A 36-year-old woman comes to the emergency department due to weakness and dizziness on and off over the last 3 days. She has had no chest pain or shortness of breath. The patient has no significant medical history. She does not use tobacco, alcohol, or illicit drugs. The patient's father had a myocardial infarction at age 68. BMI is 23 kg/m². Physical examination shows normal breath sounds. There are no heart murmurs. The extremities are normal. ECG is shown in the exhibit. Bedside transthoracic echocardiogram shows no structural abnormalities. Which of the following is the most appropriate next diagnostic study for this patient?

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11ec25c8_d179_57af_bc81_77a35510a3bf_MD0003_00 This patient's ECG shows an irregularly irregular rhythm (ie, varying R-R intervals) and an absence of organized P waves consistent with atrial fibrillation (AF). AF can be asymptomatic or present with symptoms, including palpitations, weakness, lightheadedness, dyspnea, and chest pain. ECG In a patient with new-onset AF, a cause of the episode should be sought because it may be reversible. Advanced age and comorbidities that contribute to atrial dilation (especially hypertension, mitral valve disease, and heart failure) are important risk factors that encourage atrial remodeling and create a substrate for AF to develop. Atrial remodeling makes it easier for a trigger (eg, excessive alcohol use, acute illness) to initiate AF but is not always necessary. This young patient has no known risk factors for AF and her echocardiogram shows no valvular disease or left ventricular dysfunction that may be contributing. The best next step is to check a serum TSH level to evaluate for hyperthyroidism, which can increase atrial conduction automaticity and is a potential trigger of AF. In some cases, AF may be the only apparent clinical manifestation of hyperthyroidism. Because hyperthyroidism is readily treatable and treatment commonly leads to resolution of AF, serum TSH and free T4 levels should be checked in all patients with new-onset AF (Choice D). (Choice A) CT angiogram of the chest is used to evaluate for pulmonary embolism (PE). Acute PE is a potential trigger of AF, but patients usually have dyspnea and pleuritic chest pain, and echocardiography commonly shows evidence of right ventricular dilation or strain. (Choice B) Exercise stress testing is used to evaluate for coronary artery disease (CAD), which can contribute to atrial remodeling and encourage the development of AF. However, significant CAD is highly unlikely in this otherwise healthy young woman. (Choice C) Hearing evaluation is indicated in patients suspected of having the autosomal recessive type of congenital prolonged QT syndrome (ie, Jervell and Lange-Nielsen syndrome), which can result in intermittent weakness and dizziness (eg, due to arrhythmias). However, the sensorineural hearing loss associated with this condition usually presents during childhood, and this patient's ECG shows a normal QT interval (ie, <440 ms). Educational objective: Atrial fibrillation (AF) is recognized on ECG by irregularly irregular R-R intervals and an absence of organized P waves. Hyperthyroidism is a potential trigger of AF; therefore, it should be ruled out with serum TSH and free T4 testing in all patients with new-onset AF. References: Hyperthyroidism and risk of atrial fibrillation or flutter: a population-based study. (http://www.ncbi.nlm.nih.gov/pubmed/15302638)

A 36-Year-Old Woman Comes to the Emergency Department Due to Weakness