Review Question - QID 216560

In this patient with palpitations, fatigue, shortness of breath, a history of levothyroxine use, signs of volume overload (bibasilar crackles, elevated jugular venous pressure, pitting lower extremity edema), and irregularly irregular tachyarrhythmia with narrow QRS, atrial fibrillation (AF) with rapid ventricular response (RVR) leading to heart failure is the most likely diagnosis. This was likely triggered by thyrotoxicosis from exogenous levothyroxine use.

AF is characterized by numerous dyssynchronous reentrant circuits that usually originate near the roots of the pulmonary veins that override normal electrical impulses from the sinoatrial node. Causes of AF are numerous and include cardiovascular disease, hypertension, diabetes, obstructive sleep apnea, alcohol use, and thyrotoxicosis. Thyrotoxicosis, or the clinical manifestation of excess thyroid hormones, may lead to AF by shortening action potential duration through modulation of the expression of atrial ion channels. In addition, these changes to atrial ion channels predispose to AF and make reentry more likely by shortening the effective refractory period and slowing conduction. Chronically elevated thyroid hormone levels can also lead to demand ischemia from increased resting heart rate and elevation of left atrial pressure due to increased left ventricular mass from hypervolemia, both of which can predispose to AF.

Figure/Illustration A is the rhythm strip of an ECG showing an irregularly irregular rhythm (orange box), tachycardia to approximately 145 beats per minute, no P waves, and a narrow QRS complex (blue box) most consistent with atrial fibrillation with rapid ventricular response.

Reddy et al. review the relationship between atrial fibrillation and hyperthyroidism including epidemiology, pathophysiology, and management.

Incorrect Answers:
Answer 1: Aberrant electrical circuit through the cavotricuspid isthmus refers to atrial flutter. Atrial flutter often appears as organized saw-toothed waves of electrical activity on ECG and most often leads to a regular rhythm, neither of which is evident on this patient’s ECG. Furthermore, atrial flutter is typically not associated with thyrotoxicosis.

Answer 3: Myocardial infarction leading to ventricular tachycardia may be consistent with this patient’s history of palpitations, fatigue, and shortness of breath. However, ventricular tachycardia arises from reentry circuits below the bundle of His and leads to electrical activity conducting from one ventricular myocyte to the next. On ECG, this would manifest as a wide complex tachycardia, which is not seen in this patient.

Answer 4: Panic attacks may lead to sinus tachycardia. Sinus tachycardia has numerous causes including pain, infection, stress, anemia, medication or drug use, and thyrotoxicosis. However, as electrical impulses in sinus tachycardia originate from the sinoatrial node, ECG would show P waves that are correctly oriented: positive in lead I, II, and aVF and negative in lead aVR.

Answer 5: Thyrotoxicosis leading to a reentry circuit in the AV node describes AV nodal reentrant tachycardia (AVNRT). AVNRT may also present with palpitations, shortness of breath, and fatigue. However on ECG, although AVNRT also leads to a narrow complex tachycardia as it originates above the bundle of His, it is caused by a single reentrant circuit around the AV node and would thus show a regular rhythm.

Bullet Summary:
Thyrotoxicosis is a cause of atrial fibrillation, which is often characterized by abnormal electrical activity around the pulmonary veins.