointes. Amiodarone has a chemical structure that closely resembles thyroxine, which replaces thyroxine at its receptor. The molecule also inhibits the action of type 1 5'-deiodinase that is responsible for the peripheral conversion of T4 to T3. Several other pathological effects on the thyroid gland and its hormones have been described including cytotoxic effects, and inhibition of feedback regulation. Clinically, these effects may manifest as either hypo- or hyperthyroidism, which is the main reason for the close follow-up of thyroid function among patients receiving amiodarone. Amiodarone also has several other side-effects including interstitial fibrosis, corneal deposits, peripheral neuropathy, and skin discoloration. The bluish skin discoloration (ceruloderma) associated with skin deposits of amiodarone is also known as "blue-man syndrome". It most commonly involves the face and is also associated with severe corneal deposits. It resolves within weeks to months of amiodarone discontinuation. Educational Objective: Amiodarone is a class III antiarrhythmic associated with hypo or hyperthyroidism and bluish-grey discoloration with prolonged use. References: Zimetbaum P. Amiodarone for atrial fibrillation. N Engl J Med. 2007;356(9):935-41. Enseleit F, Wyss CA, Duru F, Noll G, Ruschitzka F. Images in cardiovascular medicine. The blue man: amiodarone-induced skin discoloration. Circulation. 2006;113(5):e63. First Aid 2013 page 303]]
Approved	Approved::Yes
Keyword	WBRKeyword::Amiodarone, WBRKeyword::Blue man syndrome, WBRKeyword::Hypothyroidism, WBRKeyword::Hyperthyroidism, WBRKeyword::Antiarrhythmics, WBRKeyword::Class III, WBRKeyword::Side effects
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