A radiative-convective model is used to compute dynamical heating rates in the polar lower stratosphere. In contrast to previous studies, full annual cycles are considered, and zonal asymmetries in the Arctic are also taken into account. Moreover, the impact of dynamical heating on the polar tropopause is examined. It is found that substantial zonal asymmetries are present in the Arctic lower stratosphere except in summer when heating rates are generally close to zero. Between September and January, the heating rates outside the polar vortex are at least twice as large as in the polar vortex region. Between February and April, when the polar vortex is eroded by stratospheric warmings, zonal asymmetries decrease. However, meridional heating rate gradients are maximum in February and March since the strongest heating occurs at high latitudes. At 100 hPa, heating rates up to 1 K d-1 are found. In the Antarctic, dynamical heating in the lower stratosphere exhibits a double wave with maxima in spring and autumn and minima in summer and winter. The maximum heating rates are substantially lower than in the Arctic and amount to ~0.4 K d-1 at 100 hPa. However, dynamical heating is significantly positive even in summer. The simulations indicate that dynamical heating has a fundamental impact on the tropopause, particularly in the north polar region. There, the annual cycle of the tropopause pressure would be entirely different without dynamical heating.