I am not a cardiologist but I would be a little concerned with congestive heart failure with these results. I am not totally sure but I would think your heart rate could get that high if your heart is under stress. Mine can jump to the 160s doing simple things when I am having a lot of pvcs thus causing my heart confusion. The best way to distinguish the type of rhythm it is would be if the rate starts and stops sudden then it is likely an svt of a structural nature if however, it ramps up and down with a change in activity level then it is more sinus related. In any event I would definitely follow up with the cardiologist when he gets back. Take care.
I have gotten my sinus rhythm heart rate to nearly 225bpm under extreme physcal output as measured on my Polar monitor. I was able to sustain this for only about 30 seconds, and this was perhaps 15 years ago, when I was in my mid-40's. During a stress test before my ablation last year for AVRT, I had gotten my sinus rate up to nearly 190 when my heart started to fire salvos of SVT which nearly blended into the sinus rhythm. I didn't feel them, but my cardiologist could clearly see them on the monitor. So yes, you can get sinus rates that high.
Of course we're not MD's but merely speculating here. I suspect that your overall heart condition is within normal parameters. 25mg of Metoprolol (Toprol) is a very small dose when you consider that dosages can run into the several hundred milligrams. So apparently you are either predisposed to low BP, or the medication has a profound effect on your BP. A couple of thoughts: Have you ever been tested for Neurally Mediated Hypotension? Is it possible that you are exercise intolerant, and just plain old out of shape? Could it be POTS? It would be interesting to measure your BP when your rate is high. I hope I've given you some stuff to Google. In the meantime, until you can see your cardiologist, be careful. Low BP and exercise don't mix well, and can even be dangerous!
Not sure if that would be NSR or not. Whenever I go into a-fib I get rapid ventricular response and a rate of 160 +. Have you been checked for a-fib?
An enlarged left atrium can be from a-fib as well.
Info on a-fib can be found here: http://www.stopafib.org/what.cfm
Good luck to you. Steve