Recent experiments by Jones, Fu, and Gallagher [J. Chem. Phys. 106, 3578 (1997)] designed to mimic collisional effects on the lifetimes of ZEKE Rydberg states find that, in a circularly polarized microwave field, autoionization lifetimes of atomic Rydberg states exceed those in a linearly polarized field. Surprisingly, however, the lifetimes not only do nor display the expected n(5) scaling but are essentially independent of n for most of the states prepared in the experiment. Similar observations were made by Vrakking and Lee [J. Chem. Phys. 102, 8818 (1995)] in studies of NO Rydberg states. We present a classical model that predicts n-independent behavior of autoionization lifetimes over essentially the same range of It-values as in the experiments of Jones, Fu, and Gallagher. The origin of this behavior is traced to the particular combinations of field strengths and frequency used in the experiment.