I am a nurse, and with my postpartum patients I start worrying about what their blood pressure is doing when they start getting tachycardic....I almost ALWAYS see a significant drop in blood pressure (to which I start watching to make sure they are not bleeding too much).
The relationship between blood pressure and heart rate should be inverse, meaning that as BP drops, HR increases in order to maintain a constant cardiac output. For example, in patients who are bleeding out (less blood=less pressure being exerted by it=less cardiac output per beat), they have a compensatory rise in heart rate as a means of trying to maintain their cardiac outputs (which is the volume of blood pumped per minute....if the volume per beat decreases, the number of beats must increase to maintain that output). That works for awhile until of course, the heart CAN'T beat any faster...which is why replacing fluids is necessary.
I'm sure I made that relationship about clear as mud, huh?