According to the sources, an electrocardiogram (ECG) is a graphical representation of the heart's electrical activity during a cardiac cycle . The P-wave represents the electrical excitation (depolarisation) of both atria, which leads to their contraction . The QRS complex represents the depolarisation of the ventricles, which initiates ventricular contraction . Crucially, the sources state that by counting the number of QRS complexes in a given time period, one can determine the heartbeat rate, which corresponds to the pulse . The T-wave represents ventricular repolarisation (return to a normal state), marking the end of systole . Mastering these wave interpretations is a high-priority strategy for medical aspirants, as NCERT textbooks serve as the definitive source for approximately 95% of NEET Biology questions .