48-year-old man presents to you complaining of palpitations

Obtain a cardiac consult.

The above findings on ECG show a patient with Wolf-Parkinson White (WPW) and rapidly conducted atrial fibrillation (AF) via antidromic conduction. This rhythm can be easily misdiagnosed and then treated with drugs that can lead to a preventable fatality. The treatment of choice for AF with WPW is procainamide, amiodarone, or cardioversion. Diltiazem or betablockers, which would have been an acceptable treatment for AF, could have resulted in the aforementioned fatal outcome for this particular patient with AF and WPW with antidromic conduction.

As long as a patient with an irregular, wide-complex tachycardia is not "crashing," it is reasonable to collect more data than presented in this question to increase the certainty of the diagnosis. This is important because, more than in any other tachycardia category, incorrect ECG interpretation in the wide-complex tachycardia group can lead to fatal treatment errors. This is because the pharmacologic treatment for each subgroup of irregular, wide-complex tachycardias is diametrically opposed to at least 1 other subcategory within this same group of arrhythmias. Current guidelines from the American Heart Association formally recommend obtaining a cardiac consult for relatively stable patients with wide-complex tachycardias, especially if they are irregular.

Unstable patients are defined as patients with signs of shock, heart failure, or cardiac ischemia. Patients with low blood pressure values (systolic in the 80s) but with a normal mental status and no chest pain or respiratory distress are not considered unstable for the sake of immediate cardioversion vs cardiac consult.

Though the rate of this patient's ECG puts the correct diagnosis toward the top of the differential, the delta waves are difficult to appreciate with certainty (they mimic other categories of aberrant beats). In this original case, the diagnosis of WPW was confirmed from a preexisting ECG that showed normal sinus rhythm with a short PR interval and delta waves. The patient received midazolam and was cardioverted.

Neither amiodarone nor magnesium is a first-line agent for treating WPW with irregular, wide-complex conduction (though amiodarone can be used as a second-line agent after procainamide in narrow-complex WPW)

Guidelines from the American Heart Association state: "A wide-complex irregular rhythm should be considered pre- excited atrial fibrillation. Expert consultation is advised. Avoid AV nodal blocking agents such as adenosine, calcium channel blockers, digoxin, and possibly beta-blockers in patients with pre-excitation atrial fibrillation because these drugs may cause a paradoxical increase in the ventricular response."

References:

American Heart Association. CPR & ECC Guidelines. Accessed January 4, 2016.

Link MS. Clinical practice. Evaluation and initial treatment of supraventricular tachycardia. N Engl J Med. 2012;367(15):1438-1448.

Simonian, S M, et al. “Challenging the Superiority of Amiodarone for Rate Control in Wolff-Parkinson-White and Atrial Fibrillation.”Internal and Emergency Medicine., U.S. National Library of Medicine, Oct. 2010.

Yealy DM, et al. Dysrhythmias. In Marx JA, et al. Rosen's Emergency Medicine: Concepts and Clinical Practice. 8th ed., 2014.