What is the diagnosis?
The ECG reveals an irregular wide complex tachycardia, with some variability in QRS width and a rate of approximately 270 BPM. While ventricular tachycardia (VT) may be an initial consideration, this ECG reflects atrial fibrillation (AF) with ventricular pre-excitation over an accessory pathway in a patient with Wolff-Parkinson-White Syndrome (WPW). The variability of QRS width which is typical of pre-excited AF is due to occasional fusion of accessory pathway and A - V nodal conduction. In many cases, some narrow complexes are seen as well, reflecting block in the accessory pathway and conduction over the normal conducting system into the ventricle.
Why can't this be AF with simple aberrancy?
Several reasons. First, ventricular rates this rapid during AF are extremely unusual in the absence of WPW. Secondly, the variability in QRS width would be unusual for aberrancy. Finally, the QRS pattern recorded is not a typical pattern of aberrancy; there is positive concordance across the precordium.
Could this be VT?
While VT can be somewhat irregular, it is rarely this grossly irregular. QRS variability during VT is usual due to fusion with sinus complexes; minimal or no fusion would be anticipated with VT at this rate. Lastly, it would be decidedly unusual for a 16-year-old to have hemodynamically unstable VT at this rate. Idiopathic VT does not look or behave like this; VT at this rate could be possible in a young patient with significant structural heart disease, though.
Ventricular rates during AF can be extremely rapid in patients with WPW and even degenerate into ventricular fibrillation (VF) and cause cardiac arrest. The reason is simple. The A - V node displays decremental conduction, which limits ventricular rates during AF. Accessory pathways typically do not have decremental conduction, which can result in much more rapid rates during AF.
This patient is unstable, and would best be treated with direct current cardioversion, synchronized if possible. If he were hemodynamically stable, then intravenous procainamide would be the drug of choice. Even if the diagnosis were uncertain (i.e., VT vs. pre-excited AF vs. AF with aberrancy), the treatment would be the same - countershock if unstable, procainamide if stable. Patients with pre-excited AF should not be treated with adenosine, calcium blockers, beta-blockers, or digoxin. By slowing A - V nodal conduction, these agents can facilitate more rapid accessory pathway conduction and increase the risk of VF.
A single 200 joule discharge restored sinus rhythm, and this ECG was recorded: