Stroke volume has a wide individual variation. There is good evidence that it does not plateau at 40-50% VO2 as previously thought. Also, the work of the heart (and HR)is not really in steady state (nor is the distribution of Q), so these calculations are more difficult than you might imagine. Even if we set this up properly, looking at say the integral of SV, HR, Q etc there would be some debate as to what work rate-duration would provide optimal stimulus for the heart, as there are neurological and systemic, and nutritional factors which contribute to cardiovascular performance and adaptations -- and more is not always better.
http://jap.physiology.org/cgi/pdf_extract/104/1/275?rss=1
There are several other heart variables that have importance in physiology and deal more directly with total work of the heart muscle. You can find those is a good medical physiology text and and a good bit of it in Wasserman's text on exercise testing and prescription. Oxygen pulse is especially interesting. Mean arterial pressure, ejection fraction, and contractile performance/ muscle mass. Also sounds like you'd enjoy reading about O2 desaturation in maximal exercise.
http://www.hinduonnet.com/thehindu/seta/2003/10/23/stories/2003102300050200.htm
http://en.wikipedia.org/wiki/Mean_arterial_pressure