The problem of predicting the performance of Time-Reversal Transmissions (TRT) in a time-varying complex medium is addressed in this paper. The loss of coherence in the propagation medium and its nominal energy contrast are proven sufficient to predict the average loss of quality in received signals. In particular, it is shown how a perturbation in the medium affects in a different way coherent focusing and background fluctuations. In the extreme case of a diffusive medium (e.g., Rayleigh channels), the fluctuations intensity is unaltered. The predictions of the proposed theoretical models are validated against experimental results measured in a reverberation chamber , where a mechanical paddle (stirrer) acts as the source of perturbation in the medium. It is confirmed that, depending on the medium statistics, perturbations can have a varying impact on TRT performance.