Techniques for determining paced cardiac depolarization waveform morphological templates are described. For example, an implantable medical device (IMD) may sense a cardiac electrogram of a heart, identify cardiac depolarizations within the cardiac electrogram, and determine that the cardiac depolarizations are paced cardiac depolarizations resulting from delivery of a pacing pulse to the heart by another IMD without detecting the pacing pulse and without communicating with the other IMD. The IMD may identify paced cardiac depolarization waveforms of the paced cardiac depolarizations, determine a paced cardiac depolarization waveform morphological template based on the identified paced cardiac depolarization waveforms, determine a normal cardiac depolarization waveform morphological template based on the paced cardiac depolarization waveform morphological template, and compare the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms. The IMD may detect a cardiac tachyarrhythmia based on the above comparison.
