This pair of ECGs feature one of our recurring themes: wide-complex tachycardia (WCT). It is a fascinating topic, as tachycardia has many causes and many mechanisms, and wide QRS also has many causes, with the mechanism being slow conduction through the ventricles.
Sometimes, it is not possible to diagnose the true origin of a WCT from one ECG, or even serial ECGs. Is the tachycardia due to increased sympathetic activity (fear, dehydration, exercise, hypoxia, hypovolemia, etc.)? Or is the fast rate due to reentry, where one impulse gets “caught” in a loop, repeating itself rapidly, and depolarizing the myocardium with each pass? What is the location of the pacemaker that is responsible for the rhythm? Is it a supraventricular rhythm that has suffered an intraventricular conduction delay, widening the QRS? Or is the rhythm originating in a ventricular pacemaker, without the ability to travel on the fast highway that is the intraventricular conduction system?
If you or your students work in an acute care setting, such as pre-hospital or emergency department, you may not be with the patient long enough or be able to conduct enough tests to determine without a doubt the answers to the above questions. Some WCTs cause such severe symptoms that they must be dealt with quickly, to avoid rapid deterioration to ventricular fibrillation. For that reason, there is a widely-accepted rule for WTC treatment:
TREAT ALL WIDE-COMPLEX TACHYCARDIA AS IF IT IS V TACH UNTIL PROVEN OTHERWISE.