First of all, when you have someone who has a HR above 180, acording to every class I have ever been to says, while you are getting the IV prepared have the patient do "Vagal maneuvers FIRST" If that doesn't slow the HR then look to Adenosine, but since the patient is obviously is symtomatic, the look of the patient gives you that, you need to go to direct cardoversion. First, Valium or Versed, your local protocols take affect, then shock. The use of "Amiodarone 150mg over 10 min., repeat x1" what does Ami do? It helps speed up the heart, exactly opposite of what your trying to do, slow do the heart.