Electrocardiogram (ECG) of a normal heart rate, artwork. An ECG measures the electrical activity of the heart. Contractions are caused by electrical signals between the upper (atrial) and lower (ventricular) chambers. The P wave represents the contraction, or depolarisation, of the atria. The QRS wave complex represents the contraction of the venrticles. The T wave represents the repolarisation, or the relaxing, of the ventricles. The wave appears upward or downwards depending upon whether the electrical activity is going towards or away from the area of the body the electrode is placed on.