  We derive a system of moment-based dynamical equations that describe the 1+1d
space-time evolution of a cylindrically symmetric massive gas undergoing
boost-invariant longitudinal expansion. Extending previous work, we introduce
an explicit degree of freedom associated with the bulk pressure of the system.
The resulting form generalizes the ellipsoidal one-particle distribution
function appropriate for massless particles to massive particles. Using this
generalized form, we obtain a system of partial differential equations that can
be solved numerically. In order to assess the performance of this scheme, we
compare the resulting anisotropic hydrodynamics solutions with the exact
solution of the 0+1d Boltzmann equation in the relaxation time approximation.
We find that the inclusion of the bulk degree of freedom improves agreement
between anisotropic hydrodynamics and the exact solution for a massive gas.
