I found a lot of studies in the different medical journals.
It turns out that taijiquan is an excellent way to train the whole cardiovascular system. I had read (but can't remember where) that for all round health "because taiji is so slow" you had to add an aerobic exercise form as well—something a little more rigorous. Well, it turns out that this is not quite right.
The studies demonstrate that taiji lowers blood pressure, but during practice raises the pulse to aerobic levels.
So I checked this out. For aerobic exercise take 220 bpm minus your age. That is your maximum safe heart rate. Aerobic rate is then 50-75% of that number. For me that is a range of 82-124. So, my 99 bpm (after a couple minutes slow belly breathing) turns out to be aerobic. My guess is that during the forms the bpm would be somewhat over 100. Of course, that's not as fast heart rate as when I jog or play squash, but it is moderately aerobic, and therefore good for the heart and whole cardiovascular system. The bonus is that taiji is low impact—less chance of running into the wall of the squash court, or damaging knees on pavement—and has so many other health and fitness benefits (flexibility, strength, balance etc).
The studies if you are interested:
Heart rate response during taiji practice.
Changes in heart rate.
Harvard Medical School Study.
These are just a sample of the many.
Play taiji ... be well!