Final answer:
Adenosine is the preferred drug for treating regular narrow complex tachycardias when vagal maneuvers fail, due to its rapid action and brief duration.
Step-by-step explanation:
Adenosine is the drug of choice to treat regular narrow complex tachycardias after vagal maneuvers have been unsuccessful. This medication is effective at slowing certain types of irregular heartbeat (arrhythmias), particularly atrial fibrillation. It temporarily slows down the heart's conduction through the atrioventricular (AV) node. Beta-blockers like metoprolol and propranolol may also be used in certain situations for their ability to block beta-receptors associated with vasoconstriction and cardioacceleration, which can improve cardiac function. However, for the acute termination of paroxysmal supraventricular tachycardia (PSVT), adenosine is preferred due to its rapid onset and short half-life.