A characteristic response of a medium to an excitation transient of predetermined duration which causes the medium to emit a series of signals over a period of time which is long relative to the duration of the excitation transient is assessed. The container has a front panel and a back panel, each having an exterior surface. In various embodiments, the implantable medical device produces morphological parameters indicative of the likeliness of success of the ATP therapy and selects an anti-tachyarrhythmia therapy mode based on the morphological parameters.