Tracing of the electrical activity of the heart. This endosomatic waveform consists of the P, Q, R, S, T, and U waves. The search for diagnostic information in the ECG for PDD purposes has not been fruitful to date. There are preliminary data that suggest that the pre-ejection period (PEP), which is derived from the ECG and the impedance cardiograph (ICG), is a reliable gauge of sympathetic nervous system arousal. Inter-beat interval has also shown promise. See: Kircher, Packard, Bell & Bernhardt (2003).