  We show that by requiring positivity of the longitudinal pressure it is
possible to constrain the initial conditions one can use in 2nd-order viscous
hydrodynamical simulations of ultrarelativistic heavy-ion collisions. We
demonstrate this explicitly for 0+1 dimensional viscous hydrodynamics and
discuss how the constraint extends to higher dimensions. Additionally, we
present an analytic approximation to the solution of 0+1 dimensional 2nd-order
viscous hydrodynamical evolution equations appropriate to describe the
evolution of matter in an ultrarelativistic heavy-ion collision.
