  A hybrid model is put forward for describing relativistic heavy ion
collisions. The early interaction stage responsible for entropy creation is
calculated within the transport Quark-Gluon String Model resulting in an
initial state. The passage to subsequent isoentropic expansion proceeds with
exact account for all conservation laws. Relativistic 3D hydrodynamics with the
mixed phase equation of state is applied to this expansion stage. Essential
differences in evolution of a fireball, described within this model and one for
the Bjorken regime, are noted. The in-medium modified $e^+e^-$ spectra are
studied and confronted with the recent CERES/NA45 data for 8% central Pb+Au
collisions at the bombarding energy 158 AGeV.
