Tachycardia is a rapid heart rate that is most commonly noticed during excitement or exercise, although it can also be caused by medicines, high temperature, sympathetic stimulation, shock, or cardiac disease.
It's characterized by an absence of P, Q, S, or T waves on the ECG, as well as a high intensity of upward or downward deflections.
Fibrillation:
Fibrillation is characterized by a rapid but uncoordinated heart rhythm caused by myocardial regions that relax and contract separately.
If the ventricles are functioning normally, atrial fibrillation is not a serious condition.
Ventricular fibrillation is a life-threatening condition in which blood is not correctly pumped into the body's tissues and lungs.
On the ECG, the waves are unusually uneven, and S and Q waves are absent.