What Causes Irregular Heart Beats?

Hi, Dr. Rynne? I am 71-year healthy male. What are the causes of irregular heart beats that causes unconsciousness for a minute or two? It happened to me when I was resting in bed, like during a nap or night sleep. A few times it started with a feeling of a sharp pain moved from left side of chest and close to the heart, to the right side of the chest. Then, I was unable to breathe and could hardly feel my heart beat during that time. When conscious, I was gasping for fresh air). Several heart-related tests have rejected the heart attack or disease such as cardiac fibrillation. I have mild asthma and sometimes experienced anxiety in the past. Other than this episode of heart complication, I am a healthy person. Your comment is appreciated.