An implantable medical device includes leads having electrodes that are positioned within a heart. The electrodes sense signals derived from the heart that include waveform segments. The device includes a timing module that determines when the waveform segments cross a threshold and measures time intervals between at least two threshold crossings by the waveform segments. The device also includes event identification module that compares the time intervals to a predetermined pattern associated with a cardiac event. The event identification module identifies the cardiac event based on the time intervals and the predetermined pattern.