Actually, a maximum heart rate of 200, at your age isn't too far off the mark for many of the formulas out there.
I can't comment on the presyncope except I'd look to see if it happens again.
It is possibly you had a momentary dip in blood pressure due to any number of factors. This has happened to me on a few occasions when I was young and anemic due to a health problem. If you are not fully hydrated or didn't eat well that morning and having just been sick after a longer break from your regular routine then it could just be your body got overloaded. I would suggest making sure you drink a lot of water whenever you work out. Especially have some right before you start to prevent dehydration. As well if you have stop for a time from exercising then ease into the intense stuff a bit more slowly maybe waiting a few days before going all out. Either way I agree with Tom and suggest you keep tabs on this and if it persists see a doctor. A check up wouldn't hurt either but it sounds a lot like a simple dip in blood pressure (some sort of random vasovagal response) and may have been a transient thing that isn't any concern but do keep an eye on things.
It isn't necessarily a high number of beats per minute that causes insufficiency. If the rhythm is not normal, the heart may pump less blood for the same number of beats.
Some heart rythms are sensitive to exercise. I know I am sure for a maximum stress test myself. We know that mine is triggered by exercise, and are intentionally trying to set it off by doing this.
When I still had long spells of svt, my symptoms matched your description. That too was exercise triggered, occuring after a layoff at about 10 minutes into a workout.
As far as"pre syncope" goes. My SVT Caused my hearing to dim, and I felt light headed. There are several potential causes of syncope. Normal vasovagal syncope comes on slowly, and a person is usually aware of a problem of some type. SVT comes on suddenly, and leaves suddenly. You are generally not aware of a problem until it happens. You feel lousy suddenly, then better almost instantly, and I could swear I felt the blood filling my brain again once it stopped.
Thanks everyone for your responses they've been really helpful!
I think I'll just monitor it at my next few sessions as you say and if it happens again I'll see a doctor. I was panicked as it seemed so extreme for the type of exercise I was doing and I've felt pretty fit for years now so it came as a shock! It probably was a combination of everything as you say that resulted in low blood pressure. I've also been going through an emotional time lately, so could that have had an impact?
Bromley, that's interesting to hear that you have very similar symptoms. I'll see if it happens again and if it follows the same pattern then maybe that's what it is. So many of my family members have problems with their heart, at least 5 I can think from the top of my head! So I'll keep an eye on it.
This happened to me today after only 10mins warming up on the bike. I too am used to doing more than this, often gymming 3/4 times a week. I have been on holiday so havent gymmed for two weeks and you have also mentioned having a break from exercise when this happened. Just wondering if you have had any similar episodes since? Thanks