<prompt> Acute symptomatic sinus bradycardia usually responds to-AdrenalineaDopamineAtropineNorepinephrines  <response>: Sinus bradycardia: A regular but unusually slow hea beat 50 beatsminute or less at rest. Fainting can occur with sinus bradycardia if the hea slows down even more. This can occur, for example, when the vagus nerve sends a signal to the hea that suddenly drops the hea rate Ref Harrison 20th edition pg 1456 