I'm curious why you had the stress test? I'm not a doctor, but I have read that the Maximum Heart Rate is generally considered to be 220-age. /en.m.wikipedia.org/wiki/Heart_rate
The above Wiki page covers several formulas used for the Maximum Heart Rate. This is also, I believe, the maximum rate to be obtained during prudent stress testing and not necessarily the maximum the heart can achieve.
In any case, heart rate is also influenced by conditioning, medications, and a number of other factors, there are no absolutes. After 5 minutes to be at 210 briefly, depending on those other factors, does not sound unreasonable.
At 55, I had a stress test, reaching 75% of the expected rate. If they calculated it as 220-age, 165 would have been my target. 75% of that is 124. My most recent stress was done under my normal medication with a Beta Blocker, and after 5 minutes on the treadmill, my heart was only at 91. I stopped it at 5 minutes, my legs were giving out, and though I could have made another minute, didn't want to take the chance.
What is the nature of the abnormal EKG? Also, how long did you stay on the stress test? Are you male or female? Overweight by more than 20 pounds? Tall, short, average? Conditioned or sedentary? Smoke? Drug use, including prescript
ion, supplements, caffeine? If you use illicit drugs, be sure to let your doctor know.