An implantable medical device (IMD) monitors at least one characteristic of P-waves within an electrogram signal. By monitoring changes in the characteristic over time, the IMD detects degradation of the atrial myocardium. In some embodiments, the IMD detects ischemia, an increased probability of the occurrence of angina, or a risk of future atrial fibrillation based on changes in the characteristic over time. The IMD can include a memory to store information relating to the detection and other diagnostic information for retrieval and evaluation by a clinician, and/or an alarm to alert the patient of the detection. In some embodiments, the IMD switches out of an atrial-tracking ventricular pacing mode, decreases the maximum tracking rate for an atrial-tracking ventricular pacing mode, increases the aggressiveness of rate-responsive atrial pacing, and/or initiates or modifies delivery of a therapy, such as a drug or neurostimulation, in response to the identification.