The<unk>  starts at the beginning of the atrial excitation (beginning of the P wave and ends at the beginning of the ventricular excitation (beginning of the QRS complex. It is termed the<unk>  if a Q wave is<unk> esent. The<unk>  measures the time required for an electrical impulse to travel from the atrial myocardium adjacent to the sinoatrial (SA) node to the ventricular myocardium adjacent to the fibers of the Purkinje network.
The duration of the<unk>  is normally from 0.10 to 0.21 s. A major portion of the<unk>  reflects the slow conduction of an impulse through the AV node, which is controlled by the balance between the sympathetic and parasympathetic divisions of the autonomic nervous system. Therefore, the<unk>  varies with the heart rate, being shorter at faster rates when the sympathetic component<unk> edominates, and vice versa. The<unk>  tends to increase with age; 
In childhood:   0.10 - 0.20 sec
In adolescence: 0.12 - 0.16 sec
In adulthood:   0.14 - 0.21 sec
PR interval
QRS complex
ST interval
P wave
PP interval
T wave
V wave
QT interval