1. A method for determining the location of a portion of a medical device within a body, the method comprising: inserting at least part of the medical device into the venous vasculature of the body; measuring an initial electrocardiogram waveform using an intravascular electrocardiogram electrode following the insertion; advancing the medical device within the venous vasculature of the body; measuring a second electrocardiogram waveform using the intravascular electrocardiogram electrode following the advancement of the medical device; extracting an initial amplitude of the R wave and an initial amplitude of the S wave from the initial electrocardiogram waveform; extracting a beat-to-beat amplitude of the R wave and a beat-to-beat amplitude of the S wave from the second electrocardiogram waveform; determining a first difference between the initial amplitude of the R wave and the initial amplitude of the S wave; determining a second difference between the beat-to-beat amplitude of the R wave and the beat-to-beat amplitude of the S wave; determining a ratio of the first difference between the initial amplitude of the R wave and the initial amplitude of the S wave and the second difference between the beat-to-beat amplitude of the R wave and the beat-to-beat amplitude of the S wave; comparing the ratio with one or more predetermined threshold feature values; and determining whether the portion of the medical device is at the superior vena cava of the body based on the comparison of the ratio with the one or more threshold feature values.