An implantable medical device detects a tachyarrhythmia of a heart. During the detected tachyarrhythmia, the device determines a local myocardial impedance. Using the local myocardial impedance, the device determines whether there is sufficient perfusion to the heart. The device can then either deliver a less aggressive device therapy in response to the detected tachyarrhythmia when there is sufficient perfusion to the heart, or deliver a more aggressive device therapy in response to the detected tachyarrhythmia when there is insufficient perfusion to the heart. The perfusion information can also be used to alter tachyarrhythmia detection or classification.