I read your question carefully and I understand your concern. Ativan is the brand name of a drug called Lorazepam. Lorazepam is not used to treat heart arrhythmia itself, but it is a drug used to treat anxiety and reduction of anxiety itself can help to control the tachycardia. However while it can be used for anxiety episodically if that is thought to contribute to tachycardia it is not a long term solution, there are other appropriate drugs for that, like beta blockers or other anti-arrhythmics which should have been prescribed to you by the cardiologist for your condition of atrial fibrillation with high heart rate. I actually never prescribe lorazepam long-term for any condition since it has high potential to create tolerance (meaning in time same dose won t be enough to achieve effect and higher doses will be required) as well as dependance. I hope to have been of help.