An implantable medical device, the IMD comprising: a housing configured to be implanted in a patient external to a rib cage of the patient; one or more electrodes configured to be disposed at an extravascular location; a sensing module configured to obtain a cardiac electrogram of a heart of the patient sensed using the one or more electrodes; and a tachyarrhythmia detection module configured to: identify a plurality of cardiac depolarizations within the cardiac electrogram; and characterized in that the tachyarrhythmia detection module is further configured to: determine that one or more of the cardiac depolarizations are paced cardiac depolarizations resulting from delivery of a pacing pulse to the heart by another implantable medical device without detecting the pacing pulse and without communicating with the other implantable medical device; identify one or more paced cardiac depolarization waveforms of the one or more paced cardiac depolarizations; determine a paced cardiac depolarization waveform morphological template based on the one or more identified paced cardiac depolarization waveforms; determine a normal cardiac depolarization waveform morphological template based on the paced cardiac depolarization waveform morphological template; compare the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms; and detect a cardiac tachyarrhythmia based on the comparison of the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms. a housing configured to be implanted in a patient external to a rib cage of the patient; one or more electrodes configured to be disposed at an extravascular location; a sensing module configured to obtain a cardiac electrogram of a heart of the patient sensed using the one or more electrodes; and a tachyarrhythmia detection module configured to: identify a plurality of cardiac depolarizations within the cardiac electrogram; and characterized in that the tachyarrhythmia detection module is further configured to: determine that one or more of the cardiac depolarizations are paced cardiac depolarizations resulting from delivery of a pacing pulse to the heart by another implantable medical device without detecting the pacing pulse and without communicating with the other implantable medical device; identify one or more paced cardiac depolarization waveforms of the one or more paced cardiac depolarizations; determine a paced cardiac depolarization waveform morphological template based on the one or more identified paced cardiac depolarization waveforms; determine a normal cardiac depolarization waveform morphological template based on the paced cardiac depolarization waveform morphological template; compare the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms; and detect a cardiac tachyarrhythmia based on the comparison of the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms. determine that one or more of the cardiac depolarizations are paced cardiac depolarizations resulting from delivery of a pacing pulse to the heart by another implantable medical device without detecting the pacing pulse and without communicating with the other implantable medical device; identify one or more paced cardiac depolarization waveforms of the one or more paced cardiac depolarizations; determine a paced cardiac depolarization waveform morphological template based on the one or more identified paced cardiac depolarization waveforms; determine a normal cardiac depolarization waveform morphological template based on the paced cardiac depolarization waveform morphological template; compare the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms; and detect a cardiac tachyarrhythmia based on the comparison of the normal cardiac depolarization waveform morphological template to subsequent cardiac depolarization waveforms.