1. A method for determining the location of a medical device within a body, the method comprising: inserting the medical device into the venous vasculature of the body; measuring an initial ECG waveform using an intravascular ECG electrode after the insertion; advancing the medical device within the venous vasculature of the body; measuring a second ECG waveform using the intravascular ECG electrode after the medical device has been advanced; extracting one or more feature values from the initial ECG waveform and the second ECG waveform, the one or more feature values including one or more initial feature values from the initial ECG waveform and one or more beat-to-beat feature values from the second ECG waveform; determining one or more ratios between the one or more initial feature values and the corresponding one or more beat-to-beat feature values; comparing the one or more ratios with corresponding one or more threshold feature values; and determining whether the device is at a target location based on the comparison of the one or more ratios with the one or more threshold feature values.