Describe in detail: $ECG$.

(N/A) $ECG$ is a graphical representation of the electrical activity of the heart during a cardiac cycle.
$A$ patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity.
Each peak in the $ECG$ is identified with a letter from $P$ to $T$ that corresponds to a specific electrical activity of the heart.
The $P$-wave represents the electrical excitation (or depolarisation) of the atria,which leads to the contraction of both the atria.
The $QRS$ complex represents the depolarisation of the ventricles,which initiates the ventricular contraction. The contraction starts shortly after $Q$ and marks the beginning of the systole.
The $T$-wave represents the return of the ventricles from an excited state to a normal state (repolarisation). The end of the $T$-wave marks the end of systole.
By counting the number of $QRS$ complexes that occur in a given time period,one can determine the heart beat rate of an individual.
Any deviation in the shape of the $ECG$ indicates a possible abnormality or disease. Hence,it is of great clinical importance.