Devices and methods obtain and use endovascular electrograms in a number of clinical applications and settings. In one embodiment, a method for locating an indwelling medical device within a vasculature of a patient includes identifying an endovascular ECG waveform complex from an endovascular ECG signal associated with the indwelling medical device, then calculating an absolute value of the energy of the endovascular ECG waveform complex over a predetermined segment thereof. A position of the medical device within the vasculature is then determined by observation of the absolute value of the energy of the predetermined segment of the endovascular ECG waveform complex.