Unable to complete Tempo run,

Last week I did 40 mins at my Daniels altered tempo pace of 7:30 m/m.  Today I got to 3 miles and my Heart rate went sky high and had to stop.

Is there any reason for this?   Or is it just one of them days.

Any one?

Sorry I did not mention.    I was not using my HR belt.   I could feel it in my chest get that really horrible feeling.

If it happens once, it's "just one of those things". If it happens repeatedly, go see your GP sooner rather than later. If you have a HR monitor, may be worth wearing it regularly so that if you do get this again you can take the HR trace with you when you go to see the GP (together with normal traces from runs when it didn't happen, so the GP knows it's not just that you're unfit...).

If it's anything other than just being knackered then I would err on the side of caution and see your GP in case you have an arrhythmia.

If you're sure it's just being knackered and definitely not anything odd then stop reading now.

If not...

High heart rate could be this:

http://en.wikipedia.org/wiki/Supraventricular_tachycardia

Was your HR simply very fast, or was there any fluttering/palpitation? That can sometimes feel super fast as your heart beats out funky disco rythmns instead of the proper one, or there's also a type that makes your heart race.

I had this a few years ago:

http://en.wikipedia.org/wiki/Atrial_fibrillation

It felt very odd, but it was discernable in my pulse (I also got the missus to listen to my heartbeat). Because the heart isn't beating efficiently you'll have less puff and will struggle to run like you normally would.

If you do detect (or suspect) anything like a palpitaiton, deinfitely see your GP ASAP as you'll  be at higher risk of stroke whilst it's palpitating. There are a number of types of arrythmia and they are often treatable with drugs or surgery. The only way to really tell is to reproduce the arrythmia whilst connected to an ECG.