A 68-year-old man comes to the clinic due to palpitations that have been especially prominent over the past 2 days. Medical history is significant for myocardial infarction and heart failure with reduced ejection fraction. Medications include aspirin, digoxin, furosemide, sacubitril-valsartan, metoprolol, and atorvastatin. Blood pressure is 126/74 mm Hg, pulse is 102/min, and respirations are 15/min. Physical examination reveals no murmurs or extra heart sounds. The lungs are clear to auscultation. No peripheral edema is present. ECG shows atrial tachycardia with second-degree atrioventricular block. Which of the following is most likely responsible for this patient's presentation?

Question

A 68-year-old man comes to the clinic due to palpitations that have been especially prominent over the past 2 days.  Medical history is significant for myocardial infarction and heart failure with reduced ejection fraction.  Medications include aspirin, digoxin, furosemide, sacubitril-valsartan, metoprolol, and atorvastatin.  Blood pressure is 126/74 mm Hg, pulse is 102/min, and respirations are 15/min.  Physical examination reveals no murmurs or extra heart sounds.  The lungs are clear to auscultation.  No peripheral edema is present.  ECG shows atrial tachycardia with second-degree atrioventricular block.  Which of the following is most likely responsible for this patient's presentation?

Quizplus · Accepted Answer

Explanation: Digoxin toxicity can cause a wide range of cardiac arrhythmias (eg, sinus bradycardia, junctional rhythms, ventricular tachycardia). Although most of the arrhythmias can occur in a variety of clinical settings and are nondiagnostic of digoxin toxicity, atrial tachycardia with atrioventricular (AV) block is specific for digoxin overdose and is generally considered diagnostic of toxicity in patients taking the medication. The effects of digoxin toxicity on the heart are complex but can be summarized as follows: Atrial tachycardia is likely due to increased automaticity of conduction in the atria. The focus of increased automaticity may be located within the sinus node, resulting in P wave morphology similar to that of a normal P wave, or within another part of the atrial myocardium, resulting in different P wave morphology. The rate of atrial contraction is typically around 200/min, which is slower than atrial flutter (typically ~300/min). AV block likely results from directly increased vagal tone, causing slowed conduction through the AV node. The AV block can be first, second, or third degree. Mobitz type II second-degree AV block is almost never seen with digoxin toxicity, so when second degree AV block is present it is typically Mobitz type I. Other arrhythmias also involve similar mechanisms of increased automaticity in the myocardium (eg, ventricular tachycardia) and/or increased vagal tone (eg, sinus bradycardia). Because digoxin competes with potassium to bind to the myocardium at the sodium-potassium ATPase, hypokalemia increases digoxin binding and can worsen toxicity or cause it to occur at lower-than-expected serum digoxin levels. (Choice A) An accessory conduction pathway is present in Wolff-Parkinson-White syndrome. Patients typically have slurred upstroke of the QRS complex on resting ECG and can develop episodes of AV reentrant tachycardia. Wolff-Parkinson-White syndrome (Choice B) Age-related cardiomyocyte fibrosis is likely the primary contributor to sick sinus syndrome. ECG commonly shows sinus bradycardia with sinus pauses and sometimes sinus arrest. sinus pauses (Choice C) Ischemic myocardial scarring occurs following myocardial infarction and can predispose to monomorphic ventricular tachycardia. (Choice D) Atrial wall stretching caused by left atrial enlargement predisposes to atrial fibrillation. Educational objective: Digoxin toxicity can cause a wide range of cardiac arrhythmias, typically resulting from increased automaticity of myocardial conduction and/or increased vagal tone. Atrial tachycardia with atrioventricular block involves both of these mechanisms and is the most specific arrhythmia for digoxin toxicity. References: Relationship between digoxin-induced cardiac arrhythmias and serum-digoxin levels. (http://www.ncbi.nlm.nih.gov/pubmed/66122)

A 68-Year-Old Man Comes to the Clinic Due to Palpitations