Rapid Heart Beat, Dehydrated

Hi, My mother (healthy, exercises regularly, normal weight, 61 yrs old, slightly elevated blood pressure - managed with medication and proper eating) went to the ER yesterday because her heart starting racing for about an hour when my parents were shopping yesterday. She had tanned about 1 hour prior to the rapid heart and said the tanning bed seemed warm. Her EKG came back fine along with the blood work and they did say she was dehydrated. The doctor said she was fine and that he didn t see any issues, but if it happened again to go see her primary care physician . Does this all sound normal and could dehydration really have caused this? I don t think she drinks enough water...