Why do stage 2 dengue fever victims need to take blood pressure?

I'm going to have to make a few assumptions here: * I assume you're discussing the Dengue Hemorrhagic Fever (DHF) and not just Dengue Fever (which manifests without the haemorrhagic features). * I further assume you're identifying "stage 2" as the point in DHF, after about 10 days, when hemorrhagic symptoms manifest. If that's the case, hypovolemia is one of the more dangerous effects and presumably could be diagnosed by a lowered BP, at which point you'd consider transfusing serum or perhaps whole blood, as conditions allow. Fluid replacement is key to the patient's survival in this phase. More, while DHS has no direct antagonist, palliative measures are very effective, with CDC saying that untreated DHS has an M&M of nearly 85%, while with treatment, this drops to 1%. DHS is managed by vector control.