Hi, may I answer your health queries right now ?  Please type your query here...I was using a tredmill, was on it for approx 2 minutes, at 3 Kilometers per Hour the machine shut down with a message unstable HR. Does this mean I may have some issues with my heart. I am 56 and was the first time I used a tredmill in years
Unstable heart rate means that you developed irregular heart rate due to exertion .please go for cardio assessment ...go for ECG .chest x-ray .and BP. Definitely you are having some problem ...go to general practitioner please ...