Rate - almost 300 per minute. Antidromic AVRT, Isthmus VT Antidromic and orthodromic tachycardias refer to AV reentry where the conduction is antidromic if the conduction is along the AV node from the ventricle to the atrium and orthodromic if the conduction is from the atrium to the ventricle. An orthodromic tachycardia can have a narrow QRS complex or with a BBB while antidromic conduction shows maximal ventricular pre-excitation and is always wide. This regular, wide-QRS tachycardia with a rate of almost 300 bpm suggests antidromic AV reciprocating tachycardia. There was no history of pre-excitation.
Treatment depends on stability.
If the blood pressure is near normal, intravenous procainamide is a reasonable choice. If you were to try a medication lidocaine might be safer as it has less of a negative ionotrope than procainamide. VT most likely. I would like to know about his vital signs,blood pressure,respiration,and so on. I'll do cardioversion with low energy,100-150mWsec.